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California killifish have it rough. The picture of the isopods that were as big as their host’s brain came from the killies and that is only one of many parasites which plague them regularly. Here is another!
A few months ago I was doing some killifish research and found a dead killie in the tank. I’m really concerned with the health of my animals because, 1) I appreciate the sacrifice they’re making for my research, 2) I got into this field originally because I’m a big animal lover, and 3) (most importantly) unhealthy fish yield less accurate data (unless you’re studying disease). I noticed that another fish in the tank had a milky white patch on its skin and was looking lethargic. I popped the fish under the microscope briefly and took this photo:
The infected area is the milky white patch on the left and you can see healthy scales on the right. The two neon green lines are an elastomer compound that I injected underneath their skin so I could keep track of individuals throughout my experiments. Neat, huh?
I removed some scales and identified the culprit, CILIATES! Here is a video that I took at 400X. You can see the little buggers picking food off of the fish scales:
Ciliates have a feeding stage (called trophozoites) and a cyst stage, and these are the trophozoites. Ciliates have a macronucleus and a micronucleus, and you can kind of see what looks like a big sac and smaller sac in some of the ciliates in the video.
Other common ciliate parasites include ich, which infects fish, and Balantidium coli. B. coli is an intestinal parasite of pigs, but can infect humans (who have “balantidiasis”). Ciliates are not all bad though. In fact some are gut mutualists, aiding in digestion.
This story ends well. A quick dosing with ectoparasite medication knocked out the infection and the killies recovered quickly. Interestingly, those isopods were also on the killies when I dumped in the antiparasite meds, and they were apparently unfazed by the treatment. They’re somewhat internal to the fish (being underneath the operculum and living on the gills), but still would have been subjected to the medication in the water as it passed over the gills. Hardy little guys.