Cohort Structure and Habitat Association of the Pinna Nobilis Population in Northern Corsica at STARESO Abstract Introduction

Cohort structure and habitat association of the Pinna nobilis population in Northern Corsica at STARESO

Kenan Chan & Emma Chiaroni

Abstract

The Mediterranean fan mussel, Pinna nobilis can grow to be over a meter in length and are typically found half submerged in the sand within Posidonia oceanica. In recent years, P. nobilis has been under increasing pressure due to collection and habitat destruction. Previous studies have examined P. nobilis population structure elsewhere in the Mediterranean, however, our study investigated the specific demographics of the population at STARESO Marine Research Station, located on the Northern coastline of the French island of Corsica. We created an equation that enabled non-lethal age determination based on the maximum width of the individual. Our study was based on principles of the intermediate disturbance hypothesis and aimed to quantify environmental disturbances. We looked at depth and substrate as an indicator of disturbance, with shallower regions being more exposed to high wave action and composed of rocky substrate. We found evidence that suggest that P. nobilis associates with the environmentally stable P. oceanica meadows at STARESO and that there exists distinctive cohorts within the population.

Introduction anthropogenic activity. In order to develop conservation strategies there must be

scientific knowledge on the effects of The intermediate disturbance environmental disturbances on species hypothesis (IDH) predicts that levels of assemblage and diversity (Richardson et al. species diversity will be maximal in 2004). environments experiencing intermediate The Mediterranean fan mussel, levels of disturbance (Aronson et al 1995, Pinna nobilis, can grow to be over a meter Connell 1978, Connell 1979, Sousa 1979). in length and is typically found half Elevated disturbance environments favor submerged in the sand within Mediterranean species with high reproductive capacity and seagrass, Posidonia oceanica, (Katsanevakis low competitive ability, whereas low et al. 2008). Fundamental knowledge of P. disturbance environments allow for some nobilis biology is limited, however, studies species to become competitively dominant, have revealed a depth-related size causing the formation of a climax segregation (Garcia-March et al. 2007 A, B). community (Connell 1979). Studies performed off the Mediterranean Although the IDH was based off coast of Spain suggest that smaller P. nobilis terrestrial ecological succession, it can be were found in sandy sheltered areas and applied to the marine environment. rocks at shallow depths, while large Environmental disturbances in the marine individuals were are found to be associated environment manifest in the form of with P. oceanica meadows at deeper levels hydrodynamic forces, shifting substrate and (Garcia-March et al. 2007 A). There is also

1 little knowledge about the recruitment of P. protection of this species. This study nobilis, however evidence suggests that investigated the role of environmental settlers recruit over a depth gradient. The disturbance, by using depth as a proxy for majority of settlement occurs in late autumn disturbance, with the age related segregation and winter (Garcia-March et al. 2007 A, of individuals within the P. nobilis population Richardson et al. 2004). on the Northern coast of Corsica at the Habitat destruction, pollutants, and STARESO Marine Research Station. Our collection have caused P. nobilis to be listed study looked at four main hypotheses: (1) as an endangered species in the Does the STARESO Pinna nobilis population Mediterranean (Richardson et al. 1999, show cohort structure; (2) Do P. nobilis show Richardson et al. 2004). P. nobilis used to be a substrate preference with respect to cobble, widely distributed within the shallow coastal boulder, sand, and P. oceanica; (3) Do P. nobilis show an association for more stable waters of the Mediterranean, but in recent communities (3a) Is there a positive years, increases in anthropogenic activity, association with deeper depths; (3b) Does the including the development of resorts and the age vary between the different subregions at destruction of P. oceanica meadows, have STARESO; North, Harbor and South; and (4) caused P. nobilis populations to decline. Is there a shared trend in orientation between Expanding on the limited scientific individuals? knowledge of P. nobilis ecology will aid in making informed decisions regarding the

