A purple-spined sea urchin (Arbacia punctulata) at the Flower Garden Banks National Marine Sanctuary, l ocated 70 to 115 miles off the coast of Texas and Louisiana (NOAA picture).
aboral, oral, lateral views and ambulacrum details from Cotteau, G. H. & Triger, J. 1857-1869. Échinides du département de la Sarthe, considérés au point de vue zoologique et stratigraphique. J.-B. Baillière et Fils, Paris, xv+457 pp.
Sea cucumbers are caught and eaten as beche de mer. They eat sand and detritus, digest the detritus and expel "clean" sand at the other end. One of the strangest symbiotic relationships in the ocean is with the pearl fish which lives inside the cucumber's anus during the day, and emerges at night to feed. They need to move to a better neighbourhood...
Coscinasterias muricata, Eleven-armed Seastar. Location: Australia, Victoria, Cape Paterson, Bunurong Marine National Park, Eagle's Nest. Photographer: Mark Norman