Thorny oyster and red sponge. Thorny oyster (Spondylus varians) with a red sponge (red, bottom) covering its shell. Thorny oysters are filter feeders and are common on steep reefs and walls. Along the edge of their mantle (orange and white) are simple eyes (small dots). When these detect an approaching threat, the oyster quickly closes its shell. Photographed off the Solomon Islands, West Pacific.