Dinoflagellates are single-celled organisms that are related to ciliates (like Paramecium) and apicomplexans (like Plasmodium and Toxoplasma). Many are mutualists of other organisms - Symbiodinium in corals is a good example. Some, however, are parasites. Piscinoodinium limneticum is one such parasitic dinoflagellate. These protists infect fish and cause a disease known as piscinoodiniosis - or, if your tongue gets too wrapped up on that term - velvet disease. P. limneticum can infect a wide variety of both aquarium and food fishes. The parasites attach to the skin and cause it to weaken and some observations indicate that the parasites may cause physical damage from increasing drag - essentially the force from the water "catching" on the parasites as the fish swims does more damage. And it also appears that damage attracts more parasites to glom on, producing a clustered pattern of parasites on a fish's epidermis (see photos). Young fish may actually die from an infection of these nasty little dinoflagellates.
The image comes from this paper and shows both clusters of the parasites and a close-up of one.
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