MECHANISMS OF COEXISTENCE BETWEEN TWO OCTOPUS SPECIES IN
A SOUTH FLORIDA LAGOON
by
Chelsea Bennice
A Dissertation Submitted to the Faculty of
The Charles E. Schmidt College of Science
In Partial Fulfillment of the Requirements for the Degree of
Doctor of Philosophy
Florida Atlantic University
Boca Raton, FL
May 2019
Copyright 2019 by Chelsea Bennice
ii MECHANISMS OF COEXISTENCE BETWEEN TWO OCTOPUS SPECIES IN
A SOUTH FLORIDA LAGOON
by
Chelsea Bennice
This dissertation was prepared under the direction of the candidate's dissertation advisor, Dr. W. Randy Brooks, Department of Biological Sciences, and has been approved by all members of the supervisory committee. It was submitted to the facu1ty of the Charles E. Schmidt College of Science and was accepted in partial fulfillment of the requirements for the degree of Doctor ofPhilosophy.
SUPER~ORY ~E: f;V.r~ W. Randy Brooks, Ph.D. Disserta · Advisor ._,l£11111r~. 4~~ Roge:7 anion, Ph.D. ~Q
Colin Hughes, Ph.D.
ta.J!mtlrR"' ~Cimi, Ph.D. ean, The Charles E. Schmidt College of Science
/}WeJ, t1.,. l 1d!'J Khaled Sobhan, Ph.D. Date Interim Dean, Graduate College
iii ACKNOWLEDGEMENTS
I express sincere gratitude to my committee members for all of their guidance and support, and special thanks to my major advisors for their persistence, patience, and encouragement during the completion of my PhD project. I thank Florida Atlantic
University’s scientific divers for accompanying me on research dives throughout this study; specifically, Liana Houston, Jessica Pate, Rachel Shanker, Danielle Bartz, Jeanette
Wyneken, and Marianne Porter. Thank you to FAU’s diving and boating safety program for providing SCUBA tanks and compressed air. Laz Ruda discovered Macrotritopus defillippi at BHB and invited Roger Hanlon to dive this site, which led to this research project. I am grateful for funding from the Broward Shell Club, Palm Beach Fishing
Club, and the Animal Behavior Society. Underwater photographs and octopus observations were also acquired from SCUBA divers: Sandra Edwards, David Sanchez,
Anne DuPont and Linda Ianniello. We are grateful to Stephanie Farrington for her guidance on CPCe software, Andrew Rayburn for his guidance on spatial statistics, and
Conor Sullivan for the development of the octopus 24-h camera. Thank you to my family and friends for their love and support.
iv ABSTRACT
Author: Chelsea Bennice
Title: Mechanisms of coexistence between two octopus species in a South Florida Lagoon
Institution: Florida Atlantic University
Dissertation Advisor: Dr. W. Randy Brooks
Degree: Doctor of Philosophy
Year: 2019
Theoretically, sympatric species must partition resources or space to allow for coexistence. Determining empirically the specific resources each species exploits and species’ interactions (e.g., intra- and interspecific competition) can sometimes be challenging, thus the data are relatively sparse for certain taxa. This paucity of data exists for octopuses. Therefore, I chose to study niches of two sympatric octopuses (Octopus vulgaris and Macrotritopus defilippi) in an intracoastal habitat. Specifically, I assessed
(1) spatial distribution of octopus home or “den” space, (2) habitat association, (3) octopus abundance, (4) foraging activity periods, (5) diet, and (6) associated substrates and behaviors used during foraging events. Octopus den locations were marked by GPS to quantify spatial patterns of both species and their
v spatial relationship to each other. Habitat associations were measured by quantifying photoquadrats of den and surrounding habitats. For foraging activity periods, a video camera was placed near an octopus den for 24-h observation to determine when each octopus species leaves/returns from foraging. Underwater video recording was used to determine associated foraging substrates and behaviors for both species. Prey remains from octopus’ dens and video recordings indicating prey consumption were used to determine diets of the two octopus species. Video recordings from the 24-h camera and foraging behavior events also provided observations of intra- and interspecific interactions. Results revealed that the two species are interspersed throughout the shallow Florida lagoon and are both abundant during the spring months
(March, April, May). Although both species are interspersed throughout the lagoon, their den and surrounding habitat association differed. O. vulgaris was associated with hard bottom and M. defilippi was associated with soft bottom, thus they may not compete strongly for habitats. Each species used different foraging strategies and different primary prey, which may also lessen competition and facilitate coexistence. O. vulgaris had peak foraging activity during night hours, foraged mostly on hard bottom and mainly consumed bivalves while M. defilippi had peak foraging activity during day hours, foraged mostly on soft bottom and mainly consumed crustaceans. Octopuses also had species-specific foraging behaviors, with O. vulgaris using parachute attack and M. defilippi using flounder swimming and tripod stance. Additional intra- and interspecific interactions were video recorded and included: fishes following octopuses, predation attempts, agonistic encounters, cannibalism, and tactile communication. This study identified ecological and behavioral components that may facilitate coexistence of these
vi sympatric species, provided insight into cephalopod niches and ecology, and provided baseline conservation requirements for sand-dwelling cephalopods, both of which may be using this site as a mating and nursery habitat.
vii MECHANISMS OF COEXISTENCE BETWEEN TWO OCTOPUS SPECIES IN
A SOUTH FLORIDA LAGOON
LIST OF TABLES ...... xi
LIST OF FIGURES ...... xii
INTRODUCTION ...... 1
General overview of species coexistence and niche partitioning ...... 1
Cephalopod niche partitioning ...... 3
CHAPTER I: FINE-SCALE HABITAT PARTITIONING FACILITATES SYMPATRY
BETWEEN TWO OCTOPUS SPECIES IN A SHALLOW FLORIDA LAGOON ...... 5
ABSTRACT ...... 5
INTRODUCTION ...... 6
MATERIALS AND METHODS ...... 8
Description of species ...... 8
Study site ...... 8
Spatial distribution ...... 9
Habitat association ...... 10
Octopus abundance ...... 13
RESULTS ...... 13
Spatial distribution ...... 14
Habitat association ...... 15
viii Octopus abundance ...... 16
DISCUSSION ...... 17
CHAPTER II: OCTOPUS COEXISTENCE FACILITATED BY FORAGING AND
DIET STRATEGIES IN A SOUTH FLORIDA LAGOON ...... 30
ABSTRACT ...... 30
INTRODUCTION ...... 31
MATERIALS AND METHODS ...... 32
Description of species ...... 32
Study Site ...... 33
Foraging activity periods ...... 33
Diet ...... 35
Foraging strategies (foraging substrates & behaviors) ...... 36
RESULTS ...... 39
Foraging activity periods ...... 39
Diet ...... 40
Foraging strategies (substrates & behaviors used during foraging events) ...... 41
DISCUSSION ...... 44
CHAPTER 3: IN SITU VIDEO RECORDINGS REVEAL RARE BEHAVIORAL
INTERACTIONS FOR TWO SYMPATRIC OCTOPUS SPECIES IN A SHALLOW
FLORIDA LAGOON ...... 61
ABSTRACT ...... 61
INTRODUCTION ...... 61
DESCRIPTION OF INTERACTIONS ...... 62
ix Fishes following octopuses ...... 63
Predation attempts ...... 63
Agonistic encounters ...... 64
Cannibalism ...... 66
Tactile communication ...... 66
DISCUSSION ...... 66
APPENDIX I ...... 78
Permission letter ...... 78
REFERENCES ...... 79
x LIST OF TABLES
Table 1: Number of dive hours spent searching for octopuses in the Blue Heron
Bridge study site...... 23
Table 2: Octopus foraging behaviors and foraging substrates ...... 52
Table 3: Average percentage of time for each of the eleven foraging behaviors used
during foraging for Octopus vulgaris and Macrotritopus defilippi...... 53
Table 4: Diet composition for Octopus vulgaris and Macrotritopus defilippi...... 54
xi LIST OF FIGURES
Figure 1: Two sympatric octopus species at Blue Heron Bridge, Phil Foster Park
within the Lake Worth Lagoon, FL...... 24
Figure 2: Blue Heron Bridge (BHB), Phil Foster Park, Lake Worth Lagoon, FL. .... 25
Figure 3: Octopus kernel density maps at Blue Heron Bridge based on occupied den
GPS data for individual years 2014-2016 ...... 26
Figure 4: Percent substrate category for den habitat for Octopus vulgaris,
Macrotritopus defilippi, and general substrate composition for Blue Heron
Bridge ...... 27
Figure 5: Percent substrate category for surrounding habitat for Octopus vulgaris,
Macrotritopus defilippi, and general substrate composition for Blue Heron
Bridge...... 28
Figure 6: Average seasonal octopus abundance for Octopus vulgaris and
Macrotritopus defilippi...... 29
Figure 7: Illustrations of assembled 24-h octopus monitoring gadget ...... 55
Figure 8: Average percentage hourly foraging activity and diurnal verus nocturnal
average percentage activity for Octopus vulgaris and Macrotritopus
defilippi ...... 56
Figure 9: Average percentage of time spent foraging across four substrate categories
for Octopus vulgaris and Macrotritopus defilippi ...... 57
xii Figure 10: Average percentage of time spent using behavior during foraging for
Octopus vulgaris and Macrotritopus defilippi...... 58
Figure 11: Average percentage of time Octopus vulgaris used foraging behaviors
with related substrates...... 59
Figure 12: Average percentage of time Macrotritopus defilippi used foraging
behaviors with related substrates ...... 60
Figure 13: Predation attempts on octopuses recorded using a 24-h camera...... 72
Figure 14: Octopus intra- and interspecific agonistic encounters...... 74
Figure 15: Cannibalism by Macrotritopus defilippi ...... 76
Figure 16: Octopus intra- and interspecific tactile communication ...... 77
xiii INTRODUCTION
General overview of species coexistence and niche partitioning
Mechanisms responsible for species’ coexistence have remained a central question in evolutionary biology and ecology. Species coexistence is important for understanding biodiversity and community structure and, therefore, is the conceptual framework for much of community ecology (Chesson 2000, Siepielski & McPeek 2010).
In a community, many species interact with one another either directly (e.g., via interference competition, predation, parasitism, mutualism) or indirectly (e.g., altering abiotic conditions, resource competition, apparent competition) (Siepielski & McPeek
2010). A species has a niche that refers to the environmental requirements (abiotic and biotic resources) of that species (Schoener 1974, Pulliam 2000). Mathematical models
(Lokta-Volterra equations) provided the framework for field tests of competition and suggested that species cannot coexist for long if they are complete competitors (principle of competitive exclusion), thus species must exploit different resources for stable coexistence (niche partitioning hypothesis) (Gause 1934, Hardin 1960, Gotelli 2008).
Niche (or resource) partitioning mechanisms have gained much support for being responsible for lessening competition between species; thus facilitating ecological coexistence.
Coexistence among competing species may be a common phenomenon (Gravel et al. 2011, Boeye et al. 2014); however, environments are constantly changing and it is crucial to determine mechanisms of coexistence to protect biodiversity. The three most
1 important niche dimensions are habitat, diet, and activity time (Vieira & Port 2007,
Ashrafi et al. 2011, Ashrafi et al. 2013). Mechanisms for resource partitioning can include: (1) habitat partitioning, (2) temporal partitioning, (3) diet partitioning.
Habitat partitioning is viewed as one of the major mechanisms of coexistence, which can result from spatial and structural partitioning (i.e., habitat heterogeneity) of the habitat and can occur at different scales (e.g., microhabitat partitioning) (Schoener 1974,
Jones et al. 2001). Additional abiotic factors such as water velocity and depth of water
(Bergeron & Bourget 1986, Weir et al. 2009, Connan et al. 2014) can also drive habitat partitioning.
Temporal partitioning can occur when two species have different diel cycles, when there is a change in species’ seasonal abundance, when species shift diets depending on food availability, or when prey species have different activity times
(Schoener 1974, Gutman & Dayan 2005, Fossette et al. 2017).
Diet partitioning could result from two or more species consuming different prey species or may result from behavioral differences in foraging strategies or feeding biology or anatomical differences, which allows species to forage in different habitats or at different trophic levels (Young & Winn 2003, Weir et al. 2009, Voight 2013, Albo-
Puigserver et al. 2015). Closely related, sympatric species can coexist by utilizing one or multiple niche partitioning mechanisms, or by having high overlap in one niche dimension and low overlap in another (i.e., niche-complementarity hypothesis) (Jimenez et al. 1996, Platell et al. 1998, Barnes 2002, Vieira & Port 2007, Fossette et al. 2017).
2 Cephalopod niche partitioning
Niche partitioning mechanisms have been well studied in a wide range of taxa; however, certain taxa have received little attention, including cephalopods (coleoids: cuttlefishes, squids, and octopuses). Related field research has focused on adaptive life history traits or strategies that influence trophic interactions (Aronson 1986, Aronson
1991, Mather & Odor 1991, Staudinger et al. 2011, Staudinger et al. 2013) behavioral ecology (Ambrose 1984, Ambrose 1986, Forsythe & Hanlon 1988, Leite et al. 2009a,
Leite et al. 2009b), and camouflage (Hanlon et al. 2009, Allen et al. 2014). Most cephalopod studies focus only on a single species; however, more than one octopus species can often be found inhabiting an area.
