Sipuncula

(Peanut Worms)

Phylum Sipuncula

Number of families 6

Thumbnail description
Unsegmented marine worm-like animals with a body divided into a trunk and retractable introvert

Evolution and systematics

No unambiguous fossil Sipuncula are currently known. Ottoia prolifica from the Burgess Shale has been proposed as a fossil sipunculan, but might also be an aschelminth or Priapulida. The paleozoic Hyolitha has a mix of attributes of sipunculans and mollusks, suggesting a close phylogenetic relationship with both. Fossilized burrows possibly created by sipunculans in soft sediments are known from early and mid-Paleozoic times. More recent Mesozoic and Cenozoic fossil burrows have also been attributed to sipunculan worms. Other sipunculans appear to have lived in association with corals and in vacated mollusk shells since the mid-Paleozoic, throughout the Mesozoic and Cenozoic.

In the early seventeenth century, Sipuncula were considered close relatives of holothurians. In 1847 Quatrefages erected the group Gephyrea, which he considered an intermediate between worms and holothurians and which also contained echiurans, sternaspids, and priapulids. Since the 1990s, there is general agreement that sipunculans are protostomes and closely related to annelids and mollusks, but their exact position still remains unresolved.

The phylum Sipuncula contains two classes, four orders, six families, 17 genera, and 147 species.

Physical characteristics

The sipunculan body is divided into trunk and retractable introvert. The ratio between introvert and trunk length varies among species. The mouth, at the anterior end of the introvert, is surrounded by an array of tentacles in the Sipunculidea. In the Phascolosomatidea, the tentacles are arranged in an arc around the nuchal organ, also located at the tip of the introvert. The anus lies dorsally, usually at the anterior end of the trunk, except in some species where it is shifted anteriorly onto the introvert. The nephridiopores lie ventrolaterally, typically at the level of the anus. Proteinaceous, nonchitinous hooks are often present on the distal part of the introvert and are either arranged in rings or scattered. Numerous papillae may be present on the trunk and introvert.

Trunk length varies from a few millimeters to about 11.8 in (30 cm) in exceptionally large specimens. Colors are usually shades of gray or brown, with occasional reddish, purple, or green pigment in the papillae and/or tentacles.

The body wall musculature is composed of an outer layer of longitudinal and an inner layer of circular muscles. One or two pairs of prominent introvert retractor muscles are present. A large coelom represents the main body cavity. The tentacles and the contractile vessel (fluid reservoir for tentacle extension) contain a second coelomic compartment. The intestine is characteristically U-shaped, with the ascending and descending branches coiled around each other in a double helix. A spindle muscle runs through the gut coil. It is attached anteriorly to the body wall near the anus and posteriorly to either the body wall or inside the gut coil. The sipunculan nervous system consists of a cerebral ganglion and a ventral nerve cord. Two nephridia are present, except in the genera Phascolion and Onchnesoma, which have only a single nephridium.

Distribution

Sipunculans occur in cold, temperate and tropical marine benthic habitats. They have been found in all depths from the intertidal zone to 22,510 ft (6,860 m).

Habitat

Some sipunculan species inhabit semi-permanent burrows in coarse or silty sand, and some live in crevices under rocks. A number of species bore into dead or, more rarely, live coral or other soft rocks, while one species even bores into a whale skull. Others inhabit empty mollusk shells, polychaete tubes, foraminiferan tests, or barnacles. Algal mats, large sponges, root mats of mangroves or sea grass, and byssal threads of bivalves also serve as habitats for some species.

Behavior

Relatively little is known about the behavior of sipunculans. Most species retract their tentacles and introvert quickly following a tactile stimulus. Many species are negatively phototactic and retreat into sediment or rock when given the opportunity. Burrowing and crawling are accomplished by utilizing the introvert hooks as anchors and the introvert musculature to pull the body forward. Phascolion strombus, an inhabitant of gastropod shells, is able to irrigate its shell to increase oxygen content by contractions of the body wall musculature. Swimming has only been reported in Sipunculus and consists of non-directional thrashing of the trunk.

Feeding ecology and diet

Most sipunculans are deposit feeders, except representatives of the genus Themiste, which have elaborately branched tentacles used for filter feeding. Sand-dwelling species ingest sediment and associated biomass that they collect with their tentacles. The tentacles are rarely visible above the seafloor during the day, but may be extended at night to probe the surrounding sediment for food particles. Rock-dwelling species use their introvert hooks, mostly at nighttime, to scrape sediment and epifaunal organisms from the surrounding rock surface.

