POPULATION DYNAMICS and LIFE HISTORY of the Mangrovecrabaratuspisonll (BRACHYURA, GRAPSIDAE) in a MARINE ENVIRONMENT

BULLETIN OF MARINE SCIENCE, 45(1): 148-163, 1989

POPULATION DYNAMICS AND LIFE HISTORY OF THE MANGROVECRABARATUSPISONll (BRACHYURA, GRAPSIDAE) IN A MARINE ENVIRONMENT

Humberto D[az and Jesus Eloy Conde

ABSTRACT Aratus pisonii commonly inhabits the supralittoral zone of roots, branches and canopy of the red mangrove, Rhizophora mangle. Population dynamics and life history of this crab were studied during a 2-year investigation in a marine environment. Adult crab abundance was estimated with a mark-recapture program at 13 sites. Males attain greater carapace widths (CW) than females. The CW frequency distributions were similar through time, being more symmetrical for females, but strongly skewed to the left for males. Females were significantly more abundant. Sex ratios fluctuated within the year; however, there were no significant differences between years or seasons. A V-shaped sex ratio curve as a function of size was found. A continuously low juvenile abundance was detected throughout the study. Ovigerous females were continuously present in the population during the study. Their percentage fluctuated through time, with the highest value in the last trimester, coinciding with the rainy season. Number of eggs per female increased as a linear function of carapace width, having a mean of 11,577 eggs and extremes of 3,724 and 27,134. No conspicuous crab migrations were observed; marked individuals reappeared consistently at the sites where they had been marked even 5 months after marking. Carapace width and length showed a high isometric correlation for both sexes. Females tend to have a longer intermolting time than males, although the difference is not statistically significant.

Ara/us pisonii (Milne Edwards) is a grapsid crab commonly found from the supralittoral zone of roots to the canopy of Rhizophora mangle, in the borders of mangrove forests. On the Atlantic coast it is distributed from eastern Florida (USA) to northern Brazil and throughout the Caribbean islands. On the Pacific coast it ranges from Nicaragua to Peru (Rathbun, 1918; Chace and Hobbs, 1969). Despite this species' notorious abundance, wide distribution and potential role as a regulatory agent of the mangrove root fouling community, life history and dynamics of south Caribbean coastal populations have not been adequately studied. Hartnoll (1965) and Warner (1967, 1977) studied most aspects of its life history and some features of the population dynamics at Port Royal, Jamaica. Warner (1967), reported growth rate, fecundity, and reproductive output and proposed a lunar breeding rhythm. Von Hagen (1977) reported additional life history features of this crab in Trinidad, The complete larval development of A. pisonii from the same area was described by Warner (1968), who reported one prezoea, four zoeae and one megalopa; no variability in the number of stages was found. On the contrary, Diaz and Bevilacqua (1987) found distinct larval developmental sequences in A. pisonii larvae from marine and estuarine locations. In such sequences, one and sometimes two zoeal stages had been suppressed. Fur- thermore, Diaz and Bevilacqua (1986) found differential survival of A, pisonii larvae when reared under different salinity conditions, using larvae originating from marine and estuarine mangrove swamps. Warner (1970) studied the behavior of A. pisonii and Young (1972) studied some aspects of its physiological ecology. To partially fill the gap in information on A. pisonii, and at the same time provide a biological data baseline for a national park which sustains an ever increasing tourist pressure, we undertook a long term study of the population dynamics and life history of A. pisonii at Morrocoy National Park, Falcon, Ven-

148 DiAZ AND CONDE: ARATUS PlSONl/ IN MARINE MANGROVES 149

~""" \ 1 i j / i" i, ..,i

~ MANGROVE FOREST

o 1 Km

10"46' 68° 20' Figure I. Locations of study sites at Morrocoy National Park and its relative position on the Ven- ezuelan coast. ezuela. This paper describes the annual and seasonal variations in abundance, sex ratio, fecundity, size distribution, recruitment and biometry of an A. pisonii population over a 2-year span in a marine mangrove system.