Octopuses live in a diversity of habitats ranging from coral reefs, seagrass beds, sand plains to polar ice regions, grazing on lower trophic levels (shelled mollusks, crabs, polycheates) and are often the main food source for top predators (Young & Winn 2003,
Taylor & Bennett 2008). It is important to understand the underlying mechanism(s) for octopus coexistence because of their importance in many marine food webs, and because such information can provide additional support or novel information to cephalopod and community ecology literature and conservation strategies.
Similar morphology and ecological requirements among various species of shallow-water octopuses suggest that competition for resources (e.g., habitat, prey, and space for den sites) influence their distribution, density, and overall coexistence. For example, three sympatric species of shallow-water Hawaiian octopus may persist due to differences in temporal spacing and microhabitat preferences (Houck 1982). Activity rhythms of the three species were examined in a laboratory and compared to field
3 observations; however, the number of field observations was not expressed and only descriptions of activity levels and microhabitats were given. Coexistence of Octopus vulgaris, Octopus briareus, Callistoctopus macropus, and Amphioctopus burryi on
Caribbean coral reefs was documented from descriptive observations of different foraging behaviors, prey, den types, and temporal spacing (Hochberg & Couch 1971,
Hanlon 1988). Feeding morphologies, stomach contents, and stable isotopes of deep-sea octopuses demonstrated diet partitioning thereby allowing increased octopus species diversity (Voight 2013, Valls et al. 2017). Three sympatric, sand-dwelling mimic octopus species (Thaumoctopus mimicus, ‘Blandopus’ or white V octopus, Octopus sp. 18’, and
Wunderpus photogenicus) were observed to overlap greatly in their niche requirements
(foraging, habitat, activity period) (Norman et al. 2001, (Hanlon et al. 2008); however, their mechanisms of coexistence have not been thoroughly examined.
These octopus coexistence studies were either conducted in the laboratory or based on descriptive field observations of potential resource partitioning mechanisms with little empirical data (with an exception of Voight 2013, Valls et al. 2017). Another study mentioned multiple octopus species in one location, but did not investigate mechanisms of species coexistence (Anderson et al. 2008). To help advance the understanding of species’ coexistence, I have executed an in situ, robust study on octopus coexistence that examined mechanisms of coexistence in the following three chapters.
4 CHAPTER I: FINE-SCALE HABITAT PARTITIONING FACILITATES
SYMPATRY BETWEEN TWO OCTOPUS SPECIES IN A SHALLOW FLORIDA
LAGOON
ABSTRACT
Species coexistence is a critical determinant of biodiversity and community structure, yet resource partitioning mechanisms that facilitate coexistence remain understudied for many taxa, including cephalopods. Octopus vulgaris and Macrotritopus defilippi cohabit a shallow-water lagoon in South Florida. Temporal and spatial distribution as well as habitat association were examined as potential resource partitioning mechanisms to facilitate coexistence between these species. Methods included in situ visual observations, marking locations of octopus-occupied dens, and photoquadrats of octopus’s den and surrounding habitats. Octopus den locations were marked year-round for 3 consecutive years to determine consistency of spatial distribution and temporal trends. Octopus abundance was the highest during spring and lowest during fall for both species, indicating that temporal partitioning (in terms of seasonality) is likely not a mechanism of coexistence. O. vulgaris and M. defilippi had the highest densities of occupied dens in the same general shallow areas of the lagoon, thus there was no evidence of spatial partitioning. Although octopuses spatially overlapped in the same general area, multiple substrate categories were available and each species’ den and surrounding habitat were associated with different substrates. O. vulgaris was associated mostly with hard bottom and inhabited hard-structured dens
5 while M. defilippi was associated with soft sandy bottom and inhabited burrows in the sand as dens. Fine-scale habitat partitioning is made possible by this lagoon’s heterogeneous microenvironments, which aid in explaining coexistence of these two octopus species.
INTRODUCTION
In studies concerned with the coexistence of similar sympatric species, the importance of analyzing habitat type and spatial distribution patterns has been emphasized (Edington & Edington 1972). Also, information about relative abundance and fluctuations in biomass are important for understanding the ecology of species
(Gordon 2000, Kaiser et al. 2011). Here we examined coexistence of two octopus species
(O. vulgaris and Macrotritopus defilippi) in a South Florida shallow-water environment for three consecutive years by determining each species spatial distribution, den and surrounding habitat, and seasonal abundance (Fig. 1).
O. vulgaris is found in sand, rock, rubble, seagrass, and coral reef environments.
It generally makes its home or “den” in a hole in the substrate littered with shells or by excavating sand under a boulder (Woods 1965, (Mather 1991a, Mather & Odor 1991,
Meisel et al. 2006, Anderson et al. 2008, de Beer & Potts 2013, Guerra et al. 2014,
Mehner et al. 2014). O. vulgaris is known to feed mainly on crustaceans, bivalves, and gastropods (depending on geographical location) and has varying activity patterns in different habitats (Woods 1965, Meisel et al. 2013, Hanlon & Messenger 2018). Off the east coast of South Africa, O. vulgaris’ laboratory and field observations suggested that mating, gonad maturation, and egg laying lack seasonality; however, there was a suggestion for seasonality in female maturation and sex ratio (Smale & Buchan 1981).
6 M. defilippi is known to inhabit sandy plains, making a new or pre-existing sand burrow its den, but substrate information about this species’ den and microhabitat have never been quantified. It can also bury directly into the sand and use flounder mimicry to escape predators (Hanlon 1988, Hanlon et al. 2010). These species overlap in geographical range; however, their habitat coexistence has not been studied. The coexistence of O. vulgaris and M. defilippi (and two others: O. filosus and O. briareus) was documented in Bonaire, but the focus of the study was the diet of O. vulgaris
(Anderson et al. 2008). Although there are habitat differences between these two species, there is potential habitat overlap because O. vulgaris is found in an array of environments. The habitat of O. vulgaris has not been studied at this South Florida lagoon and has never been studied in the presence of M. defilippi. Also, there have been few studies of M. defilippi, and little is known about the ecology (spatial distribution, activity time, diet, foraging behaviors) and intra- and interspecific octopus interactions of this species.
We posed the following questions: (1) Do the two species overlap temporally? (2)
Do they overlap spatially? (3) If the latter, are there differences in habitat associations between the two species? If species are abundant at different times of the year, we would hypothesize the potential use of temporal partitioning to lessen competitive interactions.
If species are abundant during the same time of year, we would hypothesize within- species aggregation, but also between species over-dispersion (spatial partitioning). If species overlap spatially, we would hypothesize species to associate with different habitat compositions (habitat partitioning).
7 MATERIALS AND METHODS
Description of species
Octopus vulgaris has a world-wide geographical distribution that includes subtropical and tropical waters (Warnke et al. 2004). This is a medium-large octopus species that can weigh up to 5kg. The average mantle length (ML) of O. vulgaris is
250mm. This species has stout arms (equal length and thickness) with an arm length of 3-
5x times its ML. It has highly variable body patterns often exhibiting a reddish-brown reticulated pattern (Hanlon 1988, Humann & DeLoach 2013). Macrotritopus defilippi has been documented in the Caribbean, Atlantic Ocean, and Mediterranean (Hanlon et al.
2010, Crocetta et al. 2015). M. defilippi is a small-medium sized octopus with a ML approximately 90mm. A specimen collected in the Canary Islands weighed 50g with a
ML 41mm (Guerra et al. 2013). This species has long, slender arms with an arm length up to 6x its ML. Its most distinguishing characteristics are its dark bars and white spots down each arm, long narrow mantle, very long arms, and small head with protruding eyes
(Hanlon 1988, Humann & Deloach 2013). These characteristics were used to aid in species identification. From field observations, ML size was estimated to range from
12mm-178mm for O. vulgaris and 12mm-90mm for M. defilippi. Majority of octopuses observed were not at the extreme ends of reported size estimates.
Study site
Spatial distribution, habitat association, and relative abundance were examined at
Blue Heron Bridge (BHB), within the Phil Foster Park portion of the Lake Worth
Lagoon, Riviera Beach, FL, USA (Fig. 2). The study site has a heterogeneous benthic environment, including mainly sandy plains, but also rock, shell, rubble substrate,
8 anthropogenic materials (e.g., glass bottles, cans, cement blocks, pipes, and sunken boats), and a mean depth of 3m. Water visibility at BHB is heavily influenced by the tidal cycle because of its close proximity to the Atlantic Ocean via the Palm Beach Inlet. The majority of octopus observations took place within 1-2 h of high, slack tide to minimize tidal current (which often peaked at 0.77m/sec) and maximize water visibility and, thus, opportunities of locating these cryptic animals. Observations were made at high slack tides that were between 6:00-21:00. In a few rare cases, observations were done at low tide if water visibility was ³ 3m to increase chances of locating octopuses.
Spatial distribution
Visual census during SCUBA dives was used to locate occupied dens and collect habitat association data. This method is commonly used to measure abundance of benthic animals and other ecological aspects of various octopus species (Aronson 1986,
Katsanevakis & Verriopoulos 2004a, b). For maximal search coverage, the study site was divided into three areas relative to the direction of BHB: southwest, south, and southeast
(Fig. 1). Each dive was designated to one of these areas, and north-south swim paths were used to survey the entire area for octopus-occupied dens. The numbers of dives in each area were kept approximately the same to ensure equal amounts of search time. Search time or dive hours were kept approximately the same for each dive. Two to four dives per week were conducted year-round between January 2014 and December 2016. Spatial distribution of occupied dens was recorded over three years (2014, 2015, 2016) to determine if intra- and interspecific patterns were consistent across years.
Once an occupied den was located, the latitudinal and longitudinal coordinates
(decimal degrees) were recorded on a Garmin eTrexÒ 10 GPS device that was kept in a
9 dive float. ArcGIS 10.2 was used to overlay occupied octopus den GPS points for visual representation of octopus spatial distribution. Spatial data on occupied dens were analyzed using base functions and the spatstat package (Baddeley & Turner 2005) in program R (R Core Team 2017). First, kernel density maps were created as a data exploration method to depict any areas of high or low octopus density for both species.
Next, for each year, univariate Ripley’s K functions (Ripley 1981) were used to test for non-random spatial patterns (intraspecific aggregation or over-dispersion) up to 30 m for each octopus species independently, and bivariate Ripley’s K functions were used to test for significant spatial interactions between species (interspecific aggregation or over- dispersion). Ripley’s K is a standard second-order spatial statistic that evaluates the number of points within a certain distance I of a randomly chosen point relative to expectations based on the density of points in the study area. Significant deviations of the
K-statistic indicate either regularity or aggregation at scale r in a spatial point pattern dataset. To stabilize variance and aid in interpretation, K-statistics were square-root transformed to L-statistics (Besag 1977) as is common practice. Monte Carlo permutation procedures (Nsim=999; Ripley edge correction) were implemented to generate simulation envelopes that allowed for detection of non-random univariate and bivariate den spatial patterns.
Habitat association
After locating and recording an occupied den, a photoquadrat was used to collect substrate composition data for each octopus species’ den and surrounding habitat. The quadrat (0.13 m2) was first placed directly over the octopus’s den, which was defined as
“den habitat.” A Canon Powershot D20 camera was used to record the substrate within
10 the quadrat. In addition to the single den photoquadrat, 8 additional photoquadrats were collected around the octopus’s den (~1 m2) and that area was defined as the octopus’s
“surrounding habitat” (total 9 photoqudrats). Thirty occupied dens were sampled for each species to determine possible significant differences in habitat association between
O. vulgaris and M. defilippi for den and surrounding habitats. Photoquadrat samples were recorded from octopuses of similar size. Along with comparing habitats between species, we also wanted to determine if the octopus’ habitat differed from the general substrate composition of the BHB study site (i.e., if an octopus species selected an area of BHB where only a specific substrate was located and not generally represented). Therefore, thirty random samples of the BHB study site, defined as “BHB,” were collected using the same methodology as den (1 photoquadrat) and surrounding (9 photoquadrat) octopus habitat. Random GPS locations for BHB sampling were generated in ArcMap and were made sure to be distributed throughout the study site to allow the most accurate representation of BHB’s general substrate composition.
Percent substrate composition was calculated using Coral Point Count with Excel extensions (CPCe) software for each species’ den (one photoquadrat per sample), surrounding habitat (nine photoquadrats per sample) habitat, and BHB random substrate samples (one and nine photoquadrats per sample) (Kohler & Gill 2006). To determine percent substrate category of each species habitat and BHB, four major substrate categories were defined for CPCe code: hard bottom (rock, rubble, shells), soft bottom
(sand), human debris (referring to anthropogenic materials such as aluminum cans, glass bottles, cement blocks, pipes) and fauna and flora (e.g., algae, sponges, hydroids, seagrasses). Each photo was overlaid with 50 random points for substrate coding to
11 achieve an accurate frequency for each substrate category for den habitat, surrounding habitat, and BHB (Pante & Dustan 2012). Each random point was coded with a specific substrate category and a percentage for each substrate category was calculated. For surrounding habitat, the percent substrate category was averaged for the nine photos. This was also done for the nine photos at each of the thirty BHB random substrate sample locations.
