Mystery Circles on Seahorse Solved? Watch out for the Asterina Stars

By: | Date: 04/20/2015 | 1 Comment |

When are spots on a seahorse not spots? When they’re starfish bites. Recently, a fellow seahorse keeper Adrienne Smith asked about some unusual markings on her seahorses. As a seahorse mom should be, she was concerned that they might be a sign of illness.

Everyone seemed to agree they were unusual, but what struck most people was what a perfect circle they were. Normally, a seahorse will either it’s natural markings, which are profuse and somewhat regular. Or they may have an injury or start of a bacterial infection, which is going to be asymmetrical.

Not these. These were perfect little circles.

Three perfectly round white circles on a seahorse's tail.

Mysterious white circles. Photo by Adrienne Smith


A Surprising Answer

Another seahorse keeper had the answer: asterina seastars.

Amanda Whitman captured a before and after picture of an Asterina seastar on one of her seahorses. The after showed a perfectly round spot of discoloration.

Small starfish on Seahorse leaving small white circle.

Before and after Asterina star. Photo by Amanda Whitman

Asterina stars are small seastars that often hitch a ride on live rock or algae; and feed off of algae in an aquarium. Some people argue whether or not they’re harmful, especially towards coral, but the general consensus is that they’re pretty benevolent, although they can get to plague numbers in the right aquarium environment.

Several people commented they had these stars in their tanks with seahorses and never had this problem. Even Adrienne had said that while the marks were new, the seastars were not. They had been in the tank for quite some time, which is why she never suspected them.

Why now then? It’s not clear – perhaps they ran out of the food they were eating previous? Some other change in the aquarium?

A Hypothesis

Unlike most fish, seahorses lack scales and just have a soft dermal layer over the bony plates that protect their body. Seahorses can host algae on their skin; they have special flame cone cells which allow algae to live on a seahorse and help provide camouflage.

While it may be impossible to determine why the stars started to feed on some seahorses, a good guess would be it was incidental damage in an attempt to feed on algae that was growing on the seahorses. Because seahorses support the growth of algae, it shouldn’t be surprising that an algae eater would decided to snack on one.

Indeed, most grazing fish can’t be kept with seahorses because of the damage they cause trying to eat algae off of seahorses.

What Makes A Good Star Go Bad?

But for as many people that chimed in with this problem, many more said they did not have a problem with asterina. And Adreinne didn’t have any problems until very recently. There are numerous species of seastar in the Asterina genus, and there has been ample suggestion that only some species of asterina cause problems in corals. The same could hold true for seahorses.

It’s also possible that if their food source changed, they may have become more desperate for food; choosing to nibble algae off wherever they could, in this case a seahorse. It’s also possible it’s not incidental damage, but intentionally targeting seahorses.

It’s likely that we won’t really know, we just know it happens.

To Remove Or Not Remove

The question on whether or not to remove these stars is a tough one. Most people don’t have this problem. Some reported noticing asterina stars on their seahorses but not noticing any damage.

The injury caused is probably a superficial one. The same flame cone cells that support algae growth can be shed easily by seahorses. It’s possible that if these are the ones being damage, they cause no significant concern. But on the flip side, seahorses are prone to bacterial infections, and any injury could be a site for infection to take hold.

To further complicate matters, there aren’t a lot of natural controls for asterina stars, fewer that are safe with seahorses. Harlequin shrimp will eat them; but once the tank is devoid of seastars, you have a species of shrimp that only eats stars and must keep feeding more, or pass them on to another aquarist in need. The only other option is manual removal, which is a time consuming process.

My feeling is that if you’re not having problems; don’t worry about it and just keep an eye on your seahorses. The occasional night-time viewing is probably a good idea as it’s when seahorses will be the most vulnerable.

Even if you have noticed this problem, so far there aren’t an reports of problems occurring with these marks. If you decide the risk is something you don’t want to deal with, I still wouldn’t think it’s necessary to tear them out all at once – just remove them as you see them.

What have my readers seen. Anyone else with asterina stars with their seahorses? Any problems? Post your experiences in the comments below.

One Response to “Mystery Circles on Seahorse Solved? Watch out for the Asterina Stars”

  1. ortho lab Says:

    What a great article, and the pictures are amazing thanks for the educational piece!

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