The discovery of acanthocephalans parasitizing chaetognaths.

A comprehensive literature review shows that 12 types of pathogens, micropredators and parasites are reported to interact with chaetognaths, mostly digenean trematodes, cestodes and nematodes larval stages. Through analysis of 78,152 chaetognaths from a monthly zooplankton time series (Jan 1996-Dec 1998) collected in the Mexican Central Pacific twelve acanthocephalan larvae parasitizing chaetognaths were discovered. This is the first record of an acanthocephalan parasitizing chaetognaths, raising to 13 the types of symbionts known to interact with them (excluding predators). Cystacanth larval specimens of Corynosoma sp. (Polymorphidae) were observed parasitizing the head, trunk and caudal cavities of three of the eight chaetognath species inhabiting this tropical coastal region (Flaccisagitta enflata, Flaccisagitta hexaptera and Zonosagitta bedoti). Because Corynosoma sp. parasitized chaetognaths during different months and years (Jan-Feb 1996, Mar and Jul 1997, Jan, Jun, Aug-Sep 1998) and because the total length of these cystacanths varied between 165-480 µm, suggesting growth inside the hosts, we conclude that chaetognaths are intermediate hosts of Corynosoma sp. The twelve parasitized chaetognaths were juveniles (without gonads) or immature adults (none in reproductively mature stage IV); therefore Corynosoma may have a negative influence on host reproduction. Marine crustaceans (amphipods, decapods, copepods, mysids and euphausiids) and fishes are common intermediate or paratenic hosts of acanthocephalans. Fish, sea birds and marine mammals are definitive hosts for marine Corynosoma species. The present discovery implies that acanthocephalans are transmitted trophically through different intermediate hosts (crustaceans, chaetognaths and/or fish); thus chaetognaths can also be part of the marine acanthocephalan life cycle.