ASPECTS OF THE POPULATION ECOLOGY AND
LIFE HISTORY OF THE CO-OCCURRING CRABS
EURYPANOPEUS DEPRESSUS (SMITH) AND
EURYPANOPEUS DISSIMILIS (BENEDICT AND RATHBUN)
(CRUSTACEA: DECAPODA: XANTHIDAE)
by
Catherine Baylor Owen
A Thesis Submitted to the Faculty of the
College of Science in Partial Fulfillment of the Requirements for the Degree of
Master of Science
Florida Atlantic University
Boca Raton, Florida
August 1990 ASPECTS OF THE POPULATION ECOLOGY AND LIFE HISTORY OF THE CO-OCCURRING CRABS Eurypanopeus depressus (Smith) AND Eurypanopeus dissimilis (Benedict & Rathbun)
by
Catherine Baylor Owen
This thesis was prepared under the direction of the candidate's thesis advisor, Dr. Sheldon Dobkin, Department of Biological Sciences and has been approved by the members of her supervisory committee. It was submitted to the faculty of the College of Science and was accepted in partial fulfillment of the requirements for the degree of Master of Science.
~~y COMMITTEE: ~~ Dr. Sheldon Dobkin Thesis Advisor
Dr. Michael ical Sciences
Dean, ience
Dr. Wi~Bradshaw Date Dean of Graduate Studies
i i ACKNOWLEDGEMENTS
I sincerely thank Dr. Sheldon Dobkin, my major professor and mentor, for his guidance and endurance throughout this project, as well as financial assistance during my time as a graduate student. I would also like to thank the members of my thesis committee - Drs. R.M.
Adams, W.R. Brooks, and G.A. Marsh - for their comments on the manuscript. My very special thanks to Dr. W.R.
Courtenay, for the use of his calipers, computer, and lab space. A great deal of thanks are given to Drs. M.L. Boss and G.R. Bourne for their help in data analysis. I would like to thank the following persons for their assistance in the field andjor the lab: E. Barham, A. Broadwell, E.
Duda, J. Hansen, M. Mihalik, C. Perretta, D. Rumbold, and
S. Staiger. I would also like to thank Rose Kritzer for typing all page numbers as well as the signature page. My dearest friend Eileen L. Garcia, has my most sincere appreciation, for all of her invaluable assistance in the field and in the lab, collecting and sorting samples, as well as lending vital aid in data analysis and computer usage. Finally, my deepest gratitude goes to my parents, for their never-ending patience, love, and support.
iii ABSTRACT
Author: Catherine Baylor Owen
Title: Aspects of the Population Ecology and Life History of the Co-occurring Crabs Eurypanopeus depressus (Smith) and Eurypanopeus dissimilis (Benedict and Rathbun) (Crustacea:Decapoda: Xanthidae)
Institution: Florida Atlantic University
Thesis Advisor: Dr. Sheldon Dobkin
Degree: Master of Science
Year: 1990
The xanthid crabs Eurypanopeus depressus and E. dissimilis co-occur among oysters on seawalls in southeastern
Florida. Population dynamics and aspects of the life history of thes e two species were compared during a 1-year
period, at two study sites. There were no interspecific
differences in adult size for either sex. For both
species, females were significantly more abundant than males. E. depressus was more abundant at one study site, E· dissimilis at the other. ovigerous females were found year-round. Number of eggs per female of both species
increased with increasing carapace width. Recruitment of young into the population occurred year-round.
iv TABLE OF CONTENTS
Page
ACKNOWLEDGEMENTS • . . . • . . . . • . • • . . . • • ...... • . . iii
ABSTRACT . . • . • . • . . . . . • . . • • ...... • • • . • i v
LIST OF TABLES ...... • • . • • . vi
LIST OF FIGURES . • • • . . . . • . . . • • . • . . • . . • • • . . . . . • . . . . vii
INTRODUCTION • • • • • • . • • • • • . . • ...... • . . . . • • • • • • • • • . • 1
MATERIALS AND METHODS
Study Sites . • • . . . • ...... • . . . • • 5
Sample Collections ...... • . . . . • . . . . • ...... 5
Measurements . . • ...... • . • ...... 6
Data Analysis . . . • . . . • ...... • • . 7
RESULTS
Environmental Conditions ...... •• 8
Population Structure •.....•...... ••..••...•• 8
Sex Ratio ...... • . . . • . • . • ...... • . . . • • . • • . . . 10
Reproduction ...... • • . 11
Recruitment • . • • ...... • . • . 13
DISCUSSION • • • . • ...... • . . • • • 15
CONCLUSIONS • . • . . . . . • ...... • . . . • • . • • 26
LITERATURE CITED • • . . . • ...... 46
v LIST OF TABLES
Table
1. Associated species (transient and permanent) collected in Crassostrea virginica clumps in Boca Raton and Deerfield Beach, Florida, from February 1988 through January 1989 ...... 28
2. Life history and reproductive parameters for Eurypanopeus depressus and ~. dissimilis, collected from Boca Raton and Deerfield Beach, Florida, from February 1988 to January 1989 ... 29
vi LIST OF FIGURES
Figure
la-d. Mean CW of male and female Eurypanopeus depressus collected from February 1988 through January 1989, at Hillsboro Bridge, Deerfield Beach, and Royal Palm, Boca Raton, Florida. (Bars indicate 95% confidence intervals) ...... 30
2a-d. Mean CW of male and female Eurypanopeus dissimilis collected from February 1988 through January 1989, at Hillsboro Bridge, Deerfield Beach, and Royal Palm, Boca Raton, Florida. (Bars indicate 95% confidence intervals) ...... 31
