Abstract
Pelseneeria castanea is a small parasitic snail found on the sea urchin Strongylocentrotus nudes in Japan. From 100 to 282 individuals of S. nudus were collected at intervals of one to four months from June, 2001, to September, 2003, in coastal waters of the Sea of Japan at Tomari in the Shiribeshi District of western Hokkaido, Japan, in order to determine the body size and age of infected hosts, the breeding season of P. castanea, and annual and seasonal changes in infection level. Snails were observed only on small-sized S. nudus of less than 50 mm in test diameter. Infected hosts were all one or two years old in 2001 and one to four years old in 2002. No snails were found on urchins aged three to eight years hosts in 2001, nor on those aged five to eight years in 2002. It thus appears that P. castanea infects smaller and younger S. nudus and induces the mortality of infected hosts. The highest prevalences of infection each year were 94.7% in September, 2001, 16.4% in November, 2002, and 10.6% in June, 2003. Mean intensity of infection sharply increased in spring and summer and reached the highest level in autumn, but declined abruptly in early winter to the lowest level in late winter. Frequency of occurrence of urchins with snail egg capsules showed a similar trend. Egg capsules were most frequently found from August to November, which indicates that this is the breeding season of P. castanea.
