Clam sand and tubeworms

Phylum Annelida Class Polychaeta Number of families 86

Thumbnail description

Segmented worms with numerous bristles and one pair of parapodia per segment

Photo: Bristle or fire worms (Chloeia sp.) primarily come out at night and are scavangers. (Photo ©Tony Wu/www.silent-symphony.com. Reproduced by permission.)

Phylum Annelida Class Polychaeta Number of families 86

Thumbnail description

Segmented worms with numerous bristles and one pair of parapodia per segment

Photo: Bristle or fire worms (Chloeia sp.) primarily come out at night and are scavangers. (Photo ©Tony Wu/www.silent-symphony.com. Reproduced by permission.)

Biology Pictures Various Worms

Evolution and systematics

Polychaetes do not fossilize very well as they are soft-bodied animals. There are few fossil records from an entire worm; these have been found from the Pennsylvanian fauna. The oldest fossil records, dating to the middle Cambrian of the Burgess Shale, include the aciculates Wiwaxia and Canadia, which have a prostomium with appendages, well-developed parapodia, and different kinds of chaetae.

A considerable diversification among polychaetes occurred in the Middle Cambrian, with six genera represented in Burgess Shale (Canada). The genera Wiwaxia and Canadia do not have jaws, but some later forms possess hard jaws that could mineralize with iron oxide. Such polychaete fossils are known as scolecodonts and have been described from the Ordovician, Silurian, and Devonian periods. Other poly-chaete fossils include tubes and burrow structures produced by some sedentary forms that secreted a mucous lining for their burrow.

Polychaeta and Clitellata, which includes the classes Oligo-chaeta and Hirudinoidea, are the two great lineages within the phylum Annelida. As the marine polychaetes fossils that appeared in Middle Cambrian are the earliest record of Annelida, they are considered to be the earliest derived group within the phylum. The exact nature of the ancestral group from which Polychaeta and Clitellata arose is still obscure, but it was likely a homonomous, metameric burrower that possessed a compartmentalized coelom, paired epidermal chaetae, and a head composed of a presegmental prostomium and a peris-tomium much like those found in modern polychaetes. Due to some recent molecular data and anatomical and developmental evidence, many scientists believe the pogonophorans and vestimentiferans should be placed within the Annelida as a specialized polychaete family, either Pogonophoridae or

Siboglionidae. The class Polychaeta is divided into 24 orders and 86 families, with more than 10,000 described species.

Physical characteristics

Polychaetes range in length from <0.078 in (<2 mm) to >9.8 ft (>3 m). The majority of them are <3.9 in (<10 cm) long and between 0.078-0.39 in (2-10 mm) wide. The morphology is greatly variable. The majority of polychaetes have a cylindrical and elongated body. The parapodia can be unir-ramous or biramous, with a dorsal lobe (notopodium) and ventral lobe (neuropodium).

The body morphology of polychaetes usually reflects their habits and habitats. Often, active forms have a more homo-nomous body construction than sedentary ones, which possess some degree of heteronomy, with differentiation in body regions that are utilized for particular functions.

Mobile forms usually have well-developed parapodia, eyes, and sensory organs. Additionally, in some species, the mouth has chitinous jaws and an eversible pharynx.

Sessile forms live in permanent burrows or tubes and have the parapodia reduced or absent; some of them possess special tentaclelike appendages projecting from the tube to collect food from the surface and also for aeration.

Polychaetes are extremely variable in color, varying from light tan to opaque, but most are colored red, pink, green, yellow, or a combination of colors. Some species are iridescent.

Polychaetes of the family Aphroditidae have the dorsum covered with scales (elytra) that can be overlaid by a hard hairlike layer. Because of these characteristics, one species is commonly called the "sea-mouse." Some planktonic species are

Eunice Aphroditois

Eunice aphroditois is a predatory worm found in the Indo-Pacific. Not much research has been done on this particular species, though it was first described in the late 1700s. This and related worms are found all around the world in tropical and semitropical waters. It is a "sit and wait" predator, waiting in ambush for victims. (Photo ©Tony Wu/www.silent-symphony.com. Reproduced by permission.)

