Parasitological studies on farmed sea bass (Dicentrarchus labrax) and gilthead sea bream (Sparus aurata) in Malta

Abstract

The objectives of this study were twofold: i) to compare the parasitic fauna of gilthead sea bream Sparus aurata which were being reared in offshore cages with those which were being reared in land-based ponds. ii) to compare the parasites of gilthead sea bream with those of sea bass Dicentrarchus labrax, which species were both being reared in offshore cages. The correlation between the incidence and prevalence of parasites and consequential pathology was also studied. A total of 310 fish distributed between two species were sampled from different systems. Upon parasitological screening, 37 different species of parasites were identified; viz., 18 protozoans, 8 monogeneans, 2 digeneans, 3 crustaceans, 2 cestodes, 2 metacercariae (eye fluke), 1 unclassified nematoda and epitheliocystis (Chlamydia - like organisms). In a parasitic survey of 155 gilthead sea bream from the offshore sea cages in local waters in Malta, seventeen different ectoparasites were identified, with the most commonly occurring being Furnestinia echeneis, Lamellodiscus ignoratus, Cryptocaryon irritans and Amyloodinium ocellatum. Eight different species of endoparasites were recorded, with Ceratomyxa sparusaurati, being the most common. In a parasitic survey of 64 gilthead sea bream from land-based ponds, fourteen ectoparasites were identified, of which Furnestinia echeneis, Amyloodinium ocellatum and Lamellodiscus ignoratus, were the most commonly encountered. Eight species of endoparasiites were recorded, with Diplostomum sp., Diplostomum-like, Myxidium leei and Certomyxa sparusaurati species being the most common; the incidence level of endoparasites was however generally low. In a parasitic survey of 91 cultured sea bass twelve species of ectoparasites were identified, with Diplectanum aequans, Diplectanum laubieri, Cryptobia sp., and Trichodina sp. being the most common. Four endoparasite species were isolated, with Ceratomyxa labracis and Ceratomyxa diplodae being very common. When results from identical sampling periods (June to August 1998) for pond and cage gilthead sea breams were analysed it was observed that the parasite diversity did not differ substantially. In terms of level of incidence the parasite counts were, in the main, higher in pond fish as compared with cage fish; this was particularly evident for Gyrodactylus sp. However, these differences were not found to be statistically significant. When results from identical sampling periods (October 1998 to March 1999) for gilthead sea bream and sea bass from cages were analysed, it was observed that the parasite diversity does not differ substantially. In terms of level of incidence the parasite counts were, in the main, higher for sea bass than for sea bream. This was particularly evident in the case of Trichodina sp. and Cryptobia sp. which were much more abundant in sea bass than in sea bream resulting in the overall level of parasites being significantly higher in sea bass than in sea bream. A significant number of unusual findings are also described. For instance, the parasite Diplectanum laubieri was commonly found on the gills of sea bass sampled from the offshore cage farm. From the published information that exists so far it was expected to be a rare find. By and large the observations resulting from the examination of the histological sections corroborated those from fresh tissue smears. Wherever high parasite levels had been observed in fresh smears, the same was observed in tissue section with accompanying histopathological changes. It is a well documented fact that many parasites abandon the host fish soon after the latter die and this therefore has to be taken into consideration. Histopathological changes in the gills consisted largely of depletion of the mucus layer and damage to the primary and secondary lamellae. The changes in the secondary lamellae ranged from mild hyperplasia largely at their bases such as in Amyloodinium infections to virtually total destruction such as in Cryptobia infections. More extensive hyperplasia throughout the entire length of the secondary lamellae led to the complete fusion of these lamellae such as was observed in infestations by Furnestinia echeneis and Chlamydia-like parasites. Clubbing of the periphery of the secondary lamellae was also fairly commonly observed such as in Amyloodinium ocellatum infections. Another frequent observation was the detachment of the lamellar epithelium presumably due to the rupture of the pillar cells such as was observed in some cases of Amyloodinium infections. Obviously some of the above could well have represented different stages of one and the same pathological process. Histological observations also permitted the study of certain parasites which would otherwise prove difficult if not impossible to study in smear preparations such as (Chlamydia-like organisms). It is also possible to pickup parasite which for some reason rather may have been missed in fresh smear preparations as was the case with Henneguya sp. in this study.