“I’ll just feed my tanks before bed.” Famous last words. I spent the next two hours with a microscope and the glow of my computer screen instead of my nice, cozy bed. Even then, it took quite a while to find someone to identify them as likely the cercariae stage of fluke parasites in my reef tank.
A few minutes after feeding my reef tank, I noticed some rather large “pods” swarming around the top of the still water (all pumps were in “Feed Mode” – off). Odd, but nothing too weird. After all, I thought they were Ostracods at first (seed shrimp). On second glance, they looked too big. I paused, and then I realized what really caught my eye. They were red. Other than “red bugs” (Tegastes acroporanus), I’ve never seen red pods. I sighed in hesitation, then accepted my fate. Bed would have to wait.
I took a sample. There was no doubt; these were truly red. Ok, red Ostracods, right? I wouldn’t have to get out my microscope and prepare a slide, right? Wait. Ostracods don’t have tails. Noooooo.
Up Close with a Microscope: Fluke Parasites
I prepared a slide, and nothing could have prepared me for what I would find. I expected something in the definitive worm or pod realm. But not this. This was respiring. It was opening and closing its… mouth?
Close-up of its anatomy… for what it’s worth.
Up until this point, they looked all cute. Like, little swimming strawberries, right? Nooooope. More like Sauron’s Eye (check out the video below).
So what are they? I have no idea. Every idea leads me down the wrong path. The closest I’ve landed is maybe some sort of larval sponge? (See update below.)
Identification: Fluke Parasites
Update (12/27/2020): Thanks to Reeffraff on Reef2Reef back in July 2020, it appears these are likely the cercariae of a trematode or fluke (in other words, they’re likely parasitic). Their life cycle probably involves snails and fish. I was right: these fluke parasites in my reef tank were the thing of nightmares! My treatment was to run fishless for several months (as the snails can shed for weeks/months). Another option, which I did not utilize, was to run PraziPro. I decided against that approach, as the number of worms (e.g., spaghetti, bristle) in my tank would probably cause a crash. I plan to quarantine all future snail purchases in a fishless system to avoid these.
This is likely a species of Bevisicula, such as B caribbensis described by Cable from Curaçao. I’ve found these infections recently in dissected snails. Your pics and video are great. The cercaria stage needs to be eaten by fish – hence the conspicuous color and appearance.
Thank you so much Pieter! This is fantastic information; hopefully it will help others too!