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RESEARCH PRODUCT

Effects of the gill parasite Zeuxapta seriolae (Monogenea: Heteraxinidae) on the amberjack Seriola dumerili Risso (Teleostei: Carangidae)

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Zeuxapta seriolae (Monogenea: Heteraxinidae) infection was associated with important mortalities of amberjacks (Seriola dumerili) reared in tanks in the experimental facilities of the Spanish Institute of Oceanography (Western Mediterranean) during the period 1998–2000. Fish infested by this parasite presented characteristics similar to those described for Seriola quinqueradiata parasitized by Heteraxine heterocerca in Japan. All dead amberjacks had high parasite abundance (mean abundance±S.D.=686.7±125.4) and egg strings entangled in gills. The parasitological analysis of 17 live amberjacks collected from the infested tanks showed that 41.2% of fish harboured Z. seriolae with intensities ranging between 5 and 731 parasites per fish. No apparent relationship was observed between the abundance of the parasite and the condition factor (rS=0.175, n=17, p>0.5). However, the haematocrit values were significantly lower in the fish infested with monogeneans. The relationship between parasite abundance and haematocrit values was negative and statistically significant (rS=−0.625, n=17, p<0.01). The light and scanning electron microscope studies of affected gills revealed that the parasites attached to the host grasping one or two lamellae with each clamp of the haptor, which led to lamellar synechiae, lamellar clubbing and disruption of epithelial and vascular structures. A mild to moderate epitheliocystis infection was also detected in gills. Combined effects of gill damage and parasite blood feeding could be related to the anaemia and, finally, the death of the fish.