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

Resources

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.

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.

[Article by: Anja Schulze, PhD]

A phylum of sedentary marine vermiform coelomates that are unsegmented, but possibly distantly related to the annelids; they are commonly called peanut worms. Two classes are defined: Sipunculidea and Phascolosomatidea. In all there are 17 genera and approximately 150 species living in a wide variety of oceanic habitats within the sediment or inside any protective shelter such as a discarded mollusk shell, foraminiferan test, or crevice in rock or coral.

Adult sipunculans range in trunk length from 2 to over 500 mm (0.08 to over 20 in.). The shape of the body ranges from almost spherical to a slender cylinder. Sipunculans have a variety of epidermal structures (papillae, hooks, or shields). Many species lack color, but shades of yellow or brown may be present. Internal anatomy is relatively simple. The digestive tract has a straight esophagus and a double-coiled intestine extending toward the posterior end of the body and back terminating in a rectum, sometimes bearing small cecum. A ventral nerve cord with lateral nerves and circumenteric connectives to the pair of cerebral ganglia are present. Two or four pigmented eyespots may be present on the cerebral ganglia, and a chemoreceptor (nuchal organ) is usually present.

Knowledge of the reproductive biology of sipunculans is scanty, and good information on breeding cycles is unavailable for most genera. Most sipunculans are dioecious and lack any sexual dimorphism. These worms play a part in the recycling of detritus and probably consume smaller invertebrates in the process. They are in turn preyed on by fishes and probably other predators (including humans).

The noun has one meaning:

Meaning #1: peanut worms
  Synonym: phylum Sipuncula

Sipuncula Fossil range: Cambrian - Recent PreЄ Є O S D C P T J K Pg N
Scientific classification
Kingdom: Animalia Subkingdom: Metazoa Superphylum: Lophotrochozoa Phylum: Sipuncula Rafinesque, 1814
Classes, Orders and Families
Class Phascolosomatidea Order Aspidosiphoniformes Family Aspidosiphonidae Order Phascolosomatiformes Family Phascolosomatidae Class Sipunculidea Order Golfingiiformes Family Golfingiidae Family Phascolionidae Family Themistidae Order Sipunculiformes Family Sipunculidae

The Sipuncula or Sipunculida, sipunculid worms or peanut worms, are a phylum containing 144-320 species (estimates vary) of bilaterally symmetrical, unsegmented marine worms. Sipunculid worm jelly (土笋冻) is a delicacy in the town of Xiamen in Fujian province of China.

Contents 1 History 2 Habitat 3 Anatomy 4 Reproduction 5 Relationships 6 Fossil record 7 Species 8 References 9 External links

History

The first species of this phylum was described in 1827 by the French zoologist Henri Marie Ducrotay de Blainville who named it the name Sipunculus vulgaris. A related species was later described as Golfingia macintoshii by E. Ray Lankester. The specimen was provided by a friend of his Professor Mackintosh. The specimen was dissected by Lankester between rounds of golf at Saint Andrews golf club in Scotland from which the species derives its name. Golfingia is now the genus name and Sipuncula the name of the phylum to which these worms belong.

Habitat

Sipunculids are relatively common, and live in shallow waters, either in burrows or in discarded shells like hermit crabs do. Some bore into solid rocks to make a shelter for themselves. Although typically less than 10 cm long, some sipunculans may reach several times that length.

Anatomy

The most recognizable part of Sipunculan worms is their mouth, which is surrounded by a mass of 18 - 24 tentacles, all of which may be inverted into the body. There is no segmentation or septa present in Sipunculans. Their body consists of an introvert (proboscis) and a trunk, the introvert being retractable into the trunk. The digestive tract of Sipunculans passes from the mouth to the posterior end of the body, before twisting back around itself and ending at the anus, on the dorsal side of the body. The anus is often not visible when the introvert is retracted into the trunk. A few taxa possess a calcified plate called the anal shield. Sipunculans have a coelom. However, they do not have a vascular blood system. Instead, interstitial fluid transports oxygen and nutrients around the body. A separate cavity fills the hollow tentacles; it passes oxygen from the tentacles to the coelom. The body wall is strong and muscular; when threatened, Sipunculids can retract their body into a shape resembling a peanut kernel - a practice that has given rise to the name "Australian peanut worm".

Reproduction

Both asexual and sexual reproduction can be found in Sipunculans, although asexual reproduction is uncommon. Sipunculans reproduce asexually via transverse fission followed by regeneration of vital body components. As for sexual reproduction, Sipunculans are dioecious. Their gametes are produced in the coelomic lining, where they are released into the coelom to mature. These gametes are then picked up by the metanephridial system and released into the aquatic environment. Fertilization in Sipunculans is external. Once male and female matured gametes meet, a trochopore larva develops followed in turn by a pelagosphera larva, then a juvenile form and finally into an adult.

Relationships

The phylogenetic placement of this phylum in the past has proved troublesome. Originally classified as annelids, despite the complete lack of segmentation, bristles and other annelid characters, the phylum Sipuncula was later allied with the Mollusca, mostly on the basis of developmental and larval characters. Currently these two phyla have been included in a larger group, the Lophotrochozoa, that also includes the annelids, the ribbon worms and several other phyla. Analysis of the mitochondrion's DNA has since revealed that the Sipuncula are clearly related to the Myzostomida and Annelida (including echiurans and pogonophorans).^[1]

Fossil record

The fossil record of the Sipuncula is, not surprisingly for soft-bodied animals, sparse. Some scientists, however, consider that hyoliths, operculate shells from the Palaeozoic may be related to the sipunculids, with the only remnant of the shell in extant forms being the anal plate.

Even excluding hyoliths, fossils of sipunculans are known extending back to the Cambrian Period. Fossils of the genera Archaeogolfingia and Cambrosipunculus from China are not drastically different from members of the Sipunculidea living today. ^[2]

Species

• Golfingia minuta
• Golfingia vulgaris
• Phascolosoma arcuata
• Phascolosoma cumanense Keferstein 1867
• Phascolosoma esculenta
• Phascolosoma lurco
• Sipunculus nudus
• Sipunculus robustus

References

1. ^ Shen X., Ma X., Ren J., Zhao F. (2009) A close phylogenetic relationship between Sipuncula and Annelida evidenced from the complete mitochondrial genome sequence of Phascolosoma esculenta. BMC Genomics. 10(1):136
2. ^ Huang, D. Y., J.-Y. Chen, J. Vannier, and J. I. Saiz Salinas (2004). "Early Cambrian sipunculan worms from southwest China". Proceedings of the Royal Society of London Series B 271 (1549): 1671–1676. doi:10.1098/rspb.2004.2774. 

External links

• Introduction to the Sipuncula, by UCMP
• Sipuncula in "Tree of Life web project"

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