3a-d. Overall size (CW) frequency distributions of male and female Eurypanopeus depressus collected from February 1988 through January 1989, at Hillsboro Bridge, Deerfield Beach, and Royal Palm, Boca Raton, Florida ...... 32
4a-d. Overall size (CW) frequency distributions of male and female Eurypanopeus dissimilis collected from February 1988 through January 1989, at Hillsboro Bridge, Deerfield Beach, and Royal Palm, Boca Raton, Florida ...... 33
5a. Monthly size (CW) frequency distributions of Eurypanopeus depressus, collected from February 1988 through January 1989, at Hillsboro Bridge, Deerfield Beach, Florida.. 34
5b. Monthly size (CW) frequency distributions of Eurypanopeus dissimilis, collected from February 1988 through January 1989, at Hillsboro Bridge, Deerfield Beach, Florida .. 35
6a. Monthly size (CW) frequency distributions of Eurypanopeus depressus, collected from February 1988 through January ~ 89, at Royal Palm, Boca Raton, Florida ...... 36
vii Figure
6b. Monthly size (CW) frequency distributions of Eurypanopeus dissimilis, collected from February 1988 through January 1989, at Royal Palm, Boca Raton, Florida...... 3 7
7a-d. Abundance (transformed counts) of male and female Eurypanopeus depressus, collected from February 1988 through January 1989, a t Hillsboro Bridge, Deerfield Beach, and Royal Palm, Boca Raton, Florida. (Bars indicate 95% confidence intervals) ...... 38
8a-d. Abundance (transformed counts) of male and female Eurypanopeus dissimilis, collected from February 1988 through January 1989, at Hillsboro Bridge, Deerfield Beach, and Royal Palm, Boca Raton, Florida. (Bars indicate 95% confidence intervals) ...... 39
9a-b. Sex ratio (expressed as a quotient, females to males) of Eurypanopeus depressus and E· dissimilis, collected from February 1988 through January 1989, at Hillsboro Bridge, Deerfield Beach, Florida. (Bars indicate 95% confidence intervals) ...... 40 lOa-b. Sex ratio (expressed as a quotient, females to males), of Eurypanopeus depressus and E. dissimilis, collected from February 1988 through January 1989, at Royal Palm, Boca Raton, Florida. (Bars indicate 95% confidence intervals) ...... 41 lla-b. Number of ovigerous females (expressed as a percentage of all females) of Eurypanopeus depressus and E. dissimilis, collected from February 1988 through January 1989, at Hillsboro Bridge, Deerfield Beach, Florida. (Bars indicate 95% confidence intervals) .... 42
vi ii Figure
12a-b. Number of ovigerous females (expressed as a percentage of all females) of Eurypanopeus depressus and E. dissimilis, collected from February 1988 through January 1989, at Royal Palm, Boca Raton, Florida. (Bars indicate 95% confidence intervals)...... 43
13. Size (CW) frequency distribution of all ovigerous female Eurypanopeus depressus and E. dissimilis collected from February 1988 through January 1989...... 44
14a-b. Regression of egg number vs. carapace width of subsamples of ovigerous Eurypanopeus depressus and E- dissimilis females (egg numbers are direct counts). Ninety-five percent confidence intervals are shown ...... 45
ix INTRODUCTION
The Xanthidae (mud crabs) historically have been a taxonomically difficult group, containing more genera (ca.
150) than any other brachyuran family (Rathbun, 1930;
Stevcic, 1971). Many of the genera are composed of closely related species with great morphological similarity, causing an ongoing controversy in identification and classification. There are approximately 1,000 species in the family (Martin, 1984), with the number constantly changing as revisions are being made. This heterogeneous assemblage has been organized and reorganized based traditionally on adult external morphology (Rathbun, 1930; Williams, 1965 & 1984) and more recently, on characteristics such as the position of genital openings (Guinot, 1978) and male pleopod morphology (Martin and Abele, 1986).
Relatively little is known about the ecological requirements of the family. Xanthids are known to inhabit oyster bars, rocky areas, and fouling communities, in temperate to tropical waters of North and South America.
Eurypanopeus depressus (Smith) and Eurypanopeus
1 2 dissimilis (Benedict & Rathbun) are two morphologically similar xanthid species that inhabit intertidal seawall oyster communities in southeastern Florida (Palm Beach,
Broward, and Dade Counties) (Owen, unpubl., 1984; Blyler,
1987). The major criteria that differentiate them are carapace morphology and texture, cheliped morphology and coloration, and, size and shape of a red spot located on the inner ischial surface of the third maxilliped.
Eurypanopeus depressus is one of the most common of the xanthids, and substantially more is known about it than its congener, ~. dissimilis. Studies of ~. depressus include those on its life history (Ryan, 1956; McDonald,
1977 & 1982), larval development (Costlow & Bookhout,
1961; Sulkin et al., 1983; Blyler, 1987), predation on oysters (Menzel & Nichy, 1958; McDermott, 1960), predation by the oyster toadfish (Opsanus tau) (Bisker et al.,
1989), genetics (Turner & Lyerla, 1980; Hilbish &
Vernberg, 1987) and taxonomy (Guinot, 1978; Martin, 1984;
Martin & Abele, 1986).
