Simultaneous hermaphroditism is, at least initially, favoured by selection under low density — and therefore it can be assumed that sperm competition has little importance in this sexual system. However, many simultaneously hermaphroditic nudibranchs have both an allo-sperm storage organ (the seminal receptacle) and an allo-sperm digesting organ (the copulatory bursa), suggesting the possibility of the occurrence of sperm competition. A nudibranch, Chromodoris reticulata, autotomizes its penis after every copulation and replenishes it within about 24 h to perform another copulation. We observed that the surface of the autotomized penis was covered with many backward-pointing spines and that a sperm mass was often entangled on the spines. This suggests that the nudibranch removes sperm that is already stored in a mating partner’s sperm storage organ(s) with its thorny penis. Using six microsatellite markers, we determined that the sperm mass attached to the penis were allo-sperm originating from individual(s) that had participated in prior copulations. We revealed that C. reticulata performed sperm removal using the thorny penis. These results suggest that competition in fertilization is quite intense and mating frequency in the wild is relatively high in this species.
Sexual selection Sperm removal Sperm competition Nudibranch Simultaneous hermaphrodite
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We are grateful to K. Sakai of the Sesoko Station of the University of Ryukyus, for his help during the field survey in Okinawa. We are also thankful to A. Shimizu and M. Okubo of the National Research Institute of Fisheries Science, Fisheries Research; N. Sato of Shimane University; and S. Nanami of Osaka City University, for their help performing DNA analysis. We thank S. Shiga for her help and encouragement. This work was financially supported by a Grant-in-Aid (#22570029) for Scientific Research to Y. N. from the Japanese Ministry of Education, Culture, Sports, Science and Technology.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
This article does not contain any studies with human participants performed by any of the authors. All applicable international, national, and/or institutional guidelines for the care and use of animals were followed.
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