polychaetes

Polychaetes are a diverse class of segmented marine annelid worms. They range from intertidal zones to the deep ocean and comprise roughly 10,000 described species. A hallmark is the presence of numerous chaetae, or bristles, on paired appendages called parapodia that extend from most body segments. This arrangement gives polychaetes a distinctive, bristly appearance and supports a variety of life modes. Most species have a well-developed head with sensory structures, while some sedentary tube-dwellers have reduced heads and specialized feeding apparatus.

The body is clearly segmented, and the parapodia are often flattened and may bear hooks or blades of chitin. Polychaetes show a broad range of lifestyles: errant, free-swimming predators with active movement; and sedentary, tube-dwelling forms that filter or deposit-feed. Some build calcareous tubes and live attached to substrates. They inhabit most marine environments, from shallow coastal pools to the deep sea, with a few species tolerating brackish water; freshwater polychaetes are rare.

Reproduction is typically sexual, with separate sexes and external fertilization in many species, followed by trochophore or other free-swimming larvae. Some undergo epitoky, a seasonal transformation of body segments that accompanies swarming reproductive events. Asexual reproduction by fragmentation or fission occurs in a minority of species.

Ecologically, polychaetes are important benthic organisms that contribute to sediment mixing (bioturbation), nutrient cycling, and serve as prey for other animals. Some species are used as fish bait; others, such as Platynereis dumerilii, are model organisms in developmental biology. Notable families include Nereididae (ragworms), Sabellidae (fan worms), and Serpulidae (tube worms).