MATERIAL AND METHODS

The Morrocoy National Park, located on the northwest coast of Venezuela (loo52'N-68°l6'W), contains a system of interconnected mangrove lagoons which communicates with the open sea by means of several channels (Fig. I). The surface water salinity throughout the Park fluctuates slightly, 36 ± 2 0/00, during the year (Diaz et aI., 1985). There is no fluvial input and the rain regime has only one peak, during the last trimester of the year (Matteucci and Colma, 1986). During this peak, due to rain runoff, transient estuarine conditions might appear along certain restricted mangrove areas near the mainland. The most conspicuous marine communities of the Park are mangrove forests, coral reefs and Thalassia beds. As along most of the Venezuelan coast, tidal range varies at most 25- 30 em. From October 1977 to September 1979, a mark-recapture program was conducted at 13 sites in the Park, according to the schedule and places indicated in Table I and Figure I. At each site, five cylindric double-entrance traps made of plastic mesh and baited with fish meat were deployed at approximately l-m intervals and fixed on branches and aerial sections of roots of the red mangrove (Rhizophora mangle). Traps were set by the sea margin, where they remained for 5 consecutive days. Each day, traps were inspected for crabs; these were marked by affixing a small, sequentially numbered, autoadhesive paper tag to their carapace. Tags were coated with transparent fingernail hardener. Carapace length and width were measured to the nearest 0.05 mm using a Vernier caliper. Sex was 150 BULLETIN OF MARINE SCIENCE, VOL. 45, NO. I, 1989

Table 1. Collection schedule of Ara/us pisonii at different sites in Morrocoy National Park

1977 1978 ]979 Site Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Nov Jan Mar Jun Sep

I X X X X X X X X X X X X X X X X X 2 X X X X X X X X X X X X X X X X X 3 X X X X X X X X X X X 4 X X X X X X X 5 X X X X X X X X X X X X X X X X X 6 X X X X X X X X X X X X X X X X X 7 X X X X X X X X X 8 X X X X X X X X X X X X X X X X X 9 X X X X 10 X X X X II X X X X X X X X X X 12 X X X X 13 X X X X

recognized and the presence of eggs was also assessed. Crabs were released at points of capture. Tag numbers were recorded daily. On thc last day of the sampling schedule, ovigerous females were collected in areas close to the sampling sites and preserved in 10% formalin in seawater. At the laboratory, the number of eggs was estimated by means of a volumetric method (Diaz et aI., 1983). Adult population abundance was estimated by means of a modified Peterson estimator, under a Schnabel census sequence (Seber, 1982). To grossly estimate the intermolting times, crabs were picked up haphazardly and confined indi- vidually in cages constructed around roots, including their aerial part. Cages were inspected at irregular time intervals and molting occurrence registered.

RESULTS Population Structure. - Throughout the study 2,675 males and 3,480 females were collected and measured for the analysis of population structure. The carapace width (CW) of males ranged from 6.50 to 26.80 mm. For females it ranged from 6.85 to 24.65 mm. The average CW size of males (x ± Sx = 19.45 ± 0.06, n = 2,666) was significantly larger (t = 18.02, P < 0.001) than that of the females (y ± Sy = 18.08 ± 0.04, n = 3,458). The size (CW) distributions of males and females collected during the whole study were significantly different (G = 831.67; P < 0.001). The CW distribution of females had less range and was more symmetrical than that of males (Fig. 2), which was strongly skewed to the left. The mode of the male distribution was shifted more to the right (20-21 mm) than that offemales (17-18 mm). The male and female CW frequency distributions were analyzed month by month by means of Stem-and-Leaf graphs (Tukey, 1977; Conde et aI., 1986). They were, in general, unimodal and slightly asymmetrical (Fig. 3A, B), with more size classes to the left of the median than to the right. Except on January 1978, the size class distributions for males and females for each month were statistically different (G-test, Sokal and Rohlf, 1981). The modal size (CW) class of males fluctuated month by month between 17 and 23 mm, while for females it fluctuated in a more narrow range: 17 to 19 mm. Juveniles were scant in the population (Fig. 3A, B). These graphs, and the boxplots (Fig. 4) reflect the absence of dramatic changes in population structure. Sex Ratio. - The overall female: male ratio (1.30: 1.00) of A. pisonii deviated significantly from the normal Mendelian or 1.00: 1.00 ratio (Gp = 105.59; P < DiAZ AND CONDE: ARATUS PlSONl/ IN MARINE MANGROVES 151

