OPPOSITE TRENDS IN BODY SIZE VERSUS LENGTH-WIDTH RATIOS OF CRABS THROUGHOUT THE MESO-CENOZOIC

Cope’s rule states that animals evolve toward larger body sizes through time, and genus-level analyses for marine animals throughout the Phanerozoic are consistent with this rule (Heim et al., 2015, Science). Species-level analyses and patterns for complete time series of individual clades remain rare, however. Brachyuran decapods (true crabs), a highly diverse clade originating in the Early Jurassic with > 6500 extant and > 2000 fossil species, provide a suitable model system to analyze body size trends and length-width ratios over time. Based on analyses of a preliminary dataset consisting of > 1250 species-level measurements of marine crab carapaces, the median maximum body size has increased through time regardless of the size metric. This result is consistent with Cope’s rule and the best-supported model of morphological evolution is an unbiased random walk. More derived clades are larger than primitive crabs in the Jurassic, but primitive crabs also became larger through time. Body size trends are habitat-dependent: body sizes in coral- and sponge-associated environments remain small throughout the Meso-Cenozoic, while those in all other environments combined show a pronounced increase. Carapace width increased at a faster rate than length, so that length-width ratios gradually decreased through time, a trend best fit by a directional trend model. This trend provides the first quantitative support for one of the cornerstones of the process of carcinization: the evolution from a lobster ancestor with a long carapace to a proportionally much wider crab body form. This change contributed to the evolution of complex modes of crab locomotion such as walking and swimming sideways in both directions, behaviors that are likely to have been beneficial in, for example, escaping from predators. The increase in median body size (and thus per-capita energetic needs) in combination with their rise in diversity and presumably abundance, implies that the impact of these crustaceans on marine ecosystems is likely to have risen during and after the Mesozoic Marine Revolution.