Asterina starfish

Identification

A tiny cushion star, usually 5-15 mm across, off-white, tan or mottled brown-grey, with five to seven short, stubby arms that are often uneven because the star splits in half to reproduce. Found on glass, rock and coral bases, most visible early in the morning or at night. Irregular arm counts and sizes in the same tank are normal for these fission reproducers.

Often confused with

Baby brittle stars, which have a small central disc with long, thin, clearly jointed arms and move quickly. Also confused with genuinely coral-eating small sea stars from other families; the plump cushion body, stubby webbed arms and slow grazing pace mark an asterinid.

What it targets

Usually algae, biofilm, coralline algae and detritus. Contested reports involve zoanthids, soft corals and SPS tissue, most often when the population has reached plague density and grazing food runs short, and some experts argue the stars were cleaning already-dead tissue.

How it spreads

Hitchhikes on live rock and coral frags, then multiplies by fission — one star literally tears itself into two regenerating halves — so numbers can climb fast in well-fed tanks.

Control (safest first)

Watch and count: leave small numbers alone and note whether any star is found sitting on coral tissue with damage beneath it. Most tanks need no action at all. Manual harvest at lights-on or with a siphon during water changes: stars on glass and rock siphon off easily, and a few minutes per water change holds numbers down. Safe, free and chemical-free; it manages rather than eradicates. Trim feeding: populations track available food, so reducing overfeeding slows the fission rate and keeps a harmless population harmless. Harlequin shrimp for confirmed problem populations: extremely effective starfish specialists (see predators), but they eat only sea stars, will also take desirable stars and urchin tube feet, and need a continued starfish food supply or rehoming once the asterinas are gone.

Natural predators

• Harlequin shrimp→
• Harlequin shrimp→

Field notes

A Reef2Reef keeper in a 10-gallon tank first heard the white ones were harmless algae eaters, then reported picking stars off zoanthids and finding a hole where each star had sat, and removed at least twenty by hand in one session with night hunting planned for the rest. Other keepers in the same discussion separated them by colour, holding that plain tan-brown stars had never bothered anything in their tanks while more strongly coloured ones were the coral botherers. A WetWebMedia expert answer took the opposite long view: thousands of these stars across facility and home tanks over ten years, with a single observed nibble on a soft coral and no real harm, plus the argument that stars found on dying SPS are usually scavenging necrotic tissue the keeper blamed on the star.