All analyses for octopus habitat association were conducted in the ecological software program PRIMER-7 (Clarke & Gorley 2015). Similarity between octopus species’ habitat association (den habitat and surrounding habitat) and substrate composition of BHB were analyzed using a resemblance-based permutation test Analysis of Similarities (ANOSIM) due to data not meeting multivariate analysis of variance
(MANOVA) test assumptions. ANOSIM was applied using a Bray-Curtis similarity coefficient for the pair-wise similarity matrix. Data were square-root transformed because samples were dominated by two substrate categories (soft bottom and hard bottom). The square-root transformation allows the less common categories (human debris, fauna and flora) to also contribute to the similarity metric. Since the ANOSIM test is sensitive to heterogeneity of multivariate dispersion, a square-root transform was also appropriate to decrease heterogeneity of dispersion (Anderson & Walsh 2013). Homogeneity of multivariate dispersion was met for den habitat and surrounding habitat similarity data
(PERMDISP; p = 0.634, p = 0.139, respectively). ANOSIM provides a global permutation based test that reports both overall probabilities of differences between groups (O. vulgaris, M. defilippi, BHB) and probabilities between group pairs. For groups that were significantly dissimilar, an Analysis of Similarity Percentages
12 (SIMPER) was conducted to determine the relative contribution of each substrate type to dissimilarity between groups and similarity within groups.
Octopus abundance
Octopus-occupied den counts were used as an estimate of octopus abundance to determine if there was seasonal variation of octopus abundance within and between species. Octopus-occupied den counts were grouped into one of four seasonal categories
(winter, spring, summer, fall). Seasons were defined as winter (December, January,
February), spring (March, April, May), summer (June, July, August), and fall
(September, October, November). We did not resample the same den location twice for any year and we assumed these recorded dens were by different individuals. A Chi-
Square statistic (significance level set at alpha = 0.05) of a 2 x 4 contingency table
(octopus species x seasonal categories) was used to determine seasonal variation in octopus abundance within and between species for each year (2014-2016).
RESULTS
In total, 249 SCUBA dives (371.4 dive hours) were conducted over three years
(2014-2016). The average number of dives per month was 7.0 (S.D. ± 2.3 dives). The number of monthly dives was lower when there was poor water visibility, poor water quality (high levels of indicator bacteria), or severe weather conditions (tropical storms or hurricanes). Although total dive hours varied year to year, diving remained relatively consistent between seasons for each year (see Table 1 for a breakdown of dive hours).
Four hundred thirty-four occupied dens were found: 296 for Octopus vulgaris and 138 for
Macrotritopus defilippi. For breakdown by year, in 2014 a total of 80 O. vulgaris and 23
M. defilippi occupied dens were recorded. For 2015, 134 O. vulgaris and 44 M. defilippi
13 dens were recorded, and for 2016 there were 82 O. vulgaris and 71 M. defilippi occupied dens recorded. Unoccupied octopus dens were not counted because they could be used by a variety of other species. Also, other species make similar homes (e.g., mantis shrimp hole in sand) that could be mistaken as an octopus den if an octopus was not present.
Spatial distribution
Pooled across years, most occupied dens for both species were found at the southwest and southeast areas of Blue Heron Bridge (Fig. 2). M. defilippi had more occupied dens scattered throughout the south sandy area than O. vulgaris. Kernel density maps illustrate the fine-scale distributions and densities of occupied dens for each year and for each species (Fig. 3). Occupied den density for O. vulgaris across the study area ranged from 0.0–13.8 dens/1000 m2 in 2014, from 0.0–18.1 dens/1000 m2 in 2015, and from 0.0–15.0 dens/1000 m2 in 2016. Occupied den density for M. defilippi across the study area ranged from 0.0–4.8 dens/1000 m2 in 2014, 0.0–6.6 dens/1000 m2 in 2015, and
0.0–14.7 dens/1000 m2 in 2016. Over three years, there was a visual trend of higher occupied den density in southwest and southeast areas for both species.
For all three years, occupied dens for O. vulgaris were significantly aggregated at scales ³ 2.0 m, especially on the southwest end. Conversely, the spatial patterns of M defilippi occupied dens were statistically random except in 2016 where M. defilippi dens were significantly aggregated at scales ³ 2.0 m. There was no evidence of significant interspecific aggregation or over-dispersion between occupied dens of the two species in any year.
14 Habitat association
Den habitat for O. vulgaris was composed of all four substrate categories and had a larger percentage of hard bottom, human debris and fauna and flora than den habitat for
M. defilippi. Den habitat for M. defilippi was mainly soft bottom with a low percentage
(or zero) of the other substrate categories (Fig. 4). In areas inhabited by both species, there were patches of both sand and hard substrate. Although the main substrate category for both species’ den habitat and general substrate composition of BHB was soft bottom, there was a significant difference between groups (octopus species’ den habitat and BHB)
(ANOSIM R = 0.149, p = 0.001). Pairwise tests revealed significant differences between
O. vulgaris and M. defilippi for den habitat (R = 0.264, p = 0.001) and between O. vulgaris den habitat and BHB (R = 0.221, p = 0.001). There was no significant difference between M. defilippi den habitat and BHB (R = - 0.017, p = 0.913). Soft bottom was the largest contributor for similarity among the three groups (SIMPER, O. vulgaris 60%, M. defilippi 93%, and BHB 88%). Hard bottom was the largest contributor of dissimilarity
(37%) between the species’ den habitats and between O. vulgaris and BHB (36%)
(SIMPER, Fig. 4).
Similar trends were seen for both species’ surrounding habitat with O. vulgaris associated with a larger percentage of hard bottom, human debris, and fauna and flora than M. defilippi. Yet for O. vulgaris’ surrounding habitat, there was a decrease in hard bottom and human debris and an increase in soft bottom. Surrounding habitat for M. defilippi still had a larger percentage of soft bottom (Fig. 5). Results for general BHB substrate composition (9 photoquadrats/ BHB sample) remained almost the same as previously reported (1 photoquadrat/ BHB sample). There was a significant difference
15 between groups for octopus species’ surrounding habitat and BHB (ANOSIM, R = 0.131, p = 0.001). Pairwise tests revealed significant differences between O. vulgaris and M. defilippi for surrounding habitat (R = 0.253, p = 0.001) and O. vulgaris surrounding habitat and BHB (R = 0.172, p = 0.001). There was no significant difference between M. defilippi surrounding habitat and BHB (R = - 0.003, p = 0.480). Again, soft bottom was the major contributor of similarity for each group (SIMPER, O. vulgaris 67%, M. defilippi 89%, BHB 85%) and hard bottom contributed most (46%) to differences in surrounding habitat between octopus species and between O. vulgaris and BHB (43%)
(SIMPER, Fig 5).
Octopus abundance
Both species showed a seasonal trend of highest octopus abundance during spring, then decreasing abundance through summer and fall, followed by an increase again during the winter (Fig. 6). There was among-season variation in octopus abundance between years, yet the trend was similar for all three years. There was no difference in seasonal octopus abundance between species for years 2014 (X2 = 4.804, p = 0.187) and
2016 (X2 = 5.098, p = 0.165 ) and only a slight difference in 2015 (X2 = 8.548, p =
0.036). A seasonal change in octopus abundance within species was detected for O. vulgaris and M. defilippi in 2014 (X2 = 43.900, p < 0.0001; X2 = 15.9783, p = 0.0013, respectively), 2015 (X2 = 44.806, p < 0.0001; X2 = 15.000, p = 0.0011, respectively), and
2016 (X2 = 39.444, p < 0.0001; X2 = 71.704, p < 0.0001, respectively). During the spring, there were single 1-2 hr. dives where 5-12 O. vulgaris and 5-7 M. defilippi were recorded. These high numbers of octopus were never observed on a single dive during another season.
16 DISCUSSION
Given that sympatric species do not necessarily partition all their critical resources, focusing on one resource may underestimate the importance of resource partitioning mechanism(s) responsible for coexistence (Limbourn et al. 2007). This study examined temporal distribution (seasonal variation in abundance), spatial distribution of occupied dens, and habitat association as potential resource partitioning mechanisms responsible for the coexistence of two octopus species.
We have provided the first report on spatial distribution and densities for these sympatric species. Both Octopus vulgaris and Macrotritopus defilippi showed intraspecific aggregation of occupied dens at the Blue Heron Bridge ecohabitat.
Aggregation, clumping, or overlapping of den distribution have been reported in previous studies for O. vulgaris (Mather & Odor 1991, Guerra et al. 2014) and Octopus insularis
(Leite et al. 2009b). These spatial patterns were seen with densities ranging from 1.2-8.8/
1000 m2 in O. insularis in Brazil (Leite et al. 2009b) and 3.8-3.9/ 1000 m2 in O. vulgaris in NW Spain (Guerra et al. 2014) (Katsanevakis & Verriopoulos 2004a)reported the density of O. vulgaris in Greek coastal waters on soft sediment to range from 0-6.9/ 1000 m2 and (Aronson 1986) reported a mean density of Octopus briareus to be 7.9/1000 m2 in a saltwater lake on Eleuthera Island, Bahamas, which he termed high density. Densities in our study for O. vulgaris and M. defilippi in South Florida were similar to these previous studies suggesting that BHB can have high densities of these two octopus species. This
South Florida lagoon is the only location known at which O. vulgaris and M. defilippi coexist in high densities. Since both species were most abundant during the same season
(spring) we suggest temporal partitioning does not facilitate coexistence.
17 Benthic octopuses are typically known as solitary animals not living near each other (Boal 2006, Guerra et al. 2014). There was evidence of intraspecific aggregation for both species. Due to the high density of both octopus species, we anticipated species to aggregate in different areas at BHB; however, this was not the case and both species aggregated in the same general area at BHB. There was no evidence of interspecific aggregation (spatial overlap) or over-dispersion (spatial partitioning) between species. If the two species were actively positioning their dens at certain distances (near or far) from dens of the other species, the signal in the spatial data would have been much stronger.
Instead, our results suggest that spatial partitioning (in terms of den location) is not a resource partitioning mechanism that facilitates coexistence between these two species.
In high densities, den spatial distribution is a compromise because it is crucial for these soft-bodied invertebrates to have shelter for survival. The importance of habitat heterogeneity for species coexistence has been documented in other taxa and is gaining support in cephalopods. Substrate type and den availability are two factors responsible for octopus distribution in multiple octopus species (Aronson 1986, Leite et al. 2009b,
Guerra et al. 2014) and octopus coexistence (Hochberg & Couch 1971, Houck 1982).
This is also the first study for M. defilippi that has quantified associated habitat (den and surrounding) and compared it to the associated habitat of a sympatric octopus species (O. vulgaris).
Octopus species coexist in the same general areas of BHB (i.e., southwest and southeast areas) due to this lagoon’s fine-scale habitat heterogeneity. The substrate category that contributed to habitat (den and surrounding) dissimilarities between species was hard bottom. O. vulgaris can be found in many sub-habitats on and around coral
18 reefs and seagrass beds throughout Florida and the Caribbean, and only inhabits sand plains when they are adjacent to substrates that contain natural dens of hard materials
(Hanlon 1988, Katsanevakis & Verriopoulos 2004a, b). This species requires hard objects for their dens and such hard objects are absent from the open sand plains, which dominates BHB habitat (especially in the south area, Fig. 2). The majority of hard and 3D substrates (rock, rubble, fauna and flora, and human debris) were concentrated at the southwest and southeast areas of BHB. The distribution of these materials is a factor of den selection and therefore is most likely responsible for the aggregate den distribution of
O. vulgaris.
This spatial pattern was stronger for O. vulgaris occupied-dens than for M. defilippi occupied-dens likely due to their differences in habitat association. M. defilippi is a sand-dwelling species similar to the octopuses in the “Long-Armed Sand Octopus” clade, which includes the Indo-Pacific mimic octopuses (Thaumoctopus mimicus,
Wunderpus photogenicus, White V’ octopus, and Hawaiian Long-Armed Sand octopus), that require sand habitat (Hanlon 1988, Hanlon et al. 2008). Since the general substrate composition of BHB is sand, this could explain the weaker aggregate den distribution for
M. defilippi. However, lack of aggregated den distribution for M. defilippi for years
2014–2015 could also be due to fewer occupied dens recorded. By recording occupied den locations over a 3-year period we were able to detect if spatial distribution trends were consistent. In the case of M. defilippi, although there was a weak trend, 2016 showed evidence of intraspecific aggregation in the same general locations of BHB as O. vulgaris. Even though M. defilippi were sometimes found in near proximity to O.
19 vulgaris, they were exclusively on sand substrates; thus, we use the term “fine scale” habitat partitioning to explain species coexistence.