Eurypanopeus depressus lS distributed from
Massachusetts Bay through Florida to Texas (including
Bermuda) and through the Dutch West Indies to Uruguay
(Benedict & Rathbun, 1891; Hay & Shore, 1918; Rathbun,
1930; Cowles, 1930; Behre, 1950; Ryan, 1956; Williams, 3
1965 & 1984; Felder, 1973; Powers, 1977).
In Florida, ~- depressus occurs on both coasts (Wass,
1955; Tabb & Manning, 1961; Dragovich & Kelly, 1964; Rouse, 1969; Grizzle, 1974; Camp et al., 1977 ; Dugan &
Livingston, 1982; Abele & Kim, 1986). Most of these distributional studies report a preference of ~- depressus for an oyster substrate, although it has been found in a variety of other habitats, such as pilings, eelgrass
(Rathbun, 1930), in tunicates (Wass, 1955), in sponges
(Ryan, 1956) and among rubble in a brackish water canal
(pers. obser.).
Rathbun (1930) reported the distribution of ~ dissimilis to extend from the west coast of Florida to
Santa Catarina, Brazil, including Cuba, Jamaica, Trinidad and Nicaragua. However, none of the Florida west coast surveys listed above for ~- depressus reported the occurrence of ~- dissimilis.
Studies of behavior, habitat zonation, and niche relationships have been published for a number of xanthids
(Preston, 1973; Turoboyski, 1973; Swartz, 1976a, 1976b, &
1978; Lindberg, 1980; Menendez, 1987), including
McDonald's (1977 & 1982) work on the co-existence of ~ depressus and Panopeus herbstii. However, Blyler's (1987) study on the comparative larval development of ~- 4
depressus and ~- dissimilis 1s the only report that investigates the possible mechanism(s) that allow these congeners to co-exist.
Eurypanopeus depressus and ~- dissimilis occur sympatrically in close proximity. This study compares aspects of the life history of these two species with an attempt to identify parameters that allow them to co-exist. Comparisons are made of relative abundances, size distributions, sex ratios, fecundities, and recruitment, during a one year period. MATERIALS AND METHODS
Study Sites
The study sites consisted of vertical concrete
seawalls colonized by the American oyster, Crassostrea
virginica (Gmelin). Oysters extended vertically for
approximately 0.5 m and horizontally for as much as 400 m
in some locations. The first study site was located on
the eastern side of the Intracoastal Waterway, directly
beneath the Hillsboro Boulevard Bridge in Deerfield Beach,
Broward County, Florida (26 °18'N, 80 °06'W). The second
study site was located in a canal branching off the
western side of the Intracoastal Waterway, in the Royal
Palm subdivision of Boca Raton, Palm Beach County, Florida
(26 ° 20'N, 80 ° 04'W).
Sample Collections
Two samples were collected at 4 to 5 week intervals
at each of the two sites, on the same day or within a 24 hour period, from February 1988 through January 1989.
Each randomly selected sample consisted of one five-gallon
bucket (ca. 0.019m3 ) filled with oyster clumps removed
5 6 with a hammer and crowbar from the seawall. All collections were made at dead low tide, when the oyster clumps were completely exposed. Salinity and water temperature measurements were taken at these times.
Salinities were measured with a refractometer, water temperatures with a hand-held stern thermometer.
Measurements
In the laboratory the collected oysters from each sample were broken apart. All crabs were extracted from the crevices. Due to the large collection size, a representative subsarnple, consisting of 50% of the crabs in each sample, was chosen for analysis, by a random number generator. All crabs in each subsarnple were then identified to species and enumerated. Sex was determined for all but the smallest individuals (ca. < 2.5 rnrn), and ovigerous females were recorded. Maximum carapace width for each crab (including anterolateral spines) was measured to the nearest 0.1 rnrn with vernier calipers. For each species, direct egg counts were made on a subsarnple of randomly chosen ovigerous females (6 for E. depressus, 4 for E. dissirnilis). 7
Data Analysis
To analyze various life history and reproductive parameters, size-frequency histograms were constructed based on carapace width. Various analyses were performed concerning the effects of the samples, locations, collection dates, and sex on each of the two species, according to statistical methods in Sokal & Rohlf (1981). RESULTS
Environmental Conditions
During the sampling period, water temperature ranged
from 19-30 °C, highest in the summer months. Salinity
ranged from 1-31%., with lower salinities recorded during
the summer months. Similar monthly water temperatures and
salinities were recorded at both study sites. Water
depths at both sites ranged from ca. 15 em at mean low
tide when oysters were completely exposed, to ca. 82 em at
mean high tide when oysters were completely immersed. A
semi-diurnal tidal cycle exposed the oysters for
approximately 2-3 hours at each low tide.
Population Structure
During the study, a total of 9,302 Eurypanopeus
depressus and ~- dissimilis were collected, together with varying numbers of associated species (Table 1). As a
result of the 50% sample reductions, 4,651 crabs - 2,795
~- depressus and 1,856 ~- dissimilis - were measured for the analysis of population structure.
