Benign effect of the fish parasitic isopod Ceratothoa cf. imbricata on Selenotoca multifasciata (Scatophagidae) from Australia.

The tongue-biter cymothoid isopod Ceratothoa cf. imbricata is nearly ubiquitous in buccal cavities of the banded scat Selenotoca multifasciata (Scatophagidae) from Waterloo Bay, south-east Queensland. To test whether infestation affects fish growth or condition significantly, we explored parasitism and condition in 122 S. multifasciata specimens. The internal area of the buccal cavity and that occupied by ovigerous female isopods were measured, allowing the relative proportion of free internal area of the buccal cavity (PFIAO) to be calculated. Of 122 fish, 119 (97.5%) were infected; 35.3% had large female isopods, the remaining infections comprised much smaller mancae, juveniles and adult males. Mean intensity of infection was significantly correlated with fish total length (TL). In some fish, the female isopod occupied up to 80% of the buccal cavity area. There was little evidence of attachment damage in the buccal cavity; only 9 of 43 hosts analysed had restricted damage to the tissues at the points of attachment of the female isopod. Condition factor, food intensity index and stomach weight did not differ between fish with and without female C. cf. imbricata. The relative proportion of free internal area of the buccal cavity with respect to the fish total length (PFIAO/TL2 ratio) of fish infected with females correlated with food intensity and condition factor. Although the correlation was significant, the actual effect was not large because more than 70% of these 2 indices was not explained by the PFIAO/TL2 ratio (r2 < 0.3 in both cases). Despite the dramatic appearance of infestations and the high prevalence of C. cf. imbricata in the population, the near-absence of pathological alterations and the limited effect of the isopod on the condition indices and food intensity suggest that this isopod is relatively benign for S. multifasciata.