Let's clarify. Is it a sand sifting brittle star or is it a serpent star? There's a difference.
Can you post a photo?
I never heard of a SAND sifting brittle star, I thought serpent and brittle stars were meat eaters, that fed on the dead, dying and uneaten food that drifts into the rocks. I say THOUGHT because I lean new stuff everyday, so maybe they sift sand or there is a type that does, nothing surprises me anymore.
A sand sifting star is what I have, but it would not feed on krill or mysis shrimp. it feeds on the fauna in the sand, they dig, and you seldom if ever see them, since they live under the sand. Mauler said his star had a broken leg, so I thought maybe it suffered some damage in a rock slide, even rock shifting a little can harm diggers.