Vmenkov (CC BY-SA 3.0)
Peanut Worms
Phylum Sipuncula (sipunculids)
Unsegmented deposit and suspension feeders that live inside rock and eat detritus; they have no jaws, bristles, or sting.
Identification
Smooth, leathery, unsegmented worms in dull brown, grey, or tan, commonly a few centimetres long (occasionally around 10 cm / 4 in). The body pairs a bulbous trunk with a narrower front section, the introvert, which can pull back inside the trunk; a disturbed worm contracts into a short, fat shape recalling a peanut kernel. They live in holes in live rock and sand and show mainly at night, stretching out to scrape or sweep up food.
Often confused with
Pest worms such as eunicids and large bristleworms. The separating tell is what a peanut worm lacks: no segments, no bristles, no legs, no antennae, and no jaws — just a smooth, soft, slightly rough-skinned body that shortens and fattens when touched. Eunicids show obvious segmentation, five short head tentacles, and grasping jaws; bristleworms show segments plus rows of bristles.
Natural predators
No natural predators are listed.
Field notes
Quietly beneficial rock cleaners that work through detritus trapped in rock pores and sand. They startle keepers on night-time flashlight rounds, then go back to cleaning — leave them where they live.