The Benthic Annelids: Polychaetes, Beard Worms, Girdle Worms, and Sea Leeches by Sean Chamberlin
Bristle worms (polychaetes), girdle worms (oligochaetes) and beard worms (pogonophora) belong to the same phylum as earthworms but most hardly resemble earthworms. All are members of the taxon Annelida, the segmented worms, though the pogonophorans were previously thought to represent their own phylum. Segmentation in these animals, including arthropods, is thought to confer several advantages but agreement concerning the evolutionary significance of this body plan is lacking. Be that as it may, the diversity of these animals and their importance to biogeochemical cycles is evidence of their success, regardless of the reasons.
Polychaetes are among the most beautiful and diverse worms in the ocean; some are even named after Greek goddesses. Familiar ones, like the suspension-feeding Christmas tree worm or feather duster, live in tubes. Others, like the predatory sand worm, Nereis, are free-roaming. Polychaetes may be found inhabiting a wide range of habitats from the shallow intertidal to deep-sea muds.
The body plan of a motile polychaeate resembles an earthworm except for the numerous bristle-covered flaps, called parapdodia, that extend from each side of its segments. A prominent proboscis, an eversible mouth, acts like a vacuum cleaner to feed on sediments and may, in some species, contain powerful chitinous jaws for subduing prey. In the polychaete, Glycera convolute, the proboscis wields four fangs that inject a neurotoxin called glycerotoxin. This compound, like many naturally occurring neurotoxins, has been especially useful in studies of synaptic transmission (e.g., Meunier et al., 2002). Tube-dwelling polychaetes differ largely by modifications to their heads, namely, the presence of colorful or elongate tentacles used in suspension or surface deposit feeding. Eudistylia, a common species of feather duster worm along the Pacific coast, bears tentacles that resemble something you would see at a show in Vegas. The Christmas tree worm, Spirobranchus, exhibits a wide range of exotic colors in a pair of tentacles shaped like a well-trimmed blue spruce.
Feeding in polychaetes presents interesting challenges for studies of the flow of energy and materials in benthic food webs. Fauchald and Jumars (1979) categorized several feeding modes in polychaetes as a means of distinguishing their potential ecological roles. These include the raptorial feeders, the motile polychaetes who actively seek food as carnivores, herbivore, detritivores, or omnivores; the filter feeders, usually tube-dwelling polychaetes who remove and/or select suspended particles; selective deposit feeders, tube-dwelling or burrowing polychaetes who collect and sort food particles at the surface using tentacles or other specialized body parts; and non-selective deposit feeders, the “earthworm” types, many motile but some tube-dwelling, who ingest sediments non-discriminately at the surface or within the sediments. Because the feeding behavior of many soft-bottom organisms is so difficult to observe, discriminating between these types of feeders and their role in digenesis remains difficult. Surface deposit feeders, like the cirratulid poychaetes, have been shown in laboratory and field studies with glass beads ingest sediment particles at the surface and egest them at depth, a process known as downward sediment transport (e.g., Shull and Yasuda, 2001). Some researchers have suggested that such processes may be important in the burial of carbon (e.g. Emerson et al., 1985).
Given their diverse lifestyles and forms of feeding, it should be no surprise that polychaetes often dominate soft-bottom communities. In estuarine, coastal and deep-sea sediments, they may comprise between 24-73% of all the species present (Hutchings, 1998). Nonetheless, the factors that influence species diversity in sediments are hotly debated, particularly in the deep sea where species diversity appears higher than coastal sediments (e.g., Levin et al, 2001). Also uncertain is the role that polychaetes (and other dominant benthic organisms) play in the global carbon cycle and the biogeochemical cycles of nitrogen, sulfur, iron and other elements. Research using benthic landers, which enclose a portion of the sea bottom for measurements of exchanges of gases and chemical compounds, indicate that bio-irrigation is an important component in the flux of biologically important nutrients from sediments on the Monterey Bay shelf (e.g. Berelson, 2003).
Polychaetes also play an important role on hydrothermal vents where they are renowned for their thermal tolerance. Lee (2003) reports that one species, Alvinella pompejana, can tolerate temperatures from 50 - 56° C (122-132° F), making it the “hottest” animal on Earth (sorry, Britney). Alvinella also boasts one of the most highly developed gills among polychaeates and a high-temperature hemoglobin with a very high oxygen binding constant that allow it to survive in poorly oxygenated waters (such as those leaving a vent). The dorsal surface of this polychaete apparently serves as a microhabitat for many different species of bacteria, some visible to the eye. This epibiotic association (epi = living on; biotic = life) has been called “the most highly evolved…among all invertebrates” (Van Dover, 2000; following Desbruyères et al. 1998). Numerous other polychaetes make their home on hydrothermal vents and likely serve as a link between chemosynthetic primary production and higher trophic levels.
