Larval patterns in the life histories of brachyuran crabs (Crustacea, Decapoda, Brachyura)

Developmental and larval patterns of brachyuran crabs are compared using data extracted from the literature for complete larval descriptions of 47 species and partial larval descriptions of 154 species. Focus was on four areas of interest: (1) Relationship between egg size, larval growth and size of first crab at settlement. Species with direct or abbreviated development had large eggs. Megalopal size and first crab size were positively correlated with adult size; egg size and zoeal size were not. Zoeal growth increased significantly with number of zoeal instars. There were no significant effects of egg size or adult size on incubation and larval periods. Species which produced only one brood per year had significantly longer brood incubation times, longer zoeal and total larval periods, and larger sizes of last zoea, first crab and adult size than multiple brood species. (2) Patterns indicating evolutionary constraints by lineage (family) on larval biology. Among the seven families with data for the most species (Cancridae, Grapsidae, Majidae, Ocypodidae, Portunidae, Pinnotheridae, and Xanthidae), there were significant differences in: adult size, incidence of species with single broods per year, brood incubation period, egg size, first zoea size, number of zoeal instars, first crab size, zoeal period, and total larval period. However, there were no significant differences in last zoeal size or megalopal size, except for large megalopas of Ocypode spp. (3) Patterns of larval biology with respect to adult habitat and climatic zone. Cold-water species (polar, cold-temperate, and deep water) had significantly longer brood incubation times, longer zoeal and total larval periods, and larger adult size than warm-water species (tropical, subtropical, and warm-temperate species dwelling shallower than 100 m). Species with life cycles restricted entirely to freshwater/terrestrial habitats had significantly larger eggs than marine species. Ocypode spp. on sandy beaches had large megalopas, and intertidal species had significantly smaller eggs and adult size than subtidal species. (4) Relationship between species geographic range and length larval period as a measure of dispersal potential. Species with greatly reduced or no larval duration in terrestrial or freshwater habitats had very small ranges. However, among marine species there was no significant relationship between extent of the range and duration of the larval dispersal period.

The patterns described in this paper should be accepted with caution, because larvae have been described for only a small proportion of species in any habitat, family, or the Brachyura as a whole. However, this analysis provides a preliminary synthesis of patterns in the context of brachyuran life histories based on a more extensive data set than available for most groups of invertebrates.