World Aquaculture Magazine - September 2021

58 SEP TEMBER 2021 • WORLD AQUACULTURE • WWW.WA S .ORG gnathiid parasites under natural conditions as there were no external lesions or any abnormal morphological changes observed on the fish (Cruz-Lacierda and Nagasawa 2017b). In another study, however, there was positive correlation between sizes of the fish host scad species S. crumenophthalmus and Decapterus kurroides and the parasitic isopod N. indica , suggesting that infection occurs early and parasites grow along with the hosts (Cruz-Lacierda and Nagasawa 2017a). Moreover, there was an absence of correlation with ovigerous females which indicates that no further growth of the isopod occurs if the host has already reached its maximum size (Cruz-Lacierda and Nagasawa 2017a). Generally, cymothoid isopods affect fish host physiology and behavior (Sikkel andWelicky 2019). Male and female five-lined cardinal fish Cheilodipterus quinquelineatus , when infected by Anilocra apogonae , had lower gonad weights in comparison to non- infected fish (Fogelman et al. 2009). Moreover, males were shown to be more inclined to cannibalize their young while females had diminished production of ova. Several physiological changes, such as reduced energy levels evident from the fish’s slower swimming speed were also observed (Östlund-Nilsson et al. 2005). The energy drain caused by the parasite resulted in restricted growth and inability to flee from predators, both of which could indirectly cause higher mortalities in the fish host population. Meadows and Meadows (2003) explained various host behavioral changes in four-eye butterflyfish Chaetodon capistratus after infection by the ectoparasitic isopod Anilocra chaetodontis . The parameters evaluated included interactions with fellow C. capistratus , interactions with damselfishes, rate of feeding and size of territory. Parasitized fish spent greater time interacting with other parasitized fish. This behavior was attributed to the isopods altering the mate-seeking behavior of the host for the isopod to transfer its offspring to other potential hosts or find mates of its own. Moreover, parasitized fish spent more time hiding underneath corals and overhangs. Such behavior allowed less feeding time for the fish, increased hostile interactions against damselfishes and reduced the size of their territories (Meadows and Meadows 2003). Isopods as Pathogen Vectors The family Cymothooidae is arguably composed of the most notorious aquatic parasitic isopods that can also serve as intermediate hosts for nematode roundworms such as Hysterothylacium sp. and cod or seal worm Pseudoterranova decipiens (Busch et al. 2012). These roundworms also parasitize members of the Malacostraca family of isopods, another group closely related to Cymothooidae. Occurrence of Hysterothylacium spp. was studied in Atlantic chub mackerels Scomber colias along the Libyan coast, and the mean prevalence rate for the roundworms was 44 percent (Cavallero et al. 2019). In another study, P. decipiens was observed in Baltic cod Gadus morhua with the mean prevalence rate of roundworms of around 29 percent (Mehrdana et al. 2014). Both Hysterothylacium spp. and P. decipiens infect humans and can cause gastrointestinal problems, such as diarrhea and nausea. In addition, both are co-parasitic with Anisakis sp., a genus known for widespread roundworm diseases in humans such as anisakiasis, compounding their present zoonotic risks (Mehrdana et al. 2014, Cavallero et al. 2019). Bacterial and fungal secondary infections commonly occur in fish infected by cymothoid isopods. When compared with non-infected fish, three types of carangid fishes ( Carangoides sp., Carangoides malabaricus , and Selaroides leptolepis ) infested by isopods ( N. phaeopleura , N. indica , and Nerocila depressa ) had greater total counts of bacteria and fungi in their tissues (Rameshkumar et al. 2013b). This was attributed to attachment of isopods on the fish host, causing damage and lesions on the body surface that enabled entry of different bacterial and fungal pathogens. Isopods also often changed attachment sites within the body of the same fish host which contributed more bodily damage. Eleven species of bacteria The cymothoid isopod Norileca indica is the most common species among the members of the cymothoid genus Norileca. N. indica infects the branchial area of many pelagic marine fishes. Females (left) are usually larger than males (right) (Photo: Erlinda Cruz-Lacierda). A group of cymothoid isopods were removed from a batch of freshly bought bigeye scad fromMuñoz Market in Manila, Philippines. These isopods appear morphologically similar to Norileca indica (Photo: J.A. Ragaza).