Occurrence of imposex and seasonal patterns of gametogenesis in the invading veined rapa whelk Rapana venosa from Chesapeake Bay, USA

The prevalence and intensity of imposex and observations on the seasonal gametogenic cycle are reported for the Asian veined rapa whelk Rapana venosa (Valenciennes, 1846), an invader of the Chesapeake Bay on the North American Atlantic coast. Adult whelks from opportunistic col- lections by commercial fishermen demonstrated inactivity from mid-November through early March, copulation from February through mid-November, and egg-laying activity from late May through August. All imposex females examined did not have a functional vas deferens (below Stage 3 on the vas deferens scale of Gibbs et al. 1987; J Mar Biol Assoc UK 67:507-523) and were expected to func- tion normally in reproduction. Although a preponderance of imposex females over 'normal' females by a ratio exceeding 2:1 was observed in 4 out of 5 regions examined in lower Chesapeake Bay, there was no significant difference in the numbers of functional females (imposex females and normal females) observed in any region. Shell lengths and wet tissue weights (g) of both imposex females and males were approximately the same and sexual dimorphism was not apparent at the population level. However, true female rapa whelks had lower wet tissue weights than either imposex females or males. Individual rapa whelks did not appear to be food-limited in the Chesapeake Bay, and the described reproductive activity suggests continued invasion within Chesapeake Bay habitats.