When these little hitchhiker starfish started appearing in our tanks all the info I could find on them said they were harmless diatom eaters. I thought Yeah! Something good to help keep our tanks clean ... until... I started seeing where they had been sitting on a coraline covered surface was now a white spot where they'd been. I started to look more closely over time & found a couple that were sitting on my corals like a Toadstool & found they had bored a hole into the tissue of my coral! Then I was hearing reports that folks were associating them with not only coral irritation/closing but decimating colonies of Zoo, polyps & more! Between my own observations & the reports of my people I started to recommend the removal of them when we saw them in our tanks. Much like Aiptasia, almost every hobbyist will encounter these little critters so I ask people not to be freaked out about them but to be proactive in your hobby & remove them when seen. They appear on the glass mostly once the tank is dark so snapping on your tank lights or being there just as they're about to come on, prepared with a net or tweezers or even just fingers to remove them.  It's easy to knock their numbers down so that they can't do any real harm. This being called a "hobby" means our tanks need hands on from time to time. I rarely see these Asterina stars in my tanks & again, when I do, I just net or suck them out when I do water changes. I'd love to think that keeping the beautiful Linckia Starfish would naturally keep the Asterinas under control. :c)

First pic is one unusually large type that shows an adult with many babies.  Most variants of Asterinas average 1/8 to 1/4 inch so you can see how large this one is.  Most are light colored looking whitish to silver to light brown, some more patterned like these pictured here.  Note how their legs can be different sizes depending on what phase of reproduction they're at.  Most divide into 2 or more "pieces" then develop additional legs etc.  The second pic is what looks to be my Orange Linckia Starfish eating an Asterina!  Yeah!!