Fish or Fishes? Shoal or School? A Few Fishy Terms Defined

school-shoal-reef-fishIn my nearly two decades as an aquarium writer and editor, I’ve noticed that certain terms routinely used in hobby literature when discussing our fine, finny friends are often a source of confusion to readers. So for today’s post, I thought it might be nice to clarify a few of these terms for those who are curious.

What’s the plural of fish?

If your answer to this question is either “fish” or “fishes,” you may be correct depending on the context. “Fish” is considered the correct plural form if you’re referring to a group of specimens all belonging to the same species. “Fishes” is the way to go if you’re referring to a group consisting of more than one species.

For example, if you have an aquarium housing five specimens of Heniochus diphreutes (the schooling bannerfish), your tank can be said to contain five fish. If, on the other hand, your tank holds a yellow tang (Zebrasoma flavescens), tomato clownfish (Amphiprion frenatus), bird wrasse (Gomphosus varius), niger trigger (Odonus niger), and sixline wrasse (Pseudocheilinus hexataenia), then you have five fishes.

So, if we’re being really nitpicky about it, that old adage “There are plenty of fish in the sea” would more accurately be written “There are plenty of fishes in the sea.”

Shoal vs. school

These two terms are commonly used interchangeably, but there actually is a distinction between them. A shoal is a group of fish congregating together to benefit from “safety in numbers” but not moving or behaving in unison. They may be facing every which way with one specimen snatching plankton from the water column, another showing off to the opposite sex, still another browbeating a subordinate, and so forth.

A shoal becomes a school when all of the members of the group coordinate their movements and behavior—for example, swimming tightly together on the same heading, changing direction in unison, etc. I suppose one could define a school as an organized shoal.

Mysis vs. mysid

Moving on to a fish-food-related term, we have the use of either “mysis shrimp” or “mysid shrimp” when referring to those tiny crustaceans so many of us feed to our fish. Is one of these forms correct and the other a misspelling? Actually, for our purposes, either version is appropriate. “Mysid” makes sense because these crustaceans belong to the family Mysidae (just as cichlids belong to the family Cichlidae for all you freshwater enthusiasts), and “mysis” is fitting because the type of mysid typically sold in the hobby is a member of the genus Mysis (Mysis relicta).

There are lots of other genera in the family Mysidae, so you could say that all mysis are mysids, but not all mysids are mysis—though I doubt it’ll ever come up in casual conversation.

Photo credit: Arthur Koch


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About Jeff Kurtz

Jeff Kurtz is the Co-founder/Editor of Saltwater Smarts, former Senior Consulting Editor for Tropical Fish Hobbyist Magazine, and the aquarist formerly known as “The Salt Creep.” He has been an aquarium hobbyist for over 30 years and is an avid scuba diver.


  1. parul kumar says

    Thank you share all this as I was searching for this for so long finally got it through u

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