Reproductive biology

Most sipunculan species are dioecious. Only one species, Nephasoma minutum, is known to be hermaphroditic. Themiste lageniformes is facultatively parthenogenetic. Asexual reproduction by budding has been reported in Aspidosiphon elegans. No sexual dimorphism is known in Sipuncula. Gonads are only prevalent during the reproductive period. Gametes are released into the coelom where maturation proceeds. Mature gametes are taken up by the nephridia and released into the water through the paired nephridiopores.

The four developmental modes include:

• Direct lecithotrophic development without a pelagic stage.
• Indirect development with a lecithotrophic trochophore larva.
• Indirect development with two lecithotrophic larval stages: trochophore and pelagosphera.
• Indirect development with a lecithotrophic trochophore and a planktotrophic pelagosphera. The planktotrophic pelagosphera lasts up to six months in the water column before settling.

Conservation status

No sipunculan species are currently on the IUCN Red List. Because of their long-lived larval stages, many sipunculan species seem to be very widespread. Abundance ranges from rare to extremely common (e.g., the density of Themiste lageniformes can reach more than 2,000 individuals/10 ft² (m²). Habitat destruction (e.g., mangroves, sea grass beds) can endanger regional populations.

Significance to humans

Fishermen in various parts of the world use sipunculan worms, mostly the larger sand-dwelling species, as bait. In Java, in the western Carolines, and in some parts of China, sipunculans are eaten by the locals.

Species accounts

List of Species

No common name

Antillesoma antillarum

order

Phascolosomatiformes

family

Phascolosomatidae

taxonomy

Antillesoma antillarum (Grübe and Oersted, 1858), West Indies.

other common names

None known.

physical characteristics

Up to 7.25 in (80 mm) long. Introvert 65–75% of trunk length. Body covered with large, dark papillae; numerous tentacles with purple or green pigment patches or stripes. Introvert hooks absent in adults.

distribution

Cosmopolitan in intertidal and shallow tropical and subtropical waters. (Specific distribution map not available.)

habitat

Lives in crevices or burrows into dead coral or soft rock.

behavior

Nothing is known.

feeding ecology and diet

Nothing is known.

reproductive biology

Dioecious; indirect developer with lecithotrophic trochophora and long-lived planktotrophic pelagosphera.

conservation status

Not listed by the IUCN.

significance to humans

None known.

No common name

Sipunculus nudus

order

Sipunculiformes

family

Sipunculidae

taxonomy

Sipunculus nudus Linnaeus, 1766, type locality unknown, but perhaps Mediterranean Sea.

other common names

None known.

physical characteristics

Commonly up to 6 in (15 cm) long, sometimes reaching 10 in (25 cm); introvert up to one-third of trunk length. Longitudinal and circular body wall musculature in bands. Distinguished by number of longitudinal muscle bands: 24–34. Introvert hooks absent.

distribution

Cosmopolitan in temperate, subtropical, and tropical waters in subtidal zone to 2,953 ft (900 m) depth. (Specific distribution map not available.)

habitat

Semi-permanent sand borrows.

behavior

During daytime, usually hides in burrow but might extend tentacles at night.

feeding ecology and diet

Ingests sediment and utilizes associated organic material.

reproductive biology

Dioecious; indirect developer with lecithotrophic trochophore and long-lived planktotrophic pelagosphera.

conservation status

Not listed by the IUCN.

significance to humans

Best examined sipunculan species; model organism for anatomy, physiology, biochemistry, and ecology. Used as bait in some parts of the world.

Resources

Books

Cutler, Edward, B. The Sipuncula. Their Systematics, Biology, and Evolution. Ithaca, NY: Cornell University Press, 1994.

Edmonds, Stanley J. "Phylum Sipuncula." In Fauna of Australia: Polychaetes and Allies, The Southern Synthesis, Vol. 4A, edited by Pamela L. Beesley, Graham J. B. Ross, and Christopher J. Glasby. Melbourne, Australia: CSIRO Publishing, 2000.

Rice, Mary E. "Sipuncula." In Microscopic Anatomy of Invertebrates: Onychophora, Chilopoda, and Lesser Protostomata, Vol. 12, edited by Frederick W. Harrison and Mary E. Rice. New York: Wiley-Liss, 1993.

Stephen, A. C., and S. J. Edmonds. The Phyla Sipuncula and Echiura. London: Trustees of the British Museum (Natural History), 1972.

Periodicals

Maxmen, Amy B., Burnett F. King, Edward B. Cutler and Gonzalo Giribet. "Evolutionary Relationships Within the Protostome Phylum Sipuncula: A Molecular Analysis of Ribosomal Genes and Histone H3 Sequence Data." Molecular Phylogenetics and Evolution 27 (2003): 489–503.

Rice, Mary E. "Larval Development and Metamorphosis in Sipuncula." American Zoologist 16 (1976): 563–571.

Anja Schulze, PhD