Swarming, diel movements, feeding and cleaning behavior of juvenile venomous eeltail catfishes, Plotosus lineatus and P. japonicus (Siluriformes: Plotosidae)

Eugenie Clark, Diane R. Nelson, Mary Jane Stoll and Yasumasa Kobayashi

aqua, International Journal of Ichthyology, pp. 211-239, Volume 17, Issue 4 – 15 October 2011

Juveniles of the venomous striped eeltail catfishes, Plotosus lineatus and Plotosus japonicus, were studied by scuba divers in Papua New Guinea, the Solomon Islands, Indonesia, Mabul (Malaysia), Japan and the Red Sea. Dozens, up to thousands, of juveniles form swarms or “balls”, often touching each other, unlike typical fish schools where adjacent individuals swim close but not touching. Feeding swarms can move across large stretches of sand in a steamroller-like movement as fish from the rear constantly speed up and advance over those in front. Lead fish use head barbels to probe the sand for food. Over algal beds and coral reefs, catfish swim in smaller and looser formations. Swarms may be followed by other fishes: shrimpfish, Aeoliscus strigatus, barred soapfish, Diploprion bifasciatum, or the puffer, Arothron manilensis, which may enter and feed with the swarm. Pomacentrids, guarding their demersal eggs on dock pilings or rocks, may attack and chase Plotosus swarms away. At Izu Peninsula, Japan, juvenile P. japonicus clean the boxfish, Ostracion immaculatus, the zebra morwong, Goniistius zebra, and the head and inside the mouth of the moray eel, Gymnothorax kidako. Plotosus lineatus juveniles also exhibit intraspecific cleaning behavior. This facultative cleaning seems to result from accidental encounters, not at established cleaning stations. Swarms of Plotosus juveniles can move hundreds of meters in one hour while feeding over sand. Near sundown they retire for the night under a reef ledge or into an artifact (e.g. hollow log, abandoned car tire), not necessarily to the same place on consecutive nights nor into home burrows with adult fish. In appearance and swarming behavior, juvenile Pholidichthys leucotaenia are Batesian mimics of the venomous Plotosus lineatus.

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