ABSTRACT

" THE EVOLUTION OF COLOR PATTERNS IN OPISTHOBRANCHS "

Opisthobranch gastropods exhibit a dazzling array of color patterns that are presumed to have an aposematic function in deterring predation. It has been suggested that opisthobranchs sequester secondary toxic compounds as a basis for this protection from predators. Opisthobranchs appear to be common members of Müllerian mimicry complexes. In many cases, these complexes involve numerous species of opisthobranchs and may also include polyclad flatworms and other organisms. In some cases, similar color patterns between opisthobranchs are a result of convergent evolution. In other instances, similarity in color pattern may be related to common descent from a common ancestor. Convergence is clearly a plausible explanation of similar color patterns in opisthobranchs that are more distantly related to each other than to other opisthobranchs that have divergent color patterns. In other cases, where members of sister taxa have the same color pattern, evolution of that pattern from the common ancestor of these taxa is a more parsimonious explanation of the observed patterns. Examples from several opisthobranch clades where the phylogeny has been hypothesized are presented. Within Thuridilla, more basal members of the genus have more cryptic color patterns as in other members of the Plakobranchidae, while more highly derived members have bright, aposematic patterns. In Flabellina, some sister species have strongly divergent color patterns while others appear to have retained the basic pattern inherited from the common ancestor. Hypselodoris species shown strong cases of inheritance of color patterns within distinct major clades. Even within minor clades, more closely related species have similar color patterns. In a few instances, some species of Hypselodoris exhibit cases of convergent color patterns with other sympatric members of the genus. In Pectenodoris, the two sister species have color patterns that more closely match those of other more distantly related chromodorids, although some components of the color pattern appear to be derived from their common ancestor. Within the genus Halgerda, evolution of color patterns appears to have a strong phylogenetic component rather than appear as a result of convergence. Phylogenetic analysis proves to be powerful tool in increasing the understanding of color patterns in opisthobranch gastropods.

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