We were coyly floating among the reef near some crevice with soft corals, watching unmindful before us when suddenly our DM pointed something attached to the coral. He poke carefully with his pointer and slowly it changes to white, and I was wondering what it was! It was our first encounter with such critter.
When we surface while still on the waters, I reminded the DM about it and told us it’s a cowrie shell. The black is part of the mollusk which slowly hides when disturb showing its white shell. Such a wonder!
The shells of the egg cowries reach 12 cm in length. In the adult the mantle covers the entire shell and is black with raised yellow tubercles and white spots. The juvenile resembles a toxic species of nudibranch. Unusually the mantle is kept out most of the time, even during daylight. The egg cowries are only seen out at night, usually on soft corals. There is evidence that they are territorial and that they return to the same hiding place just before sunrise.
The egg cowrie feed on soft corals, and are often seen feeding on leather corals.