Our understanding of how the oceans operate and affect our very existence is dependent upon understanding the diversity of life in all ocean habitats. For instance, marine biologists monitoring the health of a particular region must rely on knowing what species occur in a given area and the role they play in the ecosystem. If we scoop up mud or sand from the sea floor on any part of the planet, regardless of depth, we find that polychaetes are usually the dominant organisms. As a consequence, marine biologists have found polychaetes to be excellent indicators of the effects of pollutants, as well as pointing to natural and human-induced changes in ecosystems.
In order to accurately monitor the health of the world's oceans, marine biologists must be able to correctly identify the organisms they encounter. This is one of the reasons that the Natural History Museum of Los Angeles County maintains one of the world's largest polychaete collections, and why the collection receives so much use by researchers around the world.
The term polychaete is derived from two Greek words: polys, many, very; and chaite, long hair. This refers to the fact that polychaete worms have pairs of laterally-placed bundles of bristles, called chaetae (or setae), on each of the body segments. In some individuals, these chaetae can be noticeably numerous and long, but there are an incredible variety of shapes and sizes. Because of this variation in chaetae, they function to enable polychaetes to move with great agility and speed, whether in a burrow, tube, crawling over the sea floor, or swimming.
Photo by Leslie Harris
The term annelid comes from the Latin word, annellus, a little ring, which refers to the segments that comprise most of the body, and each segment has its own body cavity, or coelomic space. We can contrast this condition with the flatworms (phylum Platyhelminthes), which are not segmented and have a solid construction; or the round worms (phylum Nematoda), which are also not segmented and have a body cavity that is very different from that of annelids.
The phylum Annelida contains all of the segmented worms, which includes two classes: Polychaeta and Clitellata. The Clitellata contains the earthworms (which includes terrestial, freshwater, and marine species) and leeches (which are also terrestrial, freshwater, and marine). The word Clitellata comes from the Latin word, clitella, a pack saddle, referring to the clitellum, a discrete glandular swelling of the epidermis that secretes a cocoon into which the eggs are deposited.
Photo credits: left, Leslie Harris; center, William Vann; right, Geoff Read
Polychaete fossils are known from rocks dated as far back as the mid-Cambrian period (500 million years ago), such as from the famous Burgess Shale Formation in British Columbia, Canada.
Another famous locality is the Francis Creek Shale Formation in Illinois, with Mazon Creek fossils from the Pennsylvanian period (300 million years ago). What is especially fascinating about Mazon Creek polychaetes, like the one shown above, is that they contain species that can be assigned to families that have members in existence to this day.
Clearly, the remarkable longevity of polychaetes has been due to their success at being able to live in every possible type of habitat in the world's oceans.
Photo by J. Kirk Fitzhugh
The size range of adult polychaetes is immense. There are some species with adults that are so small that they live between sand grains. The longest polychaetes have been known to be up to 200 feet long, although these are very threadlike. The vast majority of polychaetes, however, range from less than one third of an inch to only several inches in length. This is why they are so easily overlooked when one is casually exploring the sea shore.
Photo courtesy of Gabrielle deGroot Redford
Polychaete worms live in every type of habitat in the seas—they can be found in the sands of any beach, all the way down to the deepest depths of the oceans. There are, however, a few species that do live in freshwater. Most polychaetes make the sea floor their home, where they burrow through sand and mud, or crawl over the sediment surface. There are some species that are fully pelagic, spending their entire life swimming in the water column. Because of their vast abundance, polychaetes comprise an extremely important link in ocean food chains. As a result, polychaetes are one of the most important groups of organisms for assessing the health of marine ecosystems.
Because polychaetes are soft-bodied animals, their main mode of defense is to remain hidden either by living in a burrow or to produce some kind of tube. The tubes of most polychaetes are formed from the mucus secreted from the body surface. Once secreted, mucus hardens when coming into contact with sea water. Most species of polychaetes that produce tubes will exhibit very specific behaviors with regard to glueing sand, shell, or mud particles to the outer surfaces of their tubes. Depending on how much mucus is produced, and what kinds of materials are added to them, tubes exhibit a wide range of flexibility and shapes. Generally, tubes open at or just above the sea floor, with the remainder embedded in the surrounding sediment. There are, however, some polychaete species who have the ability to produce tubes composed of calcium carbonate. In these instances, tubes are usually produced on the surfaces of rocks or other firm surfaces.
Because polychaetes exhibit an amazing array of body forms, especially in the head region, there are a variety of ways they acquire their food. Members of some species are carvivorous, and capable of catching their prey with the use of jaws that can be extended out of the mouth. In the photo above, you can see the jaws within the front end of the body as well as extended. Other types of individuals actively consume the sand or mud in which they are burrowing, stripping off bacteria and other nutrients from the particles. For some species, the process of acquiring sea floor mud or sand can be quite intricate, with the use of simple to complex arrays of filaments coming off the head region, as in the individual shown below.
Photos by Leslie Harris
Because polychaetes are a dominent part of the bottom fauna in any ocean ecosystem, they are a fundamental component in food chains. Polychaetes are part of the diets of shrimps, crabs, a variety of fishes, and even some shore birds.
Most polychaetes reproduce sexually, but asexual reproduction is known to occur through a variety of processes. For sexual reproduction, depending on the species, there can be individuals with separate sexes or they can be hermaphroditic, with both male and female reproductive systems. Eggs and sperm are both sometimes released into the open water, where subsequent to fertilization, larval development takes place and larvae might remain in the water column for as short as hours to as long as several months depending on the species. At the appropriate time, larvae settle to the sea floor and undergo a metamorphosis to the juvenile form what will further develop into adult worms. Rather than releasing eggs into the water, the members of some species actively brood their young in the tube or burrow of the parent, with juveniles eventually crawling away.
Asexual reproduction can occur through several different processes. Individuals capable of asexual reproduction are usually also capable of sexual reproduction.
There is only one group of polychaetes that are capable of inflicting pain, and these are known as "fire worms." Fire worms are members of the family Amphinomidae, and one of the distinctive characteristics of the group is that their chaetae contain calcium carbonate, making them very stiff and brittle. Plus, the tips of the chaetae have barbs, giving them the appearance of harpoons, and chaetae are often produced in dense bundles along the body.
In subtropical and tropical waters, fire worms, of several inches and up to one foot in length, can be found crawling over the surfaces of rocks, coral, or sand. If an unsuspecting snorkeler picks up a fire worm with their bare hand, hundreds of little chaetae get embedded in the skin, breaking off like little splinters. Because the interior of the chaetae are hollow, the liquid inside causes an immune reaction in victims, resulting in swelling, blistering of the skin, and inflammation. Hence the reference to these polychaetes as "fire worms."
Photo by Leslie Harris