Cymothoid isopods, which are parasitic crustaceans specifically on fishes, can significantly impact fisheries and aquaculture, leading to substantial economic losses worldwide (Nowak et al., 2020). These parasites attack and feed on their hosts, resulting in reduced growth and survival rates and a lower market value for fish. Indirectly, they impact the fish industry by transmitting diseases and affecting the behaviour and physiology of their hosts.
These isopods are prevalent in marine, brackish, and even freshwater environments, and they can be found in a wide variety of fish species. The World Register for Marine Species, as documented by Boyko et al. (2008), has identified and listed 380 verified species in 46 genera of cymothoids, with growing evidence that they display high specificity towards their hosts and habitat. The genus representatives are usually found in the buccal cavity, attached to the gills or fins, or even burrowed into the flesh of the fish. The buccal region is the common attachment point for representatives of the genus Cymothoa, and hence these cymothoids are fondly called ‘fish tongue’ biters.
The above parasitic “tongue biter” is Cymothoa eremita collected from the host Nemipterus furcosus. The first collected specimen was from Malaysian waters (in Sarawak) by Anand et al. (2015, 2017) from the buccal cavity of Psettodes erumei. Recently, two new fish hosts were recorded for this species (Martin et al., 2022): Nemipterus tambuloides (known as the Fivelined Threadfin Bream in English and Ikan Kerisi Jalur Lima in Malay) and Nemipterus furcosus (Fork-Tailed Threadfin in English and Kerisi Merah in Malay; fish featured here). This species has also been recorded from 11 other host families: Aulopidae (flagfins), Carangidae (jacks, pompanos, jack mackerels, runners, trevallies, and scads), Haemulidae (grunts), Lutjanidae (snappers), Mugilidae (mullets), Psettodidae (spiny turbots), Serranidae (sea basses and the groupers), Siganidae (rabbitfishes), Sphyraenidae (barracudas), Stromateidae (butterfishes), Tetraodontidae (puffers) (Martin et al., 2015, 2016).
Owing to the low host specificity on a variety of fishes, the distribution of Cymothoa eremita is highly dependent on the occurrence of fish hosts, with some species being pelagic and others being benthic. To date, C. eremita has a wide geographical distribution from the western Indian Ocean (e.g., Mozambique) to the Central Indo-Pacific (e.g., Australia).
Cymothoa eremita can be identified by several distinct characteristics:
1) The front part of the body (called the cephalon) is somewhat subtruncate.
2) The first body segment (known as pereonite 1) has margins at the front that extend nearly halfway up the cephalon.
3) The body segment after the pereonite (called the pleon) is about the same width as the pereon.
4) A pair of appendages (uropods) at the back, which do not quite reach the end of the body.
5) The seventh pair of legs (pereopod 7) has a noticeable bulge on the inner part of the leg (ischium).
6) Small horn-like structures on the posterolateral margins of pereonite 1.
Cymothoa eremita, like other cymothoids, exhibits protandrous hermaphroditism. This means that all juveniles first develop into males and will only transform into adult females if they find suitable hosts without competitors of the same species. The first male to parasitise an unparasitised fish will transform into a female, while subsequent males attaching to the same fish will remain males, possibly due to a pheromone released by the female. The female then releases up to one hundred eggs into a brood pouch on the underside of its abdomen. After hatching, the eggs undergo several moults to form juveniles that are released into the water to find a suitable host.
The lifespan of a cymothoid parasite can vary depending on various factors, such as the species of the parasite, the host species that it infects, and the environmental conditions in which the parasite lives. Some species of cymothoids can live for one to two years, while others may only live for a few months. A cymothoid is unlikely to outlive its host, as it is a parasite and requires the host to live.
It is important to continually study the ecology and biology of these parasitic cymothoids, particularly the life cycle and host preference. This will allow more informed decisions towards conservation and management efforts for affected host species and ecosystems, both regionally and globally.