Eunice aphroditois is a predatory worm found in the Indo-Pacific. Not much research has been done on this particular species, though it was first described in the late 1700s. This and related worms are found all around the world in tropical and semitropical waters. It is a "sit and wait" predator, waiting in ambush for victims. (Photo ©Tony Wu/www.silent-symphony.com. Reproduced by permission.)

adapted to live in the water column by usually being transparent and flattened with fin parapodia.

It is very common to observe the gas exchange structures of some polychaetes, especially the colorful ones. The branchial morphology is greatly variable: filaments in cirrat-ulids, anterior gills in terebellids, and a tentacular or branchial crown on the heads of sabellids, serpulids, and spirorbids. The more active polychaetes possess a highly vascularized portion of the parapodia, utilizing it for gas exchange.

Habitat

Due to morphological variations, polychaetes occupy different kinds of habitats, both planktonic and benthic. They are found from the intertidal zone to the deepest depths of the ocean in sandy and muddy sediments, digging or in temporary or permanent mucous tubes that are part of the in-faunal community, or crawling on the surface of the substrate. Some species live above the sediment surface as part of the epifaunal community; for example, some nereid species live between mussel beds attached to piers.

Polychaetes are also found in coral and rocky reefs, occupying crevices or beneath stones, and some construct sandy or calcareous tubes that are often attached to coral.

Planktonic forms have adapted structures to swim, and spend all their lives in the water column. Some species tolerate the low salinity of estuaries and a few live in freshwater environments. Some spionids and a group of nereids inhabit semi-terrestrial habitats where they can be covered by freshwater during the wet season and during tidal inundations.

There are some polychaetes that have been found in deep-sea thermal vents. Some species can be commensal or even parasitic. Histriobdella homari eats incrusted bacteria and blue-green algae found on the gills and branchial chamber of Homarus americanus and Homarus gammarus. One Stratiodrilus species feeds on microorganisms in the gill chamber of freshwater crayfish or anomuran crustaceans. In the family Oenon-idae, there are parasitic forms that live in the coelomic cavity of other polychaetes, echiurans, or bivalves.

Behavior

Polychaetes exhibit few behavior patterns. Certain species are gregarious, forming dense aggregations, while others are solitary. Horizontal and vertical partitioning of space in the sediment occurs between conspecifics and closely-related species or species from the same trophic group (species that feed on the same food resources).

Some polychaetes respond to shadows passing overhead and retreat rapidly back into their tubes. Some species "smell" their prey in the sediment through chemical sensory organs. Species that are prey of birds and fishes can discard their posterior end to avoid predation and can regenerate lost parts.

Some species pair during the breeding season; however, following this phase, they are so aggressive that they often eat each other. During spawning, several species are luminescent in response to light as a reproductive strategy. Others react to changes in temperature, day length, and Moon phases. All these responses are coordinated to ensure successful spawning and reproduction.

Distribution

Polychaetes are found worldwide, living in every marine habitat from tropical to polar regions. Some species occur in brackish or freshwater environments. A few live on land, but in habitats completely inundated with water.

Feeding ecology and diet

The morphological and functional diversity of polychaetes enables them to exploit food resources in almost all marine environments in different ways. Polychaetes are usually categorized into raptorial (including carnivores, herbivores, and scavengers), omnivores, surface and subsurface deposit feeders, suspension feeders, and filter feeders.

Polychaetes can be non-selective or selective deposit feeders. The non-selective deposit feeders ingest sand or mud grains, showing little or no discrimination for the size and nutritional value of the particles, assimilating any organic material in the ingested sediment. Selective deposit feeders, however, utilize structures such as palps, tentacles, or buccal organs to select particles with high nutritional value.

The raptorial species usually have a homonomous construction, well-developed parapodia, sensory organs on the head, and a pharynx armed with hard jaws (nereids, glycerids, phyllodocids, syllids). They make rapid movements across the substratum. They often prey on small invertebrates. The prey can be located through sensorial or chemical means. Certain forms (glycerids) have poison glands associated with the jaws. Some raptorial species are not active hunters; they lie in wait for passing prey.

