POLYCHAETES

An estimated, accepted 9,000 species of polychaete worms, representing 81 families, have been recorded worldwide. To put this into perspective, global biodiversity estimates suggest the presence of only about 15,000 species of marine fish. Although estimates from 1985 place the Pacific Northwest as home to at least 836 polychaete worms, new species are continuing to be discovered and described. This is an impressive array, but much of the biology of polychaetes awaits study. Photographs and information for 79 species in 32 families of polychaetes are included herein.
Polychaete (“many bristles”) worms are under-appreciated animals that are important in the marine environment. They are particularly significant as marine benthos (bottom dwellers) living on the seabed surface (epifauna) or within the seabed sediments (infauna). They can typically comprise 45 to 50% of the species diversity within a benthic community, and as much as 80% of the total individual animals! Like earthworms in a backyard garden, they are vital to the health of the sediments. By helping to decompose and bioturbate (turn over) the organic matter on the seabed, polychaetes actually recycle nutrients within the sediments as well as vertically into the water column. Many of these worms can rapidly recolonize and rehabilitate sediments drastically altered by waste-water disposal (such as sewage), oil spills, aquaculture-generated waste and even severe ocean storms.
A healthy polychaete community represents a major contribution to the food web. Polychaetes eat other polychaetes; other animals such as sea stars eat polychaetes; many demersal fish (those living at or near the bottom) eat polychaetes; gray whales eat polychaetes by the mouthful in sediments that they scoop from the bottom. The larvae of many polychaetes spend time in the plankton, feeding and developing to a particular stage before they can finally “settle out” onto the seabed and grow into adults. During this process, the larvae are a prime food source for other pelagic animals such as zooplankton, fish and baleen whales. Along the Pacific Northwest coastline, intertidal polychaetes are a nutritional resource for many shorebirds. And yes, there are even some species of polychaetes that are considered tasty morsels by humans in some parts of the world! What would our world be like without polychaetes?
Since the mid-1700s, the morphology (external characteristics) of polychaete worms has been used to describe and name the diversity of species. Gradually investigations shifted, and efforts focused on the relationships among these polychaete species and their families. One result of this process was an organizational scheme that divided the polychaete families into two major groups: the Errantia or errantiate polychaeta (free swimmers or burrowers) and the Sedentaria or sedentariate polychaeta (tube dwellers). This historic scheme is now considered to be an arbitrary and inconsistent division. The recent addition of research tools that incorporate evolutionary and molecular perspectives have greatly improved the understanding of polychaetes. Leading international experts have developed a new organizational scheme for polychaetes based on a combination of these perspectives that produces a more holistic approach.
According to this systematic approach, polychaetes are divided into three main groups. The simple-bodied polychaete has a noticeable head but it lacks obvious structures. The first segment is similar to all those that follow, bearing similar paired appendages for the entire body length as shown in photograph A, a Pacific Neapolitan lugworm (AN7). The sensory-palp polychaete has paired palps (anterior sensory outgrowths) but they are uniform, relatively short and positioned ventrally on the head. The usually visible, tapering and tactile palps are used for sensory perception. In addition, each member of this group of worms has a pair of antennae on the head. Finally, its parapodia (fleshy, flap-like appendages) are often well developed. Photograph B shows an example in the form of a red-and-white-banded sea-nymph (AN47). The feeding-palp polychaete has long palps on its head, in pairs, multiples or as a “tentacular” crown. Each of these palps has a ciliated (hair-lined), longitudinal groove that transports food to the mouth, as illustrated by the beautiful, delicate tentacular crowns of some tubeworms. Photograph C is an image of a feather-duster worm removed from its tube, p. 151.
Within these three categories, the polychaete worms are generally organized alphabetically by family, and within family groupings. Some exceptions apply where similar-looking polychaetes are grouped together for easier comparison. Where possible, the establishment of common names was based on the root derivative or origin of the scientific name.
In general, each segment of a polychaete worm is adorned with a pair of parapodia that bear the characteristic chaetae (bristles; occasionally written as “setae”). Often the parapodia are subdivided into two branches with the upper notopodia (near the back) carrying notochaetae, and the lower neuropodia (near the belly) carrying the neurochaetae. The individual shape of the chaetae, their combination and configuration, are usually species-specific. The anatomy of these bristly appendages is reflective of the lifestyle or “modus operandi” of each polychaete species.
Polychaete worms are a challenge to identify—living or preserved. In many cases, specific identifications require microscope examination, which unveils the fascinating and exquisite shapes of polychaete chaetae! However, the observant beachcomber or diver may, with some experience and the information here, recognize numerous commonly encountered species.