Reproduction in many polychaetes involves the bizarre transformation of a benthic, non-reproductive atoke to a pelagic, gonad-bearing epitoke with enlarged eyes and parapodia modified for swimming. Like the mass spawning of corals, polychaetes spawn synchronously under specific conditions and at particular times of the year. In a phenomenon known as swarming, thousands of epitokes rise from the sea floor, usually in response to light cues (most being negatively phototaxic). At the surface, they release their eggs and sperms, oftentimes conducting a kind of nuptial dance, like the swarming palolo worms who twirl and twist as they spawn. American Samoans celebrate the night of the palolo by gathering along the shore with nets to catch and eat the annelid caviar (raw, fried with eggs or spread on toast). At least one species, the Bermuda fireworm (Odontosyllis enopla), employs bioluminescence in a mating ritual between “steady-glow” females and “blinker” males, ending, as one author puts it, “in a stunning luminescent final” of spewed eggs and sperm (Ruppert et al, 2004).
Beard worms (Giant tube worms, vent worms or lipstick worms)
Beard worms are named for the beard-like “tentacles” (palps) that line the anterior part of their body (prostomium). They gained their fame in 1977 when the deep-sea submersible Alvin discovered colonies of “giant tubeworms” on hydrothermal vents of the Galapagos Rift. These vent worms (more aptly called lipstick worms for the vivid red prostomium that extends and contracts from a tube like lipstick) were initially placed in the Phylum Pogonophora but since have been classified as polychaetes (Black et al., 1997). Nonetheless, their legendary status in the annals of oceanographic history give them special attention here.
When first discovered, Riftia pachyptila (as it is known) was a biological oddity for its extreme size (animals up to nine feet have been found, but typically they range from 2-4 feet); its lack of a mouth, digestive system or anus; and its specialized organ, the trophosome, packed with bacteria. The first question scientists asked: how does an animal get so large without any obvious means of nutrition? The answer was found in anatomical studies of the worm, revealing four distinct body regions that support their symbiotic relationship with chemoautotrophic bacteria (see Figure ##). The plume of the worm (the lipstick “beard”) extends into vent water containing hydrogen sulfide and other nutrients which can be absorbed and transferred via blood vessels to the internal compartments of the animal, namely the trophosome. Here reside trillions of bacteria (several trillion bacteria per gram body weight of trophosome have been observed) that produce ATP by oxidizing hydrogen sulfide (chemoautotrophy) and use that energy to supply sugars (presumably) to their worm host. Rates of chemoautotrophy in Riftia rank highest among chemosymbiotic species and are more than sufficient to meet the metabolic demands of the organism by at least twofold (Van Dover, 2000; following Childress et al. 1991).
Within the class Oligocheata (Phylum Annelida), are around 200 species of girdle worms that live interstitially or in tubes in beach sediments although some deep-sea species are known. Girdle worms, which include the familiar earthworm (and are sometimes called aquatic earthworms), display a girdle-like “gland”, called a clitellum, that serves various reproductive functions and give these animals their name. Otherwise, their body plan closely resemble burrowing polychaetes.
Among the most conspicuous girdle worms are the tube-dwelling ones (tubificids), notably Tubifex. These hardy animals can be found in the mud flats of estuaries, especially where human impacts are high, like San Francisco Bay and New York/New Jersey harbor. Able to withstand large fluctuations in salinity, temperature and water conditions, these animals are an important indicator species of “pollution” in shallow water, soft-bottom habitats. They also serve as an important source of food for bottom-feeding fishes. Unfortunately, the accumulation of toxic chemicals in their tissues leads to biomagnification of those toxins by fish who feed on them. Tubifex has also been shown to feed selectively on silts and clays (which are the most susceptible to metal and toxin binding) and egest them on the surface, which has important implications for ecotoxicological studies and possibly, for sediment transport in the sediments.
See Hydrobiologia, November 2001, Vol. 463, 1-3, for an entire series of articles on aquatic oligochaetes. Google-search keywords: tubificids, marine oligochaetes, girdle worms
Made infamous by Humphrey Bogart in The African Queen, the leeches—a taxa within Annelida—often evoke sounds of disgust at their mention. Nonetheless, leeches continue to serve an important medical role for humans, being used to reattach severed appendages (like fingers and toes).
In the ocean, only a few marine leeches are known and these act as parasites on sharks and rays. Should you ever lose a finger to a shark and can retrieve it, the shark’s companion leech could come in handy.