Bristle worms paddle every ocean depth from surf to trench
Polychaetes are mostly marine annelids—“bristle worms”—with parapodia bearing chitinous chaetae on each segment. More than 10,000 species span planktonic surface lives to an undescribed form glimpsed by ROV in the ocean’s deepest region. Lugworms, bloodworms, and ragworms bait fishers worldwide.
Polychaetes, or bristle worms, are a class of mostly marine segmented worms within the annelids, though not a single natural lineage. Every segment carries a pair of fleshy, paddle-like flaps called parapodia, studded with bundles of chitin-like bristles, the chaetae that give the group its name. The parapodia are richly supplied with blood vessels, serving for locomotion and, in many species, as the main breathing surface.
Over 10,000 species are known, and they turn up everywhere in the sea: drifting near the surface, across abyssal plains and coral reefs, as parasites, occasionally in fresh water, and even in Challenger Deep, the deepest point in the oceans, where a remotely operated vehicle filmed a small undescribed species. Most are under 10 cm long, but they range from 1 mm to about 3 m in Eunice aphroditois, and some are brightly coloured, iridescent or luminescent. Familiar shore species such as lugworms, bloodworms and ragworms are dug up for fishing bait. Stranger relatives include stinging fireworms, the predatory bobbit worm, the culturally important palolo worm, bone-eating worms, and giant tube worms that tolerate near-boiling water at hydrothermal vents.
Body plans vary as much as lifestyles. The most generalised species crawl over the seabed, while others burrow, swim in open water, live in tubes they build or bore, share hosts' homes or live as parasites. The head, or prostomium, is better developed than in other annelids and overhangs the mouth. Diets range from hunting and grazing to filter feeding and scavenging, but most species have paired jaws of hardened collagen and a pharynx they can shoot out to seize food, sometimes lengthened into a proboscis.
Heads may carry two to four pairs of eyes, usually simple light detectors, though the lensed eyes of the Alciopidae rival those of squid and vertebrates. Antennae, tentacle-like palps and ciliated pits called nuchal organs help them smell out food. The body wall is a single layer of columnar cells under a thin collagen cuticle.
Source: Polychaete
