Lives in an empty barnacle shell with only the spiny head out, and withdraws the moment a camera approaches.
To 6 cm. Elongate with a short blunt head. The head is grey-brown with yellowish tentacles, the throat and chin carry yellow spots, and the iris is red. The body is reddish-brown with white spots along the side, pale diamond-shaped marks at the base of the dorsal fin, and numerous blue dots.
It is the only blenny of its genus in the Galapagos, so the red iris and the yellow chin spots confirm it without needing to work through relatives.
It lives in empty barnacle shells and worm tubes on rocky reef between 1 and about 25 m, usually between 6 and 12, and it perches in the opening with only its head showing, darting out to take passing plankton and small crustaceans and reversing straight back in. It is seldom seen in the open. The male guards the eggs, which are stuck to the walls inside the shelter.
It is endemic to the Galapagos and is listed as vulnerable, which is unusual for a small blenny. It is named after Miguel Castro, the first conservation officer at the Charles Darwin Research Station.
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.