World Aquaculture Magazine - September 2021

WWW.WA S .ORG • WORLD AQUACULTURE • SEP TEMBER 2021 57 Two preserved specimens of the giant isopod Bothynomus sp. exhibited in the National Museum of the Philippines. These carnivorous deep sea isopods resemble wood lice (Photo: Janice A. Ragaza). over three months, with prevalence rates of 0, 9.5 and 3.9 percent in February, March and April, respectively (Perdana et al. 2019). In a separate and year-long study, Indian mackarel Rastrelliger kanagurta harbored N. indica in their gill chambers and skin, with some isopods infecting the host in pairs. The overall prevalence was 28 percent, with April having the lowest at 21 percent and August with the highest at 36 percent (Jemi et al. 2020). The difference in distribution was attributed to the breeding seasons of the fish host, which happens fromMay to July during the monsoon. In addition to abiotic environmental factors, host population is the main influence of isopod distribution (Jemi et al. 2020). Although there was an increase of parasite intensity per fish host, with the highest record of six parasites attached to a single host during summer, there was no marked increase or decrease in snapper-choking isopod Cymothoa excisa infection of Atlantic croaker Micropogonias undulatus over a 22-mo period (Cook and Munguia 2015). Effects of Cymothoid Infections The reports on the effects of parasites on fish hosts are conflicting. In some studies, there were no external manifestations of the infection; however, in others, adverse changes on the host’s body were evident. Orange-spotted grouper Epinephelus coioides from Lingayen Gulf, Philippines tolerates the praniza larvae of ( C O N T I N U E D O N P A G E 5 8 ) the host muscles and attach to the body surface strongly. Ryukyua circularis , Mothocya renardi and Joryma hilsae have serrated margins on the maxilla that allow it to hold strongly to gill chambers of the host. An organism’s body size determines its survival and role in any ecosystem. Hence, there is a relationship between fish host size and parasite size. Although there have been reports of positive correlation between parasite body size and host body size, similarly sized parasites infesting hosts of different body sizes also exist (Tsai et al. 2001). Therefore, what determines parasite body size is not the host body size but the space available for the parasite to grow. Morphological changes may occur in some isopod species upon infecting a fish host. The first C. excisa isopod to infect M. undulatus changes to female while succeeding isopods remain male (Cook and Munguia 2015). There was also a positive correlation between female isopod size and host size, and this was attributed to the isopod infecting the host early to grow alongside it. This way, the females will have a large body size, increasing its potential for reproduction. Similarly, fecundity in isopod species depends on the body size of the female while male size had no direct effect on fecundity (Tsai et al. 2001). Seasonal variation affects prevalence of isopod infection of fish hosts. An average prevalence of 4.5 percent of Norileca indica in bigeye scad Selar crumenophthalmus gill chambers was found A cymothoid isopod is attached to the left brachial cavity or gill of a freshly caught bigeye scad Selar crumenophthalmus . At times, a pair of isopods can be seen attached to both sides of the branchial cavities (Photo: J.A. Ragaza).