The combination of high octopus abundance, accessibility to the study location, and extensive dive time, permitted us to conduct the first 3-year study on octopus resource partitioning mechanisms. These species exhibit spatial-temporal overlap by occurring in high densities in the same general area during the same season. Coexistence could occur by having high overlap in previously mentioned niche dimensions and low overlap in latter niche dimension- habitat type (i.e., niche-complementarity hypothesis,
(Jimenez et al. 1996).
Sympatric species may also be partitioning other resources at varying degrees to facilitate coexistence (Jimenez et al. 1996, Barnes 2002, Vieira & Port 2007). Food abundance and availability are factors reported to influence octopus density and distribution (Guerra et al. 2014). Diet, activity period(s), and foraging strategies should be examined for these coexisting species to determine if they assist in coexistence. The diet of M. defilippi has never been reported and we are unaware of reports on these aforementioned topics as resource partitioning mechanisms for octopuses under natural conditions.
Many ecological coexistence studies (including this study) assume that species coexist due to partitioning resources, thus lessening competitive interaction. However, alternative explanations for species coexistence may not be from competitive pressure, but from environmental preferences and tolerances of each species, relating to dispersal and establishment of each species, or source and sink resource dynamics (Gordon 2000,
20 Kirol et al. 2015). Therefore, similar species may have increased chances of coexisting because of these shared environmental and ecological attributes.
Many of the octopuses observed during the spring were juveniles and are likely responsible for the high density of octopuses during spring months. BHB’s close proximity to the Palm Beach Inlet would make dispersal/migration and recruitment possible. Recruitment of juvenile octopuses has been reported to peak in spring and summer (Aronson 1986, Katsanevakis & Verriopoulos 2004a) and was responsible for the overall increase in the octopus population. It appears that water temperature is one parameter that is correlated with octopus density . Small octopuses may prefer shallow, warm water to achieve a greater growth rate and shorten the period in which they are most vulnerable to predation (Forsythe 1993). Once adults, medium and larger octopuses have been reported to abandon warm waters for deeper, cooler waters to reduce the energy cost of a higher metabolism (Katsanevakis & Verriopoulos 2004a). After using this shallow, warm water habitat to speed growth, medium – large sized octopuses could migrate to deeper, cooler waters and then potentially return to mate. Mating events (both species) and females with eggs (only O. vulgaris) have been observed; therefore, the lagoon may function as a nursery and mating habitat (e.g., source habitat acting as a population refuge). More observations are needed to determine the reproductive period of these two species in the western Atlantic and octopus size-class recordings to confirm
BHB as a nursery/recruitment habitat.
We encourage future studies on ecological coexistence of cephalopods to measure additional resource partitioning mechanisms mentioned (i.e., diet, activity time, foraging strategies) and the influence of abiotic factors on octopus’ spatial distribution and
21 abundance. Since this is a shallow area, heavily influenced by tidal flow and freshwater input, temperature and salinity should be measured. Salinity was reported to influence octopus presence; low salinities are associated with octopuses being absent or their restriction to areas of normal salinity (Hartwick et al. 1984). Octopus tracking would be instrumental to determine if these species have a seasonal migration pattern to and from
BHB via the Palm Beach Inlet. By further identifying mechanisms of coexistence, we can provide insight into cephalopod coexistence, and conservation strategies to maintain or increase cephalopod diversity, an important group in many marine food webs.
22 Table 1: Number of dive hours spent searching for octopuses in the Blue Heron Bridge
study site each season (winter, spring, summer, fall) over a 3 yr period (2014–2016).
Seasons were defined as: winter (December, January, February), spring (March, April,
May), summer (June, July, August), and fall (September, October, November).
Dive hours
2014 2015 2016 Total
Winter 21.1 27.9 33.6 82.6
Spring 19.5 35.8 43.3 98.6
Summer 25.4 45.6 37.0 108.0
Fall 21.7 39.0 21.5 82.2
Total 87.7 148.3 135.4 371.4
23
Figure 1: Two sympatric octopus species at Blue Heron Bridge, Phil Foster Park within the Lake Worth Lagoon, FL. Top: Octopus vulgaris, Bottom: Macrotritopus defilippi
24
Figure 2: Blue Heron Bridge (BHB), Phil Foster Park (26.7843˚N, -80.0427˚W) located within the Lake Worth Lagoon, FL. Location within Florida indicated by star in inset image. Study location was in BHB waters (outlined in white, ~62,000 m2) and was divided into three survey areas (white-dashed lines) relative to BHB: southwest (SW), south (S), and southeast (SE). Occupied den locations for Octopus vulgaris (green circles, n=296) and Macrotritopus defilippi (pink diamonds, n=138) were recorded via GPS for years 2014-2016.
25
Figure 3: Octopus kernel density maps at Blue Heron Bridge based on occupied den GPS data for individual years 2014-2016 (top-bottom) and octopus species (Octopus vulgaris, left; Macrotritopus defilippi, right). Kernel density map scale: occupied octopus dens/1000m2. Blue-purple colors represent low occupied den densities, pink-red colors represent medium occupied den densities and orange-yellow colors represent high occupied den densities.
26
Figure 4: Percent substrate category (±SE) for den habitat for Octopus vulgaris
(stippled), Macrotritopus defilippi (stripes), and general substrate composition for Blue
Heron Bridge (BHB) (grey). Photos represent the den habitat for each octopus species and BHB substrate composition with a white arrow indicating octopus in photo. There was a significant difference between O. vulgaris and M. defilippi (ANOSIM, R = 0.264, p
= 0.001) den habitats and O. vulgaris den habitat and BHB (ANOSIM, R = 0.221, p =
0.001). Percent contribution of dissimilarity (SIMPER) between O. vulgaris and M. defilippi den habitats & between O. vulgaris den habitat and BHB is listed above the respective bracket for each substrate category.
27
Figure 5: Percent substrate category (±SE) for surrounding habitat for Octopus vulgaris
(stippled), Macrotritopus defilippi (stripes), and general substrate composition for Blue
Heron Bridge (BHB) (grey). Photos represent surrounding habitat for each octopus species and BHB substrate composition. There was a significant difference between O. vulgaris and M. defilippi (ANOSIM, R = 0.253, p = 0.001) surrounding habitats and O. vulgaris surrounding habitat and BHB (ANOSIM, R = 0.172, p = 0.001). Percent contribution of dissimilarity (SIMPER) between O. vulgaris and M. defilippi surrounding habitats & between O. vulgaris surrounding habitat and BHB is listed above the respective bracket for each substrate category.
28 O. vulgaris 60 M. defilippi 50
40
30
20
Octopus AbundanceOctopus 10
0 Winter Spring Summer Fall
Figure 6: Average seasonal octopus abundance (±SE) across three consecutive years
(2014-2016) for Octopus vulgaris (dashed line) and Macrotritopus defilippi (solid line).
Seasons were defined as: winter (December, January, February), spring (March, April,
May), summer (June, July, August), fall (September, October, November).
29 CHAPTER II: OCTOPUS COEXISTENCE FACILITATED BY FORAGING AND
DIET STRATEGIES IN A SOUTH FLORIDA LAGOON
ABSTRACT
Octopuses play a key role in many marine food webs yet little is known about resource partitioning mechanisms that facilitate octopus coexistence and thus diversity.
Octopus vulgaris and Macrotritopus defilippi coexist in high densities at a South Florida lagoon. I examined foraging strategies (activity periods, foraging substrates, and foraging behaviors) and diet as potential resource-partitioning mechanisms. Twenty-four-hour video recording was used to determine octopus’ foraging activity periods. Methods for determining diet included: (1) collection of prey remains, (2) video tracking of foraging octopuses, and (3) 24-h video recording of octopus dens. Method 2 was also used to determine potential differences in species’ foraging substrates and foraging behaviors. O. vulgaris had nocturnal peak foraging activity at 2200 h while M. defilippi had diurnal peak foraging activity at 1300 h. Diet for O. vulgaris was mostly bivalves (55%), followed by gastropods (30%), and then crustaceans (15%). M. defilippi diet was almost exclusively crustaceans (94%) with few bivalves (6%). O. vulgaris foraged mostly on hard bottom (48%) while M. defilippi foraged mostly on soft bottom (69%). Both species’ foraging behaviors were dominated by speculative bottom searching (39%, 45%; respectively); however, species-specific behaviors were parachute attack (O. vulgaris), and flounder swimming and tripod stance (M. defilippi). Differences in foraging strategies and diet may promote coexistence of these sympatric species.
30 INTRODUCTION
When studying species’ coexistence it is crucial to study multiple mechanisms of coexistence because sympatric species may be partitioning resources at varying degrees
(Jimenez et al. 1996, Barnes 2002, Vieira & Port 2007). Two octopus species (Octopus vulgaris and Macrotritopus defilippi) occur in high densities and exhibit spatial-temporal overlap in the same general areas of a South Florida lagoon (Fig. 1 & 2). These high- density octopus populations coexist in part by fine-scale habitat partitioning (Bennice et al. 2019). I proposed there are additional niche partitioning mechanisms responsible for the coexistence of these two species at this location.
I chose to examine diet and activity periods for each species as potential niche partitioning mechanisms because they are the primary niche dimensions usually partitioned between species (along with habitat, which was examined in Chapter 1)
(Gutman & Dayan 2005, Vieira & Port 2007, Sbragaglia et al. 2019). Foraging strategies are an important niche partitioning mechanism operating simultaneously with other mechanisms (Fossette et al. 2017). Octopuses have an extensive behavioral repertoire; thus, it is important to examine their foraging strategies (i.e., foraging substrates and foraging behaviors) with the two niche dimensions (i.e., diet and activity period). The diet, activity period, and foraging strategies of O. vulgaris have not been studied at this
South Florida lagoon. Also, foraging strategies for O. vulgaris have never been studied in the presence of M. defilippi. For M. defilippi, there are no reports on diet or foraging strategies and opposing observations for activity period (Hanlon et al. 1985, Humann and
DeLoach 2013).
31 I posed the following research questions: (1) Do the two species overlap temporally (i.e., activity periods)? (2) Do their diets overlap? (3) Do they forage on different substrates? (4) Do they use different foraging behaviors? and (5) Are foraging behaviors associated with distinct substrates for each species? Although there is fine- scale habitat partitioning between these species in this South Florida lagoon, they still inhabit the same general areas (Bennice et al. 2019). Therefore, given the close proximity of these two species in this habitat, I hypothesized temporal partitioning (different activity periods) to facilitate coexistence. Knowing these two species are associated with different substrates for den and surrounding habitat, I hypothesized that octopuses would forage on different substrates and use different foraging behaviors. Also, I hypothesized that certain foraging behaviors are more advantageous on specific substrates, thus there would be an association between substrates and foraging behaviors for each species.
Furthermore, I hypothesized that octopuses would consume different prey (diet partitioning) to facilitate coexistence.
MATERIALS AND METHODS
Description of species
Octopus vulgaris inhabits subtropical and tropical waters world-wide. Defining characteristics of this species include stout arms of equal length and thickness, an arm length 3-5x its mantle length (ML), and highly variable body patterns often exhibiting a reddish-brownish reticulated pattern. Macrotritopus defilippi inhabits the Caribbean,
Atlantic, and Mediterranean and has defining characteristics that include: long slender arms with an arm length up to 6x ML, dark arm bars with white spots down each arm, small head with protruding eyes, and narrow mantle. A more detailed description of each
32 species can be found in Chapter I under the Materials and Methods section “Description of species”.
Study Site
Activity period, diet, and foraging strategies were examined for each species at
Blue Heron Bridge (BHB), within the Phil Foster Park portion of the Lake Worth
Lagoon, Riviera Beach, FL, USA (Fig. 2). BHB has a heterogeneous benthic environment, mainly consisting of sandy plains, but is also comprised of rock, shell, rubble substrate, and anthropogenic materials (e.g., cans, glass bottles, cement blocks, pipes, artificial snorkel trail, and sunken boats). Due to BHB’s close proximity to the
Atlantic Ocean via the Palm Beach Inlet, this shallow lagoon’s (average depth ~3m) water visibility is heavily influenced by the tidal cycle. The majority of scuba dives took place within 1-2 h of high, slack tides (between 6:00-21:00) to minimize tidal current
(often peaked at 0.77m/sec) and maximize water visibility. This increased the likelihood of locating octopuses.
Foraging activity periods
Once an octopus in its den was located, a 24-h octopus monitoring gadget (OMG) was deployed to record foraging activity periods of the octopus. The OMG was composed of a GoPro Hero 3 camera, a 20 mAH external battery, and a red LED light fixture, all encased within a waterproof housing that had a plexi glass viewing area for the GoPro camera (Fig. 8). GoPro Hero 3 video settings were adjusted to 420 resolution to allow for video storage on memory card. An external battery was necessary to record video for 24 h and operate the red LED light. Red light (630 nm) was used for capturing night video because octopuses are insensitive to red light, thus; not disturbing their
33 natural behaviors (Weiss et al. 2006, Sinn 2008). To secure the OMG 0.3 m from the octopus’s den, a 1.8 kg weight was attached to each side of the underwater housing and the OMG was anchored into the sand. The OMG was deployed during June 2015-
December 2016 for O. vulgaris and August 2015 - June 2017 for M. defilippi. I did not tag or track individual octopuses daily; however, I deployed the OMG at different occupied dens and made the assumption that all 24-h videos were of different octopuses.
