Effects of imposex and parasite infection on the reproductive features of the Neptune whelk Neptunea arthritica

Abstract

The effects of imposex and parasite infection on the reproductive cycle of Neptunea arthritica as well as sexual maturity size were examined from April 2003 to August 2004 in Saroma Lagoon, Hokkaido, Japan. Samples were sexed and divided into two groups (normal and abnormal), based on the degree of imposex and parasite infection. In normal whelks, males reached sexual maturity at a smaller size (60 mm in shell length) than females (75 mm). Normal whelks copulated from March to August, with a peak between May and July, and spawned from May to August, with a peak in June–July. In all groups, the female ovary and pallial oviduct, and the male seminal vesicle developed synchronously, and inverse to the male testis. Abnormal whelks follow a normal reproductive cycle. However, significantly lower reproductive organ index values, modifications in the number of individuals and periods extension in each developmental stage were observed. Gonads of individuals with severe imposex (stage 3) and parasite infection were reduced to a thin layer of tissue or eliminated. Imposex (stage 3) and parasite infection had serious effects on the population reproductive cycle of N. arthritica, but infection by parasites seemed to be the more disruptive phenomenon according to its proportion (25%).

Keywords:

• Neptune whelk
• imposex
• parasite
• reproductive cycle
• sexual maturity size

Published in collaboration with the University of Bergen and the Institute of Marine Research, Norway, and the Marine Biological Laboratory, University of Copenhagen, Denmark

Published in collaboration with the University of Bergen and the Institute of Marine Research, Norway, and the Marine Biological Laboratory, University of Copenhagen, Denmark

Acknowledgements

We would like to thank Mr K. Maekawa and through him the Aquaculture Cooperative of Lake Saroma for the authorization for the execution of this investigation and their participation during this process. Also our thanks go to the valuable comments and suggestions of Professor S. Goshima, Professor John Bower, Professor M. Shimizu and also to Dr Sachindra Nakkarike, Dr Kazuyoshi Nakata, Dr K. Baba and Mr Y. Sakai for their valuable assistance and support.

Notes

Published in collaboration with the University of Bergen and the Institute of Marine Research, Norway, and the Marine Biological Laboratory, University of Copenhagen, Denmark