Acanthemblemaria hancocki
A pinhead-sized blenny peering from tubeworm holes on reefs in Guanacaste, Costa Rica, in water around 27 to 29C, year-round.
A tiny fish, barely 3cm long, that spends almost all its time with just its head sticking out of an old worm tube or hole in the rock. Look for a stubby, blunt-faced head with large eyes and branching skin flaps (cirri) above them, giving it a slightly spiky, punk-rock look. Body colour is mottled brown to olive, but you will rarely see more than the front third of it, since it retreats fully into its burrow the moment it feels threatened.
Spend the dive scanning old barnacle shells, worm tubes and small holes in rocky or coral substrate at close range, since this blenny never strays from its burrow entrance. It will hold still and watch you with its head poking out, then vanish inside in a blink if you get too close or move too fast. Slow down, hover, and let your eyes adjust to the scale, most divers swim right past them until they learn to look for the tiny cirri catching the light.
Like other tube blennies, it never really leaves its burrow, it feeds, defends territory and mates all from that one fixed hole, darting out only briefly to snatch passing plankton before retreating.
No clear seasonal pattern shows up in sightings, and its warm-water niche of roughly 27 to 29C holds steady year-round on these Pacific coasts, so any month is workable.
Most reliably found on rocky and coral reefs in Guanacaste, Costa Rica, with further sightings along Costa Rica's South Pacific and Central Pacific coasts. Panama's Pacific Coast, especially around Coiba and the Gulf of Chiriqui, is the other strong option for this species.
Mapping where Hancock's Blenny is logged…
Guanacaste in Costa Rica is the strongest area, with further sightings along Costa Rica's South Pacific and Central Pacific coasts and on Panama's Pacific Coast.
There is no strong season in the sighting data, and water temperatures stay in the warm 27 to 29C range year-round, so it can be found on any month's dive.
Look for a tiny, 3cm head poking out of a worm tube or small hole in the reef, with large eyes and branched skin flaps above them. The body is mottled brown and almost never fully visible since it stays tucked in its burrow.
They max out around 3cm, making them one of the smallest fish you can realistically go looking for on a reef.
It is a tube blenny, a group that lives permanently inside old worm tubes or small burrows in the rock rather than swimming freely. It feeds, defends its spot and breeds from that fixed home, only darting out briefly for passing food.
No, they pose no threat to divers. They are too small and too shy to do anything but retreat into their burrow if you get close.
Site and sighting data is aggregated from diver logs, iNaturalist and GBIF. For taxonomy and conservation status:
Yellowtail Clownfish
3,523 sites