Foraging activity periods were recorded for each video and foraging was defined as any time an octopus left its den or was returning to its den and showed one of the foraging behaviors described in Table 2. I established additional criteria for scoring octopus foraging activity periods in the event of an octopus leaving the camera’s field of view (FOV). During video tracking events, octopuses were observed foraging up to 2-3 h. Therefore, if the octopus left the FOV, that hour and an additional hour were recorded as foraging activity time. Also, when the octopus returned to the FOV, that hour and the hour before the return were recorded as foraging activity time. For example, if an octopus started foraging and left the camera’s FOV at 12:00 and returned to FOV at 15:00 we would score that entire time as a foraging activity period. If the 24-h octopus video did not meet these criteria (i.e., out of FOV for ≥ 3 h or octopus never returns to den) it was not used to analyze activity periods.
First, hourly percentage activity time was calculated for each species to determine if data should be separated into two (day vs night) or four (dawn, day, dusk, night) temporal categories for further statistical analysis. Due to peak foraging activity occurring during day or night hours, data were separated into the two categories: diurnal
(6:00-17:59) and nocturnal (18:00-5:59). The frequencies of diurnal foraging activity
34 events (number of diurnal foraging events/ total number of all foraging events) were compared between species using the non-parametric Mann-Whitney U test since data did not meet parametric test assumptions (Shaprio-Wilk, p < 0.05; Levene’s Test, p < 0.05).
Diet
The diets of O. vulgaris and other shallow-water octopus species are commonly studied by collecting midden contents (piles of prey remains left around the octopus’s den) (Dodge & Scheel 1999, Kuhlmann & McCabe 2014). However, this method has its limitations. Any prey with a light-weight exoskeleton or is soft-bodied and completely consumed will be missed from the diet analysis. Octopuses may also collect old shells to
“decorate” their dens for protection from predators or consume the prey away from their den. Also, physical forces such as currents or other organisms may remove or add items
(Ambrose 1984, Mather 1991a) from the surrounding den area. For these reasons, three methods to collect information on each species’ diet were used: (1) collection of newly discarded prey remains, (2) video tracking of foraging octopuses, and (3) 24-h OMG video.
To be classified as recently discarded, the prey remains must be free of algal growth and/or worm casings. Shells with growth, worm casings, or are worn down are likely old and collected for the purpose of constructing the octopus’s den, rather than eaten as prey (Katsanevakis & Verriopoulos 2004b). Prey remains were also examined for presence of octopus’ drill hole. For video tracking (method 2), octopuses were followed on foraging events in case they consumed prey away from the den or soft- bodied animals. If a prey item was captured and consumed it was recorded using a Canon
Powershot D20 camera. If the octopus spat out prey remains during the foraging event,
35 those remains were collected. Additionally, 24-h video recording (method 3) was used to capture the octopus consuming soft-bodied animals, taking prey back to its den, or spitting out prey remains. These last two methods provided confirmation that the prey species was consumed. BHB underwater photographers also contributed their photos and videos of octopuses capturing or consuming prey, which were also used in the diet analysis.
All prey items collected or recorded (via photos or videos) were separated into one of three prey categories: (1) bivalves, (2) gastropods, or (3) crustaceans. Prey items were identified to species when possible. Due to multiple collection methods used, the total number of prey items for each prey category were compared between species using a 2x3 contingency table. Theoretical counts for Chi Square statistic were lower than five; therefore, the Fishers Exact Test was performed to determine any differences in prey categories between octopus species.
Foraging strategies (foraging substrates & behaviors)
Method 2 (video tracking of foraging octopuses) was also used to determine if species differed in foraging strategies (i.e., foraging substrates and foraging behaviors).
For this study, foraging was defined as the time away from the den and exhibiting an observed behavior which illustrated the octopus searching for prey or an attempt attack on prey. During octopus foraging events, diver (CB) followed and continuously recorded foraging strategies from a distance of approximately 1 m to not disturb the octopus’s natural behaviors. Video recording was done during the day under natural light and at night with artificial light (video lights) and took place from August 2013-November
2015.
36 Eleven foraging behaviors and four substrate categories were the focus of video scoring and are listed with a detailed description in Table 2. This allowed the testing for association between: (1) octopus species and foraging substrates, (2) octopus species and foraging behaviors, and (3) foraging behaviors and substrates for each species. Foraging strategies were continuously scored to calculate duration (min) of time on each substrate category and duration of time each foraging behavior was used. Foraging videos had to be at least 1 min in total duration to be scored. Frequencies observed on substrate category types (duration of time spent foraging on specific substrate/ total duration of foraging substrate video) and for foraging behaviors (duration of time spent using foraging behavior/ total duration of foraging video) were used to determine differences in foraging substrates and behaviors between species. The same videos were used to score foraging substrates and behaviors separately and in association with each other.
Foraging substrate and behavior data did not meet parametric test assumptions
(normal distribution and homogeneity of variance), thus non-parametric tests were used to analyze data. Mann-Whitney U tests were used for comparing foraging substrates between species and foraging behaviors between species. After calculating frequencies of the eleven foraging behaviors, five behaviors represented an average of 88% of all foraging behaviors observed for O. vulgaris and an average of 94% of all foraging behaviors observed for M. defilippi. Three of these five behaviors were frequently used by both species (speculative bottom searching, sitting, and groping) and two of the five behaviors differed between species (O. vulgaris: parachute attack and crawling; M. defilippi: flounder swimming and tripod stance) (Table 3). Since five foraging behaviors comprised the majority of all behaviors used, I only analyzed these behaviors between
37 species (three shared behaviors and two species’ specific behaviors for a total of seven behaviors). The Z statistic and exact p-value were reported from the Mann-Whitney U tests. A Bonferroni correction was applied to compensate for multiple comparisons on the substrate data set and the foraging behavior data set, thus not increasing the chances of a
Type I error (incorrectly rejecting null hypothesis). Adjusted p-value for each octopus’ foraging substrate test was 0.0125 (0.05/4). Adjusted p-value for each octopus’ foraging behavior test was 0.007 (0.05/7).
To determine if there was an association of foraging behavior and substrate for each species, frequencies of observed foraging behaviors on substrate categories
(duration of time spent using foraging behavior on substrate/ total duration of time spent on substrate) were calculated. This allowed testing whether the behavior is used differently on each substrate (e.g., frequency of parachute attack behavior on soft bottom vs hard bottom vs fauna and flora vs human debris). Friedman Tests were used to compare each foraging behavior across substrate category types for each species because the data did not meet test assumptions and there were multiple observations from the same octopus video (scoring behavior on different substrates for same octopus). The
Friedman Test is the non-parametric equivalent to a one-way repeated measures
ANOVA, which looks at differences between dependent groups where more than two dependent variables were measured in a sample. Only the five most frequently used foraging behaviors (Table 3) were analyzed for each species. A Friedman Test for each behavior across substrates was used, therefore, a Bonferroni correction was applied to compensate for the multiple comparisons on the foraging substrate and foraging behavior
38 data set. The adjusted p-value for each test was 0.01 (0.05/5). The exact p-value for each
Friedman Test was reported.
RESULTS
Foraging activity periods
The 24-h octopus monitoring gadget (OMG) was deployed a total of 19 times for
Octopus vulgaris and 28 times for Macrotritopus defilippi. From these deployments, 14 of the 19 videos for O. vulgaris and 11 of the 28 videos for M. defilippi met the criteria for data analysis (i.e., full 24-h foraging activity video). Videos where the octopus did not return to the den or OMG was obstructed from viewing the octopus (e.g., moved by fauna, human, or current), which resulted in a time frame ≥ 3h from observing the octopus in field of view, were not used.
Peak foraging activity for O. vulgaris was during nocturnal hours whereas for M. defilippi peak foraging activity was during diurnal hours (Fig. 8). Largest percentage hourly foraging activity observed for O. vulgaris was during 2200 h with 71% (10/14 octopus) and the smallest hourly percent foraging activity was during 1200 h with 7%
(1/14 octopus). M. defilippi’s largest hourly percentage foraging activity was during 1300 h with 64% (7/11 octopus) and the smallest hourly percent foraging activity was 0% and occurred during multiple times: 0-0400 h, 0600 h, 1900-2300 h. When octopus activity periods were separated into four categories (dawn, day, dusk, and night), O. vulgaris demonstrated foraging activity during all four categories. From the total number of O. vulgaris observed (n = 14), six O. vulgaris were active during dawn and day, four were active at dusk, and all octopus were active at night. M. defilippi (n = 11) demonstrated little foraging activity for dawn (1/11 octopus), dusk (2/11 octopus), and night (2/11
39 octopus), and was almost exclusively active during the day (10/11 octopus). After analyzing foraging activity periods between species for two categories (day vs night) we found a significant difference (Z = -3.830; p < 0.0001, Figure 8).
Diet
There were 139 prey items recorded with 29 species identified (17 bivalve species, 8 gastropod species, 4 crustacean species) through a combination of collection methods (midden collections, foraging tracking video, and 24-h video). For breakdown by octopus species, a total of 121 prey remains were recorded from 32 O. vulgaris and a total of 18 prey remains were recorded from 16 M. defilippi (Table 4). The majority of prey samples recorded for O. vulgaris were from recently discarded prey remains while the 24-h camera and foraging video tracking methods were the most effective in recording the diet of M. defilippi.
Two of the three prey categories overlapped between octopus species. Despite diet category and prey species overlap, the prey category that contributed most to each species diet was significantly different (p < 0.0001). The diet of O. vulgaris was mainly composed of bivalves (55%) followed by gastropods (30%) and crustaceans (15%). The dominant prey from each category was Chione elevata (bivalve), Bulla occidentalis
(gastropod), and Calappa spp. (crustacean). The diet of M. defilippi was almost exclusively crustaceans (94%). Only one bivalve and zero gastropods were recorded for
M. defilippi. The dominant prey from each category was Calappa spp. (crustacean, excluding unidentified crustaceans) and Laevecardium mortoni (bivalve). Octopus drill holes were present in all prey categories, but only in certain prey species collected from
O. vulgaris’ dens (Table 4). The average shell thickness for bivalves with no drill hole
40 was 0.413 mm (S.D. ± 0.325 mm) and the average shell thickness for bivalves with a drill hole was 0.741 mm (S.D. ± 0.255 mm). All gastropod species reported with a drill hole were also species with an operculum. For crustaceans, a drill hole in the claw was found for three of the Calappa spp. remains collected from O. vulgaris.
Foraging strategies (substrates & behaviors used during foraging events)
There were 25 octopuses of each species recorded and analyzed to determine differences in substrates used during foraging events between species. A total of 194.22 min of video for O. vulgaris and a total of 351.97 min of video for M. defilippi were recorded. Foraging substrate videos ranged from 1.97 - 29.58 min (average video 7.77 min) for O. vulgaris and 1.09 – 47.28 min (average video 14.08 min) for M. defilippi.
Foraging substrate for O. vulgaris was composed of all four substrate categories and had a larger percentage of hard bottom, fauna and flora, and human debris than M. defilippi.
Hard bottom was the substrate O. vulgaris spent most of its time on during foraging events. Foraging substrate for M. defilippi was mainly soft bottom with a low percentage
(or zero) of the other substrate categories (Fig. 9). Although both species were found on
3 of the 4 substrate categories, there was a significant difference between species for substrate categories during foraging. O. vulgaris spent more time on hard bottom (Z = -
2.629, p = 0.008) and M. defilippi spent more time on soft bottom (Z = -3.826, p <
0.001). There was no significant association with fauna and flora (Z = -2.148, p = 0.032) and human debris (Z = -2.013, p = 0.026) between species (Bonferroni adjusted p-value p
= 0.025).
A total of 187.61 min of foraging behavior video from 25 O. vulgaris and a total of 344.80 min of foraging behavior video from 24 M. defilippi were analyzed. Foraging
41 behavior videos ranged from 1.57 - 28.66 min (average video 7.50 min) for O. vulgaris and 1.09 - 29.06 min (average video 14.37 min) for M. defilippi. The two most frequently used behaviors during foraging events for both species were speculative bottom searching and sitting. The next three most frequently used behaviors (in decreasing order) for O. vulgaris were groping, parachute attack, and crawling and for M. defilippi were flounder swimming, tripod stance, and groping (Fig. 10). With the adjusted p-value (p = 0.007), three of the species’ specific behaviors were significantly different with O. vulgaris using parachute attack more than M. defilippi (Z = -4.268, p < 0.001), and M. defilippi using flounder swimming (Z = -5.260, p < 0.001) and tripod stance (Z = -3.075, p = 0.002) behaviors more than O. vulgaris during foraging events (Fig 10). There was not a difference in speculative bottom searching (Z = -1.260, p = 0.212), sitting (Z = -1.062, p
= 0.293), and crawling (Z = -1.785, p = 0.076) behaviors between species.
