Seasonal feeding-mode changes in the marine facultative kleptoparasite Trichotropis cancellata (Gastropoda, Capulidae): trade-offs between trophic strategy and reproduction

摘要

I report the first field study of seasonal patterns of host use by a marine kleptoparasite. Trichotropis cancellata Hinds, 1843, a marine snail, is a facultative kleptoparasite, able to suspension-feed and steal food from hosts. I investigated the incidence of kleptoparasitism and suspension feeding in T. cancellata populations at six sites around the San Juan Islands, Washington State, USA, through field surveys spanning 2 years. Facultative kleptoparasitism was the dominant feeding mode in the summer (>65% of the population) for T. cancellata of all sizes at all sites, with the highest frequency of parasitism in the summer and the lowest in the winter (the snail mating season). Sexually immature snails (smaller than 5 mm) formed the only group that predominantly did not leave hosts in the winter. Females returned to hosts after their egg masses hatched (late March to early April), while males returned earlier. Tube-dwelling worms from three families are the usual hosts of T. cancellata. Snail andworm host densities were correlated. Surprisingly, host size was not a primary trait determining host use. Prevalence of infection was less than 40%, suggesting that hosts are not in limited supply. Single-snail infections were the most common, althoughmultiple-snail infections occurred. Parasite load was not randomly distributed; an excess of infections with three or more parasites suggests that snails nonrandomly cluster on hosts. Understanding the population dynamics of this facultative kleptoparasite may shed light on the selection pressures driving the evolution of kleptoparasitism and suspension feeding and of snail adaptations to exploit their most common hosts.