Table 2 lists various life history and reproductive
8 9 parameters of the two species. The carapace width (CW) of male~- depressus ranged from 2.5 to 17.0 mm, females from
3.0 to 15.5 mm. The cw of male~- dissimilis ranged from 2.8 to 18.3 mm; for females from 3.1 to 14.7 mm. At no time during the study was the mean CW of one sex significantly larger than the other, for either species
(Kruskal Wallis 1 way ANOVA). For males and females respectively, the values were 6.7 mm and 6.8 mm for~ depressus, and 7.1 mm and 7.5 mm for ~- dissimilis.
However, the mean CW of ~- dissimilis was significantly larger (3 way ANOVA, p < 0.05) than that of ~- depressus, for both sexes (Figs. la-d, 2a-d). In addition, a significant difference in the mean CW (3 way ANOVA, p <
0.05) was found between study sites, in that both species grew to a slightly larger size at the Royal Palm site
(Table 2) (Figs. la-d,2a-d).
The CW distributions of males and females collected during the entire study (Figs. 3a-d, 4a-d) were significantly different (3 way ANOVA, p < 0.05). For both species, the CW distributions were skewed to the right, although the distributions of the females had less range and more symmetry than those of the males, which were more strongly skewed to the right.
The monthly CW frequency distributions were analyzed 10
in part by size- frequency histograms (Figs. 5a-b, 6a-b).
They were, in general, unimodal or bimodal, asymmetrical
(i.e., skewed to the right), with more individuals smaller
than the median, where more size classes occurred among
larger individuals.
For both species, females were more abundant than
males (3 way ANOVA, p < 0.05) (Figs. 7a-d, 8a-d). In addition, both sexes of E. depressus were more abundant at the Royal Palm site, whereas both sexes of E. dissimilis were more abundant at the Hillsboro Bridge site (3 way
ANOVA, p < 0.05) (Table 2) (Figs. 7a-d, 8a-d). Also, both
species were more abundant during the summer and fall
months, and least abundant during the winter months.
sex Ratio The overall female to male ratio was 1.7:1.0 for E. depressus, and 1.3:1.0 for E. dissimilis. These ratios deviated significantly from the Mendelian 1.0:1.0 ratio
(Chi Square, p < 0.001), at both study sites. Monthly sex
ratios favored females of both species (3 way ANOVA, p <
0.05), with relatively few exceptions, in which the female to male ratio approximated 1.0:1.0 or favored males to a
small degree (Figs. 9a-b, lOa-b). Sex ratios of females to males were always greater than 1.0:1.0 for E. depressus 11
at both sites, but occasionally males of E. dissimilis outnumbered females at both sites. Overall, peak sex
ratios were generally found during the spring and summer months for both species, and in January for E. depressus
only.
For both species, at both sites, females were
significantly more abundant than males in most size
classes (Chi Square, p < 0.05). Females dominated smaller
to intermediate size classes (4.0-11.9 mm CW). Males
tended to be more abundant than females only in the
largest size classes (12.0-17.0 mm CW), with both sexes
approximately equal in abundance in the smallest size
classes (2.5-3.9 mm).
Reproduction
Reproductive activity, as indicated by the presence
of ovigerous females in the population, was continuous during the period of study, with highest peaks occurring during the summer and early fall (Figs. 11a-b, 12a-b). For E· depressus, the percentage of ovigerous females (expressed as a percentage of all females) reached their maxima in August(34 %), March(28%), and October(23%) at
Hillsboro Bridge (Fig. 11a), and in February(20%) and June (21%) at Royal Palm (Fig. 12a). For E. dissimilis, maxima 12
were reached in August(33%), October(23%), and June(20%)
at Hillsboro Bridge (Fig. 11b), and, in July(27 %) at Royal
Palm (Fig. 12b). At the Hillsboro Bridge site, both
species showed strong reproductive peaks in August and October. Also at this site, E. depressus ovigerous females peaked in March, when the abundance of E. dissimilis ovigerous females was very low (Figs. 11a-b). Ovigerous females of E. depressus were more abundant than those of E· dissimilis in the smaller size classes (6.0-9.9 mm CW). Ovigerous females of E. dissimilis outnumbered those of E. depressus only in the larger size classes (10.0-12.9 mm CW) (Fig. 13). Ovigerous females of
E. depressus were most abundant in the 7.0-7.9 mm size class. Ovigerous females of E. dissimilis were most abundant in the 8.0-8.9 mm and 10.0-10.9 mm size classes
(Fig. 13). The smallest ovigerous females collected during the entire study were 5.6 mm for E. depressus and 5.8 mm for E. dissimilis. Regression analyses (Figs. 14a-b) indicated that the number of eggs per female increased with increasing carapace width (r 2 = 0.995 and 0.991, p<0.001, for E. depressus and E. dissimilis, respectively). For E. depressus, fecundity ranged from 270-1730 eggs per female, with a mean value of 847 eggs (Fig. 14a). The CW of 13
ovigerous females ranged from 5.6-13.4 mm, with a mean
value of 8.3 mm (Table 2). For ~- dissimilis, fecundity
ranged from 280-1210 eggs per female, with a mean value of
802 eggs (Fig. 14b). The CW of ovigerous females ranged
from 6.3-12.4 mm, with a mean value of 9.7 mm (Table 2).
Recruitment
Recruitment seemed to be continuous, since most size
classes, especially the smaller to intermediate classes,
were well-represented throughout the sampling year (Figs.
5a-b, 6a-b). This occurred for both species, at both
study sites. No conspicuous winter die-offs were noted.
