Xanthidae

Abstract: Electrophoretically detectable variation in 38 proteins and color morphology were used to determine the evolutionary relationships of crabs of the genus Menippe (Xanthidae) in the southeastern USA. Both allele frequencies (=genotype) and color morphology (=phenotype) showed that one species, Menippe mercenaria, is probably a taxonomic supergroup composed of two taxa (semispecies). One taxon (the western Gulf form) is distributed from northwest Florida westward through Texas. The second (the peninsular Florida form) ranges through the Florida peninsula from northwest to east central Florida, and in North Carolina. The taxa appear to have hybridized in two discrete regions: in the Gulf of Mexico (northwest Florida) and in the Atlantic Ocean (east central Florida to South Carolina). The agreement of patterns of geographic variation in genotype and phenotype with the geological record and estimated times of divergence based on genetic distances suggests that the observed patterns are the product of the influence of Late Cenozoic changes in climate and geology. The Atlantic zone of hybridization was formed prior to the closure of the seaway across north Florida connecting the Gulf of Mexico and the Atlantic, and the northwest Florida zone at some time subsequent to the closure. The present distribution, and the location of zones of hybridization, between the two semispecies of M. mercenaria illustrate the importance of the interaction of historical geological and climatic events with ecological boundaries in determining the distribution and interactions of shallow water marine species.

Abstract: LITT LE is known of the influence of physical factors on the complete larval development of Xanthid crabs. In reports on the successful rearing of the larvae in the laboratory, data on salinity and temperature are either omitted (Hart, 1935; Knudsen, 1958) or, when given, are limited in range (Chamberlain, 1957). Connolly (1925) mentions that the larvae of Rhithropanopeus harrisi were obtained from waters of different salinity but does not give the hydrographic data for the collecting stations. Bousfield (1955), concerned primarily with distribution of barnacle nauplii in the Miramichi Estuary, does indicate the range of salinity in which he found adults and zoeae of R. harrisi. Studies on the development of the Xanthidae have been primarily confined to descriptions of larval stages based on reconstructions from the plankton (Birge, 1883, Neopanope texana sayi; Hyman, 1925, N. texana sayi, Panopeus herbstii, Eurypanopeus depressus and a few stages of other Xanthids; Connolly, 1925, R. harrisi). More recently the larvae of some species of this family have been reared successfully in the laboratory (Hart, 1935, Lophopanopeus bellus; Chamberlain, 1957, N. texana sayi; Knudsen, 1958, 1959a, 1959b, L. leucomanus leucomanus, L. bellus diegensis, Paraxanthias taylori; and Costlow and Bookhout, 1961a, E. depressus, and 1961b, P. herbstii).

Abstract: By examining the morphological basis of force generation in the chelae (claws) of both molluscivorous and non-molluscivorous crabs, it is possible to understand better the difference between general crab claw design and the morphology associated with durophagy. This comparative study investigates the morphology underlying claw force production and intraspecific claw dimorphism in six brachyuran crabs: Callinectes sapidus (Portunidae), Libinia emarginata (Majidae), Ocypode quadrata (Ocypodidae), Menippe mercenaria (Xanthidae), Panopeus herbstii (Xanthidae), and P. obesus (Xanthidae). The crushers of the three molluscivorous xanthids consistently proved to be morphologically ‘strong,’ having largest mechanical advantages (MAs), mean angles of pinnation (MAPs), and physiological cross-sectional areas (PCSAs). However, some patterns of variation (e.g. low MA in C. sapidus, indistinguishable force generation potential in the xanthids) suggested that a quantitative assessment of occlusion and dentition is needed to understand fully the relationship between force generation and diet. Interspecific differences in force generation potential seemed mainly to be a function of differences in chela closer muscle cross-sectional area, due to a sixfold variation in apodeme area. Intraspecific dimorphism was generally defined by tall crushers with long in-levers, though O. quadrata exhibited an extreme dimorphism suggesting that factors unrelated to a speed–strength dichotomy (e.g. sexual selection) have shaped dimorphism of that species. It is concluded here that: (1) the majority of interspecific claw strength variation is a function of closer muscle cross-sectional area; (2) variation in claw morphology related to force production and transmission does have some relation to hardness of diet; (3) claw dimorphism in many species does seem to be related to strength and speed trade-offs; (4) factors besides molluscivory must be considered to understand claw evolution fully; (5) a quantitative assessment of force distribution, lacking in the literature, is essential for a more complete understanding of the relationship between claw design and ecological function.