Isopod – Sphaeromatid

sphaeromatid-leftThese guys sometimes cause confusion in our hobby due to their similarities in appearance to another isopod the Cirolanid. The Sphaeromatid is a harmless herbivore, while the Cirolanid is a fish parasite. Once you know what you’re looking for, telling the difference between a Cirolanid (bad) versus a Sphaeromatid (good) becomes a lot easier. It’s all in the eyes…..

  • The eyes of the Cirolanid are generally closer together and towards the front of the head. They always look angry because of the slant of their eyes. Think of them as needing to look forward to “capture” its prey. The Sphaeromatid eyes are generally more to the side of the head. They always remind me of baby seals with their big black harmless looking eyes. Looking at the side by side pictures I’ve included of both isopods may help with this visualization.
  • Sphaeromatids also have the common name “Roly Poly” as they, unlike the Cirolanid, roll up in a defensive position when they feel threatened. The Cirolanids cannot do this. Think of it like this: the Cirolanid is mean and will never back down from a fight. The innocent little Sphaeromatid rolls up in a tiny little ball saying “Please, don’t hurt me!”.
  • Where you find them may also help decipher which is which. If it’s attached to your fish, it’s probably pretty safe to say it’s a Cirolanid. If there are a few scurrying around when you lift up a rock, lean towards it being a Sphaeromatid.



The following video states they are Cirolanids, but their ID is incorrect.



The following video states they are Cirolanids, but their ID is incorrect.


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Links for Further Reading


– Pills, Parasites, and Predators; Isopods in the Reef Aquarium – Reefkeeping Magazine