Asterina Starfish

Asterina sp.

Identification	Tiny star, usually under 1/2 inch, often with unequal or oddly numbered arms (a sign of reproduction by splitting). Colors vary — tan, gray, green, or mottled. Frequently seen on glass and rock at night.
Origin	Cosmopolitan — enters tanks as a hitchhiker on live rock and coral frags
Maximum Length	Usually under 0.5 inch
Status	Nuisance hitchhiker — monitor and control

Behavior

Asterina stars are small starfish that hitchhike into reef tanks on live rock and coral frags. They reproduce asexually by fission — literally tearing themselves in half — which is why colonies can explode from a handful to hundreds seemingly overnight, and why individuals often have mismatched arm lengths. Most are harmless nocturnal grazers that clean film algae and detritus from rock and glass, and in small numbers they are a beneficial part of the cleanup crew. The problem is population: unchecked, they can overrun the tank, and certain strains develop a taste for coralline algae or even coral polyps and zoanthids.

Diet & Feeding

Omnivore/detritivore. Grazes film algae, diatoms, detritus, and biofilm. Some strains also eat coralline algae and may nip corals, particularly zoanthids and SPS bases. There is no need to feed them — they thrive on tank film, which is exactly why they multiply so fast.

Control & Removal

Reproduces rapidly by splitting — a small population can become an infestation. Watch for damage to coralline algae, zoanthids, or coral bases; harmful strains should be controlled. Control options: manual removal (pick them off at night when active), and the Harlequin Shrimp (Hymenocera picta), which eats starfish and will clear an Asterina outbreak — though it then needs a continuing supply of stars or other seastar food. Avoid introducing them intentionally, and inspect new frags and rock before adding.