METHODS

Study Area to replicate those established in 2012 STARESO Marine Research Station (Elsmore and McHugh 2012). The Harbor is located on the Northern coastline of the transect extended from the ladder within the French island of Corsica. The station is a harbor (N42.58026, E8.72418), to a relatively protected site allowing for the permanent PVC pipe (N42.57988, observation of species that inhabit the E8.72449) 50 meters out at a heading of 150 prolific sea grass meadows. There is a small degrees. The North transect extended 85 breakwater that creates a shallow sheltered meters from the North PVC pipe boat harbor. To the North and South of the (N42.58004, E8.72499) inshore (N42.58075, station are steep cliffs that line the coastline, E8.72503) at a total distance of 85 meters. creating unique geological formations The South transect ran between the South including cobble fields and vertical walls. PVC pipe offshore (N42.57951, E8.72475) We used SCUBA for all aspects of and continued for 60 meters (N42.57934, the field data collection. In order to find E8.72410) inshore. The shallowest end of individual P. noblis previously found in the each transect was assigned meter mark 0. 2012 BIOE 159 course, permanent transect lines were set up in three locations (Map); (1) the STARESO harbor (2) North of the STARESO harbor and (3) South of the STARESO harbor. All transects were set up

Map: Map of STARESO with 3 permanent transects Figure 1: Total length, maximum exposed length and (marked in red), North individuals (blue) Banana submerged length shown. individuals (purple), Harbor individuals (yellow) and South individuals (green) plotted in Google Maps. orientation, arc thickness, maximum width Each of the three study locations had (figure 2B), distance from transect and their own distinguishing features. The meter-mark on the permanent transect for Harbor’s substrate included cobble and each found individual. Depth was measured boulders, man-made jacks, and P. oceanica by placing a dive computer where the patches. Depth ranged from 0 to about 10m. mussel met the substrate (depth The North location was lined with large measurements were taken using the Imperial boulders and cobble mounds that dropped system due to our dive computers settings). off quickly to a P. oceanica dominated landscape. The North location also included a few unique formations dubbed the Bananas for their sickle, banana-like shape. The Bananas were regions where P. oceanica receded away from bare sand due to erosion. The South was composed of steep rocky walls that met contiguous P. oceanica meadows. Both the North and A. South study sites typically sloped from 5 to 20 m offshore.

General Data Collection

To test whether P. nobilis abundance and age are higher in areas of low disturbance, we recorded the depth, B. maximum exposed length, submerged length (figure 1), Figure 2A/B: Figure 2A depicts the measurement of the arc width while Figure 2B shows the maximum width.

The maximum exposed length and maximum width were measured with rulers while the submerged length (figure 1) was

3 measured with a PVC pipe that was inserted run a Chi-Squared test in to test for an into the substrate until it met resistance association. We set our critical p-value to (submersion technique), where the byssal .05. threads of the P. nobilis meet the rhizomes (Richardson et al 1999). Orientation was determined with a compass, using reciprocal headings along the margin where the two valves meet. The thickness of the valves was measured with calipers from the top of the P. nobilis valves (figure 2A). A meter tape was used to measure the distance from the permanent Figure 3: 1m² with 9 UPC points. transect line to the mussel. We used this data to run a Binomial Demographics: Pinna nobilis age Probability test with a critical p-value of distribution and cohort structure at 0.05 to determine if a difference existed STARESO between shallow and deep depth zones. To determine the average age of the mussels at In order to relate age to a each location, we ran an ANOVA test using morphometric measurement, we established a critical p-value of 0.05 in JMP Pro 11 a total length to maximum width (JMP Pro 11 was used in all subsequent relationship using measurements collected statistical tests). To test for a shared from dead P. nobilis. Shells collected for orientation between individuals at this purpose ranged from 11.8 cm to 69.5 STARESO, we used a Chi-Squared test with cm. Total length, maximum width, shell a critical p-value of 0.05. thickness, arc length, shell volume, adductor Once the mussels were sized and scar length, and adductor scar ring (figure 4) their positions noted, GPS coordinates were number were recorded. taken for each individual via a float and surface support. We then plotted GPS coordinates using Google Earth and Google Maps to visualize the distribution of the mussels.

