A tightly packed swarm of juvenile Striped eel catfish (Plotosus lineatus) is a common sight in Indo-Pacific coral reefs, but how these large aggregations (up to 2,700 individuals) behave has not been well studied until now.
Recent research by Eugenie Clark and colleagues published in the most recent issue of the journal Aqua documents the unusual biology of these swarms.
Basing their results on more than 5,000 hours spent observing and studying swarms of up to 100 Striped eel catfishes (and the closely related Plotosus japonicas) in Japan, Indonesia, Malaysia, the Solomon Islands and Papua New Guinea, the authors document the behaviour of the catfish in the swarm, the occasional cleaning of larger fish, as well as the mimicry of the catfish by the Convict blenny (Pholidichthys leucotaenia).
The individual catfishes in each swarm were found to be in physical contact with each other, which is unlike typical fish schools, where adjacent individuals swim close but do not physically contact one another.
Feeding swarms were found to move across large stretches of sand in a steamroller-like fashion with the fish in front packed closely together and their barbels probing the substrate for food.
The fish from the rear constantly sped up and advanced over those in front.
The catfish swam in smaller and looser formations over algal beds and coral reefs.
The feeding swarms were sometimes followed by other fishes such as Shrimpfish (Aeoliscus strigatus), Barred soapfish (Diploprion bifasciatum) or Narrow-lined pufferfish (Arothron manilensis), which fed on invertebrates stirred up by the swarm.
The juvenile Striped eel catfish were diurnal (in contrast to the adults, which are nocturnal) and retire for the night under shelters such as reef ledges or in sunken car tyres.
The closely-related Plotosus japonicas was found to be a facultative cleaner of other fishes such as morays, boxfish, and morwongs. The catfishes were also found to clean each other.
The authors also observed schools of Convict blennies, noting their similarity in shape and colour pattern to the catfish. Based on this and the fact that predation on the blennies was rarely observed, they concluded that the blennies were protected by their mimicking the highly venomous catfishes.
For more information, see the paper: Clark, E, DR Nelson, MJ Stoll and Y Kobayashi, 2011. Swarming, diel movements, feeding and cleaning behavior of juvenile venomous eeltail catfishes, Plotosus lineatus and P. japonicus (Siluriformes: Plotosidae). Aqua 17, pp. 211–239.
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