Key-hole Sand Dollar - Echinodiscus truncatus
Using its velvet-like covering of small, short spines, the Key-hole Sand Dollar, Echinodiscus truncatus (Clypeasteroida - Astriclypeidae), burrows just beneath the surface of intertidal sands in the Indo-west Pacific waters.
This is a large sand dollar about 8-9cm across, with a pair of elongated holes or lunules close to the disc edge. Various explanations for the adaptive value of the lunules have been put forward. One theory if that the lunule spins assist in burrowing while another ideas is that the holes have a hydrodynamic function in reducing lift in strong currents and thus preventing dislodgment.
Photo credit: ©Loh Kok Sheng | Locality: Tanah Merah, Singapore (2008)