There are apparently a few things that can cause a biscuit starfish to be square, with a birth defect being one of them. It can also be caused by one losing a limb.
But the search results for it are all pretty meh in reliability, so take that as you will.
So there are starfish that are meant to be squarish or diamond or roundish in shape. They’re called cushion stars.
The “defect” is that they’re flat instead of bulbous.
They’re beautiful
There are apparently a few things that can cause a biscuit starfish to be square, with a birth defect being one of them. It can also be caused by one losing a limb.
But the search results for it are all pretty meh in reliability, so take that as you will.