Parasites of the chaetognath Sagitta setosa in the western English Channel

Abstract

The temporal occurrence of parasites in Sagitta setosa J. Müller, caught one to three times a month during 1982 and 1983 off Plymouth, England, was investigated. S. setosa was infected by eight parasitic species: one nematode, five trematodes, one cestode and, possibly, one protozoon. Infection by a single parasite, normally found in the body coelom, was the rule. The nematode Hysterothylacium aduncum, as III-stage larva, was by far the most common parasite (56% of all parasites found), followed by the metacercariae of three non-encysted trematodes: Derogenes varicus, a didymozoid species and a Lecithochirium species. Two trematode species new to Chaetognatha were found. The percentage of infected S. setosa specimens ranged from 0 to 7%. Absence or low numbers of parasites from June to November were correlated to the disappearance of large S. setosa and the appearance of a new S. setosa generation. Trematodes may show great annual and seasonal differences in occurrence in S. setosa. No injuries due to parasites were seen. The reproduction of the S. setosa populations off Plymouth did not seem to be affected by parasite infections during 1982–1983.