Skimming the surface: a larval window on the biodiversity of nemertean worms

By Christopher Laumer, Department of Organismic & Evolutionary Biology, Harvard University

Go to the marina and gently lower a fine net into the frigid Pacific. Pace the docks a few times, trailing your net behind, just below the choppy water. In the small jar at the end of your net, you’ll find all the usual suspects – miniscule flitting crustaceans, kaleidoscopical green cells, perhaps a few fledgling fish. Look closely, however, and you may also see several of what for all the world appear to be tiny transparent helmets, blown off the heads of Lilliputian jockeys. The ocean can seem an alien universe.

Separate them, feed them, and watch them grow. Over time, from the surface of the helmet, small blobs of tissue coalesce to form a worm, the true helmsman of this UFO. Once complete, the worm will separate from its amniotic fetters, crawl about restlessly inside, and in a fateful minute, pierce through the edge of the helmet and writhe its way out, devouring the vehicle which was until now its entire world. Thus ends the life of a “pilidium” larva, and begins the life of an adult nemertean, or ribbon worm, a member of one of Earth’s most remarkable invertebrate groups.

Keep away your crabs! This swift larva is common in Coos Bay, OR. Its DNA match is Carcinonemertes errans, a hoplonemertean parasite and egg predator of the Dungeness crab, a commercially important species. As if in a Greek tragedy C. errans falls prey to another nemertean – Riserius.

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