NICHE MODIFICATION IN CHAETOGNATHA INFECTED WITH LARVAL TREMATODES (DIGENEA)

Infection by larval trematode parasites caused sympatric chaetognath species to live nearer the surface than normal, hence to select more upper water prey than did non‐parasitized chaetognaths of the same species. This was associated with contagious distribution of parasites within the population. Trematode larvae appear as opaque objects in their almost transparent chaetognath hosts. Since they also induced their hosts to be larger and to live in better‐lit habitat than non‐parasitized ones, the parasites render their hosts more conspicuous and thus probably more susceptible to sight hunting fish predators. Available data from seasonal cycles of parasitism in chaetognaths confirms that trematode‐parasitized chaetognaths suffer higher mortality than normal, probably ascribable to predation. However, the size increase also may discourage predation by smaller, incorrect final host species. Localization of parasitism (which lowers individual reproductive potential). and predation in the same segment of the population should minimize loss of reproductive potential for the chaetognath population as a whole. This conservation of host resources can be considered primarily of benefit to the trematodes. Knowledge of parasite‐induced host modification should enable prediction of mode of parasite transmission, and vice‐versa. Contagious distribution of parasites in their host populations can help damp population cycling. Copyright © 1979 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim