''Listriolobus pelodes'' is a species of marine

spoon worm The Echiura, or spoon worms, are a small group of marine animals. Once treated as a separate phylum, they are now considered to belong to Annelida. Annelids typically have their bodies divided into segments, but echiurans have secondarily los ...

. It is found in shallow seas in the North East Pacific off the coast of California. It lives in a burrow in soft sediments.

Description

''Listriolobus pelodes'' has a plump, sausage-shaped body about long and wide. An extensible spoon-shaped proboscis projects from the anterior (front) end of the body and the mouth is at its base on the ventral side. There is a pair of hooked setae (bristles) projecting from the ventral surface of the body behind the mouth and a pair of nephridiopores nearby. The

anus The anus (Latin, 'ring' or 'circle') is an opening at the opposite end of an animal's digestive tract from the mouth. Its function is to control the expulsion of feces, the residual semi-solid waste that remains after food digestion, which, d ...

is at the posterior end of the body. There are two anal vesicles on either side of the rear end of the gut which are used for respiration. The body is a translucent green colour and there are eight longitudinal bundles of muscle in the body wall.

Distribution

''Listriolobus pelodes'' is found along the coast of North America ranging from northern California as far south as Baja California at depths between about . It lives in a burrow in fine grained sands and mud with a high content of organic matter.

Biology

''Listriolobus pelodes'' lives in a U-shaped burrow in the sediment. Its body remains below the surface while it extends its flexible proboscis across the

substrate Substrate may refer to: Physical layers *Substrate (biology), the natural environment in which an organism lives, or the surface or medium on which an organism grows or is attached ** Substrate (locomotion), the surface over which an organism lo ...

with the ventral side upwards. Sediment is scooped up by the proboscis and is wafted along a central groove to the mouth by the action of

cilia The cilium, plural cilia (), is a membrane-bound organelle found on most types of eukaryotic cell, and certain microorganisms known as ciliates. Cilia are absent in bacteria and archaea. The cilium has the shape of a slender threadlike projecti ...

. The sediment passes through the gut where the nutritive parts are digested and absorbed, and the residue is ejected through the anus as faecal pellets. The worm creates a current of water through the tube by

peristaltic Peristalsis ( , ) is a radially symmetrical contraction and relaxation of muscles that propagate in a wave down a tube, in an anterograde direction. Peristalsis is progression of coordinated contraction of involuntary circular muscles, which ...

movements and this washes the pellets out of the second aperture of the tube. They accumulate nearby in little piles, the presence of which may show where these spoon worms are buried. The faecal pellets get ingested during further feeding activities and it is probable that rapidly growing bacteria and other micro-organisms form part of the diet. Other movements of the body wall draw water into and out of the anal vesicles and respiratory gases are exchanged. The worm periodically reverses its position in its burrow. The proboscis can extend to a length of and, feeding from the two burrow apertures, each worm can sweep an area of sediment of . The feeding activities of this worm, occurring as it does in large numbers, has a considerable effect on the seabed ecosystem. The sexes are separate in ''Listriolobus pelodes''. The

gonad A gonad, sex gland, or reproductive gland is a mixed gland that produces the gametes and sex hormones of an organism. Female reproductive cells are egg cells, and male reproductive cells are sperm. The male gonad, the testicle, produces sper ...

s are located beside the anal vesicles and liberate gametes into the

coelum The coelom (or celom) is the main body cavity in most animals and is positioned inside the body to surround and contain the digestive tract and other organs. In some animals, it is lined with mesothelium. In other animals, such as molluscs, it re ...

or body cavity. Here they mature and are then stored in the nephridia before being liberated into the sea in the late winter or spring. After fertilisation, the eggs hatch into planktonic trochophore larvae. After several developmental stages over a period of about six months, the larvae settle on the seabed and undergo

metamorphosis Metamorphosis is a biological process by which an animal physically develops including birth or hatching, involving a conspicuous and relatively abrupt change in the animal's body structure through cell growth and differentiation. Some inse ...

into juvenile spoon worms. They seem to be attracted to settle in areas of fine, nutrient-rich sediment but the mechanism for this discrimination is unclear. The juveniles become sexually mature in another six to twelve months.

Ecology

A number of other invertebrates take up occupation in this spoon worm's burrow and live there as commensals. These include the small

crab Crabs are decapod crustaceans of the infraorder Brachyura, which typically have a very short projecting "tail" (abdomen) ( el, βραχύς , translit=brachys = short, / = tail), usually hidden entirely under the thorax. They live in all the ...

'' Pinnixa schmitti'', the bivalve mollusc '' Mysella tumida'' and the polychaete worm '' Hesperonoe laevis''. These probably benefit from the protection from

predators Predation is a biological interaction where one organism, the predator, kills and eats another organism, its prey. It is one of a family of common feeding behaviours that includes parasitism and micropredation (which usually do not kill the ...

that the burrow provides and the nutrient rich stream of water passing through the burrow. The spoon worm is

preyed Predation is a biological interaction where one organism, the predator, kills and eats another organism, its prey. It is one of a family of common feeding behaviours that includes parasitism and micropredation (which usually do not kill t ...

on by bottom feeding fish such as

flounder Flounders are a group of flatfish species. They are demersal fish, found at the bottom of oceans around the world; some species will also enter estuaries. Taxonomy The name "flounder" is used for several only distantly related species, thou ...

s, Dover sole ('' Microstomus pacificus'') and bat rays ('' Myliobatis californicus''). In the 1970s, ''Listriolobus pelodes'' was found near wastewater discharge outlets off Los Angeles at densities of up to 1,500 individuals per square metre (11 square feet). The burrowing and feeding activities of these worms redistributed and aerated the sediment and promoted a more diverse community of fauna than would otherwise have existed in this heavily

polluted Pollution is the introduction of contaminants into the natural environment that cause adverse change. Pollution can take the form of any substance (solid, liquid, or gas) or energy (such as radioactivity, heat, sound, or light). Pollutants, the ...

area. The worms flourished for several years before disappearing from the area but their legacy of improved

benthic The benthic zone is the ecological region at the lowest level of a body of water such as an ocean, lake, or stream, including the sediment surface and some sub-surface layers. The name comes from ancient Greek, βένθος (bénthos), meaning "t ...

diversity remained.

References

{{Taxonbar, from=Q2339954 Echiurans Animals described in 1946