Parasite communities of Alepocephalus rostratus Risso, 1820 off the deep Mediterranean Sea: natural variability and influence of parasites on fish health

Abstract

Alepocephalus rostratus Risso, 1820 is the second most important fish species, in terms of biomass, inhabiting the deep slope of the Catalan Sea. However, its parasitic fauna remains understudied. In July 2010, specimens of A. rostratus (n=82) were collected on board the R/V García del Cid in the Catalan Sea (Western Mediterranean) with a semi-balloon otter-trawl (OTSB-14). A subsample of 10 individuals within this load was exclusively devoted to histopathological analyses. A portion of musculature of fishes was taken for biochemical markers determination. Environmental parameters (temperature in °C, salinity in psu, oxygen concentration in ml*l-1 and turbidity in voltage units) were recorded with a SBE25 CTD profiler at 5 m above the bottom. Eight different parasite species were found: 1 digenean, 1 monogenean, 1 cestode, 4 nematodes and 1 coccidian. Four of the seven metazoan taxa were larval forms, which combined with low parasite richness, correspond to a parasitic fauna pattern more typical of a bathypelagic fish species rather than a demersal one. The larvae Tetraphyllidea indet. (Scolex pleuronectis Müller, 1788) and Cucullaninae gen. sp. were the predominant species. Differences in abundance along depth were found mainly for the nematodes Cucullaninae gen. sp. and Hysterothylacium aduncum (Rudolphi, 1802). The former was found in higher numbers at lower depths, meanwhile the latter increased its abundance at greater depths. Both may indicate a shift in A. rostratus diet due to food scarcity at increasing depths. Cucullaninae gen. sp. was also related to high water turbidity, which may be related to high proliferation of one of its possible intermediate hosts (calanoid copepods), which in turn allow the high infection rates at these waters. Five of the metazoan parasites could be related to alterations in A. rostratus muscular enzymatic activity. Scolex pleuronectis, AnisakisType II and H. aduncum infections were associated with lower activity of acetylcholinesterase (AChE), whereas higher numbers of Cucullaninae gen. sp. were related with higher AChE activity. Moreover, higher abundance of Paracyclocotyla cherbonnieri Dollfus, 1970 and S. pleuronectis were related to higher activity of lactate dehydrogenase, which may indicate stress on fish due to parasitism. Despite some degree of stress has been detected due to parasitism, it does not seem to be reflected on histological alterations since none of the parasites was related to any pathology in the present study. Except for four individuals that presented melanomacrophage centers in spleen, none of the other fish presented any histological alteration in the examined organs (intestine, gills, gonads, liver and spleen)