Asterina Starfish | Aquarium Friend or Foe?

Hey, reef keeper! If you’re reading this, you probably woke up one morning to find an uninvited mini starfish in your tank (or maybe two, or even ten). We all know the drill: various creatures can hitchhike their way into our tanks on live rock or coral. Some are welcome additions, others can wreak havoc.

Are these starfish, known as Asterina, aquarium friends or foes? Should you get rid of them, and if so, how do you do it? Let’s find out!

What is an Asterina starfish?

“Asterina” is a genus of asteroideans (better known as starfish or sea stars). It’s also the name applied to a variety of small starfish, usually reaching no more than 1″ in diameter, commonly found as hitchhikers in saltwater aquariums.

Funnily enough, as often happens in the aquarium hobby, the name used for these sea stars is actually incorrect. The species that come in on live rock almost never truly belong to the genus Asterina, which reproduces relatively slowly and therefore doesn’t tend to reach infestation-level population sizes.

The mini stars we find in our tanks tend to be species of Aquilonastra, which admittedly belong to the same family as Asterina: the Asterinidae. Unlike their cousins, Aquilonastra starfish tend to reproduce asexually by means of fissure. Since everyone knows them as Asterina, though, that’s what we’ll continue to call them here.

Many starfish can split themselves and regenerate from a single arm, but Aquilonastra does so almost non-stop. This is why the specimens you’ll find in your tank often have a completely random number of arms, ranging from two or three after splitting all the way to seven or even more.

Asterina (Aquilonastra) starfish mostly feed on detritus and algae. However, when other food sources run low—and depending on the exact species you’re dealing with—some have been said to develop a taste for coral, particularly Zoanthids.

Asterina: Aquarium friend or foe?

With most aquarium hitchhikers, it’s pretty clear whether they’re welcome friends or fearsome foes. The Stomatella snail, for example, is a clear friend. The gorilla crab, on the other hand, a destructive foe. Things aren’t quite as clear with Asterina starfish—it’s mostly a matter of opinion!

Some aquarists welcome Asterinas into their tank, considering them a part of their aquatic janitorial crew and noting no issues with their corals. Others are sure they’ve seen them munching on polyps (or just find them unsightly) and remove the stars as soon as they pop up.

Additionally, whether an Asterina infestation gets out of control and takes over your tank depends on the amount of food (mostly detritus) available. Aquarists who diligently clean their tanks and feed sparingly might find only one or two Asterinas at a time, which is tolerable. In messier aquariums, the population can seemingly explode to alarming levels overnight.

To remove, or not to remove?

If you spot Asterina stars in your tank, we recommend monitoring them. A rapidly expanding population can indicate you’re overfeeding, and you may be able to keep their multiplication somewhat under control by cutting back.

Also keep an eye on your corals. White nibble marks and irritated, closed-up polyps suggest coral munching, which warrants eradication efforts. Picking off all those tiny sea stars by hand would be exhausting, but luckily, help is available.

Getting rid of Asterina starfish

If you’re wondering what the heck you’re looking at in the photo above, you’re not alone. Is it a praying mantis? An orchid flower? Nope, it’s the harlequin shrimp (Hymenocera picta), your greatest ally in the battle against unwanted Asterina starfish.

This shrimp isn’t just special in terms of looks, although it certainly delivers in that department. The main reason aquarists are so fond of it is its diet: harlequin shrimp exclusively eat starfish. In fact, they starve if none are available! An Asterina infestation is their idea of a 5-star buffet, and they can make incredibly quick work of it.

If your tank and its inhabitants match the needs of this shrimp (you can find them in our harlequin shrimp care guide), you can expect to be rid of the mini sea stars within weeks. When this happens, remember to either return the shrimp to your aquarium store or start buying live starfish for it to eat; it’s unable to accept any other type of food.

Tip: Can’t accommodate a harlequin shrimp? You’ll have to rely on manual removal. Picking the starfish off one by one is an option, but you can also try a simple DIY trap by placing some food in a small container and waiting until the starfish flock to it before removing them. And don’t forget to limit the nutrient levels in your tank so the population can’t grow!

Need help?

A beautiful reef aquarium filled with colorful fish and gently swaying corals is a dream, but it’s not all fun and games: hitchhikers like Asterina can infest your tank and cause trouble. But did you know you can have a reef without having to worry about anything?

The FantaSEA Aquariums team can design, set up, and maintain your dream aquarium for you, so all you have to do is enjoy it. We get our hands wet so you don’t have to! Contact us with your ideas today.