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.

Effect of acanthocephalan parasites on hiding behaviour in two species of shore crabs.

The effect of acanthocephalan parasites (Profilicollis spp.) on the hiding behaviour during low tide of two species of shore crabs (intermediate hosts), Macrophthalmus hirtipes (Brachyura: Ocypodidae) and Hemigrapsus crenulatus (Brachyura: Grapsidae), was examined at Blueskin Bay, South Island, New Zealand. Exposed M. hirtipes were found to have significantly higher infection levels than did hidden conspecifics. This pattern was not observed for H. crenulatus. Mean cystacanth numbers were found to be considerably higher in M. hirtipes than H. crenulatus. Crabs exposed at low tide are at a greater risk of predation by definitive shorebird hosts than are hidden conspecifics. Preferential manipulation of one intermediate host species over another could influence diversity within ecosystems.

Inequalities in size and intensity-dependent growth in a mermithid nematode parasitic in beach hoppers.

Inequality in body sizes is a common feature in populations of helminth parasites, with potential consequences for egg production and population genetics. Inequalities in body lengths and the effects of intraspecific competition on worm length were studied in a species of mermithid nematode parasitic in the crustacean Talorchestia quoyana (Amphipoda: Talitridae). The majority of the 753 worms recovered were relatively small, and an analysis using a Lorenz curve and Gini coefficient suggested that there were no marked inequalities in body lengths among the worms. Total worm length in the 356 infected amphipods (i.e. the sum of the lengths of all the worms in a host) increased steadily as a function of the number of worms per amphipod, whereas the length of the longest worm per amphipod peaked in amphipods harbouring intermediate numbers of worms. This last result was not significantly accounted for by the observed increase in host size with increasing intensity of infection, but resulted from a correlation between worm length and host size. As the number of worms per amphipod increased, the relative sizes of the second-, third-, and fourth-longest worms per host increased markedly. This means that relative inequalities in sizes become less pronounced, i.e. subordinate worms get closer in size to the longest worm, as the number of worms per host increases. The main consequence of this phenomenon is that worm sizes in the mermithid population are more homogeneous than they would be if intraspecific competition had stronger effects on worm growth.