Common Names: Asterina Starfish, mini-star
Scientific Name: Asterina spp.
Asterina is a genus of small starfish (sea stars) that are under 1″ across and usually much smaller in the 1/4″ – 1/2″ range. They are typically light gray or light brown in color often with darker mottling. The body is large in relation to the size of the legs. The legs are variable in number and may be different sizes due to their method of production. The genus encompasses about 30 different species which leads to confusion in the hobby since the different species are not differentiated and their behavior may vary between the different species.
Asterina have been reported to feed on everything from detritus, film algae, coralline algae, snail eggs, soft coral like green star polyps and SPS corals. The stomach is extended from the body and dissolves the food external from the body. This behavior can sometimes be observed when the starfish are clinging to the glass and consuming the film algae on it.
They reproduce in the tank by fragmentation. In some cases, the body will split into somewhat equal halves and in other cases they will drop a leg which then grows a new body. This method of reproduction results in Asterina often looking asymmetrical or with different numbers of legs. Some species seem to reproduce much more quickly in the tank than others.
Good or Bad?
Many hobbyists including myself have had Asterina starfish in their tanks for many years with no evidence that they eat anything worth caring about nor do they reproduce enough to become unsightly.
A number of hobbyists have observed them apparently eating coralline algae in their tanks. Whether this is a concern or not would depend on their numbers. A few hobbyists have observed them eating their soft corals like green star polyps or button polyps. Many warn that Asterina are avid consumers of SPS corals, but actual first hand reports of SPS damage seem to be hard to come by. There was one report by GARF.org many years ago which seems to be the main root of this claim that keeps getting repeated. A few other hobbyists have also reported SPS damage.
There is no doubt that the rapidly reproducing varieties can reach plague numbers and become somewhat unsightly in the tank. There is also some theories that like many reef tank inhabitants, once they exhaust their normal food supply, they start to look for other things in the tank to munch on so tanks that have large populations (or are kept scrumptiously clean) may be more at risk for bad behavior from the Asterina in their tank.
Something else that may be at play here which is similar to bristleworms is that they may be more likely to attack dying coral. They get blamed for the corals death when actually they are just scavenging dead tissue the coral that was dying from other causes.
In general, as long as the Asterina are observed hanging out on the glass or rockwork and not on coral, I leave them alone. After much time in the hobby, I have also come to realize that having something in the tank that may on occasion eat one of the rapidly reproducing polyps is not necessarily a bad thing.
If you want to control the Asterina in your tank for whatever reason, complete eradication can be difficult, but manual removal is generally sufficient for small tanks.
The only other option is to use biological control. The obvious candidate for this job is the Harlequin shrimp Humenocera elegans, which feeds exclusively on starfish. This approach works as long as you don’t have other starfish in the tank that you want to keep. You also then are faced with the issue of how to feed the Harlequin shrimp once the Asterina are eliminated. Some enterprising hobbyists have raised the fast reproducing varieties of Asterina in separate tanks to feed their Harlequin shrimp.
The Jewel starfish Nardoa spp. have also been reportedly observed eating Asterina, but it is unclear whether they would provide a viable complete control solution or not.
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