Habitat Association

In order to quantify P. nobilis habitat association with certain substrates, we carried out Uniform Point Contact (UPC) surveys at each mussel using 9 total points Figure 4: Adductor mussel scars on the internal side of a P. on a 1 square meter PVC quadrat (figure 3) nobilis valve. Photo: Kenan Chan (note: center mark placeholder was always P. nobilis individual). This data was then By graphing measurements of total length 1 used with a wider spread UPC data set to against maximum width from the dead individuals, the following linear relationship 1 An expansive UPC data set was conducted by J. Harrison, L. was determined: Hernandez, and E. Williams that consisted of both onshore and offshore transects every 10 meters at each of the 3 permanent transect (appendix 1).

4 Equation 1 measurements of maximum width to be converted into age as displayed in Equation Total Length = 2.8836 (Max Width) – 5.5516 3 below. In the past, P. nobilis had to be removed from the substrate to examine the The total length of each live P. adductor muscle scar rings in order to nobilis was determined using Equation 1, determine age. The examination of the and then compared to the lengths obtained adductor muscle scar rings necessitated the by the submersion technique practiced in the lethal removal of the animal from its valves. field. Total length and maximum width data The von Bertalanffy equation developed in were utilized in constructing a von this study is a non-lethal tool that allowed us Bertalanffy growth equation for the to obtain estimates of age. STARESO population. The von Bertalanffy growth equation: Equation 3

퐿(푡) = 퐿 (1 − 푒−푘(푡−푡0)) (2.8836푊 − 5.5516) ∞ 푙푛 [1 − ] 69.5 푡 = + 3 Drawing upon previous studies on P. −0.1823 nobilis age and growth (Richardson et al 1999), the upper limit was set as the largest 푤ℎ푒푟푒 푊 𝑖푠 max 푤𝑖푑푡ℎ individual in the population; a dead individual from STARESO with a total The results of Equation 3 were input length of 69.5cm. The growth constant k into JMP to create a histogram displaying was determined through the relationship the age distributions. This allowed for the between total length and the number of identification of possible cohort structure adductor scar rings. Adductor scar rings within the STARESO P. nobilis population. acted as a measurement of age (Richardson To analyze the age distribution produced by Equation 3, we ran a cluster analysis. et al 1999, Richardson et al 2004). 푡0 is the age at which the organism is size zero; this was set as the youngest and smallest Results individual, which was a dead individual from the STARESO harbor, that was 3 years Pinna nobilis Cohort Structure at old and 11.8 cm in total length. By STARESO customizing the von Bertalanffy growth equation to the STARESO P. nobilis Through the use of the modified von population, we produced Equation 2. Bertalanffy equation, we were able to establish an estimated age for each of the P. Equation 2 nobilis individuals. We were then able to group them into cohorts. Four distinct [69.5(1 − 푒−0.1823(푡−3))] + 5.5516 cohorts were found (Appendix 2). The first 푊(푡) = 2.3836 cohort ranged from approximately 3-6 years, the second from 8-11 years, the third from 푤ℎ푒푟푒 푊 𝑖푠 max 푤𝑖푑푡ℎ 푎푡 푡𝑖푚푒 푡 (푦푟푠) 12-15 years, and the last cohort ranged from approximately 20-22 years. Using a cluster To determine the age distribution of analysis in JMP (figure 5), we could see the STARESO population, Equation 2 was how the cohorts were divided and the rearranged to solve for age as a function of relative sizes of each cohort. maximum width. This allowed for field

5 higher at 239 observations. With boulder, we found the opposite, with an expected of 88.57 and only 3 observed points. The observed values for sand and cobble were consistent with the expected values. Cobble had 16 observed points and an expected of 14.35 points while sand had 22 observed points and an expected 14.6 points.

Depth and Age Distribution with Respect to Disturbances

We ran a Binomial Probability test between shallow (0-25ft) and deep (26-50ft) depth zones using a critical p-value of 0.05 Figure 5: P. nobilis cohorts at STARESO. Each separate cohort is separated by a different color. The cohort age and yielded a significant p-value of 0.04. increases from top to bottom. We determined that the mussels exhibited a higher association with the deeper depth The general age distribution of the entire zone (26-50ft). STARESO population was graphed (figure Using an ANOVA we tested the ages 6) and showed peaks and valleys, indicative of individuals with respect to the location. of cohort structure. We found the North individuals to be significantly older (P=.0486, df:2) than the individuals in the South and the Harbor (figure 7) at an average age of 13.9 years old.

