Body size diversity across the fish tree of life and the impact of environment on maximum body size: Does water and macrohabitat type shape the distribution of size?

Background/Question/Methods

A number of hypotheses and assumptions of the distribution of body size across the phylogeny of fishes and across habitat has been made in the literature. Relatively high occurrence of body size reduction has been identified in fishes as compared to all other vertebrates. Similarly it has been stated that within fishes, the phenomenon of body size reduction most frequently occurs in freshwater systems relative to marine lineages. In addition, within freshwater fishes, body size reduction is more prevalent in tropical habitats. We used a dataset comprised of distribution, environment (i.e. water type) and macrohabitat data for approximately 32,000 species of fishes, to test for divergence of body size among broad taxonomic groups (class, order) as well as across environment and macrohabitat. We tested specific hypotheses and assumptions of body size distribution across major lineages, geographic regions and environments, and explored potential divergence in habitat type. Distributions of fishes within taxonomic groups, environments and habitats were compared to pseudo-distributions based on the distribution of all fishes, major lineages and log-normal distributions. 

Results/Conclusions

Divergence in body size among environments (i.e. freshwater vs. marine) is strong in fishes, and body size distributions within a given environment appear to be similar across major lineages. Fishes in freshwater environments typically show reduced body size as compared to marine fishes, with an increased shift towards right-skewed distributions despite showing reduced body size diversity overall. Divergence is also apparent among macrohabitats (i.e. river, lake, reef, non-reef). Riverine and lacustrine fishes showed reduced mean body size and body size diversity in association with a shift in the range towards reduced body size and right-skew. In contrast, reef and non-reef fishes showed increased mean body size and body size diversity, shift of the range toward larger body size and left skew. These patterns are generally consistent across major lineages. Differences in the body size frequency distribution characteristics of fishes appear to be influenced predominantly by affiliation to freshwater or marine environments, and influences of macrohabitat may be acting in a narrower taxonomic context.