Juvenile striped eel catfish form a coordinated, roiling ball to avoid predation

January 27, 2020 • 1:15 pm

Both Matthew and reader Dom sent me this link to The Colossal describing and showing another amazing animal behavior, this time in the striped eel catfish (Plotosus lineatus), a smallish species (average size about 14 cm) found in the Indian and Pacific Oceans, as well as the Mediterranean where it’s a recent invader. It’s the only catfish associated with coral reefs.  It’s also venomous, with a poison spine on the dorsal fin and each of the pectoral fins, which is reported to have been “fatal”, apparently to at least one human.

While adults are solitary or occur in small groups, the juveniles form schools like the one shown below, comprising 100-150 individuals.  As the report below recounts (click on screenshot), and the videos underneath show graphically, the youngsters swim in roiling balls of fish, with each individual heading towards the bottom and then back up again. (The video was filmed in Bali.) They apparently gather like this for protection, as younger fish are only lightly venomous and could be taken by predators who apparently avoid adults. (Note the striking striped coloration, which may well be “aposematic“: a pattern warning that the bearer is dangerous or distasteful.)

Why they move down and up again is anyone’s guess. I had two ideas: it gives everyone a chance to forage on the bottom while they stay together as a mass, or it makes their pattern more obvious to predators. Or it could be both.

The video comes from the Abyss Dive Center in Bali:

I found another two-minute video that also suggests that they move like this so all individuals can forage.  And be sure to see what happens one minute in, when they rise from the sea floor all together, writhing like a giant jellyfish:

Here are two photos from the Monaco Nature Encyclopedia. The first is an adult, the second a school of juveniles hiding in a reef.

© G. Mazza


© Sebastiano Guido

20 thoughts on “Juvenile striped eel catfish form a coordinated, roiling ball to avoid predation

  1. For me, the effect in that first video is like some large, multi-legged animal walking along the bottom. If predators that might take individual catfish see it that way, you can understand why that might be a pretty effective time deterrent.

  2. Looking at some of these photos and videos (and many others on this site) I get the feeling that Sci-fi writers and directors lack imagination.’

  3. I was stung in the thumb by one of these beasties in the early days of my two-year stint on a prawn trawler which trawled along the east coast of Australia, just north of Byron Bay. The catfish were known locally as “footballers”, because “they wear a striped jacket.”

    The pain from the sting was like how it feels after you have accidentally struck your thumb with a hammer, for the second time.

    There was only paracetamol in the first aid kit, and the skipper said that some people have been known to go into the engine room and work battery acid into the wound in an attempt to do something, anything, to mitigate the pain. I declined to follow that advice.

    Unfortunately, people would break into the trawlers when moored in harbour and steal the stronger pain-killers, and even destroy inflatable life-rafts in their search.

    1. Catfish can sting? Charming!

      Oh, I see you said it was in Australia. That explains it. In Oz, every bloody thing can sting, bite or otherwise ruin your day… 😉


  4. Why speculate when researchers can introduce predators and answer why the formation. It may not even be a defense mechanism.

  5. Maybe 60 years ago, when I was a teenager, I occasionally saw a similar ball of tadpole size catfish moving over shallow water in a fresh water lake in central Minnesota. Not foraging but moving languidly in a pretty tight ball. Surely fresh water ichthyologists must know something about this behavior. Got any at UC?

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