Figure 6: Age structure of the mussel population. Number of individuals in each age class vs. Years (age).

Habitat Association of Pinna nobilis at STARESO

We found that the mussels showed a stronger association with P. oceanica than Figure 7: Mean age of P. nobilis individuals at their with any other substrate or primary respective locations; North, Harbor, South. placeholder (sand, boulder, and cobble) (Chi-Square: 122.65, df: 3, P< .0001). The expected P. oceanica count was Orientation of Pinna nobilis 162.45, normalized from the general UPC We constructed a Mosaic Plot (figure 8) and data1, but the observed value was much ran a Pearson Chi-Square Test (Chi-square:

6 2.48, df: 3, P= .7795) and found there to be cold (~12°C) seawater at STARESO no trend in valve orientation. The expected (Richardson et al. 2004). values for each 30 degree increment was While we attempted to maintain 13.3 individuals. equal levels of effort when searching for small and large individuals in the seagrass, the density of P. oceanica during the study period made it difficult to locate small individuals. Smaller P. nobilis were easier to find in areas with reduced seagrass cover. The lower number of small individuals involved our study could be a potential source of error when investigating the existence of the cohort structure. A study by Katsanevakis (2005) investigated the size frequency distribution of P. nobilis in a Grecian marine lake. Katsanevakis identified three size classes which corresponded to three cohorts within the Expected Quantity Actual Quantity lake population: maximum width 1-7 cm, 9- 15 cm, and 15-21 cm. Using Equation 3 to Figure 8: Expected Quantity (left column) vs. Actual convert maximum width into age, these Quantity (right column) of P. nobilis individuals who fall under their respective 30-degree orientation range. three size classes convert into the age groups 0-4 years, 5-7 years, and 7-12 years old. The cohorts found by Katsanevakis are similar in Discussion structure to the cohorts we found at STARESO, suggesting that fluctuations in Pinna nobilis Cohort Structure at P. nobilis recruitment may correspond to STARESO environmental fluctuations (Richardson et al. 2004.) In our study we were able to construct a von Bertalanffy growth equation Habitat Association of Pinna nobilis at for the STARESO Pinna nobilis population. STARESO The results of the equation suggested that the P. nobilis population at STARESO does The UPC habitat association results in fact display cohort structure (Appendix suggest that P. nobilis displays a strong 2). Using maximum width as a measurement association with P. oceanica (Garcia-March for size (Katsanevakis 2005), we were able et al. 2007 A, B, Richardson et al. 1999, to determine that larger individuals were Richardson et al. 2004). P. oceanica inherently older (Richardson et al. 1999). meadows stabilize the hydrodynamic Out of a total of 40 mussels, four environment by creating a dampening effect cohorts were identified: 3-6 years, 8-11 from waves and currents. The P. nobilis that years, 12-15 years, and 20-22 years old. The inhabit P. oceanica meadows benefit from largest cohort was 8-11 years old. This the reduced water movement created by the suggests that there may have been favorable densely packed blades and are protected oceanographic conditions 8-11 years ago for from potentially harmful wave action P. nobilis recruitment, such as an influx of (Coppa et al. 2010, Hendricks et al.