At Hillsboro Bridge, juvenile ( <5 mm) ~- depressus were
most abundant in June and July (Fig. Sa), whereas juvenile
( <5 mm) ~- dissimilis were most abundant in March,
November, and December (Fig. 5b). At Royal Palm, juvenile
~- depressus were abundant in all months except February,
March, and January (Fig. 6a), whereas juvenile ~ dissimilis were most abundant in November, December, and
January (Fig. 6b).
In some cases, recruitment ''peaks" of small ( <5 mm) individuals appeared 30-90 days after the peak appearance of ovigerous females (Figs. 5a-b, 6a-b; 11a-b, 12a-b).
However, in most cases, rate of growth was difficult to 14 ascertain from the size frequency histograms, as new recruits were rapidly absorbed into the population. DISCUSSION
It is obvious from the high densities of crabs
9,302 individuals in a total volume of less than 1 m3 of
oysters that the intertidal "seawall-oyster reef"
habitats have abundant space and food. Bahr (1974)
calculated that every 1 m2 of horizontal mud flat oyster reef in Georgia provided 50 m2 of total surface area. He
found that this type of habitat provided food, shelter,
and substrate for many different species, as was seen in the present study in which 25 associated species were
found.
Indeed, oyster reefs provide much spatial and biological heterogeneity. Dampness and shade provide a measure of relief from high temperatures (Grant &
McDonald, 1979). McDonald (1977) found that~. depressus and the co-occurring xanthid Panopeus herbstii used the oyster reef as refuge from potential predators and from the effects of desiccation at low tide. He explained the co-occurrence of these two species by a combination of trophic and microhabitat factors. The larger £. herbstii occupied depressions and grottoes at the mud-oyster
15 16
interface, while the smaller, flatter E. depressus was restricted to the narrow spaces within dead shells and
between living oysters (McDonald, 1982).
In the present study, both species of Eurypanopeus,
being approximately the same size, were found to occupy
the same microhabitat. There was no sexual dimorphism in
size, in contrast to the findings of McDonald (1977) who found that male E. depressus were larger, on the average, than females. I found no apparent differences in microhabitat utilization by either sex of either species.
Interspecific communication via agonistic visual displays,
such as body position or cheliped posture, has been shown to be important in determining patterns of microdistribution within a restricted habitat (Hazlett,
1976). Hazlett (1976) also stated that the coloration pattern on the major chela appears to be significant. E. depress us is an omnivorous crab, consuming primarily algae and detritus scraped from the substrate with its specialized minor cheliped. In addition, stomach contents analyses revealed the presence of shell fragments, polychaete setae, sponge spicules, pieces of crustacean exoskeleton, and amphipods (Bahr, 1974;
McDonald, 1977). In the present study, no stomach contents analyses were performed, nor any microscopic 17
examination of mouthpart setation, which could indicate a
partitioning of available food resources by the two
species. However, it should be noted that their cheliped morphology is extremely similar, both species having the
finger of the minor chela modified into a hollowed out spoon-like apparatus. Differences in mouthpart morphology and food habits would be expected in situations where both competitors occur in close proximity and exploit identical microhabitats (McDonald, 1977). During the present study, an overall female-biased sex ratio was found for both species. Wenner (1972) showed that deviations from the expected 1:1 sex ratio are widespread among marine crustaceans. He found four patterns resulting from the relationship between sex ratio and size: standard, sex reversal, intermediate, and anomalous. The species in my study fit the anomalous pattern: females dominated smaller to intermediate size classes and males were most abundant only in the larger size classes, with the sex ratio near unity in the smallest size classes. Swartz (1976b) also reported this pattern, for the xanthid Neopanope sayi.
Wenner (1972) proposed five explanations for the anomalous pattern - sex reversal, differential life span, migration, mortality, and growth rate. Of these, 18
differential growth rate is the most attractive
hypothesis, and can be explained in part by differences in
energy expenditure between the two sexes for both species.
Once a female reaches sexual maturity (ca. 5.5-6.0 mm),
much of her energy is allocated to vitellogenesis and egg
brooding, rather than somatic growth. Swartz (1976b) also found this to be true for N. sayi females, which had a longer intermolt time and thus were most abundant in
intermediate size classes, while males were able to
complete an additional 2-3 molts, and thus grow larger.
Nearly continuous breeding by mature intermediate-sized
females, typical of tropical populations, might retard
carapace growth (Diaz & Conde, 1989).
Swartz (1976b) also suggested that because larger
crabs are usually males, they may be more susceptible to
xanthid predators (e.g., Opsanus tau). Indeed, I noted
that larger crabs could not fit into the oyster crevices
as well. He also observed that the relationship between sex ratio and size in N. sayi may be adaptive, because males never copulate with larger females. Since most mature males are larger than most mature females,
individuals have a greater probability of encountering a mate. (He observed copulation only once for E. depressus). Since male xanthids may mate a number of 19
times each season, a maximum number of females are
inseminated, thus increasing the reproductive potential of
the species. The greatest peak reproductive activity for the two species coincided, with the exception of one additional
peak by E- depressus when E- dissimilis activity was low. Perhaps this is a possible means of decreasing larval
competition, either in the plankton or during settling. Of particular interest was that even though the overall size ranges were the same, ovigerous females of E. depressus were in greater abundance at smaller sizes than were ovigerous females of E. dissimilis. Eurypanopeus dissimilis ovigerous females were in greater abundance at larger sizes. This pattern seems to be indicative of
either an overall later maturity or faster growth rate for E. dissimilis, and thus a further possible means of decreasing larval competition. Regression analyses indicated a strong correlation between fecundity and size, as has been previously reported (McDonald, 1977; Lowery & Nelson, 1988; Diaz & Conde, 1989).
