Environmental media and shape asymmetry: a case study on turtle shells

Although physical features of the environment are known to influence shape, to date, the effects of many environmental conditions on asymmetry remain unexamined. The present study aimed to examine the relationship between asymmetry and the fluid media (air versus water) that an organism inhabits and, subsequently, to use these findings to test a novel hypothesis that fluid environments can impose selective forces acting to constrain asymmetry to maintain or enhance biomechanical function. We examined carapace asymmetry in 114 species (69 aquatic and 45 terrestrial) from the chelonian superfamily Testudinoidea. The results obtained indicate that environment is correlated with the degree of asymmetry, but in different directions for the two clades that comprise the Testudinoidea. Within the Testuguria, aquatic turtles have lower levels of asymmetry than terrestrial turtles, which is consistent with our proposed biomechanical hypothesis. This pattern was not observed within the Emydidae, possibly due to the much shorter time that terrestrial taxa in this clade have existed. Nevertheless, the present study provides the first evidence for a relationship between fluid media and asymmetry in any taxonomic group. (C) 2008 The Linnean Society of London.