sandy beach drag net. 2. 850. Mole lagoon proper tidal flat, drag net mole, under and by hand between stones. 3. 1500. Island lagoon proper tidal flat drag net. 4.
HELGOLANDER MEERESUNTERSUCHUNGEN Helgol~nder Meeresunters. 48, 59-78 (1994)
Distribution and habitat preferences of two grapsid crab species in Mar Chiquita Lagoon (Province of Buenos Aires, Argentina) E. Spivak 1, K. Anger 2, T. Luppi 1, C. Bas 1 & D. Ismael 2 I Departamento de Biologfa, Facultad de Ciencias Exactas y Naturales, Universidad Nacional de Mar del Plata; C. C. 1245, 7600 Mar del Plata, Repfiblica Argentina 2 Biologische Anstalt Helgoland (Meeresstation); D-27483 Helgoland, Federal Republic of Germany
ABSTRACT: Cyrtograpsus angulatus a n d Chasmagnathus granulata (Grapsidae) are the two dominant decapod crustacean species in the outer parts of Mar Chiquita Lagoon, the southernmost in a series of coastal lagoons that occur along the temperate Atlantic coasts of South America. Distribution and habitat preferences (water and sediment type) in these crab species were studied in late spring. There is evidence of ontogenetic c h a n g e s in habitat selection of both species. Recruitm e n t of C. angulatus takes place mainly in crevices of tube-building polychaete (Ficopomatus enigmaticus) "reefs" and, to a lesser extent, also in other protected microhabitats {under stones). In the latter, mostly somewhat larger juveniles were found, suggesting that these are used as a refuge for growing individuals. Adults are most frequently found on unprotected m u d d y a n d sandy beaches. C. angulatus was found in all parts of Mar Chiquita Lagoon, including freshwater, brackish, a n d marine habitats. C. granutata, in contrast, was restricted to the lower parts of the lagoon, where brackish water predominates a n d freshwater or marine conditions occur only exceptionally. It showed highest population density on "dry mud" flats and in Spartina densiflora grassland, where it can build stable burrows and w h e r e high contents of organic matter occur in the sediment. Such habitats are characterized by mixed populations of juveniles (including newly settled recruits) a n d adults, males and females {including a high percentage of ovigerous). Unstable "wet mud" as well as stony sand were found to be inhabited by chiefly adult populations, with only few ovigerous females. In "dry mud" flats, the proportion of males increased vertically with increasing level in the intertidal zone, showing a significantly increasing trend also in their average body size. These observations may be explained by higher resistance of males, in particular of large individuals, to desiccation, salinity, a n d temperature stress occurring in the u p p e r intertidal. However, an opposite, or no such, tendency was found in the distribution of ovigerous a n d nonovigerous females, respectively. With increasing distance from the w a t e r edge, salinity increased and pH decreased significantiy in C. granulata burrows, whereas temperature showed no consistent t e n d e n c y within the intertidal gradient. A highly significant linear relationship (r = -0.794; P 2.0 mm, m a i n l y fragments of bivalve shells), sand (from 0.062 to 2 ram), a n d silt c o m b i n e d with clay (20 m m CW) w e r e most f r e q u e n t on o p e n m u d d y or s a n d y beaches. A d u l t a n d juvenile crabs w e r e o b s e r v e d coexisting in almost e q u a l proportions (56 a n d 44 %, respectively) at site 2, living on a n d b e n e a t h stones. The sex ratio of C. angulatus w a s in g e n e r a l a p p r o x i m a t e l y 50:50 in s m a l l crabs (4 to 11 m m CW; living in "reefs"), b u t it was significantly b i a s e d towards f e m a l e s in all l a r g e r
Crab ecology in a coastal lagoon
69
individuals, with only 19 to 35 % males (Fig. 7). The proportion of ovigerous females (as a p e r c e n t a g e of all females) varied greatly a m o n g sites, from only 12 % on the b e a c h of a freshwater creek (site 7) to 86% in a brackish tidal creek (site 4). High n u m b e r s of ovigerous females were found also on b e a c h e s near the m o u t h of the lagoon, i.e. at sites 1 a n d 2 (Fig. 7).
Chasmagnathus granulata M a x i m u m population density of C. granulata was found on a tidal flatw h o s e upper zone was covered by Spartina densiflora grassland (site 5~ 53 individuals.m-2; however, no quantitative estimate could be m a d e at site 2, where this species also occurred in high numbers. At site 3 (bare m u d flats),two different microhabitats could be distinguished within only a few meters distance from each other: on relatively dry mud, where stable burrows could be excavated, C, granulata was twice as abundant as on soft wet mud, where excavation was not possible (Fig. 5). Size frequency distribution and sex ratio varied between the three sites at which C. granulata was sampled (Figs 7 and 8). Crabs of all sizes were found together in samples from the most densely populated habitat, site 5. Deep and interconnected burrows, placed a m o n g plants, were found to be occupied by juveniles, adult males, and both ovigerous and non-ovigerous females. A similar pattern was observed in burrows at site 3 (dry mud). Both populations (more pronounced at site 3) showed a bimodal sizefrequency-distribution, with juveniles < 1 0 m m and adults > 2 5 m m C W dominating (Fig. 8). At site 2 (mole), only larger juvenile and adult individuals were found. The sex ratio of C. granulata was approximately 50:50 at site 2. A slight but statisticallysignificant (P < 0.05) deviation occurred at the "cangrejal" (site5), with 56 % males and 44 % females, whereas the population at site 3 was strongly biased towards females, in particular in the dry m u d habitat (Fig. 8). The latter showed also the highest proportion of ovigerous females, where 67 % of all females carried eggs, followed by site 5 (Fig. 8). It is interesting to note that only very few females in wet m u d (site 3) and b e t w e e n stones (site 2) were ovigerous (Fig. 8). Burrow study Burrows i n h a b i t e d by adult Chasmagnathus granulata at site 3 (dry mud) were studied on five days as to their physical conditions during low tide. During the first sampling (24 November), the distance of burrows from the edge of the tidal creek was grouped a n d recorded only in three classes (zones); no correlation analyses w e r e carried out with these data. Four individuals escaped during sampling, so that no data are available for them. Six burrows contained more than one i n d i v i d u a l (three burrows with pairs of 1 male a n d 1 non-ovlgerous female each, a n d one each with 2 males, 2 ovigerous females, or a pair of 1 male a n d 1 ovigerous [emale). These cases of double occupancy had obviously a r a n d o m nature. In all other cases (118 samples), one i n d i v i d u a l was present in each burrow. Crab size did not show a very clear t e n d e n c y in relation to the position of the burrow within the intertidal m u d flat (Fig. 9). While males t e n d e d to be b i g g e r in the u p p e r intertidal zone (significant positive correlation b e t w e e n carapace width a n d distance of the burrow from the water edge; r = 0.329; P < 0.01), ovigerous females showed a weak but statistically significant opposite trend [r = -0.341; P < 0.05). No t e n d e n c y was visible in non-ovigerous females (Fig. 9).
E. Spivak, K. Anger, T. Luppi, C. Bas & D. Ismael
70
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Crab ecology in a coastal lagoon 40
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E. Spivak, K. Anger, T. Luppi, C. Bas & D. Ismael
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