The overall size distribution (unimodal and symmetrical) obtained for both sexes of the two species is not uncommon for tropical decapod populations, reflecting continuous recruitment without size class disruptions due 20
to constant reproductive activity. This continuous recruitment and thus inferred constant mortality rates
indicates a stable population structure (Diaz & Conde, 1989). Because oyster-covered seawalls line the
Intracoastal Waterway in southeastern Florida, the areas
studied are constantly inundated with larvae from other populations to the north and south. In higher latitudes,
seasonal changes in size-frequency distributions are
common. McDonald (1982) reported that reproductive
activity of ~- depressus females was restricted to the
months of March through October, for a temperate South Carolina population. However, in subtropical southeastern
Florida, reproductive activity was constant, as evidenced
by the presence of ovigerous females year-round.
Conditions favoring the success of the Florida populations included good weather and favorable water temperatures
year-round, with no winter die-offs, as reported in McDonald's (1982) study. Monthly size-frequency histograms revealed that most
individuals belonged to smaller size classes, supporting the hypothesis that faster growth rates occur because molting frequency is increased up to 5.0-6.0 mm. At this point, sexual maturity is reached (pers. obser.).
McDonald (1977) found that growth of ~- depressus through 21
the smaller size classes was relatively rapid, but that
the rate decreased with size. Smaller crabs molt rapidly
and have a relatively large percentage increase in carapace width per molt, while larger crabs molt less
frequently with a relatively small percentage increase in carapace width per molt. Field growth rate data on crustaceans is very scarce and it is not yet possible to determine the age of crustaceans in field populations. Accurate comparisons of age and size in "natural" crustacean populations are remarkably few in number (Wenner et al., 1974). Swartz
(1976a) obtained size frequency information for N- sayi.
However, he did not study a natural oyster bar, but an artificial habitat with oyster-shell filled trays suspended beneath a pier. Marked crabs were removed and measured at intervals. Turoboyski (1973) studied the complete life history of the xanthid Rhithropanopeus harrissii for 4 years, both in its natural habitat and in the laboratory, and he was able to define molting frequency and growth rate.
McDonald (1982) estimated the CW growth rate of E depressus at ca. 10 mmjyear, with an average life span of ca. 1 year and a maximum life span of ca. 3.5 years. He reported that females reproduced twice each year. Their 22
size at first reproduction was 6 mm. I found this to be
5.6 mm and 5.8 mm for E. depressus and E. dissimilis,
respectively. Possible discrepancies exist in areas of
McDonald's (1977) study, though, since crabs under 6.0 mm
were not examined. Ryan (1956) studied the life history
of E. depressus in Chesapeake Bay, and found that immature
males ranged from 3.2-6.0 mm, while females ranged from
3.6-6.4 mm. Males attained sexual maturity at 5.1-6.0 mm,
and females from 5.5-6.4 mm. This generally concurred with values obtained for the two species in the present
study.
Martin et al. (1984) studied the development of
juvenile E- depressus up to the fifth crab stage (ca. 2.4 mm, ca. 71 days after hatching). In the present study, the smallest crabs measured were 1.5 and 1.6 mm for E. dissimilis and E. depressus, respectively. These would be equivalent to the third crab stage described by Martin et al. (1984).
Blyler's (1987) comparison of the larval development between E. depressus and E. dissimilis revealed that both species have 4 zoeas and 1 megalops. However, the developmental times to pass through these stages (17.5 days forE- dissimilis, 16 days for E. depressus), as well as the overall larval sizes, differed slightly for the two 23 species. She concluded that these factors decreased the level of interspecific competition between similar members of the planktonic community, and promoted microhabitat and resource partitioning in juveniles and adults.
Sulkin et al. (1983) demonstrated a high degree of depth regulation (upward migration) for larval E. depressus. They proposed that this enhanced the successful recruitment of the megalops to the appropriate adult habitat. If this parameter were investigated for E. dissimilis, it might indicate differing spatial positions of the two species in the water column, thus decreasing larval competition. Both species grew to a larger average size at the
Royal Palm site than at the Hillsboro Bridge site. This could have been the result of the presence at Hillsboro Bridge of significant numbers of juvenile £. herbstii, a crab that was exceedingly rare at the Royal Palm site. McDonald (1977) suggested that juvenile £. herbstii compete at the microhabitat and trophic levels with E. depressus. Additionally, the Hillsboro Bridge site has significantly more wave action and current than the Royal Palm site. The turbulence thus resulting could remove or prevent the deposition of detrital material, thus diminishing available food. Interestingly, more 24
individuals of E. depressus occurred at the Royal Palm site, while more individuals of E. dissimilis occurred at the Hillsboro Bridge site.
Collections made throughout eastern Florida indicate that E. dissimilis is restricted to a more southerly distribution than E. depressus (pers. obser.). Why does its occurrence seem to be restricted to vertical seawall
oyster reefs? Why isn't it found on typical horizontal
mud flat oyster reefs in southeastern Florida, as is E depressus (and £. herbstii)? This could reflect some sort of larval specialization or preference for a vertical substrate. Or, perhaps competitors of E. dissimilis have some advantage on horizontal reefs.
