We described and compared the mating behavior and morphology of the reproductive system in chromodorid nudibranchs, then, examined how extensively the unique usage of the penis (autotomy and sperm removal) evolved among Chromodorids. In addition to Ardeadoris egretta, all of examined five species in Glossodoris autotomized their penises at the last stage of copulation. An interspecific difference was seen in penis autotomy in Noumea and Goniobranchus. A spiral structure was recognized in the vas deferens of autotomized species. This structure is supposed to be undifferentiated “next penises” stored in the vas deferens, which ensure successive copulation in autotomized species. Though the basic mechanism of penis replenishment was consistent, the reason to autotomize their penises may not be the same. Goniobranchus reticulatus is reported to remove sperm already stored in the mating partner’s sperm storage organ(s) with backward-pointing spines on the surface of its penis. Contrary to G. reticulatus, all of ten species that autotomized their penises in the present study did not have thorny but smooth penises. When they autotomized their penises, the tip of the penises still remained in the vagina of the partners. This suggests that autotomized penises in these nudibranchs function as a kind of copulatory plugs.

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