20 FEMALES, n = 3458 18 I 14 12 10 8 <.9w 6 « 4 r- z 2 w u a::: MALES, n = 2670 w 14 0... 12 I

6 4 2 6 7 8 9 10 II 12 13 14 15 16 17 18 1920 21 22 23 24 25 26 CARAPACE WIDTH (mm) Figure 2. Size (CW) frequency distributions of males and females of Aratus pisonii collected and measured during the whole study.

0.00 1). Monthly sex ratios were consistently female biased (Sign test; SN = 17; P = 0.000069), with the exception of May 1978, when the proportion of females was 48.9% (Fig. 5A). There were no significant differences between years or seasons (time series ANOYA; Gilchrist, 1978). When all data for males were plotted, a U-shaped sex ratio curve as a function of size appeared (Fig. 5B) resembling the "anomalous pattern" described by Wen- ner (1972). The 95% confidence intervals were lengthy for the sex ratio of classes in both extremes, due to the low sampling intensity. Similarly, sex ratio curves for each sampled month also showed an "anomalous" pattern (figures available on request). Recruitment.-Juvenile and small adults showed low abundance all through the year (Fig. 4). Only 1.5% of the males and 1.1% ofthe females captured were equal or less than 10 mm (Figs. 2, 3). There were no conspicuous peaks in recruitment. Reproduction. -Reproductive activity, as indicated by the presence of ovigerous females in the population, was continuous during the whole period of study. The percentage of ovigerous females in the population ranged from 5.1 in March 1978 to 29.3 in November of the same year. Maxima were reached in November 1977 and 1978 (Fig. 6A). There were significant differences in the percentage of ovigerous females between seasons (time series ANOY A; Gilchrist, 1978), with the highest percentage (21.3) in the last trimester, October-December, the rainy season. The percentage of ovigerous females in each size class increased continually 152 BULLETIN OF MARINE SCIENCE, YOLo 45, NO. I, 1989

lOs

Figure 3. Monthly size (CW) frequency distribution of Aratus pisonii. A. Females; B. Males. Top polygon numbers indicate highest frequency for each month. Polygons express the pooled data from collections at 13 studied sites. DiAZ AND CONDE: ARATUS P1S0NIl IN MARINE MANGROVES 153

• ~!:~'m upper four'h medIan lower FEMALES lourlh 26 ~

1?2~18 I I- 14 o 0:", - 10 '-HHHHHt t ~ ~ 3: 6 w U

AUG ----OCT NOV DEC JAN FEB MAR APR MAY JUN JUL AUG SEP NOV JAN MAR JUN SEP 1977 1978 1979 TIME Figure 4. Boxplots of carapace width for male and female Aratus pisonii collected during the study. from the 11-12 mm interval to the 23-24 mm (Fig. 6B). The number of eggs per female increased with CW (Fig. 7; correlation coefficient: 0.7629, P < 0.01). Fecundity ranged from 3,724 to 27,314 eggs per female, with a mean value of 11,577. Logarithmic and square root transformations of raw data did not sub- stantially improve the correlation. For the data included in the regression, ovigerous females CW ranged from 11.40 to 23.10 mm, with a mean value of 18.47 and a mode of 17.00 mm. Migration. -No conspicuous massive movements of crabs were observed during the period of study. From all marked individuals, 12% of them reappeared in following months at the sites where they had been marked, in one instance even 5 months after being marked (Table 2). Relative Growth. - The carapace width and length in the range studied show an isometric relation for both sexes, the relation between the measures being linear and almost perfect (r = 0.9949 and 0.9935, for males and females, respectively). A principal axis analysis (Sokal and Rohlf, 1981) gave these results for males: on the major axis YI = 0.3458 + 1.l043Y2 and for the minor axis Yl = 33.6800 - 0.9056Y2- Likewise results for females were YI = 0.1456 + 1.0803Y2 and YI = 32.6688 - 0.0926Y2' Abundance. - Variations in total abundance (modified Peterson estimator) through time in every collection site (Fig. 8) do not evidence definitive patterns: maxima and minima occurred asynchronously and oscillations were not in phase. A runs test for trend data (Sokal and Rohlf, 1981) showed that variations of abundance are random mechanisms, except in site 7 (Table 3). Molting. -Although the median intermolting time of ovigerous females were greater than that of non-berried females (Fig. 9) and this even greater than the 154 BULLETIN OF MARINE SCIENCE, VOL. 45, NO. I, L 989

