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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%). [ABSTRACT FROM AUTHOR]

The state of imposex in Neptunea arthritica from seven sites along the coast of Hokkaido, Japan was examined in 2002 based on the criteria: (i) relative penis size index (RPSI); (ii) imposex frequency; (iii) stage frequency distribution of imposex in adult and immature whelks; and (iv) sex ratio. RPSI differed from site to site, although values from all sites were low (0.186-5.294). In particular, the RPSI values for four sites were very low (<1.0). In sites where immature whelks were also collected, the frequency of imposex was considerably lower in immature (7.7%-55.6%) than in adult whelks (50%-95.2%) except at one site. The imposex stage frequency distribution also differed among sites, and the trend in the adult whelks corresponded with their RPSI value. Female whelks showing severe imposex (Stage 3 and 3+) were restricted to large individuals. [ABSTRACT FROM AUTHOR]

Seasonal and tidal height variations in tissue weight and radular length of Nodilittorina radiata (Gastropoda: Littorinidae) were investigated for 14 months on a concrete breakwater at Kattoshi, Hokkaido, northern Japan. Snails were collected monthly from high, mid and low zones on the break-water. The regressions of dry tissue weight and radular length against shell height were determined monthly in each tidal zone. From spring to early summer, tissue weight in the low and mid zones increased more than in the high zone. During summer, tissue weight in the low and mid zones declined. After a small increase in autumn, tissue weight decreased until early spring in all zones, and was higher in the high and mid zones than in the low zone. Radular length increased from autumn to spring in all zones and then decreased during summer, particularly in the low zone. The variations in tissue weight and radular length are discussed in relation to temporal and spatial variations in snail activities, particularly feeding, and environmental variables. [ABSTRACT FROM PUBLISHER]