The five most frequently used foraging behaviors for each species were then analyzed with the associated substrates for a total of 166.76 min for 25 O. vulgaris and a total of 327.51 min for 24 M. defilippi. Since the same videos were used to analyze behaviors with associated substrates, video ranges and averages were similar (O. vulgaris video range 1.37-27.45 min, average video = 6.67min; M. defilippi video range 1.09-
28.49 min, average video = 13.65 min).
The most frequently used behavior by O. vulgaris was speculative bottom searching and it had the largest percentage of association on fauna and flora; however, this behavior was frequently used across all four substrates and there was no significant difference for an associated substrate with this behavior (p = 0.721). The second most frequently used behavior for O. vulgaris was sitting and it was mainly associated with
42 human debris then hard bottom. There was also no significant difference for sitting across substrates (p = 0.444). The next three most frequently used behaviors (grope searching, parachute attack, and crawling) were associated mainly with soft bottom (grope searching and crawling) and fauna and flora (parachute attack) (Fig. 11). Grope searching and parachute attack were not observed on human debris. Although these three behaviors were used a higher percentage on one substrate, there was no significant difference of association across substrates for any of these behaviors (grope searching, p = 0.265; parachute attack, p = 0.106; crawling, p = 0.053).
Like O. vulgaris, M. defilippi also used speculative bottom searching (most frequently used behavior) across all substrate categories and there was no significant difference of association across substrates (p = 0.392). Second most frequently used behavior for M. defilippi was sitting and this behavior was mainly associated with soft bottom and hard bottom and was not observed on human debris. From the three substrates sitting was observed on (soft bottom, hard bottom, and fauna and flora) there was no significant difference of association across substrates (p = 0.392). The next three behaviors observed during M. defilippi’s foraging events (flounder swimming, tripod stance, and grope searching) were associated with hard and soft bottom (both flounder swimming and tripod stance) and human debris (grope searching) (Fig. 12). Flounder swimming and tripod stance were not observed on human debris. These three behaviors did not have a significant difference of association with a specific substrate (flounder swimming, p = 0.392; tripod stance, p =0.392; grope searching, p = 0.392).
43 DISCUSSION
Understanding the underlying mechanisms of species’ coexistence is complex.
This study investigated the coexistence between two octopus species, which live in high densities and are sympatric (Bennice et al. 2019). Habitat or microhabitat partitioning has been viewed as a primary mechanism of coexistence and aided in the explanation of coexistence between these two species. However, coexistence of sympatric species can involve partitioning several niche dimensions. Here I discovered differences in foraging strategies and diet between two octopus species that reduced resource overlap, potentially lessening competition and thus promoting coexistence.
This was the first study to use in situ 24-h video recording to determine activity periods of Octopus vulgaris and Macrotritopus defilippi. I showed O. vulgaris has a nocturnal activity period. Although inhabitants of near-shore environments are subject to several environmental changes, one important factor affecting them is the 24-h alternation of night and day (Meisel et al. 2006). For cephalopods inhabiting the photic zone, light seems to act as a time cue for activity while deeper living cephalopods are little affected by light (Denton & Gilpin-Brown 1961, Cobb et al. 1995b). Usually the activity pattern of a species over a 24-h period can be described as nocturnal, diurnal, or crepuscular. However, there have been conflicting reports for O. vulgaris. O. vulgaris was reported to be nocturnal in the wild in the Mediterranean (Woods 1965, Norman
2000), but was diurnal in the Caribbean Sea and the Atlantic Ocean (Hochberg & Couch
1971, Hanlon 1988, Mather 1988). In Bermuda, juvenile O. vulgaris was found to be crepuscular (Mather 1991a). My results differed from the previous reports for O. vulgaris’ activity period in the western Atlantic.
44 For M. defilippi, no specific study about its activity pattern existed before this study. Our results show that M. defilippi has a diurnal activity period. Hanlon et al.
(1985) reported this species to be nocturnal and Humann and DeLoach (2002) reported it to be day-active. Both reports from M. defilippi were from the western Atlantic; however, one report was from rearing a female in the laboratory and the second from field observations.
Meisel et al. (2006) compared activity patterns of two species (Octopus macropus and O. vulgaris) that overlap in habitat and found that that both species had controlled circadian activity, but the importance of light as a cue was greater for O. macropus than
O. vulgaris. Their laboratory results were consistent with previous field reports for
Mediterranean O. macropus (nocturnal activity), but results for Mediterranean O. vulgaris (predominantly diurnal activity) differed from previous reports (nocturnal activity). The difference of light as a cue for activity between the species could have allowed temporal spacing of activity and reduced competition, which was also shown between 3 Hawaiian octopus species (Houck 1982, Meisel et al. 2006). The flexibility of
O. vulgaris to change its activity profile could aid in its coexistence with M. defilippi, whose activity pattern was strictly diurnal by decreasing chances of interference competition. O. vulgaris and M. defilippi showed no seasonal temporal partitioning
(Bennice et al. 2019), but may use 24-h (diel cycle) temporal partitioning to coexist.
These results also demonstrate the importance of examining temporal partitioning at different scales.
Other environmental or biological factors that could influence the activity periods of these species may be tidal flow or ontogenetic shifts. This shallow lagoon is heavily
45 influenced by the tidal cycle and could impact when octopuses leave their dens to forage.
Although I showed O. vulgaris is nocturnal, this species also showed activity during crepuscular hours. Many of the O. vulgaris I observed at this location were juveniles and may be responsible for the activity observed during crepuscular hours as reported in
Mather (1991). Hanlon et al. (1985) showed M. defilippi to have a nocturnal activity pattern; however, it became less pronounced with age. I observed on a few occasions juvenile M. defilippi active on night dives. Future studies on activity periods for these species should examine these factors.
Intensive research has been done on octopus diets, especially the diet of O. vulgaris. However, little research has examined diet partitioning (Voight 2013, Valls et al. 2017) and no research has examined foraging strategies (in terms of foraging substrates and behaviors) as mechanisms of octopus coexistence. Here I examined both diet and foraging strategies of two sympatric octopus species. I used a combination of three methods to determine the prey composition of O. vulgaris and M. defilippi. By using multiple methods for octopus diet, I was able to record soft bodied prey that are unlikely to be included when sampling an octopus’s den. Multiple methods also allowed me to gather information on the diet of M. defilippi because this species does not leave prey remains around its den. This was the first study to report on M. defilippi’s diet. Also, results using the three methods showed a greater diet overlap for prey (specifically
Calappidae crabs) between the two octopus species, but octopuses still differed in diet composition.
O. vulgaris foraged on all four substrate categories and consumed a wide variety of mollusks and crustaceans, which is consistent with other diet studies on this species
46 (Anderson et al. 2008, Leite et al. 2009a, Leite et al. 2009b, Kuhlmann & McCabe 2014) and the characterization of a generalist or opportunistic feeder (Ambrose and Nelson
1983, Kuhlmann and McCabe 2014, Hanlon and Messenger 2018). The octopus’s prey selection may also be influenced by foraging behaviors. Tactile feeding is of major importance to octopuses that feed on crabs, bivalves, and gastropods, which are seized with the arms after exploration with the suckers (Hanlon and Messenger 2018). The majority of foraging behaviors used were speculative search or speculative hunting behaviors where tactile feeding is used. Previous field observations indicated that shallow-water octopuses frequently used speculative hunting (Houchberg & Couch 1971,
Hartwick 1983, Mather 1991, Forsythe & Hanlon 1997). These hunting methods for prey in the surrounding habitat would account for the wide range of prey species consumed by
O. vulgaris (Mather 1991).
Octopuses are either visual predators (laboratory studies, Wells 1962, Wells 1978) or tactile hunters (Houchberg & Couch 1971, Hartwick 1983, Mather 1991, Forsythe &
Hanlon 1997). My results indicated that these two species were tactile feeders; primarily used speculative bottom searching to find prey. However, during foraging events, octopuses spent a large portion of time stationary (i.e., sitting and tripod stance).
Speculative hunting behaviors combined with stationary behaviors illustrated a foraging pattern of “stop- and-go” behaviors and octopuses using “saltatory search” behavior to find food (Obrien et al. 1990, Forsythe and Hanlon 1997, Baatrup et al. 2018). These stationary behaviors serve the purpose for octopuses to use their keen vision to scan their environment for potential foraging areas and predators. After an octopus has detected a new foraging area, they use repositioning behaviors to travel to this location. While
47 transitioning to the new foraging area, the octopus needs to remain cryptic from predators. O. vulgaris used crawling to move slowly across the substrate while M. defilippi used flounder swimming to move across soft bottom and mimic a flatflish
(Hanlon et al. 2010). Other behaviors such as the “moving rock trick” and “moving algae” are used as camouflage tactics (masquerade) by these species when moving across open sand plains. These were observed (CB and RTH), but not included in the foraging behavior because of their rare observed occurrences (Hanlon and Messenger 2018).
Individuals may select where to forage based on prey availability or prey preference. After using these foraging strategies in a habitat, an octopus may learn where preferred prey are available (Mather & Odor 1991). As saltatory foragers, octopuses may be mapping the surrounding environment while stationary, and selecting where to forage
(Forsythe & Hanlon 1997). O. vulgaris spent half of its foraging time on hard bottom and the other half of its time on soft bottom and fauna and flora. Although O. vulgaris consumed a variety of prey, this species diet was bivalves > gastropods > crustaceans. M. defilippi is a sand-dwelling species that primarily foraged on soft bottom and strictly selected for crustaceans (crustaceans >> bivalves > gastropods). Optimal foraging theory states that a predator will always consume its preferred prey species when encountered unless this species is rare, and its less preferred prey will not often be consumed, even if they are common. Optimal foraging theory also predicts that the preferred species should be of highest dietary value; however, this preferred rank may include shape or taste of prey species (Ambrose 1984). For octopus’ prey, shape may involve shell thickness, presence of spines, or an operculum. This would influence the handling time of the prey species and could influence the prey preference. Crabs and shrimp may be handled and
48 consumed faster than a shelled prey that requires drilling before being eaten (Ambrose
1984). The speculative search behaviors mentioned use the octopus’s chemotactile arm suckers to detect and “taste” prey, which could aid in selecting prey based on the prey’s dietary value and handling time.
Octopuses strong preference for crabs is possibly due to their optimal diet (dietary value and short handling time); however, preferred prey species are usually not as abundant in the field as other prey (e.g., bivalves) (Taki 1941, Ambrose 1984) or are also sought after by other members in the community. M. defilippi’s high selectivity for crustaceans may result in this prey being less available to O. vulgaris. Calappidae crabs are generally found in soft substrates ranging from muddy or fine sand bottoms to sand and rubble, which is where M. defilippi spent the majority of its time foraging (Bellwood
2002). If crustaceans became rare, O. vulgaris may compromise and forage on a less- preferred prey that is available, but will still consume preferred prey when encountered
(Optimal Foraging Theory). This could be the reason O. vulgaris’s diet was mostly bivalves. O. vulgaris foraged on different substrates than M. defilippi and came into contact with a variety of prey. Prey inhabiting areas with 3D structure where O. vulgaris foraged (rock, fauna and flora) may be less-preferred, but more available. The bivalve species largely consumed, even though drilling was involved, was Chione eleveta; a shallow-burrowing species found in muddy sand or coarser sediment and is frequently found with seagrass (Daley et al. 2007). When resources are potentially limited (preferred prey), O. vulgaris compromises and forages on multiple substrates and selects a potentially lower ranked prey; thus facilitating coexistence.
49 Gastropods were also consumed more than crustaceans for O. vulgaris. Anderson et al. (2008) reported that O. vulgaris consumed gastropods the less than bivalves and crustaceans in the Caribbean. This may also indicate that crustaceans were rarely encountered by O. vulgaris at this South Florida lagoon. The gastropod species largely consumed (Bulla occidentalis) does not have an operculum, which may contribute to a greater preference over other gastropods that we rarely collected. However, there was one individual octopus that was responsible for many of the B. occidentalis collected.
Octopuses have been reported to be generalist at the population level and specialist at the individual level (Anderson et al. 2008). This individual specialization may also help stabilize population and community dynamics by lessening intra- and interspecific competition (Bolnick et al. 2003).
This rigorous field study identified additional mechanisms of coexistence for two sympatric octopus species. These species inhabit the same general areas, but compensate by having low overlap in other niche dimensions (diet, activity time, and foraging strategies) supporting the niche complementarity hypothesis (Jimenez et al. 1996;
Richoux et al. 2014). Future experiments should be done to examine the prey preference of these two species in the absence of each other. Ambrose (1984) found that O. vulgaris preferred crabs in the laboratory, but mainly consumed snails in the field because the preferred prey was rare. Diet of octopuses can also be influenced by water depth
(Ambrose 1984), bottom habitat (Vincent et al. 1998), and seasonal variation (Quetglas et al. 1998). Spatial distribution (across seasons) of prey should be examined to determine prey availability at this lagoon. Foraging home ranges or foraging distances have also been shown to differ depending on octopus density (Yarnall 1969). Since this lagoon is
50 occupied by two species in high densities, we encourage mapping foraging distances or home ranges as another potential resource partitioning mechanism.
