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.
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.