size of asterina starfish

Starfish ID Please

Hitchhiker Starfish ID Please

Do you have these starfish growing in your tank?I do. They are called Asterina Starfish.

hitchhiker starfishThey are tiny starfish that grow up to be about a quarter of an inch to an inch across (Sprung 2001)—which I found out, through painstaking scientific examination, is approximately the same size as George Washington’s face on a quarter (Ulrich…being a knuckle head moments ago).

size of asterina starfish

about the size of George’s face

I typically notice them munching on algae on my aquarium glass, especially when the glass is dirty. I’m not sure if that’s when they are most active, or if that’s when I feel the most guilt for cleaning the glass.

How do I know they’re munching away? Starfish eat by extending their stomach outside of a central cavity on the under-side of their body. You can see this in action on the aquarium glass, where the soft, translucent stomach can be seen as a salmon color between the glass and the hard shell of the starfish.


In my tank, these starfish tend to be missing legs. It is rare to find them with what you may be thinking of a symmetrical. I learned that, like corals, these starfish reproduce from fragments of themselves, by a process called fission.

The big question is: are these starfish reef safe?

Are these starfish reef safe?

Whether or not these Asterina starfish are reef safe appears to be a more complicated question than you would think.
Julian Sprung, in Invertebrates, mentions that the smaller species feed on algae, but other larger species may sometimes feed on coral tissue and are considered to be dangerous in the reef aquarium, because they might feed on coral tissue and because they reproduce rapidly.

However, there appears to be a bit of an air of mystery about these starfish. There are plenty of forum threads about these little critters—and most of what I see online is people talking about proactively removing them from their tanks, in fear that these little critters will be coral predators—but I have seen very little proof of these critters in action.

Even in the Sprung book, there is an image of some Asterina starfish munching on an obviously dying stony coral, but the coral is hundreds of times larger than the Asterina starfish allegedly caught in the act of murder, but the entire coral mass has bleached…not just the parts the starfish have eaten. Which begs the question whether the starfish were attracted to the dying coral or whether the starfish caused the demise of the coral.

There was a report of these critters causing problems (eating) at the base of SPS corals on

asterina starfish is a saltwater aquarium hitchhiker

Asterina starfish are also thought to eat soft coral polyps, like green star polyps. I have kept green star polyps for years—and have yet to see anything stunt their growth, besides the air. The Asterina in my tank are always on the glass, never on the GSP, or zoanthids, for that matter.

Like the Aiptasia anemone, I think the Asterina starfish has a bad rap. Over the years, I have had a few different species (or at least color morphs) in my tank, and I’ve never noticed any problems.

Starfish are opportunistic eaters, I have no doubts about that—and in an immaculately cleaned tank, I can see a starfish resorting to whatever food source is available, but I have a hard time believing these little guys should be extinguished, en masse, if you don’t see them causing any problems.

With that said, if you have had personal, first-hand experience with asterina starfish causing problems, I encourage you to reply to this post and let us know. I will personally continue to watch these starfish with one suspiciously raised eyebrow, but for now, I am pronouncing them harmless…you know what…I’m going to elevate their status from harmless to helpful…since they appear to be cleaning up the aquarium glass, one George Washington face full at a time. For that reason, I plan to live and let live


For more information, check out these sources and decide for yourself:



  1. Well I must say I have had many of these little stars in my tank, not over abundance of them at least what I have seen. I know people were always talking about them eating corals and I have always had a hard time keeping Zoas and other Green Star Polyps. The other day I saw one what appeared to be munching on the Green Star Polyps. Since I had these Green Star Polyps they only lasted about 5 days and appear to all be dead now . Since then I have been removing them every time I see them. I will be giving it a few more weeks before I add more of the Green Star Polyps:

    1. Author

      Bob T, I am sorry to hear that. Could you clarify–are you able to keep other corals, you just have challenges with Green Star Polyps? Also, could you confirm, were the Green Star Polyps otherwise healthy before you found the stars on them? The reason I am asking is that I’m trying to determine, for myself, whether the GSP is dying and attracting the starfish or if the starfish are attacking healthy GSP and making them die. What are your thoughts about that, based on what you see?

  2. Hey there Al! I have a bunch of experience with these little guys. I first thought they were wonderful little creatures to add to a reef tank but soon realized that, in my tank at least- and I believe it is a hit
    or miss with these little guys, is that mine started irritating my zooanthids to the point where they were loosing stature and eventually melting away or flattening out upon the rocks they are on. From what I have read on them in a few marine biologists articles is that they can pull the “life” or zooxanthellae. I now pull them off whenever I see them because yes they are prolific breeders and I started with 5 and now have thousands in a very short period of time. I tend to just take them out when I see them near my zoos and any very large ones. I found this article Al if you want to take a look at it. I found it very interesting and informative:

    1. Author

      Toni, it is always great to hear from you. Thank you for sharing your experiences here. The linked article you provided is also interesting. Thanks for your help here!

  3. My asterina starfish population eventually got out of hand, and their number got into the thousands in my old 125 gallon. It was way too many to try and manually remove. They stripped my beautiful coraline algae that cover all the rocks. My tank turned ugly with no coraline. I eventually bought a Harlequin shrimp for the tank. However the shrimp disappeared and I assumed it died when I didn’t see it for a month. I added another Harlequin shrimp to my tank, and the next day my long lost shrimp was sitting beside the new addition. They ended up becoming a mated pair. It took them about 6 months to make an visual impact on the starfish population. At the year mark I didn’t see too many of the starfish left and I sold my shrimp pair to a LFS. Good luck!!

    1. Author

      Mike, thanks for sharing your experience with these fish. I love Harlequin shrimp, they are gorgeous…what an interesting way to establish a pair. It sounds like you had issues with the coralline algae in the tank–any issues with them actually attacking corals? Even at a concentration of thousands? Not that stripping coralline algae isn’t frustrating enough, just trying to get different perspectives on it. thanks!

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