w 70 <.?

o AUG OCT NOV DEC JAN FEB MAR APR MAY JUN JUL AUG SEP NOV JAN MAR JUN SEPT * *** *** •. •. .. 1977 1978 1979 T I ME

100 (f) B W -1 <.! 80 ~

~ 60

W <.9 <.! 40 f- Z W u 20 0::: W a... I I I I I I o /4 8 12 16 20 24 28 CARAPACE WIDTH (mm) Figure 5. A. Monthly variation in percentage of Aratus pisonii females. Asterisk indicates months contributing significantly to sex ratio heterogeneity. B. Mean percentage of males in each size class. All mean percentage values are related to the number of individuals of each sex present in each size class for the whole data collected through the studied period. 95% confidence intervals are shown. intermolting time for males, the differences were not statistically significant (one way ANOV A: F = 1.408; 0.25 > P > 0.10), possibly due to reduced sample size and high variability of the data. More data and rigorous experimental conditions are needed before any conclusions are reached on this feature.

DISCUSSION The global size (CW) frequency distribution for both sexes of Aratus pisonii at Morrocoy National Park were found to be unimodal and approximately symmetrical. This type of distribution is not uncommon for tropical decapod populations (Goodbody, 1965; Warner, 1967; Ansell et aI., 1972; Ahmed and Mus- taquim, 1974; Gibbs, 1974; Ahmed, 1976; Subramonian, 1977; Moreno, 1980; Hails and Yaziz, 1982; among others); although polymodal size frequency dis- DiAZ AND CONDE: ARATUS PlSONlllN MARINE MANGROVES 155

36 A 32 w 19 28

o AUG NOV JAN MAR JUN SEP 1979

en 100 B w ---l C> E5 60 C!:> >- C> ::s 40 w C!:>

Figure 6. A. Percent of ovigerous females through time. G Tests of Homogeneity reveal the following homogeneous sets: January-March April-June July-September October-December. B. Mean percentage of ovigerous females in each size class. 95% confidence intervals are shown. tributions for both sexes also have been reported (Wenner and Fusaro, 1979). In higher latitudes, seasonal changes in size frequency distributions are common, from unimodal to bimodal and vice versa (Osorio et a1., 1967; Efford, 1970; Samuelson, 1970; Smaldon, 1972; Reilly and Sai1a, 1978; Siep1e, 1979; Diaz, 1980; Simons and Jones, 1981; Thurman, 1985, among others). Bimodality or polymodality in the size frequency distribution usually reflects recruitment pulses, differential or catastrophic mortality, or behavioral differences. Unimoda1ity usually reflects a continuous recruitment without class disruptions, and constant mortality rates. Throughout the year we found for A. pisonii a continuous recruitment, and virtually constant unimodal and symmetrical size distributions, revealing a stable population structure. Disparity in sex ratios among crustaceans might result from sex reversal or 156 BULLETIN OF MARINE SCIENCE, VOL. 45, NO. I, 1989

