Swimmerpede

The swimmerpede evolved from the stingerpede. Now it's an active hunter, which will hunt for beakworms and other creatures about a beakworm's size. The spikes on its back degenerated into little bumps, while the spikes on the tail became flat and are used as a backfin to follow the swimmerpede's prey. It's about as long as a human forearm and lives in the reefs.