Trematode Accumulation by the Estuarine Gastropod Ilyanassa obsoleta

The accumulation of larval trematodes by Ilyanassa obsoleta (Gastropoda) at 2 estuarine sites in Delaware was studied. Initial infection status of snails was assayed by looking for shed cercariae. Native snails (most already infected) were deployed at sites A and B, and sentinel snails (putatively uninfected) were deployed at site B. All were individually marked and, if found, reassessed for infection after being free 1-3 summers. Himasthla quissetensis, Lepocreadium setiferoides, Zoogonus rubellus, Austrobilharzia variglandis, and Gynaecotyla adunca infections were observed in recovered snails. At site A, in 1993, 62 natives were recovered. Among the 26 initially testing uninfected, 15 had infections at recovery. Of 36 that released cercariae, 26 had the infection initially indicated. Of sentinels released at site B in 1996, 3 of 16 had infections when recollected. One probably was infected before transplant; at least 2 were infected at site B during 1996-1997. Among site B natives, 26 were later examined by dissection in 1996; 22 had the infection status initially revealed. Some site B natives (n = 35) escaped recapture in 1996 but were found in 1997, or 1998, or both, and were reexamined, most by cercarial release. The same cercariae were produced by 30. Among natives (both sites, n = 123), 27.6% exhibited some difference in infection status compared to the initial assay. This probably overestimates changes. Some differences were real but most can be discounted as cases where initial infection status was misrepresented.