Growth and development of Argulus coregoni (Crustacea: Branchiura) on salmonid and cyprinid hosts.

The obligate fish ectoparasite Argulus coregoni is strictly specific to salmonids and is very rarely found on other fish species. The ability of the parasite to grow and complete its life cycle on a cyprinid host, Rutilus rutilus, was compared with that on a typical salmonid host, Oncorhynchus mykiss. Rearing experiments were run for 42 d with newly hatched metanauplii in flow-through tanks. Body length and sex of the parasites were recorded every 5 d. Growth rates on O. mykiss exceeded those on R. rutilus from the age of 2 wk, at which time the parasites reached a length of about 3.5 mm. Males grew faster than females at the beginning of the experiment up to a length of 2.5 to 3.0 mm; thereafter, a faster growth rate was observed in females. In another experiment, association of parasites with the hosts was monitored and residence time defined as the period between attachment and first detachment from the host. Longer residence time was observed on O. mykiss than on R. rutilus; female parasites stayed on both fish species longer than did males. Faster growth of parasites could be associated with longer uninterrupted periods of attachment to hosts, since frequent detachment means higher energy losses and less time available for feeding. Despite its slower growth on R. rutilus, A. coregoni matured and laid egg clutches, but took 5 d longer than on O. mykiss. The potential of A. coregoni to complete its life cycle on cyprinids could have important ecological consequences, creating an infection reservoir when the main salmonid hosts are rare or temporarily missing.