Photo: Peanut worm (Phascolosoma sp.) with partially extended introvert. (Photo by L. Newman & A. Flowers. Reproduced by permission.)
Evolution and systematics
No unambiguous fossil Sipuncula are currently known. Ot-toia prolifica from the Burgess Shale has been proposed as a fossil sipunculan, but might also be an aschelminth or Pria-pulida. 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.
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 Sipun-culidea. 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 ventro-laterally, typically at the level of the anus. Proteinaceous, non-chitinous 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 Phasco-lion and Onchnesoma, which have only a single nephridium.
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).
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.
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 pho-totactic 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.
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.
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 tro-chophore larva.
• Indirect development with two lecithotrophic larval stages: trochophore and pelagosphera.
• Indirect development with a lecithotrophic tro-chophore and a planktotrophic pelagosphera. The planktotrophic pelagosphera lasts up to six months in the water column before settling.
No sipunculan species are currently on the IUCN Red List. Because of their long-lived larval stages, many sipuncu-lan 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.
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.
No common name
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