This study provided insight to octopus niches and ecology under natural conditions, especially for a species we know very little about (M. defilippi).
Understanding the behavioral dynamics of octopuses can help us better understand the environmental requirements multiple octopus species need to coexist. This is important because shallow-water octopuses are major predators of benthic communities and serve as prey for top predators of many marine food webs.
51
Table 2: Description of octopus foraging behaviors and foraging substrates.
Foraging Behaviors Behaviors Description Parachute attack Octopus pounces or leaps on a prospective feeding spot and simultaneously spreads and extends the arms and the extensive web, pounces with the interbrachial web spread widely. Grope searching Uses long arms (not web) to reach into small holes in the substrate Speculative bottom searching When highly extensible web and long arms completely covered on the surrounding substrate and the dexterous arm tips and suckers rooted into the substrate Sitting Stationary and alert. Motionless octopus in a head-high posture, arms are not actively extended, searching for prey. Tripod Stance Standing tall on arms and viewing habitat while foraging Crawling Continuous slow movement of the mantle from one physical location to another when the arms are in contact with the substrate. Exploring rock sides Octopus searches for food with four arms spread along both sides of a rock (not below, not in crevices), and without spreading its web Reconstructing sand Octopus digs or excavates sand or mud with some of its arms. Backward swimming Stream-lined mantle-first swimming in water column. With the mantle first and head arms trailing Forward swimming Swimming with eyes and head forward, the arms usually divided four to a side and trailing along with the mantle Flounder swimming Forward swimming in which the octopus positions its arms to attain a flounder-like appearance; sometimes with undulations of the body that are similar to flounder body movements Substrate Category Substrate Description Soft bottom sand Hard bottom rocks, rubble (mixture of broken shells and rocks), and shells Fauna and flora algae, hydroids, seagrasses, sponges, corals Human debris pipes, cement blocks, bottles, sunken boats, aluminum cans
52 Table 3: Average percentage of time for each of the eleven foraging behaviors used during foraging for Octopus vulgaris (total of 187.61 min of foraging video) and
Macrotritopus defilippi (total of 344.80 min foraging video). Five behaviors represented an average of 88% of all foraging behaviors observed for O. vulgaris and an average of
94% of all foraging behaviors observed for M. defilippi (in yellow). These behaviors were used in the foraging behavior analyses.
53 Table 4: Diet composition for Octopus vulgaris and Macrotritopus defilippi separated into 3 prey categories (bivalves, gastropods, crustaceans). Prey identified to species if possible with quantity collected for each prey genus or species. Presence of octopus’ drill hole indicated by “D” for each prey genus or species.
Bivalves Gastropods Crustaceans O. vulgaris M. defilippi O. vulgaris M. defilippi O. vulgaris M. defilippi Anadara transversa 1,D(rill hole) - Bulla occidentalis 22 - Calappa spp. 9, D 5 Argopecten gibbus 1 - Cerithium atratum 4 - Cryptosoma balguerii 1 2 Chione elevata 39, D - Cymatium cynocephalum 1 - Menippe mercenaria 1 1 Codakia orbicularis 4 - Polinices lactaeus 1, D - Portunus gibbesii - 1 Crepidula atrasolea 1 - Stigmaulax sulcatus 1, D - Pseudosquilla ciliata 1 1 Dendostrea frons 1 - Strombus alatus 2, D - Unidentified 6 7 Euvola ziczac 1 - Strombus pugilis 3 - Laevecardium mortoni 6 1 Trochomodulus calusa 1, D - Lucinisca nassula 2, D - Unidentified 1 - Macrotoma fragilis 1 - Posinia discas 1 - Puberella interapurpurea 1, D - Tagelus divisus 2 - Tagelus plebius 1 - Tellina listeria 1 - Trachycardium 1, D - egmontianum Trachycardium muricatum 2 - Unidentified 1 -
Total 67 1 36 0 18 17
54
Figure 7: Illustrations of assembled 24-h octopus monitoring gadget (top left) and its components: PVC pipe external housing, GoPro camera, red LED light, external battery, weights (bottom left). Photo on right illustrates 24-h camera anchored near (~0.3 m) an octopus to record activity periods.
55
Figure 8: Average percentage hourly foraging activity (top) and diurnal versus nocturnal average percentage activity (±SE) (bottom) for Octopus vulgaris (n =14, stippled) and
Macrotritopus defilippi (n = 11, solid). Diurnal vs. nocturnal foraging activity was significantly different between species (Z = -3.830; p = < 0.0001).
56
Figure 9: Average percentage (±SE) of time spent foraging across four substrate categories for Octopus vulgaris (n = 25, stippled) and Macrotritopus defilippi (n = 25, solid). There was a total of 194.22 min of video recorded for O. vulgaris and a total of
351.97 min of video recorded for M. defilippi. O. vulgaris spent more time foraging on hard bottom than M. defilippi (Z = -2.629, p = 0.008) and M. defilippi spent more time foraging on soft bottom than O. vulgaris (Z = -3.826, p < 0.001). There was no significant difference between fauna and flora (Z = -2.148, p = 0.032) and human debris
(Z = -2.013, p = 0.026) between species with the adjusted p-value (p = 0.025).
57
Figure 10: Average percentage (±SE) of time spent using behavior during foraging for
Octopus vulgaris (n = 25, stippled) and Macrotritopus defilippi (n = 24, solid). There was a total of 187.61 min of video recorded for O. vulgaris and a total of 344.80 min of video recorded for M. defilippi. Three behaviors were significantly different between species.
O. vulgaris used parachute attack more than M. defilippi (Z = -4.268, p < 0.001), and M. defilippi used flounder swimming (Z = -5.260, p < 0.001) and tripod stance (Z = -3.075, p = 0.002) behaviors more than O. vulgaris during foraging. There was not a difference in speculative bottom searching (Z = -1.260, p = 0.212), sitting (Z = -1.062, p = 0.293), and crawling (Z = -1.785, p = 0.076) behaviors between species.
58
Figure 11: Average percentage of time Octopus vulgaris used foraging behaviors with related substrates. There was a total of 166.76 min of foraging video recorded for O. vulgaris (n = 25). There was no association with a substrate category for any of the five frequently used foraging behaviors: speculative bottom searching (dark grey, p = 0.721), sitting (striped, p = 0.444), grope searching (light grey, p = 0.265), parachute attack
(black, p = 0.106) and crawling (stippled, p = 0.053).
59
Figure 12: Average percentage of time Macrotritopus defilippi used foraging behaviors with related substrates. There was a total of 327.51 min of foraging video recorded for M. defilippi (n = 24). There was no association with a substrate category for any of the five frequently used foraging behaviors: speculative bottom searching (dark grey, p = 0.392), sitting (striped, p = 0.392), flounder swimming (stippled, p = 0.392), tripod stance (black, p =0.392), and grope searching (light grey, p = 0.392).
60 CHAPTER 3: IN SITU VIDEO RECORDINGS REVEAL RARE BEHAVIORAL
INTERACTIONS FOR TWO SYMPATRIC OCTOPUS SPECIES IN A
SHALLOW FLORIDA LAGOON
ABSTRACT
Many octopus interactions are rarely documented under natural conditions; however, with the advancement of technology these gaps in the observation of behaviors are diminished. During a 4-yr period, including 288 SCUBA, video recordings were made of octopus foraging behaviors. A stationary 24-h underwater camera was also used to record octopus activity periods. During both kinds of in situ video recording, multiple intra- and interspecific interactions of two sympatric octopus species (Octopus vulgaris and Macrotritopus defilippi) in a shallow Florida lagoon were documented. In this
Chapter, I document rare interactions, including: fishes following octopuses, predator- prey, agonistic encounters, cannibalism, and tactile communication and how they could potentially affect population dynamics and the coexistence of these two sympatric octopus species.
INTRODUCION
The natural history, population dynamics, and community structure of octopuses is influenced by many variables including predation, food supply, and the relationship with con- and heterospecifics. In Chapters 1 & 2, I documented multiple resource partitioning mechanisms that could potentially lessen competition and promote species
61 coexistence among these two octopus species. However, variables such as intra- and interspecific interactions could also drive coexistence (produce similar results of resource partitioning) or further support that these species have limited resources and must partitioning resources to coexist. During my 4-yr period (2013-2017) at Blue Heron
Bridge within the Lake Worth Lagoon, FL (Fig. 2) octopus activity periods and foraging strategies were recorded. Exhaustive field observations (288 SCUBA dives) allowed documentation of behaviors that are rarely seen under natural conditions. Herein, I describe the different interactions that may give insight to population dynamics and community structure of two octopus species (Octopus vulgaris and Macrotritopus defilippi) that cohabit this shallow-water lagoon.
DESCRIPTION OF BEHAVIORAL INTERACTIONS
There were several interspecific interactions that involved fishes following octopuses on foraging events and predation attempts. For intraspecific interactions, there were a total of six observations for O. vulgaris and five observations for M. defilippi. I observed five interactions between the two species. Many of the interactions included deimatic behavior from the octopus. Deimatic behavior is a threat, startle, frightening, or bluff behavior and can be intra- and interspecific (Hanlon & Messenger 2018). Deimatic behavior can be expressed in various forms. This chapter will include two categories of deimatic behavior (deimatic display and flamboyant display) that I observed during intra- and interspecific interactions.
In O. vulgaris, deimatic display comprises 6 components: (1) paling skin, (2) arms curved in wide arc and web spread maximally, (3) dark eye ring, (4) dilated pupil,
(5) dark edged suckers to create dark margin to the octopus’s outline, and (6) jetting
62 water (Hanlon & Messenger 2018). Deimatic displays differ slightly for other octopus species. Flamboyant display is any display that includes widely flared/ splayed and contorted arms or mantle. This body posture is usually in conjunction with a mottled or disruptive color pattern. Flamboyant displays either function as camouflage (masquerade) or as a threat posture where the octopus is “showing the weapons” (arms and beak)
(Hanlon & Messenger 2018). Intra- and interspecific octopus interactions were separated into the categories: fishes following octopuses, predation attempts, agonistic encounters, cannibalism, and tactile communication and are described in the following sections.
Fishes following octopuses
On most foraging trips, octopuses were accompanied by one to nine fishes. As the octopus searched for food, fishes would hover close by waiting for small organisms to be flushed out of holes and crevices by the octopus. Four fish species were identified:
Diplectrum formosum (sand perch), Lutjanus synagris (lane snapper), Halichoeres bivittatus (slippery dick wrasse), Sphoeroides spengleri (bandtail puffer). For the majority of observations, the fishes did not disturb the octopus and the octopus did not react to the fishes. There were incidents where a fish attacked the octopus’s arm and where the octopus would swat at the fish.
Predation attempts
Multiple potential predation attempts, or octopus responses to potential predators were observed. These potential predators ranged from diving marine birds to fish inhabiting the water column and benthic environment. From the 24-h video, we observed two encounters of the diving double-crested cormorant (Phalacrocorax auritus). In both incidents, the marine bird made multiple passes by the octopus’s den searching for the
63 octopus. In one case the octopus retreated deep into its den. In the second case the octopus used a deimatic display and swatted back at the bird with one arm (Fig. 13A). A deimatic display was also used after the octopus was detected by a mutton snapper
(Lutjanus analis) and a yellow stingray (Urobatis jamaicensis) (Fig. 13B & C). There was one observation where O. vulgaris quickly jetted away from a potential predator. As the octopus left its den to forage at night, a purple-mouth eel (Gymnothorax vicinus) took over its den. When the octopus returned to its den, it slowly crawled near the den opening where the eel’s head was exposed. The octopus sat (remained stationary and alert) for a few seconds then used jet propulsion to quickly swim away (Fig. 13D).
Agonistic encounters
Agonistic encounters were observed within and between species. I recorded four intraspecific agonistic encounters for both O. vulgaris and M. defilippi and two interspecific agonistic encounters. For O. vulgaris, all recorded encounters occurred at night when this species is most active (see Chapter 2). One encounter involved two octopus (octopus and invader octopus) of similar size that engaged in a short arm wrestle
(grapple) with the result of the invader octopus taking over the den. (Fig. 14A). Two encounters involved an O. vulgaris fleeing its den when a second O. vulgaris approached its den or started foraging in close proximity. These encounters involved a smaller octopus fleeing from a larger octopus. The fourth encounter involved male-male aggression when a second male attempted to mate with the first male’s female partner
(Fig 14B). Male 1 was extending third right arm (hectocotylus) to mate with the female when male 2 approached the mating pair. Once male 1 visually detected male 2, male 1 left the female and approached male 2. The males oriented towards each other and
64 extended their arms and web. Male 2 moved past male 1 towards the female and extended his web to cover her. Male 1 followed male 2 and extended his arms and web over male 2 which, resulted in a short fight. Male 2 jetted off after the fight and male 1 resumed mating behavior with the female (Fig. 14B).
