A gastropod scavenger serving as paratenic host for larval helminth communities in shore crabs.

The whelk Cominella glandiformis is an important predator-scavenger of New Zealand intertidal ecosystems; a few whelks can quickly eat all the soft tissues of recently dead crabs. In this study, we demonstrate that whelks can also ingest and act as paratenic hosts for at least 4 helminth species that use crabs as intermediate hosts: metacercariae of the trematode Maritrema sp. and of another unidentified trematode, larval acuariid nematodes, and cystacanths of the acanthocephalans Profilicollis spp. Large whelks ingest disproportionately more helminth larvae than small whelks, but the survival of parasites during their short stay in the whelks is not affected by whelk size. The majority of metacercariae and nematodes are passed out in whelk feces within 3 days of ingestion, whereas the few cystacanths found did not leave whelks until after that time; no parasite was left in whelks 5 days postingestion. Survival of all 4 helminth species was generally very high, though it decreased day by day in 2 species. Given that the avian definitive hosts of all 4 helminths also eat whelks, our results indicate that alternative transmission pathways exist and that parasites can take routes through food webs that are too often ignored.

From first to second and back to first intermediate host: the unusual transmission route of Curtuteria australis (Digenea: Echinostomatidae).

The trematode Curtuteria australis uses the whelk Cominella glandiformis as first intermediate host and the cockle Austrovenus stutchburyi as second intermediate host before maturing in shorebirds. The whelk also happen to be an important predator of cockles on intertidal mudflats. In this study we show that whelks can act as temporary paratenic hosts for the trematode. A single whelk feeding on 1 cockle can ingest large numbers of metacercariae, which remain within the whelk for 1-3 days before passing out in feces. The viability of these metacercariae assessed as the percentage capable of successfully excysting under conditions simulating those inside a bird's digestive tract, is lower after passage through a whelk (48%) than before (59%). Still, given that shorebird definitive hosts prey on whelks as well as cockles, survival inside the whelk allows C. australis to complete its life cycle: overall, though, whelk predation is likely to be an important sink for the trematode population. To our knowledge, this is the first report of a trematode using a snail as both first intermediate host and paratenic host, offering an alternative transmission route for the parasite as a result of the unusual trophic relationships of its hosts.