Abstract

Ceratothoa oestroides is an isopod parasite of the family Cymothoidae, which invades several fish species in shallow waters of the Adriatic Sea.⁵ Although never reported in free-ranging populations, it is considered a common isopod parasite in farm-raised sea bass (Dicentrarchus labrax L. 1781).^1,3 Ceratothoa oestroides is a protandrous, hermaphroditic parasite. Adult male and female parasites live in the buccal cavity of the fish, and their holoxenic life cycle includes a single host.⁴ The parasite eggs are laid into the marsupium of the female, where they hatch into "pullus larvae" that actively search for a host upon exit.² The negative impact of the parasite on the fish growth and survival has been reported for several sea bass farming operations along the Croatian coast.³ Infested fish populations have significantly reduced growth and increased mortality compared with non-infested fish.³ In general, parasitic infection of fish depends on many host-related (age, size), feeding, and environmental factors. The disease prevalence is associated with the presence and numbers of pulli II larvae, fish density, and biogeographical characteristics of the farms. This study of Ceratothoa oestroides infestation in the cultured sea bass was carried out on one fish farm in Zadar County, Croatia. The aim was to provide new insights into parasite life cycle in this geographic area in order to improve existing fish farm management strategies. Samples were considered as one population since they were collected from one offshore cage, and the fish were of the same age and origin. A total of 609 parasitized fish with body weight ranging from 76.2 to 456.4 g were collected during the winter and summer harvest season (a total of 12 sampling periods). Cymothoids were removed from all infested fish, and the number of parasites and their life stages were recorded. Ceratothoa oestroides was identified according to established criteria, and the morphological parameters of each parasite and the host fish were recorded. The number of pulli larvae held in female's marsupium was counted. The results show no seasonal variation and no effect of the sea temperature or moon phases on the reproductive activity of Ceratothoa oestroides. Significant positive correlation was found between the fecundity of female parasites and the daylight time. Our study suggests several farm management strategies for the control of cymothoid infections. These include seeding of sea bass fry on farm in autumn period during the shorter daylight time, and introduction of larger fingerlings as possible ways in disease prevention.

Acknowledgments

The authors wish to thank the management of the CROMARIS fish farm company for allowing us to collect samples and providing the infrastructure needed for the research. The research was partially funded by Zadar County and City of Biograd.

References

1.  Horton T, Okamura B. Cymothoid isopod parasites in aquaculture - review and case study of a Turkish sea bass (Dicentrarchus labrax) and sea bream (Sparus auratus) farm. Dis Aquat Organ. 2001;46:181–188.

2.  Horton T, Okamura B. Post-haemoragic anemia in sea bass Dicentrarchus labrax (L), caused by blood feeding of Ceratothoa oestroides (Isopoda: Cymothoidae). J Fish Dis. 2003;26(7):401–406.

3.  Mladineo I. Prevalence of Ceratothoa oestroides (Risso, 1826), a cymothoid isopode parasite, in cultured sea bass Dicentrarchus labrax L. on two farms in middle Adriatic Sea. Acta Adriat. 2002;43(1):97–102.

4.  Mladineo I. Life cycle of Ceratothoa oestroides, a cymothoid isopod parasite from sea bass Dicentrarchus labrax and sea bream Sparus aurata. Dis Aquat Organ. 2003;57:97–101.

5.  Šarušić G. Preliminary report of infestation by isopod Ceratothoa oestroides (Risso 1826), in marine cultured fish. Bull Eur Assoc Fish Pathol. 1999;19(3):110–113.