Neither a blenny nor a goby, and the juveniles pour over the sand in their hundreds.
An eel-shaped fish in its own family, Pholidichthyidae, related to neither the blennies nor the gobies of its two common names. Juveniles are black with a single white stripe from snout to tail; adults reach about 34 cm and change to a pattern of broken pale bars on dark, becoming more banded and less striped with size.
The behaviour is what a diver sees. Juveniles form dense schools of hundreds that move over rubble and sand as a single dark ribbon, flowing rather than swimming, and pour back into a burrow entrance when disturbed. The schooling pattern has often been described as mimicry of the striped catfish Plotosus lineatus, which is venomous and schools in the same places — plausible, frequently repeated, and not demonstrated. Adults excavate and maintain extensive burrow systems under rubble, carrying spoil out in the mouth, which is where the “engineer” name comes from, and the young are fed inside the burrow on mucus produced by the parents. Recorded from about 1 to 25 m.
Ratings are per location: 1 means found on most dives that look for it, 5 means a handful of records. Where enough dives have been logged the figure is calculated from them; otherwise it is an editorial estimate, and says so. We publish habitat and destination, not the dive site.