Thyca crystallina is a species of sea snail, a marine gastropod mollusc in the family Eulimidae.

This species was first described in 1846 by the American conchologist Augustus Addison Gould as Pileopsis crystallina but was later transferred to Thyca.

[3] The mollusc larvae tend to settle on the upper side of one of the arms of the starfish, usually near its attachment to the disc.

As they grow, they migrate to the underside of the arm, settling on the right side of the starfish's ambulacral groove, and orientating themselves towards its mouth, and here they become firmly attached.

[5] The ventral surface has a central mouth and adheres to the starfish by suction created by the muscular pharynx.