Polychaeta, composed of about 10,000 species, is the larger of the two classes of segmented worms (phylum Annelida) – the other class being the Clitellata (earthworms and leeches). The polychaete worms are problematic in that molecular phylogenetic analyses carried out the last 20 years indicate that they are not a natural (monophyletic) group; rather, this clade includes the phylums Sipunculata and Echiura, as well as a small enigmatic group of worms called the beard worms (pogonophorans+vestimentiferans; about 100 species). In addition, many analyses also now suggest that Clitellata, which has long been considered the sister group to the polychaetes, is actually a derived group of worms within the polychaetes (McHugh 1997, 2005, Struck et al 2007).
The polychaete worms (in the traditional inclusion of taxa) are mostly marine. They are commonly found burrowing in sediments on beaches, or live in tubes, which in cases where many worms live together, form calcareous reef structures. Some species are free-swimming, some live as commensals or parasites. Polychaetes are the most abundant macrofauna of the deep sea, and inhabit the world’s oceans at all different depths and water temperatures. Polychaete worms are characterized by an elongated, metameric body usually bearing a pair of appendages called parapodia on each metamere (segment), as well as tufts of chaetae (spines served by muscles which typically can be extended and retracted; often the polychaetes are called bristle worms, and their name derives from the Latin for many bristles). The body segmentation is visible as lateral lines around the worm’s body, reflecting the internal separation of segments with septa (although septa are lost or reduced in some groups, especially tube dwellers, see below). Parapodia show vast diversity of form and function, serving purposes such as locomotion, gas exchange, protection, attachment, controlling water flow within a tube, or can be reduced or lost altogether. The polychaete head can be adorned with a multitude of sensory structures such as tentacular palps, antennae, and cirri. Predatory carnivores often have large pharyngeal jaws. At the end of the segmented body is the tail, called the pygidium, which houses the anus (Brusca and Brusca 2003)
A common organization of the polychaetes is to divide them into sedentary forms and free-living forms. Although this organization does not reflect their genetic relationships, it does illustrate the adaptation of their body form to their habitat and lifestyle. The free-living forms, which include families of carnivorous predators as well as direct deposit feeders – (free-living worms that burrow ingest the sediment sift out food particles in their gut) are commonly composed of a series of identical body segments (a homonomous body plan). They have well developed muscles and move by swimming, crawling, or burrowing with their parapodia adapted as paddles or legs. Burrowers often have a muscular proboscis to aid in digging. In contrast, the body segments of sedentary, tube-dwelling polychaetes show specializations for different functions (heterotomous form). These worms feed from permanent tubes or burrows, often by suspension feeding, selective deposit feeding or feeding on detritus. Their parapodia are often adapted for circulating water in the tube. Permanent tube-dwellers have softer and less muscular bodies, and frequently lose the septa between segments. This allows for adjustment of hydrostatic pressure within the worm, which is important for functions such as anchoring the end of the body housed in its tube. As well as providing protection, tubes also function as external support for these worms. Tubes can be soft, parchment-like forms constructed from sand and mucus, or hard calcareous tubes, which when many worms are together, form reef structures (Brusca and Brusca 2003)
Reflecting the large diversity of lifestyles and degree of independence of body segments, polychaete circulatory and respiratory systems also show many variations among taxa. Almost all polychaetes have a closed circulatory system. Many have distinct gills, usually adapted as highly vascularized parts of the parapodia, and circulatory systems are well-developed with a pair longitudinal vessels carrying blood in the anterior (dorsal vessel) and posterior (ventral vessel) directions along the full body of the worm. Gas exchange in others occurs across the entire body surface (especially in small or sedentary taxa with no parapodia, or in worms with no or partial internal body septa to separate coelomic spaces). Some taxa increase surface respiratory areas with feathery protrusions of the body surface through which the coelom extends. In these taxa the circulatory system is reduced, and most oxygen and nutrients are distributed in the coelomic fluid. Some taxa have pumping structures to increase blood flow, especially sedentary worms that do not use body movements to circulate the blood. Most (but not all) polychaetes have oxygen-carrying pigments in their circulatory fluid and coelomic fluid, usually a form of haeomoglobin. Some taxa have more than one pigment. The pigments that are present often have adaptive value for the animal’s lifestyle, for example intertidal dwellers have the ability to hold oxygen during high tides and release it during low tides. Almost all polychaetes have metanephridia allowing for each coelomic space to eliminate waste, osmoregulate, and spawn gametes. The nervous system includes a cerebral ganglion at the head and one or more longitudinal nerves running the length of the body with an associated pair of ganglia in each segment. Polychaetes display a large array of different sensory structures, including touch receptors; photoreceptors which may be developed into one or more pairs of anteriorly positioned eyes or distributed around the body; chemoreceptors, and statocysts.
Reproduction and development
Most polychaetes can regenerate to some degree – from regenerating lost appendages to posterior body segments. Some polychaetes reproduce asexually by breaking into two or more groups of segments, and some reproduce very efficiently this way by “multiple fragmentation”, in which each segment becomes a new individual (for example Dodecaceria (Cirratulidae)).
Polychaetes are almost all dioecious, although they do not have distinct gonads. Instead, patches of the peritoneum lining the coelem (in one or many segments, depending on the taxon) divide to produce prospective gametes, which then break off and fully mature into eggs or sperm in the coelom. Eggs and sperm are released from the worm’s coelom through nephridial or coelomic ducts to the outside, or through breaks in the worm’s body wall during spawning. Most polychaetes have external fertilization, although some species brood their young. In order to maximize fertilization some primarily-benthic polychaetes (characteristic in the families Nereidae, Eunicidae, and Sillidae) spawn while swarming at the top of the water column. To do this, these worms transform into a swimming, sexual form quite different from the benthic form (called epitoky). One way this is done, as seen in members of the families Nereidae and Eunicidae, is by completely transforming the whole body into a sexual individual. Some notable modifications include enlarging swimming parapodia at the anterior end and often developing large eyes. Epitoke formation is stimulated by hormones in the brain, which are found only in older worms. Another method is to bud off the posterior portion of the body to form the sexual epitoke. This happens in the Syllidae. A third method of forming a sexual form is for a hind portion with the gametes to break off. This is not a true epitoke because the swimming section is not a complete worm. Palola viridis (Eunicidae) is an example of a species that makes this type of swarmer. Lunar cycles trigger the swarming event, and this species swarms in such huge numbers that natives of the samoan islands, where they are found, collect and feast on them (Kozloff 1990; Brusca and Brusca 2003)
Polychaete eggs undergo spiral cleavage. Many develop into a free-swimming trochophore larva. This larva grows at the “growth zone” on the posterior end, by forming sequential serial segments in a process called teloblastic growth. After a period of larval elongation, the larvae settles from the plankton and becomes a juvenile worm. Other polychaetes undergo direct development (without a larval stage), or short, non-feeding trochophore stage (Brusca and Brusca 2003)
Fossil polychaetes have been found dating back to the early Cambrian. They are known mostly from fossilized jaws and mineralized tubes (Brusca and Brusca 2003).