Fish parasite castrates its host

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A fish parasite found in the gut of a South American freshwater fish has been shown to exhibit parasitic castration.

According to the results of a new study by a team of Brazilian scientists, the parasite Riggia paranensis causes so much internal damage to fish that it inhibits or prevents their reproduction.

The study, which has just been published in the journal Neotropical Ichthyology, looked at the presence of the crustacean parasite in the South American curimatid fish Cyphocharax gilbert, and found that 60% of specimens caught contained parasites capable of castrating their hosts.

The infected fish showed parasitic castration and lacked gonadal development and two sex-specific plasma proteins. The presence of the parasites enhanced the growth of the fish, particularly infected females, and suggest that they are capable of inhibiting the initiation of the host's reproductive cycle.

The crustacean parasite is a member of the Cymothoidea and is a protandric hermaphrodite which is born as a male and becomes female during the parasitic phase of its life cycle. Larval parasites attack fish near their gills and make a perforation in the flesh near the pectoral fin, through which they are able to obtain oxygen. The parasites live around the gills; with females feeding directly on the blood and the smaller males living in the area around them.

As the parasites grow they are attacked by the immune system of the fish, which produces a capsule around them in the pericardial area. By the time the parasites are ready to reproduce they can drink the host's blood from the comfort of the host-synthesised capsule and, when they've spawned, release their offspring out of the hole they punched in the side of the fish's body. The females can reach nearly five times the size of the males, at up to 2.2cm in length.

The study showed that of over 1300 Cyphocharax gilbert studied, a massive 60% of specimens were infested by one or two of the parasites.

The authors summarised: "Infestation pattern was investigated on 1358 specimens. Most of them were infested (57.9%), with one or two parasites; the majority (62.9%) was collected during the rainy season (spring-summer). The parasite did not show preference for sex or size of hosts. A total of 91.5% of the 511 examined parasites had a body size that represented 10.1% to 20% of host standard length.

"The reproductive condition of 311 specimens of R. paranensis was analyzed checking the presence of oocytes in the ovarian and eggs or embryos in the marsupium. Nearly 73% of them were at reproductive phase, and had a body size that represented 5.1% to 20% of host standard length. The size of the immature parasites varied from 0.1% to 5% of the host size. The results suggest that R. paranensis may adopt a fast growth rate strategy and increase the investment in reproduction when they occupy most of the host's pericardial space."

Within the family Curimatidae, Cyphocharax gilbert is believed to have the greatest fecundity - producing more eggs than any other species known, which may explain why the parasites choose the species over other members of the curimatid family.

According to the redescription of Riggia paranesis by Bastos and Thatcher, the species is one of at least 25 freshwater cymothoid isopods in South America spanning 10 genera. R. paranensis is normally seen in Cyphocharax gilberti but has also been recorded from a related species known as C. platana.

For more details on the research see the paper: de Souza Azevedo J, da Silva LG, Bizerril CRSF, Dansa-Petretski MAD, Lima NRW (2006) - Infestation pattern and parasitic castration of the crustacean Riggia paranensis (Crustacea: Cymothoidea) on the fresh water fish Cyphocharax gilbert (Teleostei: Curimatidae). Neotropical Ichthyology, 4(3):363-369, 2006.