Polychaeta

Polychaeta, or polychaete, is the name given to any of thousands of species of marine worms. The segmented worms live in virtually all areas of the ocean and range in size from about one-twenty-fifth of an inch (one millimeter) to more than ten feet (three meters) in length. They vary greatly in appearance, in the environments in which they can live, in their reproduction habits, and their food preferences. However, all polychaetes are invertebrates, meaning they do not have a spinal column, and they have bodies that are segmented into a head area, a main trunk or body, and a tail. They are also characterized by the chaetae, or bristle-like structures, that extend from their bodies. Polychaetes are important because they provide a food source for other marine animals and because their health can help scientists monitor the condition of the oceans.

Background

The name Polychaeta comes from a Greek word polychaitēs. This is a compound of two Greek words: poly, meaning "many," and chaite, meaning "hair." The name was given to the worms because they all have characteristic hairlike bristles that extend from various parts of their bodies. This led to some of the common names applied to the many species of Polychaeta, including bristle worms and feather duster worms. They are also known as fireworms, clamworms, lugworms, and sea mice. The common names reflect the vast differences in the appearance and living conditions of various polychaetes.

Fossil records indicate that polychaetes existed at least since the Ediacaran period, which occurred more than five hundred million years ago. As soft-bodied animals that do not have spines and bones to fossilize, there are fewer fossil records of ancient Polychaeta than of some other animal species. However, scientists have been able to study the fossil remains of several species as well as the fossilized remains of the tubes they dig as burrows. Researchers have also uncovered a number of fossilized jaws of the first hard-jawed polychaetes; these jaws are known as scolecodonts.

Researchers have been able to determine that there have been many different species of Polychaeta over the centuries. These include some that evolved and became extinct in prehistoric times. Others, such as those found in the Mazon Creek area of Illinois, date back about three hundred million years ago and can be identified as ancestors of Polychaeta species that still live in the twenty-first century.

Polychaeta are part of the phylum, or biological classification, known as Annelida. This grouping includes all of the segmented worms. These creatures all have long, cylindrical bodies made up of many small ring-like sections. The Annelida phylum includes two classes, the Polychaeta and the Clitellata, which is the class that includes earthworms and leeches.

Overview

Although there are many species of Polychaeta with many diverse characteristics, there are some traits that all of the species have in common. In addition to having at least two of the bristles that give them their name, all polychaetes have ring-like segments that make up the length of their bodies. There is also an area before the segments start and another one after they end. The area before the segments is divided into two parts known as the prostomium and the peristomium; these areas make up the head of the animal. The area after the segments is the postsegmental pygidium, or tail.

Inside the segmented body are rings of muscle layers that protect a fluid-filled area known as the coelom. This area generally houses digestive and reproductive systems, although some polychaetes expel their reproductive material (sperm and eggs) to be fertilized in the water. On the outside, the animal is equipped with a set of feet on each segment. These feet are known as parapodia and usually have bristles. The bristles are made of a substance known as chitin, which is also found in the exoskeletons, or armor-like external covering, found on some invertebrates.

Beyond these characteristics, polychaetes can vary widely in appearance and in physical structure. Much of this is determined by the area where the animal lives. There are about seventy-two families of Polychaeta divided into a series of groups known as clades. These clades are roughly based on the characteristics of the different species of Polychaeta. For example, nereididae spend much of their time crawling on the ocean floor and have appendages that help them sense what is around them as well as better developed feet for moving, while capitellidae, which are frequently found digging through sediment, have fewer appendages to get in the way as they burrow. Polychaetes known as Hesiocaeca methanicola live in an area of the sea full of methane deposits; they have developed the ability to go without oxygen for up to ninety-six hours.

Some species have developed unique ways to reproduce to overcome challenges in their surroundings, while others have evolved to have skin that produces an acid that allows them to burrow through bone; this allows these polychaetes to scavenge food out of bone marrow from animals that die and sink to the ocean floor. Some have survived by developing symbiotic relationships with other sea creatures; these mutually beneficial relationships provide something for each partner. For instance, the Pompeii worm, which lives in tubes burrowed into hydrothermal vents in the ocean floor, are believed to survive high temperatures there in part due to a form of bacteria that lives on its back and forms a layer of insulation. In exchange, the bacteria feed on mucus produced by the worm's skin.

Polychaetes also play an important role in the life of other marine animals. A number of species serve as food for fish; some are even used as bait by fishermen. Commercial fishermen can use the presence of a polychaete population as an indicator of good fishing grounds.

Oceanographers also use polychaetes to help monitor the health of the oceans. Their prevalence, speedy response to environmental changes, and relatively short life span mean that scientists examining the health of the animals can learn much about the condition of the water in which they live. Changes to the water and soil will be reflected in the health of the polychaetes, allowing scientists to quickly identify and respond to problems. The absence of polychaetes in an area can also be an indicator of a problem with the conditions of the sea.

Bibliography

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