For M. defilipi, all interactions occurred during the day, when this species is most active (see Chapter 2). Agnostic encounters observed (three out of four recordings) for M. defilippi involved M. defilippi visually detecting a conspecific, making a bee line for that octopus (using backward swimming) and grappling (Fig 14C). These grapples lasted a short period of time (1-2 sec); however, there were cases when one M. defilippi would follow the second M. defilippi and try to interact for a second time. There was one observation where three M. defilippi grappled with one another. The fourth encounter involved an invader octopus, similar to that of O. vulgaris’ invader interaction. After the invader octopus detected the second octopus in its den, the invader approached the second octopus, used a deimatic display, and took over its den (Fig. 14D).
Two out of the five interspecific interactions were agonistic encounters. During one foraging event, M. defilippi came across a structure (pipe) when foraging on soft bottom. While searching for food, M. defilippi extended its long arm inside the pipe where an O. vulgaris dwelled. Soon after M. defilippi’s arm reached inside the pipe, O. vulgaris jetted outside of the pipe and used a flamboyant display. The second encounter involved O. vulgaris visually detecting M. deflilippi foraging in close proximity to its den. O. vulgaris oriented towards M. defilippi and spread its arms and web in a deimatic display. M. defilippi reciprocated with its deimatic display (Fig.14E).
65 Cannibalism
Cannibalistic events for both species were observed or reported from a BHB diver. O. vulgaris was reported consuming a conspecific of approximately the same size
(personal communication with BHB diver Jeff Nelson). I observed M. defilippi during a foraging event use grope searching to extend its arm down a sand hole. When M. defilippi pulled its arm out of the hole, attached was a severed arm from a conspecific (Fig. 15A).
The octopus retreated to its den with the arm and handled the arm until it was fully under its arm web for consumption (24-h video did not show the octopus discarding the arm).
My second observation was M. defilippi occupying a den with a single arm extended from underneath the octopus. This arm was a different coloration than the octopus exposed from its den. I speculate this was cannibalism (Fig 15B).
Tactile communication
The other intra- and interspecific interactions that did not demonstrate agonistic behaviors involved tactile communication. Both intra- and interspecific tactile communication were initiated by an octopus visually detecting the second octopus followed by orienting themselves towards the octopus. Once in close proximity to each other, either one or both octopuses extended one arm towards the other octopus. Shortly after this tactile cue (~1 sec) one octopus moved away from the other (if the second octopus was in a den, (Fig 16A & B) or both octopuses quickly jetted backwards away from each other (Fig 16C).
DISCUSSION
Foraging octopuses are often followed by scavenging fishes. This has been reported for O. vulgaris in the Mediterranean (Kayes 1974) and Bermuda (Mather
66 1991b), for O. cyanea and O. macropus on coral reefs in the Red Sea (Ormond 1980,
Diamant & Shpigel 1985) and Polynesia (Forsythe and Hanlon 1997), and for O. dofleini in the Pacific (Hartwick and Thorarinsson 1979). These following fishes could make it easier for predators to find an octopus by drawing attention to it.
Field evidence has suggested that interspecific competition is rare in nature
Connell 1975, Birch 1979, Strong 1983) and other factors such as predation may be more important in limiting population growth (Gordon 2000) or altering life history traits.
Also, it has been shown that predation pressure can produce niche partitioning patterns identical to those produced by competition (Holt 1977).
Shallow-water octopuses are prey to various species ranging from birds, fishes, and marine mammals (Hanlon & Messenger 2018). There is an inverse relationship between the number of predatory teleosts and octopus population density (Aronson 1986,
Ambrose 1988) and predators can influence octopus’ foraging strategies. Mather & Odor
(1991) suggested that the trade-off for high predatioman pressure was limiting the foraging time for O. vulgaris. Amount of activity was observed to change in laboratory experiments when a diurnal or nocturnal predator was present and temporal spacing occurred only in the presence of the diurnal predator (Meisel et al. 2013).
O. vulgaris is a generalist species that shows plasticity in its circadian rhythm, thus able to switch its activity phase. Our results for octopus activity periods may fit the
“predation risk allocation” hypothesis (Lima & Bednekoff 1999). Animals that are supplementary prey to generalist predators are most likely to benefit by shifting their activity time, while a predator that specializes in a particular prey species will most likely match the prey’s activity pattern. O. vulgaris could have shifted to be nocturnally active
67 to avoid diurnal predators. O. vulgaris is known to be a common food for eels (Hanlon &
Messenger 2018) and eels also have evolved a solid skull to swim through small crevices in search of prey (Grüninger 1997), thus shifting its activity time would not aid an octopus in avoiding predation by eels. Minimizing hiding where the predator hunts (i.e., being more active during the time of hunting) may be a better adaptive response.
Inhabiting the same general area, I would speculate M. defilippi to have similar predators as O. vulgaris and shift activity time to be nocturnal. There were many observations where M. defilippi had multiple missing arms potentially from predators.
This species may be a specialist and cannot shift circadian rhythm. O. vulgaris has been previously reported as a diurnal species in the western Atlantic. Competition pressure may be greater than predation pressure, resulting in O. vulgaris shifting to nocturnal activity to avoid interference competition with M. defilippi. Additional experiments, especially for M. defilippi, need to be done to determine octopus’ response (switch in activity period) to different levels of predation pressure and in the absence of the other species.
Criteria has been established to become more convinced that there is an occurrence of interspecific competition (Wiens 1989, Krebs 2009). Suggestive evidence includes the occurrence of intraspecific competition. Intraspecific interactions for both species mainly included agonistic behavior (fights or den takeovers); potentially indicating interference competition for limited resources. Two of the five interspecific encounters included agonistic behavior; however, this only involved a deimatic display
(threat) and never resulted in a fight. This still illustrates a species’ territoriality for potentially limited resources (space and food).
68 The remainder intra- and interspecific encounters involved tactile communication by performing a delicate, brief touch of the suckers on the distal arm. Tactile communication seems to be uncommon in cephalopods due to their solitary lifestyle; however, it may be important in areas of high-density to exchange chemosensory information about the animal’s species or sex (Hanlon & Messenger 2018). After this tactile communication, an octopus may decide whether to engage in mating, to flee
(heterospecific or conspecific of same sex), or compete if the benefit out-weighs the cost
(energy gain from food or den space out-weighs energy loss of fight).
Another mechanism to aid species’ survival when food is scarce or to regulate population density is cannibalism. Cannibalism has been suggested to be an indicator for limited food (Calow 1998). Octopuses have high growth and metabolic rates, which makes cannibalism beneficial for this group in the situation where food is scarce. Under such conditions, cannibalism is a mechanism for survival for at least part of the population (Ibanez & Keyl 2010). Cannibalism could also be a strategy used to stabilize the population by decreasing competition. In the case of cephalopods, it is assumed that cannibalism occurs in populations with high densities (Ibanez & Keyl 2010). At high- densities, the probability of conspecific encounters is increased and cannibalism more likely. This was the case for high-density populations of O. briareus (Aronson 1986),
Enteroctopus megalocyathus (Ibanez and Chong 2008) and O. vulgaris (Oosthuizen &
Smale 2003).
Two types of cannibalism are recognized: intracohort and intercohort. Intracohort occurs between conspecifics of approximately the same age and has been documented, in the field for O. dofleini (Hartwick 1983) and in the field and laboratory for O. briareus
69 (Hanlon 1983b, Aronson 1989). Intercohort cannibalism occurs between conspecifics of different ages and has been documented in the field for O. vulgaris (Hernandez-Urcera et al. 2014). A large size-range of octopuses inhabit this shallow lagoon (Bennice et al.
2019), which would make both types of cannibalism possible, thus decreasing competition by reducing density. By decreasing intraspecific competition, one would suggest that interspecific competition would also decrease, thus allowing coexistence.
Cannibalism could be incidental, common, or high for an octopus population and depending on the rate could have different effects on the population. Cannibalism could be kept low if the main food source for the octopus population is abundant (Cortez et al.
1995). Additional research is needed to determine how cannibalism impacts the octopus populations at this lagoon.
The role of competition in natural populations can be investigated in several ways. I examined several resource partitioning mechanisms to determine what facilitates the coexistence of these two octopus species (Chapter 1 & 2). Although species inhabit the same general area of this shallow-water lagoon, they have different resource utilization (habitat, diet, and foraging strategies). The evolution of coleoid cephalopods has been strongly influenced by both predation and competition pressures from the
Mesozoic onwards (Packard 1972). Even if such segregation occurred by evolutionary change (predation pressure and/or the ghost of competition past) interspecific competition should still be examined today in these populations.
Competition could still be a major influence on niche structure even if it only occurs during rare and brief intervals in time (Gordon 2000). Wiens (1977) argued that competition may be rare in some populations because of high environmental fluctuation.
70 According to this argument, populations are typically below the carrying capacity of their environment and resources are abundant. Occasionally a “crunch” occurs, a period of resource scarcity in which competition happens. However, before competitive displacement could have time to occur, there is an environmental fluctuation (Hutchinson
1961). High-densities of these two species only occur in the spring, thus they may have a period of competition; however, this is temporary and competitive exclusion does not occur. Also, species utilize different resources and potentially use other mechanisms to cope with interactions during high densities (deimatic displays, tactile communication) or stabilize their population (cannibalism).
This system gave me the opportunity to study a widespread phenomenon
(resource partitioning) in a group of animals where it is poorly understood. Potential coexistence mechanisms were identified, which may play a role in determining the niche dimensions of octopus species. In conclusion, species coexistence is very complex, especially in intelligent invertebrates that are capable of adapting and learning their surrounding environments. Shallow, coastal areas may be an important nursery habitat.
This lagoon could potentially be a nursery habitat for these octopus species populations
(source habitat). Future research should focus on water temperature, salinity, water flow, reproduction, migration, predation, and cannibalism, which will help us better understand habitat use, recruitment, survival, and population dynamics of these two species that are an important mid-trophic level component in many marine food webs.
71
Figure 13: Predation attempts on octopuses recorded using a 24-h camera. (A) Diving double-crested cormorant (Phalacrocorax auritus) made multiple passes at Octopus vulgaris and O. vulgaris swatted back at the bird. (B) O. vulgaris camouflaged with surrounding 3D structure (rock covered in algae). When mutton snapper (Lutjanus analis) approached O. vulgaris, the octopus responded with a deimatic display. (C)
Macrotritopus defilippi in a tripod stance above its den (hole in sand). When a yellow stingray (Urobatis jamaicensis) swam close to the octopus, M. defilippi reacted with a deimatic display. (D) O. vulgaris left its den at night for a foraging event. When O.
72 vulgaris was foraging, a purple-mouth eel (Gymnothorax vicinus) took over the octopus’s den. The eel waited with its head exposed while the octopus approached its den. After a few seconds, the octopus jetted off.
73
Figure 14: Octopus intra- and interspecific agonistic encounters. (A) Intraspecific interaction where invader Octopus vulgaris fights a second O. vulgaris and executed a den takeover. (B) O. vulgaris male 1 extending hectocotylus to mate with female. Male 1 left female when he visually detected male 2 approaching. Male 1 and 2 oriented towards each other and both extended arms and web. Male 2 traveled over to the female and covered her with his web. Male 1 followed right behind to then cover male 2 with his arm web. This was then followed by a fight (male-male aggression). After the fight, male 2 jetted off and male 1 resumed mating behavior with the female. (C) Intraspecific interaction where two Macrotritopus defilippi visually detected each other and engaged
74 in an arm wrestle (grapple). (D) Intraspecific interaction where M. defilippi invader visually detected second M. defilippi in den. Invader shows a deimatic display and executes a den takeover. (E) Interspecific interaction where M. defilippi was foraging and was visually detected by O. vulgaris. O. vulgaris left its den, oriented towards M. defilippi and showed a deimatic display. M. defilippi reciprocated with its deimatic display. After the deimatic displays, M. defilippi continued foraging away from O. vulgaris and O. vulgaris returned to its den.
75
Figure 15: Cannibalism by Macrotritopus defilippi. (A) Octopus 1 left its den for a foraging event and used grope searching to inspect a sand hole, where octopus 2 inhabited. Octopus 1 retreated to its den with a severed arm from octopus 2 where it was potentially consumed (24-h video recording indicated the octopus never discarded the arm). (B) Octopus 1 was found in its den with a long, conspicuous arm extended from underneath it. I speculate this arm is from a second octopus and this is a second incident of cannibalism.
76
Figure 16: Octopus intra- and interspecific tactile communication. (A) Intraspecific interaction for Octopus vulgaris (V) where O. vulgaris extended its arm to touch a second octopus in its den. After tactile communication, the octopus moved away from the other octopus in the den. (B) Interspecific interaction where Macrotritopus defilippi (D) visually detected O. vulgaris during a foraging event. M. defilippi approached O. vulgaris and extended its arm to touch O. vulgaris. After tactile communication, M. defilippi continued foraging away from O. vulgaris’s den. (C) Interspecific interaction where both
O. vulgaris and M. defilippi extended an arm to touch each other. After tactile communication, both octopuses jetted backwards away from each other and moved in different directions.
77 APPENDIX
Permission letter
78
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