Herbivorous polychaetes feed by scraping or tearing plant material with their pharyngeal structures. Some scavengers feed on any dead or organic material they encounter. Omnivorous species feed on any material they find, and there are some carnivores that feed on deposits when prey are scarce.

Surface and subsurface deposit feeders ingest sandy or muddy particles, feeding on organic material attached to them. They usually have a saclike pharynx that sucks the grains from the sediment when burrowing (capitellids). Some surface deposit feeders have grooved mucous tentacles that collect particles from the surface and carry them directly into the mouth (terebellids, cirratulids).

Suspension feeders possess specialized structures such as tentacular sulcated crowns, or palps on the head, that enable them to collect suspended material in the water column. Some forms are very active, frequently moving their palps (spionids), and others simply expose their crown, waiting for particles to fall onto their surface (Oweniidae).

Filter feeders have specialized crowns with pinnated radioles that create a water current through the pinnules, collecting the particles in suspension (sabellids, sabelariids, spirorbids, serpulids).

Reproductive biology

Polychaetes have different degrees of regeneration. They regenerate lost appendages such as palps, tentacles, cirri, and parapodia. The regeneration of posterior ends is common, but regeneration of a lost head end is uncommon. Many polychaetes utilize regeneration during asexual reproduction, producing a series of individuals, a bud that grows from an individual, or new individuals that develop from an isolated fragment.

The majority of polychaetes are dioecious (gonochoristic); hermaphroditism occurs in relatively few species. Gametes usually mature inside the coelom and are released by gonod-ucts, coelomoducts, nephridia, or through the rupture of the parental body wall. The majority of species release their gametes into the water, where fertilization takes place. The larvae are planktotrophic, but some species have lecitotrophic larvae, and a few have both. Species with internal fertilization brood their eggs or produce encapsulated eggs that float or are attached to the substratum.

As a rule, segments are generated from a posterior growth zone, arising and developing sequentially from the anterior to the posterior.

Some polychaetes have evolved methods to increase the chances of fertilization. Some sexually reproductive nereids, syllids, and eunicids form an epitokous individual, wherein various body parts or the whole body become a gamete-carrying bag capable of swimming from the bottom upward into the water column, where the gametes are spread. In many poly-chaetes, the larva is a trochophore that possesses a locomotory ciliary band near the mouth region. The lifespan can range from a few weeks to several years, depending on the reproductive strategy of the particular species.

Conservation status

Polychaetes are part of many investigations that contribute to the conservation and knowledge of the biodiversity of the marine environment. No species of polychaete are listed by the IUCN. However, the Palolo worm, Eunice viridis, could potentially be in need of conservation efforts.

Significance to humans

Polychaetes play an important role in the marine benthic food chain, not only serving as food for other organisms, but also recycling organic matter within the sediment and breaking down plant material. Some polychaetes, such as nereids, are known to be important food sources for birds and for economically important fishes; polychaetes are also used as bait for recreational fishing.

Polychaetes play an important role in monitoring marine environmental quality. They respond quickly to changes in the environment, promoted by anthropogenic compounds or chemical contaminants because of their direct contact with the sediments and water column.

Some species can provide an indication of the condition and health of the sediment they live in and often occur at high densities in polluted habitats.

Some polydorids (Spionidae) bore into oyster shells, affecting their appearance and hence their market value, and cause a decline in oyster cultures or become pests. There are numerous significant fouling species that can settle and grow on the hulls of ships.

Oweniidae Regeneration

1. Sand mason (Lanice conchilega); 2. Lugworm (Arenicola marina); 3. Capitella capitata; 4. Tubeworm (Serpula vermicularis); 5. Honeycomb worm (Sabellaria alveolata). (Illustration by Amanda Humphrey)

Chaetopterus Variopedatus

1. Parchment worm (Chaetopterus variopedatus); 2. Ragworm (Hediste diversicolor); 3. Pile-worm (Neanthes succinea); 4. Fire worm (Eurythoe complanata); 5. Catworm (Nephtys hombergii). (Illustration by Amanda Humphrey)

Fire worm

Eurythoe complanata

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