Skip to content
reefcalcs

Invertebrates

beginner

Asterina starfish: harmless or coral eaters?

Asterina spp. (small fissiparous sea stars) · Asteroidea (sea stars)

Also called: asterina, asterina stars, mini starfish

Checked against 3 sources on · reefcalcs editorial

Quick reference

Care level

Beginner

Adult size

About 1/4–1/2 in, up to about 3/4 in (2 cm)[1]

Reef safe

Mostly; some eat coral polyps, especially zoanthids[1, 2]

Diet

Opportunistic: film, detritus and leftover food; sometimes coral[2, 1]

Tank size

Not applicable: they arrive on rock and coral.

Temperature

76–81 °F (general reef range)

Salinity

35 ppt (SG 1.0264 at 25 °C)

pH

8.0–8.4

What are Asterina starfish?

Asterina are small, flattened sea stars with wide arms.[2] Reef Builders' Nicole Helgason describes them at about a quarter to half an inch across, rarely more than three-quarters, and says that by and large they're innocuous members of a mature reef; telling the harmless species from the coral-eating ones, though, is hard.[1]

How to get rid of Asterina starfish

First decide whether you need to. If corals show no damage, Asterina are clean-up crew.[1] If they're eating corals:

Feed less. Jake Adams, writing in Practical Fishkeeping, says growth depends on food supply, so cut back feeding and keep enough flow that leftovers stay suspended for the filter to catch; with plenty of food they breed faster than you can remove them.[2]

Pick them off, especially near zoanthids, at night or early morning when more are out.[2]

Add harlequin shrimp. Adams suggests one or two once food is reduced, and Helgason says they will consume an infestation in weeks to months but must then keep getting starfish or they'll die; some reefers share shrimp between tanks.[2, 1, 3]

What they eat

Adams calls the genus opportunistic feeders of almost everything, including zoanthids, and says zoanthid grazing usually signals excess food in the tank.[2]

Why it's a problem

The sign of a coral eater is jagged white marks at the base of corals; Helgason advises watching coral bases during an outbreak.[1]

What eats it

Harlequin shrimp eat them and little else, so plan their next meal before you buy them.[1, 3]

Prevention

Cutting back food is the best prevention: numbers follow the food supply.[2]

How it spreads

They reproduce by splitting: the body divides and lost arms regrow, so you'll see lopsided, three-armed individuals mid-regrowth, and a handful can cover the rock seemingly overnight.[1] Food supply sets how fast.[2]

Troubleshooting

Jagged white marks at the base of a coral

Likely cause: Coral-eating Asterina.[1]

Fix: Feed less, remove stars near the coral, consider harlequins.[2, 1]

Zoanthids closing, stars on the colony

Likely cause: Zoanthid grazing, usually with excess food.[2]

Fix: Cut feeding and pick off stars at night.[2]

Hundreds overnight

Likely cause: Splitting, fuelled by food.[1, 2]

Fix: Reduce feeding; numbers follow food.[2]

Common mistakes

  • •Killing harmless Asterina that aren't touching corals.[1]
  • •Buying harlequins with no plan to feed them after.[1, 3]
  • •Overfeeding the tank.[2]

Asterina starfish FAQ

Are Asterina starfish bad?
Mostly not; Helgason calls them innocuous in a mature reef, but some eat corals, especially zoanthids when there's excess food.[1, 2]
What eats Asterina starfish?
Harlequin shrimp: Helgason says they'll consume an infestation in weeks to months.[1, 3]
Why are there suddenly so many Asterina?
They split to multiply, and food supply controls the rate.[1, 2]
How big do Asterina get?
About a quarter to half an inch, rarely more than three-quarters.[1]
How do I know if Asterina are eating my corals?
Look for jagged white marks at the base of corals.[1]

Sources

  1. Nicole Helgason — Asterina starfish, an introduction to this common reef aquarium critter (Reef Builders, 2016)
  2. Jake Adams — How can I control Asterina starfish? (Practical Fishkeeping, 2009)
  3. Tropical Fish Hobbyist — Hymenocera picta (harlequin shrimp eat only live sea stars)

Spotted something out of date? The corrections log lists every change we make.

Related

Last reviewed