@birdsandbats I don't think so, if it had lost an arm there'd be a gap, but this starfish has evenly-spaced arms, so seems likely this starfish just happens to have less than the usual number of arms. it can happen occasionally, they end up with more or less than the usual number of arms for that species. it doesnt seem to greatly disadvantage to starfish so they can still survive perfectly well in the wild. like this, 5 is the usual number for this species
