#wormcrush

Just another day in the life of an acorn worm 📅

Named for their acorn-shaped front end, acorn worms (class Enteropneusta) are actually more closely related to humans than to worms. They have a rudimentary nerve cord that is similar to ours, and they breathe oxygen using structures similar to a fish's gills.

Acorn worms have no bones, no eyes, and no brain. Their bodies have three main parts—a long, worm-like trunk, a wide collar that surrounds a mouth, and a large proboscis that fills the mouth and extends, tongue-like, beyond it. Most acorn worms use their mouths and proboscises to slurp up sediment and debris that has settled on the seafloor.

Over millions of years, acorn worms in the deep sea have evolved some very different behaviors from their shallow-water relatives. For example, shallow-water acorn worms spend almost all their time in burrows within the sediment. They rarely leave their burrows, but extend their proboscises out of their burrows to feed. Most of their deep-sea relatives, however, live on top of the seafloor sediment. With their soft, weak bodies, many deep-sea acorn worms do not seem to be able to burrow at all.

In our latest installment of things that MBARI scientists get excited about we give you—a bioluminescent arrow worm.

Arrow worms, also known as chaetognaths, are relatively small marine worms, with some species only reaching two millimeters (or nearly a quarter of a centimeter) to 120 millimeters (about four inches).