Biological Differences between Chromodoris reticulata and Chromodoris tinctoria (Opisthobranchia: Nudibranchia)

Abstract

The shape and formation of the radula have traditionally been regarded as important characteristics in classifying nudibranchs. However, we cannot use radular information to aid in the classification of living specimens, because a radula cannot be examined unless dissected. In contrast, the reliability of mantle color patterns as a characteristic for species identification has been open to debate because of the wide variations observed among individuals. When we compared various characteristics of Chromodoris reticulata and Chromodoris tinctoria (Opisthobranchia, Nudibranchia), which have similar color patterns and are sometimes confused with each other, we found considerable differences in the overall body shape, in external and internal morphologies such as the rhinophores and secondary gill, shape of the spines on the surface of the penis, formation style of the embryos and egg capsules, and in mating behavior. A consistent relationship was observed between these characteristics and the color pattern of the mantle. During crossing experiments, although members of the two species tried to mate, they stopped copulation before inserting their penes. Our results support the theory that mantle color pattern is a reliable characteristic for identification of at least some nudibranchs and is applicable to ecological studies.