The class of Polychaeta best known as bristle worms, as its name indicates, are characterized by the presence of numerous chaetas (a species of keratinous bristle or seta with structural, tactile and/or locomotive function). These animals are mostly from marine aquatic habitats, and mainly benthic (living on the sea floor), although there are also some pelagic species that are part of the plankton.
The body of bristle worms is generally elongated and thin, with a size between 5 and 10 cm in length, although there are species with extreme sizes ranging from 1 mm to over 3 meters. The segmentation of the body of bristle worms is clearly visible. These segments, called metamers, may have lateral extensions called parapods, which, in turn, end up in the previously mentioned chaetas. Along their body, three regions are distinguished: the head (prostomium) in which the sensory organs are located (in those species that have them), the middle region called soma and in whose segments the parapods are found, and finally the posterior region called pigidium.
We can distinguish two very different morphological models that correspond to the subclassification of bristle worms ...
- Errantia. Species with well-developed parapods that they use to move and actively dig through mud/sand or under rocks in search of their prey (Crustaceans, Bivalves, Sponges and other bristle worms)
- Sedentaria. They are species with undifferentiated cephalic region, and generally with absent or very diminished parapods. Many of the species are tube-dwelling, that is, they live in tubes that they make themselves. Therefore, we are dealing with sessile or limited movement species. Most of them are filtering species, which generate water currents in order to attract suspended particles, filter them with their gill tufts (often the only part of the body that protrudes from the tubes), and then ingest them. The gill plume not only allows them to filter the water, but also allows them to perform the gaseous exchange of the breath
The vast majority of bristle worms have separate sexes and lack sexual dimorphism. Their reproduction consists of the release of the gametes into the sea current at certain times of the year, and therefore fertilization occurs externally.
Bristle worms tolerate a wide range of salinities. They are relatively important species from a food chain point of view, since bristle worms are food for countless species of fish and Crustaceans. It is the most numerous group of Annelids, with approximately 9,000 species described