World Aquaculture Magazine - September 2021

56 SEP TEMBER 2021 • WORLD AQUACULTURE • WWW.WA S .ORG almost exclusively found on adult fish. Gnathiids are an exception within cymothoids, since they are only present on fish as larvae. Bopyroids are known parasites of decapods and, with cryptoniscoids, are known for using those they infest as their intermediate and definitive hosts (Williams and Boyko 2012). Of all parasitic isopods, cymothoids are the most widely observed and studied in Philippine fisheries, aquaculture and natural aquatic environments. Some gnathiids engage in conspecific klepto-parasitism. Gnathiid isopods were collected from light traps in central Philippines (Shodipo et al. 2019). One gnathiid was attached to another and both of their abdomens were enlarged. It was suspected that one of themwas feeding on the blood meal of the other because the ventral side of the gnathiid, where the mouthparts are located, laid directly on the pereon or thorax of the other gnathiid. This was confirmed when unfed gnathiids were introduced into a group of fed gnathiids. The blood meals of the fed gnathiids were “stolen,” though there seemed to be no immediate negative effects on the gnathiid hosts (Shodipo et al. 2019). Hence, it is possible to encounter an isopod feeding on a fish host’s blood while another feeds on its fellow isopod. Cymothoid Isopod Infestation The family Cymothooidae parasitizes cartilaginous fishes (Elasmobranchii) and ray-finned fishes (Actenopterygii) as hosts. There are various attachment sites for cymothoid isopods (Rameshkumar et al. 2014). Those belonging to the genus Nerocila can be found on the pectoral fin, caudal peduncle and body surfaces. One species, N. phaiopleura , can be found attached to pectoral, pelvic and anal fins. The Indo-Pacific isopod species C. indica and Joryma sawayah attach to the buccal and branchial cavities of the fish, which means that only through dissection can these parasites be found. Anilocra dimidiata can be found only on the external body surface of the host (Rameshkumar et al. 2014). The mouth parts of cymothoid species reflects their diet (Rameshkumar et al. 2013a). Cymothoa indica and Joryma brachysoma have mouthparts specialized for the transfer of host blood while attached to the buccal cavity. Nerocila phaeopleura , an ectoparasite, has broad mandibles and spines that allow it to tear Parasitic Cymothoid Isopods Parasitic crustaceans are among the most harmful parasites that affect fishes. Certain species can cause mass infection that may lead to widespread mortality in wild capture fisheries and aquaculture, resulting in serious economic losses. As some fish are cultured in open systems such as cages or net pens in rivers, lakes and seas, the concern over transmission of parasites from farmed to wild fishes and vice versa is becoming more widespread. Population density and environmental conditions are important factors in parasitic infections of cultured fishes. A favorable environment enables mass reproduction of parasites while high stocking density and population translate to rapid disease transmission (Lopez 2001). Among the most prominent parasitic crustaceans are isopods, small crustaceans with seven pairs of legs (Boyko et al. 2019). Isopods are generally dorsoventrally flattened and characterized by a lack of carapace (dorsal shell). Gills are also absent for this group but instead specialized lamellar gill-like pleopods (swimming legs or swimmerets) are used for respiration. Many cymothoids are large and can be seen easily with the naked eye. However, owing to their larval forms, gnathiids are very small and require a dissecting microscope to be clearly seen. Cymothoids are protandrous hermaphrodites; they start life as males and later develop into females, explaining why females are always larger than males (Bunkley-Williams andWilliams 1998). The main super-families are Bopyroidea, Cryptoniscoidea and Cymothooidea, the last of which includes the family Gnathiidae. These isopod super-families consist of approximately 10,300 species or 7.7 percent of all isopod species. They are most diverse in the tropics but are most numerous in temperate areas. Cymothoids are ectoparasites (i.e., they live outside the host) and most parasitize the host throughout their life cycle by feeding on blood. These isopods attach either permanently or only during their larval stages. Isopods may attach to all external, branchial and buccal surfaces of fishes. Many of these parasites are capable of inflicting large wounds on hosts that often stunt growth and sometimes lead to death. In general, cymothoids are highly host- and site-specific. They often grow along with the hosts such that adult parasites are Parasitic Cymothoid Isopods in Asian and Philippine Fish Species Tanya Faye S. Muji, Joaquin R. Sorreta, Selwyn B. Briones, Jr., Justine Oliver S. Maquirang and Janice Alano Ragaza Groupers are regarded as a high-value market fish species in the Philippines. Groupers that are reared and thrive in warm and tropical marine waters may harbor a variety of parasites (Photo: Paula Rae S. Joson).