7 2011). Little is known about the settlement nobilis is sheltered from potentially harmful processes of P. nobilis, but studies have wave energy. revealed that larva settle over a depth range. The results of the ANOVA test This means that P. nobilis do not actively confirmed that older animals inhabit the settle on a specific substrate, but rather North more so than the Harbor and the suggests that post-settlement mortality is South locations. The habitat in the North is lower on certain substrates. Since larva almost entirely composed of P. oceanica settle over a range of depths, and therefore a meadows with the exception of sloping range of habitats, there must be certain boulders and a cement jack wall at the factors of P. oceanica that promote P. border of the Harbor and North locations. nobilis survivorship. Older individuals associate more with the The mussels displayed a strong North over any other location possibly due disassociation to boulder (expected: 88.57 to the region’s low levels of environmental and observed: 3). At STARESO, boulder perturbation. Water movement decreases and cobble habitats were found in the with depth (Bryson 1961), making the shallow depth zones. Due to their shallow North, which was the deepest out of all three locations at STARESO, these substrates locations, the most hydrodynamically stable make the mussels susceptible to wave site. The low levels of environmental action. P. nobilis that settle in these shallow disturbance created by the depth and the substrates are more vulnerable to harmful dampening effect of P. oceanica, form a hydrodynamic activity than individuals who favorable environment for reducing post- settle in P. oceanica. The depth of the P. settlement mortality of P. nobilis. By oceanica beds at STARESO also plays a key associating with the hydrodynamically role in maintaining the stability of the stable P. oceanica in the North, P. nobilis habitat. are less likely to be harmed by wave action or shifting substrates allowing them to reach Depth and Age Distribution with Respect older ages. to Disturbances Orientation of Pinna nobilis In order to determine if P. nobilis associates with stable environments, we We investigated the presence of a compared depth to mussel abundance, and potential trend in valve orientation. Initially, location (North, South, and Harbor) to P. we predicted that the animals would orient nobilis age. When comparing P. nobilis themselves to reduce drag as large P. nobilis abundance to depth, it was determined that a are commonly pulled out of the substrate higher number of the individuals studied at during storms (Coppa et al. 2010, Garcia- STARESO reside within the 26ft-50+ ft March et al. 2007 B). We determined that (~7.5-15+ m) depth zone. This depth range there was no trend in valve orientation. The corresponds our observed range of P. lack of a trend is likely due to the settlement oceanica at STARESO. The shallowest processes of P. nobilis (Cabanellas- depth limit of the sea grass (~5-6 m) is Reboredo et al. 2009,Garcia-March et al. restricted by wave action (Infantes et al. 2007 A). When P. nobilis larvae settle in an 2009). The distribution of mussels in terms environment they likely settle in one of depth supports the hypothesis that P. position for their entire life span. Some nobilis associates with P. oceanica. By changes in orientation may occur due to associating with deeper water habitats, P. shifting substrates; P. nobilis most likely

8 retains its original settlement orientation. chemical cues from adult P. nobilis that aid Although we found no trend in valve in choosing an appropriate settlement habitat orientation, an interesting follow up study (Kobak 2001, Thrush 1996). However, the could examine the role of valve orientation uniform distribution suggests the existence in reducing drag (Garcia-March et al. 2007 of specific mechanisms that regulate the B). If the valve orientation of a P. nobilis number of individuals within a given area. was parallel to water flow, then it should As mentioned previously, high levels of possess better hydrodynamic efficiency than post-settlement mortality in rocky shallow a conspecific with a valve orientation that is habitats may add to the depth-size perpendicular to the water flow. segregation of animals. The intermediate disturbance Conclusion hypothesis provides insight into the distribution of P. nobilis at STARESO. Old The results of our study make it individuals associate with the most stable apparent that P. nobilis associates with P. environment at STARESO; the deeper water oceanica over any other habitat. By P. oceanica meadows. Young animals comparing age structure and distribution associate with shallow water areas with with habitat, it was revealed that diverse substrate assemblages such as the environmental stability plays a role in the harbor. Post-settlement mortality is depth-age segregation described in previous potentially lower in P. oceanica meadows, studies. Younger P. nobilis are more likely therefore strengthening the association of P. to be associated with shallow high nobilis to P. oceanica. Because it is listed as disturbance habitats. Older P. nobilis are an endangered species, it is critical to strongly associated with deeper areas with understand the population ecology of P. low levels of environmental perturbation. nobilis for continued protection efforts. While our study investigated an explanation Scientific knowledge about the habitat to the depth-size segregation, the mechanism associations of P. nobilis age groups is behind this pattern has yet to be determined. important in making conservation decisions When collecting data in the field, we noticed to preserve and rehabilitate the species back that P. nobilis possessed a relatively uniform to original numbers. distribution. It is possible that larva sense

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Appendix

Appendix 1: UPC map by J. Harrison, L. Hernandez, and E. Williams 2014. Transects done both onshore and offshore every 10 meters at each of the 3 permanent transect, North, Harbor and South.

Appendix 2: Cohort structure within the P. nobilis population at STARESO. Age based on derived equation is represented on the x-axis and clustered cohorts are shown in the rows. The bar graphs are based off the total number of individuals at a given age. Cohort age increase top to bottom.