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Polychaetes are segmented worms found mostly in marine environments, with some species adapted to wetlands. They have existed since the Early Cambrian and have diverse lifestyles and adaptations, including bone-eating flower snot and the heat-tolerant Pompeii worm. Polychaetes engage in epitoky for reproduction and some, like the lugworm and common worm, are important food sources for other marine creatures.
Polychaetes are a class of ubiquitous segmented worms, mostly marine, although some species have adapted to terrestrial life in wetlands. They are annelids, the marine counterparts of terrestrial annelids such as earthworms. “Polychete” means “many hairs,” a reference to the chitinous hairs that protrude from both sides of their bodies, with an identical set of hairs per segment. Like terrestrial annelids, polychaetes have an advantage over simpler worms due to their segmentation, which helps give them a stiffer structure more conducive to secondary adaptations.
Like a few other common animal phyla, polychaetes have existed since the Early Cambrian, about 540 million years ago. Fossils of definitive polychaetes have been found in the Sirius Passet Lagerstatte, together with primitive arthropods. They may go back even earlier, as segmented hollow skeletal tubes (Cloudina), resembling the tubes used by some modern polychaetes, have been found in the Ediacaran, although consensus is absent on the matter. The pipes from Cloudina are among the earliest mineralized skeletons and earliest fossils to show evidence of predatory pits.
There are approximately 10,000 species of polychaetes, with great diversity in lifestyles and adaptations. Some, like the lugworm, spend their entire lives in U-shaped burrows in the intertidal zone. The coiled castings discarded above the surface are clearly visible on the beaches when the tide is out, and the fisherman sometimes brings up the worms for use as bait.
Another typical polychaete is the common worm, which searches for algae and other worms on the seabed and provides an important food source for bottom-dwelling crustaceans and fish. When it’s time to reproduce, the clam worm engages in a reproductive mode unique to polychaetes, epitoky. During the epitoky, the worm’s body suddenly changes dramatically, developing stronger swimming appendages, producing ova and sperm, and enhancing sensory and motor centers at the expense of the digestive system. It swims from below to the pelagic zone, where the plankton live, and expels its gamete packet, where it mixes with the gametes of other worms to produce larvae. These larvae feed on plankton, eventually sinking to the bottom and turning into worms.
Some notable polychaetes have learned to adapt to the more unusual environments. Bone-eating flower snot, for example, is a recently discovered genus that survives whale falls, the corpses of whales that sink to the bottom of the sea. It digs into bones with the help of bacteria, with which it is so symbiotic that it requires no stomach or mouth, instead absorbing nutrients with a strange root-like structure. The Pompeii worm, another bottom polychaete that thrives on bacteria around hydrothermal vents, is among the most heat-tolerant organisms, able to withstand temperatures as high as 80 degrees C (176 degrees F). Another polychaete, Lamellibrachia, is one of the longest-lived animals in the world, with ages up to 250 years.
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