An area of discrepancy exists concerning the
function, if any, and the occurrence of the red spot on
the inner ischium of the third maxilliped. Martin et al. (1984) reported for E. depressus that no red spot had developed at the fifth crab stage (ca. 2.4 mm) and that
"this spot had not yet developed at the tenth crab stage."
However, in the present study, the red spot was
consistently found (for both sexes of both species), down
to the smallest crabs collected ( <2 mm). Also, Abele (1970) noted that the presence of the red spot in E. depressus was not consistent among specimens collected in 25
northwestern Florida, but was only found in crabs associated with oysters. I have collected and examined E. depressus from different habitat types in eastern Florida,
and they consistently possessed this mark. In preliminary laboratory observations E. depressus was found to be aggressively ''dominant" in body and cheliped posture,
although further quantitative data are needed to
substantiate these observations. It is highly probable that interspecific behavior plays an important role in this apparently successful coexistence. CONCLUSIONS
1. There was no size-based sexual dimorphism for either
species of Eurypanopeus. However, ~. dissimilis had a
larger mean carapace width than ~. depressus, for both
sexes.
2. Both species grew to a slightly larger size at the
Royal Palm site. This could have been attributed to the presence of juvenile Panopeus herbstii at the Hillsboro
Bridge site, as well as strong current and wave action at that site.
3. Eurypanopeus depressus was more abundant at the Royal
Palm site, whereas ~. dissimilis was more abundant at the
Hillsboro Bridge site.
4. Females were significantly more abundant than males, for both species. In addition, females tended to be most abundant at intermediate sizes. This probably was due to energy expenditure in vitellogenesis by the female, which suppressed growth after maturity was attained.
26 27
5. Ovigerous females were present year-round, and thus
recruitment was continuous. Ovigerous females of ~-
dissimilis were more abundant in larger size classes, but
those of ~- depressus were more abundant in smaller size
classes. Fecundity increased with increasing carapace
width, for both species.
6. There were generally no differences in the life history parameters studied for these two species.
7. The "seawall oyster-reef" habitats inhabited by these crabs provides ample food and space, as reflected by the high densities of crabs collected. 28
Table 1. Associated species (transient and permanent) collected in Crassostrea virginica clumps in Boca Raton and Deerfield Beach, Florida, from February 1988 to January 1989.
Rhodophyta unidentified red alga Platyhelminthes unidentified turbellarian Annelida unidentified errant polychaete Mollusca Gastropoda Littorina angulifera Bivalvia Brachidontes domingensis Geukensia demissa Isognomon alatus Arthropoda Insecta Collembola Anurida maritima Crustacea Cirripedia Balanus amphitrite Balanus sp. Amphipoda Melita nitida Isopoda Cirolana parva Ligia exotica Decapoda Palaemonetes intermedius unidentified alpheid Petrolisthes armatus Pinnotheres ostreum Aratus pisonii Pachygrapsus transversus Sesarma cinereum Panopeus herbstii Bryozoa unidentified bryozoan Chordata Osteichthyes Gobiidae Bathygobius soporator Lophogobius cyprinoides Blennidae Chasmodes saburrae 29
Table 2. LiFe history and reproductive parameters For Eurypanopeus depressus and E. dissimilis. collected at Boca Raton and DeerField Beach. Florida. From February 1988 through January 1989. PARAMETER Total N 2.795 1. 856 males 885 684 Fe111ales 1. 486 900 sex undeter~ined 424 272 CW range Mean CW (.,m) 6.2 6.7 111ales 6.7 7.1 Females 6.8 7.5 Ovigerous Fe•ales 156 110 CW range (tata) 5.6-14.1 6.0-12.4 Mean CW <••> 8.3 9.7 Egg nu•ber range 270-1730 280-1210 Mean egg number 847 802 HILLSBORO BRIDGE Total N 901 1. 001 males 348 421 Females 553 580 ovigerous Females 74 75 Mean CW (1!1111) 6. 