26000

22000 (j) c..? <.? w 1800 lJ... 0 14000 ... . 0:: w ~ 10000 ~ . Z . 6000

2000 , , , 14 I'5 16 17 "8 I'g 20 21 22 23 CARAPACE WIDTH (mm) Figure 7. Regression lines for the relationship between carapace width of ovigerous females Aratus pisonii and the number of eggs. The major axis equation is y = -40,998.76 + 2,846.55x and for the minor axis is y = 11,577.10 - 0.00035x. The ellipse encloses a 95% confidence region for the bivariate mean, n = 87.

differential life span, migration, mortality and growth rates (Wenner, 1972; Winget et al., 1974; Swartz, 1976; Haley, 1979; Diaz, 1980). Sex reversal has not been reported for the Brachyura (Swartz, 1976) and for A. pisonii in Morrocoy National Park migration does not seem to occur. Although the effect of predation on natural mortality rates is difficult to assess, we observed for A. pisonii that predation by birds or other crabs seems not to be an important cause of mortality. However, when individuals jump into the water as a consequence of an escape reaction, they might promptly be attacked by fishes. Warner (1967) frequently observed Goniopsis cruentata (Latreille) preying on Aratus. We found individuals of Goni- opsis preying on A. pisonii inside the sampling traps. Nevertheless, the conditions posed by this confined scenario make it difficult to extrapolate to a more natural setting. Deviations from the expected 1:1 ratio may internally regulate the size of a population by affecting its reproductive potential (Giesel, 1972). This might even be valid for a species with a differential growth rate and/or a different life expec- tancy for each sex, producing larger and older individuals in one of the sexes. There is some evidence indicating slightly male biased sex ratio, in tropical intertidal crabs (Ansell et al., 1972; Ahmed and Mustaquim, 1974; Ahmed, 1976; Subramonian, 1977). On the contrary, using a catch-effort design for A. pisonii, we have found (unpubl. data) highly female biased sex ratios in an estuarine coastal lagoon (1.61: 1.00) and in a restricted marine bay (1.71: 1.00) along the Venezuelan coast. In the present study, we found a globally female biased sex ratio (1.30: 1.00) with some variability through time, which might be induced by some transient local factors. The similarity detected for the sex ratio curve as a function of size throughout the monthly collection and its lack of periodicity through time for each size class, might reflect the relative environmental stability surrounding the crab population. Thus, as suggested by Wenner (1972), some other factors might DiAZ AND CONDE: ARATUS PISONlllN MARINE MANGROVES 157

Table 2. Permanence in time of marked individuals of Aratus pisonii at collection sites. Dashes indicate no data due to sampling design

Crabs marked Months afier marking pcr month 2 4 Total (%) Number of crabs recaptured ]39 16 3 0 0 0 19 (13.7) 319 29 7 0 0 0 36(11.3) 323 27 11 2 0 0 40(12.4) 228 13 10 0 0 0 23 (10.1) 300 27 31 3 0 0 61 (20.3) 257 34 ]5 2 1 0 52 (20.2) 267 14 16 1 1 0 32 (12.0) 258 31 14 2 0 47 (18.2) 282 29 21 2 0 52 (18.4) 315 41 ]6 0 57 (18.1) 368 44 4 0 48 (13.0) 264 26 0 26 (9.8) 392 32 I 33 (8.4) 298 17 18 (6.0) 259 2 2 (0.8) 242 6 6 (2.5) 181 - (-) 4,511 305 (6.8%) 219 (4.9%) 24 (0.5%) 3 (0.07%) 1 (0.02%) 552 (12.2)

