What are polychaetes?

The polychaete worms (class Polychaeta) are a paraphyletic group of worms annelidsalmost every marinewhose body is characterized by being segmented and having in each segment a pair of appendages, the parapodia or parapodia

Although not present in all species, each parapodium has numerous hairy structures for tactile and locomotor function called setae or bristles, hence the name polychaetes, which means «many setae«.

Class Polychaeta has more than 10 thousand species described cosmopolitan distribution. Among the most representative species we can mention arenicola marinawhich lives in the sand on beaches, and various species of the genus AlittaWhat A. succinea Y A. virens.

In addition to numerous, the polychaetes colonize very different aquatic habitats, from species that can live in the cold waters of the abyssal plains, to species that live in super-hot waters in the vicinity of hydrothermal vents.

Polychaetes are present in oceans around the world and can be found at virtually any depth. And although they are known as marine segmented worms, there are more than 160 known species that live in fresh water.

Morphology and general characteristics

The polychaetes are segmented body worms with an approximate average length of around 10-15 cm, with extreme examples as short as 1 mm or as long as 3 meters (eunice aphroditeis).

The color of polychaetes can vary considerably. They can be dull colors, they can be bright colors, they can be iridescent (change color by changing the angle of reflected light), and they can even be luminescent.

On each body segment they carry two parapodia they use to move and that in many species they also constitute their main respiratory surface. The setae, characteristic of these worms, are grouped in bundles that protrude from the parapodia and are often used as sensory organs.

This general description is the most common among the typical polychaetes that live in the sand and in the winds, but there are polychaetes that have adapted to other marine niches, from polychaetes that live suspended in the water to those that live buried, anchored to the ground. and rocks (tube worms) or those that live in the pelagic zones. There are also commensals and parasites. Each of these groups have bodily and physiological adaptations for their particular way of life.

The head of the polychaete, called prostomium, is usually well developed in all groups, almost always larger than in other groups of annelid worms. It usually has a mouth at the bottom and two or four pairs of ocelli or simple eyes capable of detecting changes in light, but not true vision. There are also completely blind species and there are also those with more complex eyes capable of more sophisticated vision.

The head usually protrudes a pair of antennae and several cilia with chemoreceptors which they use, among other possible objectives, to search for food.

anatomy and physiology

The body of polychaetes consists, as described above, of several segments, each with two appendages called parapodia, and a head or prostomium. The outer covering is a wall formed by a layer of simple columnar epithelium covered by a fine chitin cuticle and intertwined collagen fibers.

Beneath this covering, from the outside in, is a layer of connective tissue, a layer of circular muscle tissue, a layer of longitudinal muscles, and the peritoneum that surrounds the central cavity or body cavity. Attached to the parapodia may be oblique muscles that give it movement.

In some species the body cavity is continuous, but in most the cavity of one segment is separated from the cavity of the next by one or more layers of peritoneum.

Digestive, respiratory and excretory system

The mouth of these worms is located in the peristomium, the first true segment of the body, lying just below the prostomium or head. The mouth is very variable in shape, with specific adaptations to the diet of the species, since among the polychaetes there are predators, herbivores, filter feeders, detritivores and parasites.

In general, the mouth consists of two mandibles formed by plates of hardened chitin (sclerites) and a pharynx that can be everted (pulled out) and used to catch food. In some species the pharynx is modified in the form of a proboscis (elongated tubular appendage).

The digestive system continues from the pharynx as a simple tube and usually includes a stomach.

Many species have gills associated with the parapodia to breathe, but smaller species and those that live in burrows usually breathe directly through the body surface.

The excretory system usually consists of protonephridia and metanifridia, also associated with parapodia. In many polychaetes tissue similar to the chloragogenous tissue that have other annelids, such as oligochaetes, with a metabolic function that could be analogous to the liver of vertebrates.

Circulatory system

Polychaetes have a simple circulatory system consisting of two main interconnected vessels, one ventral or inferior and one dorsal or superior. Blood vessels are contractile, so don't need a heartalthough in some species there are several muscular pumps distributed throughout the body.

The blood may be colorless or present certain pigments, including hemoglobin (the most common), hemerythrin and chlorocruorin (green color).

There are also some species with a minimal or completely absent circulatory system. These species transport oxygen and nutrients through the coelomic fluid.

Nervous system

The nervous system is also quite simple. It consists of one or two nerve branches that run longitudinally along the dorsal side of the body. Smaller nerve and ganglia leave from these branches to innervate the various structures of each segment.

The brain is conspicuously large compared to other annelids and is situated in the upper part of the prostomium. Under the brain is an endocrine gland related to reproductive function.

Several sensory organs are located in the head that can also occur throughout the body, such as photosensitive ocelli, statocysts and nerve endings related to touch.


Most polychaetes have separate sexes. The most primitive have a pair of gonads in each body segment. Some species copulate but normally fertilization is external.

Gametes usually pass from the gonads directly into the body cavity. Here they mature until they are expelled to the external environment through ducts, but also in some species breaking the body wall and killing the adult.

The fertilized eggs hatch and a larva emerges that lives in the plankton and undergoes metamorphosis to become an adult. Some species do not go through the larval stage.

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