I've been making efforts to keep Linckia Starfish in my 120-gallon mixed reef for years now. Most of the time the Linckias (and also Fromias) didn't live very long. I had a blue Linckia in the past that lived for two years as I recall, and even lost a leg with it having grown it back so there apparently was nutrition available for it in my tank. That one ultimately died, which death I attribute to the dirty-work of a Xanthid Crab that was running rampant in the tank but has since been removed. But, while some of the other Linckias and Fromias that I tried to keep lived for a number of months, most died rather quickly. I can't attribute those deaths to anything in particular...it could have been lack of sufficient nutrition, shipping injuries before I acquired them, etc.
But now this: I've had mottled-blue Linckia in my reef since at least the beginning of February, 2008 and this one has grown and thrived, and I believe I've learned why and how.
In the years that I've been attempting to keep these "reef-safe" starfish, I've done as much reading as I can find, some of it in this forum. The popular...and probably reliably so...wisdom is that no one knows what they eat. One kind might eat a particular "bacteria" and another, "sponges" says the reading material. But, in any event, it isn't likely that what they eat would be in an aquarium so a prevailing opinion is that they shouldn't even be attempted because they would most likely die from starvation.
But, this mottled-blue Linckia at least has revealed itself to be eating....Asterina Starfish.
I had hundreds of them in my reef in the past. I used to have to pick them out of the tank by hand, only to have them replenish their numbers over only a few week's time. I've been wondering what happened to them. Then, one day the past-week, I noticed that the mottled-blue Linckia appeard to be "eating" one. I watched it over a few hours and sure enough, it did seem clear that it had eaten the Asterina.
Here are some photos that show it ingesting another Asterina, which I took after the first one was ingested but on the same day...You can see the Asterina decomposing if you look carefully at its outline...
Here's another example of what appears to be the Linckia ingesting another Asterina on the same day....
...and this is the residue of the Aserina left after that ingestion....
I don't know if this means that all Linckias will be able to ingest, or are dependant upon a supply of Asterina Starfish. It could be that this is the case. Or it could be that only the mottled-blue Linckias consume them, as opposed to the blue, red, brown/red, etc. Linckias which require some other (unknown) food. But, I'm just about convinced that so long as the supply of Aserina Starfish in my reef (that is seriously dwindling) is available, this one will be able to survive and flourish on them.