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Kudoa

Kudoa is a genus of Myxozoa and the only genus recognized within the monotypic family Kudoidae. There are approximately 100 species of Kudoa all of which parasitize on marine and estuarine fish. Kudoa are most commonly known and studied for the negative effects the genus has on commercial fishing and aquaculture industries.

Description

The genus Kudoa is identified by the possession of four or more shell valves composed of a fragile membrane and arranged in a or stellate pattern. The maximum number of shell valves in any described Kudoa species is 13. Each of these valves has a polar capsule. The genus Kudoa was originally part of the genus Chloromyxum because of the distribution of their polar capsules, however, it was later determined to be a separate genus. Each Kudoa has two sporoplasm cells, one enclosed by the other. Most Kudoa are histozoic parasites, with a few species being described as coelzoic. Some taxonomists question whether these coelzoic organisms belong to a separate genus. There are approximately 100 described species of Kudoa which can be found in the Atlantic, Pacific, and Indian oceans.

Species

The following species are recognized in the genus Kudoa:

Development

In Myxozoan development, the Myxosporean life-stage develops inside a fish host, while the Actinosporean life-stage develops in an annelid host. Fully-developed Myxospores are consumed by annelids and reproduce asexually via schzogony in the gut epithelium of worms. Gametes are formed in the gut of the worm and these gametes fuse together to create eight zygotes. The zygotes become spores with three valves, 3 polar capsules, and a sporoplasm that are released in the worm's feces and attach to the surface of a fish host. Once attached to the fish, the gamete injects the sporoplasm cell into the fish. The sporoplast divides forming a fully-developed Myxosporean.

Diet

Kudoa parasitize on marine and estuarine fish. Kudoa typically feed on the skeletal muscle tissue, although some may feed on other parts of the body such as the central nervous system, heart, intestines, ovaries, or gills. Most Kudoa are histozoic parasites, however, a few species are coelozoic. The genus Kudoa attack a wide range of hosts, however, individual species only feed on specific hosts. Kudoa feed via pinocytosis across the host-parasite membrane. One species of Kudoa, K. thyrsites, are distributed worldwide and are believed to have been reported in over 20 different species of fish but it is possible that the species described as K. thyrsites is actually multiple different species.

Implications for the fishing industry

Kudoa are most well known for the economic loss they cause for the commercial fishing and aquaculture industries. When Kudoa attach to hosts, they leave unsightly cysts that lower the price fish can be sold for at market. Kudoa also release proteolytic enzymes that degenerate muscle in fish to aid in their own growth and development, further decreasing the fish's value. Some species in the genus Kudoa with the most notable effects of the commercial fishing and aquaculture industries are: K. musculoliquefaciens in Broadbill Swordfish (Xiphias gladius), K. thyrsites in Atlantic Salmon (Salmo salar), K. clupeidae in Atlantic Herring (Clupea harengus), K. septempunctata in Olive Flounder (Paralichthys olivaceus), K. thunni in Yellowfin Tuna (Thunnus albacares) and K. paniformis in Pacific Hake (Merluccius productus).

Some studies have found evidence to suggest that some species of Kudoa are linked to food-borne illness in humans.

References

Encyclopedia of Life