be influencing sex ratio changes as a function of size: longevity, sex reversal, differential mortality and growth. The similarity we found contrasts with the differences of sex ratio curves "between population and at different times of the year" proposed by Wenner and Fusaro (1979), based upon results for mole crabs from Fusaro (1977). These hippids inhabit sandy beaches, thus are exposed to environmental predictable and unpredictable changes which might result in pop- ulational or seasonal changes of their sex ratio curves (Diaz, 1980). Adult A. pisonii, instead, might be somewhat more independent from environmental changes because they spend most of their time on the aerial part of roots and branches of mangroves. The V-shaped sex ratio curve found for A. pisonii in Morrocoy might result from several interacting factors: 1. Low capture of small size individuals. 2. Development of gonads and pleopods prior to the actual mature stage might have induced a slow carapace expansion in young females. 3. The continuous breeding throughout the year shown by mature intermediate size females might retard carapace growth. This seems to be supported by the detected trend of a larger intermolting time for females, being more evident for ovigerous females. 4. A differential growth rate would explain the increase of male proportion in size classes beyond the 22 mm CWo A differential growth rate for A. pisonii has been suggested by Warner (1967). Our size distribution data with males reaching greater sizes than females and different modal size classes suggest a difference in growth patterns, probably due to the allocation of an important portion of energy for reproductive purposes by the females. The occurrence of periodic reproductive patterns seems to be proximally cor- related with latitude (Pillai and Nair, 1968; Samuelson, 1970; Smaldon, 1972; Ahmed and Mustaquim, 1974), and ultimately related to environmental conditions favorable to larval survival and recruitment and also to availability of food 158 BULLETIN OF MARINE SCIENCE, VOL. 45, NO. I, 1989

450

400 SITE I SITE 2 350 300 250 200 150 100 50 a

250 SITE 3 SITE 4 200 w 150 u Z 100 300 co

SITE 10

SITE 12

SITE 13 )('37.51

I 3 6 9 10 II 12 I 2 3 4 5 6 7 8 9 II I 3 6 9 L-1979~ LI977L--J 1978------1 L-1979--l TIME Figure 8. Variation in estimate abundance of Aratus pisonii at collection sites through time. Means and 95% confidence intervals are indicated. DiAZ AND CONDE: ARATUS P1S0N/IIN MARINE MANGROVES 159

Table 3. Significance of variations of estimated abundance of Ara/us pisonii in the collecting sites, assessed by means of a runs test for trend data (Sakal and Rohlf, 1981) (tm = r - u/r) where r = number of runs (sets of like signals preceded or followed by unlike signs), u, = expected number of runs and, = standard deviation of the number of runs

Abundance: Collection site No. crabs' site-I T Probability

I 124.60 0.609 0.504 2 67.7 1.826 0.067 3 74.2 0.782 0.434 4 83.9 1.389 0.165 5 106.9 0.609 0.542 6 103.3 0.000 1.000 7 57.1 2.065 0.039* 8 67.8 0.609 0.542 9 65.6 0.534 0.596 10 35.8 0.534 0.596 II 37.6 1.934 0.054 12 22.6 0.534 0.596 13 37.5 0.534 0.596 • Signi ficant. and mild environmental conditions for adult populations. Organisms inhabiting tropical zones, usually exhibit continuous reproduction (Giese, 1959; Sutherland, 1980). The continuous breeding pattern found for A. pisonii in Morrocoy also has been reported by Warner (I967) for the population at Port Royal, Jamaica, but in this population reproduction appeared related to a lunar rhythm. In the Ven- ezuelan population the maximum number of ovigerous females occur during the rainy season. Although lunar synchronization has an adaptive advantage for lit- toral organisms in avoiding larval stranding (Hartnoll, 1965; Reaka, 1976), this advantage seems diminished when the organisms live along the mangrove forest margin and high levels of water are always available, a fact that has also been recognized by Warner (1977). Aratus pisonii has been regarded elsewhere as a herbivore (Warner, 1967; de Lacerda, 1981); however, for the present study we were able to consistently trap this crab using fish as bait. In another part of Venezuela, Laguna de Tacarigua National Park, an estuarine coastal lagoon, A. pisonii is not attracted by fish meat (Conde and Diaz, in press) and only eats filamentous algae and mangrove leaves. Thus, this species appears to be an opportunistic omnivore (Diaz and Conde, in press), at some locations. Aratus pisonii at Morrocoy is a conspicuously sedentary crab. No mass migrations were observed, and some marked individuals were found in the same site for several months. No marked individuals were found in sites other than those where they had been marked previously. Warner (1970) observed that A. pisonii migrations were not extensive. Taking into account the habitat occupied by the adult population of A. pisonii, mass migrations seem unnecessary for this species to accomplish larval release or copulation, as is the case of Cardisoma guanhumi (Gifford, 1962; Moreno, 1980). Colonies of A. pisonii living in the inner portion of the mangrove forest, would need to migrate seaward for larval release. Such colonies might be implanted in places where high tidewaters carry late larval stages up to the inner portion of forest; the stressing conditions of such habitat might drive recruits out to the forest margins. We did not observe this type of migration; however, Warner (1967) suggested that ovigerous females of this species may migrate to the mangrove fringe. . 160 BULLETIN OF MARINE SCIENCE, VOL. 45, NO. I, 1989

