Infestation of the jack mackerel, Trachurus declivis (Jenyns), with the cymothoid isopod, Ceratothoa imbricatus (Fabricus), in south eastern Australian waters

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Abstract
The tongue biter, Ceratothoa imbricatus, a cymothoid isopod, commonly found on a number of Australian inshore temperate fish species, was recorded in the pelagic jack mackerel, Trachurus declivis, in 26 of 43 trawls. The infestation rate was low (1·76%) in the 26 trawls where the association was noted. It is suggested that infestation occurs at an early age and that subsequent growth by the isopod is mediated by the host. Estimates of the growth parameters indicate that the isopods are long lived, and can remain for at least nine years on the host fish. Protandrous hermaphroditism, common in the Cymothoidae, is found in C. imbricatus with females attaining a large size in the bucco‐pharyngeal cavity while the male, with suppressed growth, is restricted to the branchial arches.Although cymothoid isopods are commonly described as parasites, observable damage on the jack mackerel was restricted to the tissues at the points of attachment of the isopods. The effect of the isopod on the jack mackerel may not be as debilitating as might be expected and it is argued that the relationship may best be described as commensalism.

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