Biogeographic Patterns of Body Size in Stomatopod Crustacea: Ecological and Evolutionary Consequences

Species of stomatopods reach larger adult body sizes in soft (mud, sand) rather than hard (coral, rock) substrata, in deep rather than shallow habitats, in temperate rather than tropical latitudes, and in cold rather than warm environments. However, the proportion of species that inhabit soft substrata increases in deep habitats, at high latitudes, and in cold environments. Also, the proportion of species that occur in deep environments increases at high latitudes. Due to these interactions, comparisons were restricted so that only one variable was considered, providing a more rigorous test of the effects of that factor upon body size. Body sizes for species of stomatopods that excavate their own burrows in mud or sand still exceed those for species that occupy pre-existing holes in rock or coral if comparisons are restricted to species in shallow tropical habitats. Taxa from cold environments still reach larger body sizes than those from warm environments when only species from soft bottoms are considered. However, depth is not significantly related to body size when only species from tropical soft bottoms are compared, and latitude is not significantly related to body size when only species from shallow soft bottoms are analyzed. The previously established overall correlations between large body size and high latitude and deep water therefore appear to be due to the disproportionate number of species that occupy soft bottoms in these cold environments. Overall, patterns of body size and habitat use do not vary significantly among species of stomatopods in the East Atlantic, West Atlantic, and East Pacific oceans. However, the East Pacific stomatopod fauna from hard substrata in shallow tropical habitats is dwarfed compared to those in the West Atlantic. Body sizes of species from soft bottoms in shallow tropical habitats do not differ between the East Pacific and West Atlantic. Of all the traits considered, postlarval size is most highly related to adult body size in species of stomatopods. Also, species inhabiting hard substrata produce significantly smaller postlarvae than those from soft bottoms. Although the relationships are more variable, latitude and temperature appear to be related to postlarval size in at least some cases. In summary, for stomatopods in general, the factors that most influence adult body size are type of substratum occupied and temperature, while the factors that most influence postlarval size are adult body size, type of substratum inhabited by adults, and possibly temperature or latitude. 210This study questions the factors that constrain body size in the active, aggressive species which occupy non-expandable holes in hard substrata compared to the larger species which burrow in mud or sand. I suggest that mechanical or metabolic constraints, predation, and possibly the nature of the substratum have limited body size in stomatopods from hard bottoms; however, these factors have not operated comparably in reef-dwelling decapods, and the question remains open. Similarly, the factors that have fostered dwarfism in East Pacific compared to West Atlantic species from hard bottoms remain to be determined. Regardless of the causes of these shifts, the patterns of adult and postlarval size in different environments and geographical regions are ecologically and evolutionarily significant. A series of species arranged along an environmental or geographical cline could produce a community of sympatric species with contant ratios of body size without the intervention of competition. Also, species characterized by relatively large body size and long lived larvae should have high larval mortality, long lived iteroparous adults, less predictable recruitment at any one site, and less deterministic community structure than would be found in an assemblage of small species. Predation by stomatopods upon members of the cryptic reef community may be more severe in the West Atlantic than in the East Pacific due to the larger sizes of reef-dwelling stomatopods in the former rather than the latter region. The postlarvae of small species with reduced dispersal are more likely to settle near relatives and to have smaller geographic ranges than species with large long lived larvae. Most importantly, reduced adult body size and postlarval size will predispose species from hard substrata, from warm or tropical environments, and from certain geographic subregions (e.g. the East Pacific) toward higher rates of evolution.