(/) ~ 70 "'0

W 60 -~ I- 50 c.9 Z I- 40 ....J :J 0 ~ 30 n::: w I- Z 20 -- -- 10

0 MALES FEMALES OVIGEROUS FEMALES Figure 9. Boxplots of intermolting times of males, females and ovigerous females of Aratus pisonii.

The regression lines for carapace width and length do not pass through the origin; this is probably due to a change in relative growth occurring during an early stage (Warner, 1977). Since A. pisonii is a sedentary crab and recruitment is continuous through the year, large variations in its estimated abundance between months at one location (Fig. 8) could be a consequence of differential attraction to changing compositions of fish meat offered as bait, more than a consequence of life history phenomena or population dynamics. We found that A. pisonii is widely distributed throughout the Morrocoy National Park, but their abundance was variable across sites. There was no apparent relation between crab abundance and distance from mainland or water current intensity and/or direction. We observed that A. pisonii abundance seems to be related to fouling community richness and Catenella repens, a red alga present on the intertidal portion of the mangrove roots. This question requires further study. Although they appear throughout the year, juveniles and small A. pisonii were uncommon throughout the study. They showed a low attraction to the fish bait. However, careful examination of the mangrove roots showed few small crabs, no more than the proportion previously detected in the regular samplings. Similar findings have been made at some other marine and estuarine mangroves along the Venezuelan coast (Conde and Diaz, in press). Aratus pisonii larvae can cope with a wide spectrum of salinities, from 15 to 350/00 (Diaz and Bevilacqua, 1986; 1987). Since the salinity at Morrocoy remains DiAZ AND CONDE: ARATUS PlSONIlIN MARINE MANGROVES 161 close to 360/00, and ovigerous females are always present in the population, there is a continuous supply of larvae which might be reflected in some population dynamics features such as continuous recruitment, stable abundance, and unimodal size frequencies. Once the larvae reach the adult stage, the crab becomes somewhat amphibious and quite independent of water salinity but becomes de- pendent on the productivity of the mangrove environment, thus affecting fecundity and reproductive intensity, which seem to be related to availability of food re- sources. However, a more complete understanding of the population dynamics and life history tactics of A. pisonii will emerge only after complementary studies determine the factors controlling abundance and distribution of this crab.

ACKNOWLEDGMENTS

This work was partially financed by the Consejo Nacional de Investigaciones Cientificas y Tecno- 16gicas (CONICIT), Venezuela (Project No. S 1-0766) and a doctoral Scholarship to the junior author. Thanks are extended to J. Mujica and M. Bevilacqua for much appreciated assistance during the field campaigns, to A. Alvarez and 1. J. Ali6 for stimulating discussion and to unidentified reviewers for valuable criticism.

LITERATURE CITED

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DATE ACCEPTED: May 1, 1988.

ADDRESS: Centro de Ecologia, Instituto Venezolano de Investigaciones Cientificas. Apartado 21827. Caracas lO2D-A. Venezuela; PRESENT ADDRESS: (J.E.C.) Centro de Investigaciones Marinas, Univer- sidad Nacional Experimental Francisco de Miranda. Plaza La AntWana, La Vela de Coro, Falcon, Venezuela.