1 6.6 111ales 6.4 6.7 Females 6.6 7.3 RO'r'AL PALM Total N 1.470 583 males 537 263 Females 933 320 ovigerous Females 82 35 Mean CW (.,m) 6.3 6.9 males 6.8 7.8 Females 6.9 7.9 30 Hillsboro Bridge 10 10 • I v • a • a I . Zl • i ll "'"" ·J.J,.eor•o .J "'"".J.J,.eor•o.J f1DNTH f1DNTH Royal Palm c> 10 d>IO ~ . • I a 0 -1..1..-L,_L.,-L,..I...,.JL..,..L... ..L.,.J..rL,-J- PIIAJ J A8011DJ I'JfA]JA80110J IKIIfTJI IKIIfTJI Fl~;ures la-d. Mean Ca..! o-f' m•le and -fem•le Eurwe•noe•u• depreasus cc11ec1'ed -from f"ebru•rw 1998 1'hrou1;h Januarw 1989,. aT Hlll•boro Brld9~ Deerfield Be•ch. and Ro'*dal Palm, Boca R•1on.. F"lorlda. Hillsboro Bridge 10 ~ . ++-i-+ ...... +r+r+ I ~ . I~ . • • •xa.J,JAIOIID,J KOIITII Royal Palm C) 10 ~. . I • •xa.J,JilftOIID.J IIOIITB f'l~ur•• 2•-d. M•an CW o., male and -f'emale Eurwpaoopeus dlss!m!Jis collec-ted .,rom F'ebruarw 1988 throu~h Janu•rld 1989. aT Hillsboro Bridge.. Deer-field B•ach,. •nd Row•l Palm. Boca Raton. F'lorlda. Hillsboro Bridge b) ID ID I ID aa•ae?aamuaaM••" aa4aa7aalouaaM••l7 CAJU~P.a.c. WTDTII < - ) CAJLAPAC. VDIT1II < - ) Royal Palm c) 30 d' ·~ I ID I ID 1)45a7aaJDa•aM••l7 11458?881DD.aH••l7 CAJlAPACII: WXDT.H < U > CAJlAPACI: NUTI'II ( 11M ) F'l~ur•• 3a-d. Ovarall •lza (CW> -fraquancw dl•frlbutlon• o-f mal• and -famale Eurweanoe•u• deer•••u• collecfed -ftrom Februarw 1988 'through Januar'-' 1989., af Hlll•boro Brld~•· O•arf'leld Beach. and Row•l Palm., Boca Rafon.. F'lorlda. 33 Hillsboro Bridge •> b) I I aac•a?aaJDIIDDM •• I? •••••.,••.,a•.:tw••n Cllll ttrac11 'MXDTil < - > CAJla.PACII NDTnl ( - ) Royal Palm 10 10 ? d' aD I ID I ID 1___, Uh-r, 0 r 0 aa4Sa?a81DUDDM •• I? aacsa?aaiDa••M••n CA&APACII vmTJI ( - ) CAaaPACII vmTJI ( 101 ) F"lsaur•• 4•-d. Ov•rall slz• CARAPACE .-JTH CARAPACE llalTH CARAPACE tmTH CARAPACE ...OTH <--> Figure 6b. Monthly size Hillsboro Bridge ., • PIIAJJA801111J ~ Royal Palm c) ., 'a ~ I PIIAJJA801111J ~ F'l-aur•• 7a-d. Abundanc:• Hillsboro Bridge ., b) J) ., d' • -a • ~ • 'i I • • r:a PIIA.JJ'A80liDJ ~ Royal Palm c> ., It • it • 'i 'i PIIAJ'J'A80liDJ' P II A J' J a 8 0 K D J' IIOIITJI IIOIITJI Fl~ur•s 8•-d. Abundanc• Ctrans-Form•d counts> of m•l• and .f:•m•l• Eur\4eanoe•u• dlsslmlli1, coll•ct•d from F•bru•r-., 1988 throu~h J•nu•r\1 1989, at Hillsboro Brid~e, D••r-FI•ld B•ac:h. and Ro\1•1 P•lm. Bee• R•ton. Florid.._ 3 2 1 P M A J J A S 0 H D J MOHTH b) li· dissimilis 3 1 P M A J J A S 0 H D J MOHTH F'IQur•• 9•-b. S1>< r•tlo (I)(Pr••••d as • quotl•nt. T'emal•• to mal••> of Eurvpanoe•u• deer•••u• and E:· dl11lmiiiL c:ol11c:t•d from F1bruarw 1988 throu-ah Januarw 1'989, at Hillsboro Brldg1, Deerfield Beac:~ F'lorlda. 3 J;. ciepr•••u• F M A J J A S 0 H D J MOHTH b) 3 li· dissimilis 2 1 F n A J J A S 0 H D J MOHTH Fl~ures 10a-b. S•x ratio ~;. depr•ssus a) P M A J J A S 0 H D J MOHTH 1;. dissimilis b) F M A J J A S 0 H D J MOHTH F'lt;;ures 11a-b. Number o-f ovigarous +'emalas 30 0 P M A J J A S 0 H D J MOHTH b) li· dissimilis P M A J J A S 0 H D J MOHTH F"l~ur•• 12a-b. Numbar o-f' ovl~arous T'amal•• <•xpr••••d as a p•rcan'ta~• o., all -f'ama1as> o., Eurweanopeus depressus and f;. dlsslm!lls. collec't•d from Februarw 1988 throu~h Januarw 1989. at Rowa1 Palm, Boca Raton.. F"lorlda. ::: Dissm1s 19 Depressus S6789Sn121314 CARftPACE WIDTH < MM ) F'"lgure 13. Size (CW> -f'r•quencw distribution o-f' all oul~iiaerous -f'em•l• Eurwpangpeus d•er•••u• and t. dl•slmlll• collected -f'rom F•bruarw 1999 ihrouliiah J•nuarw 1989. 45 ,. - rZ • 800 0 4 e 10 12 14 ' ~N:te Vt101H ~ b) t;;. d!sslmll!s 1~ ,. --681.6 + 166.8x rz• 0.991 m1000 ~ I fSOO 0 6 14 Fl~ur•• 14a-b. R•~r•••lon o., ·~g numb•r v•. c•r•p•c:• width o-f subsamp1•• o., ovh;•rou• Eurweanoe•u• d•er•••u• •nd t;;. dls•lmll!s T'•m•l•• <•;a~ numb•r• •r• dlr•ct c:ounts). Nln•tw T'lv• p•rc•nt con-fld•nc• lnt•rvals ar• shown. LITERATURE CITED Abele, L. 1970. The marine decapod Crustacea of the northeastern Gulf of Mexico. M.S. thesis, Florida State University. 137 pp. Abele, L. and W. Kim. 1986. 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