Fish are often plagued by external parasites, which are presumably difficult for them to remove.

There are well known cases where large fish come to coral reefs and allow smaller fish to pick off their parasites. See below a moray eel being "cleaned" by a shrimp.

enter image description here

Some fish, like sunfish, even get cleaned up by birds: https://www.youtube.com/watch?v=U60obmWODLQ

But on land, we have many social animals that solve this problem more directly by grooming each other. How come the fish never seem to do this? Many of them are already social, and I imagine they wouldn't have much trouble digesting the external parasites. Why don't they groom each other?

  • $\begingroup$ They probably think that's gross. $\endgroup$
    – Sparky
    Nov 12, 2014 at 7:50

3 Answers 3


All cases of same species grooming I know of involve animals with a certain amount of dexterity. For example (images from wikipedia):

  • Macaw beaks:

                            Hyacinth Macaws, Anodorhynchus hyacinthinus at the Aquarium of the Americas in New Orleans, Louisiana.

  • Primate fingers:

                            Macaques grooming each other at a foggy viewpoint near Lonavla, India.

  • Even ponies rubbing each other:

                            Mutually grooming ponies at Turf Hill, New Forest, U.K.

In all of these cases, the animals involved have the right tools for the job. fingers, beaks and prehensile lips can all close in on a parasite and remove it with precision. how exactly would you expect a shark to do this (image from the guardian)?

                            shark jaws

Sharks can and do rub themselves against rocks to get rid of dead skin but they simply don't have the tools for the kind of detail work that the smaller fish can do for them. The same applies to other fish. I think you will find that at least in most cases, fish that use client species to groom themselves wouldn't be physically able to do the same job themselves. It's all a matter of the right size and the right tools.

Note that this is pure speculation, I have no references to back this up, it just seems reasonable. I would not be surprised if some fish did actually indulge in social grooming, it's just harder for the reasons outlined above.

  • $\begingroup$ A good answer, but keep in mind that the question was about fishes in general. I assume that the OP is talking about the more subordinate species of fish. $\endgroup$ Nov 10, 2014 at 17:50
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    $\begingroup$ @VatsalManot yes, but I think it's applicable to all. In order to be able to eat parasites, you need to be small enough to get at them. Presumably, if you're the same species, you will be too big. Still, as I said, this is pure speculation. $\endgroup$
    – terdon
    Nov 10, 2014 at 18:07
  • $\begingroup$ As you correctly noted in your answer, symbiosis is a viable solution for the size to size ratio problem. $\endgroup$ Nov 10, 2014 at 18:12

Something from my imagination: Probably the first fishes were of small size with some of them evolving to be big. The variation in the sizes of the fishes is larger compared to the variation in the sizes of mammals(picture elephants and rats with whales and tiny fish). That being said, if you'd have bank of small fish staying in one place and all grooming each other, a bigger fish would swallow them with one bit(as whales eat). This does not happen on land with, say monkeys, because it would be too big of a load for a tiger or an eagle to snatch both of them with one move. In conclusion, the ways of predators in oceans(trickier ambush tactics, faster speed attacks) and variations in sizes might have made this more of a disadvantage than an advantage underwater compared to on land.

  • $\begingroup$ Regarding predators: Many fish such as herrings already live in large, dense groups - and they do get swallowed wholesale by whales. This apparently is not enough to deter the schooling behavior, though. $\endgroup$
    – Superbest
    Nov 10, 2014 at 17:28
  • $\begingroup$ But herrings constantly move, they would have to be able to move and groom at the same time. $\endgroup$ Nov 10, 2014 at 17:47

Perhaps it has something to do with fish slime coats.

Fish are covered in a thin layer of mucus which helps with immune system function, etc. According to these guys at least, who admittedly are not exactly a scientific authority.

Perhaps grooming as a social behavior would disturb the slime coat more often than it would help remove parasites? Or it's dangerous for other reasons? (Some fish are very territorial)

Some(most?) fish are just incapable of grooming other fish of the same size and shape. I mean look at this guy. The moray eel above has a jaw much larger than that of the entire shrimp that is grooming it, so perhaps picking parasites off things with such a jaw is like cutting your fingernails with a meat cleaver?

  • 1
    $\begingroup$ But the fish still get groomed, by other species. Are you saying that they would loose the layer if a fish did the grooming but not when done by a shrimp or seagull? $\endgroup$ Nov 10, 2014 at 4:15
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    $\begingroup$ You know the more I think about it the more I suspect mouthparts are to blame. A marlin for instance can't groom anything, much less another marlin. They just don't have the right teeth/jaw for it. $\endgroup$
    – Resonating
    Nov 10, 2014 at 4:20
  • $\begingroup$ Yeah... but then at some point there was a bifurcation, they could have chosen a suitable mouth for grooming or a mouth for other stuff to do. Maybe the severity of having parasites, which I don't know, versus other dangers has something to say in this. $\endgroup$ Nov 10, 2014 at 4:24
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    $\begingroup$ I mean grooming parasites off of other members of your own species can't be a viable food source for your whole species. Specialist mouthparts for eating any other food would be better, so I imagine it's just a thing that might be nice to have, evolutionarily speaking, but ultimately shrimp are "good enough" at it. $\endgroup$
    – Resonating
    Nov 10, 2014 at 4:34

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