Phenotypic differences between crab populations inhabiting close but contrasting habitats. Neohelice granulata as a case study.

SPIVAK, EDUARDO; BAS CLAUDIA; LUPPI TOMÁS

Frankfurt

Congreso; 8th International Crustacean Congress (ICC-8); 2014

Phenotype varies among natural populations of a species that occupy different environments and are affected by diverse selection pressures, probably conferring a local fitness advantage; different phenotypes could be the result of local adaptation, if gene flow among populations is restricted, or phenotypic plasticity, when genetic differentiation is absent. Two large populations of the Southwestern Atlantic burrowing crab Neohelice granulata (Brachyura, Varunidae) inhabit bays that drain into a Patagonian gulf: San Antonio Bay (SA) and San José Gulf (SJ). They are separated by only 190 Km but differ in substrate characteristics and vegetation. Both populations showed no genetic differentiation between them and the hydrological pattern suggests a larval connectivity. However, crab morphometry varied markedly between sites and dramatic differences in maximal size were detected. The main goal of this study was to examine and quantify life history traits and reproductive biology on the basis of samples (substrate and crabs) collected on successive days in the middle of the reproductive season in both saltmarshes and mudflats of each site. Granulometry and organic matter content of sediment were measured as well as burrow dimensions. The following crab life history information was obtained: density, size frequency distribution (CW = carapace width), size-weight relationship, sex ratio, percentage of ovigerous females, fecundity, reproductive effort, hepatosomatic(HIS) and gonadosomatic (GSI) index of ovigerous and non ovigerous females with ovaries in three developmental stages (early, intermediate, late) and of males, and size at morphometrical maturity of females. Substrate was hard in SA with a high proportion of gravel and scarce organic matter whereas it was soft in SJ, with twice as silt and clay and higher organic matter content. Burrows were three times longer and twice as wider in SJ than in SA. Crab density was higher in saltmarsh than in mudflat in both sites, and higher in SA than in SJ mudflats. Size frequency distributions differed markedly between populations, crabs of both sexes being larger in SJ (maximum CW: males 38.3 vs 32.8 mm; females 35.5 vs 29.0 mm); recruits were present only in SA. Size-weight relationship did not differ between sites. Sex ratio was female biased in SJ but not in SA. Percentage of ovigerous females was lower and differed between habitats in SJ. Fecundity did not differ between sites but reproductive effort was higher in SA. Percentage of females with ovaries in each developmental stage, GSI and HSI differed between sites: Male HSI did not differed between sites but GSI was higher in SJ. The size of the smallest morphologically mature female did not differ between sites but the size at maturity range was shorter in SJ. The obtained results indicate the existence of differences in several life history traits and reproductive schedule between populations that will serve as a basis for an ongoing experimental study designed to understand its causes under the hypothesis of an effect of food availability and, finally, to evaluate the effects of local adaptation and phenotypic plasticity. Funding: UNMdP EXA 618/12 and CONICET PIP 830/13