Sipuncula or Sipunculida is a phylum of bilaterally symmetrical, unsegmented marine invertebrates, characterized by a worm-like body divided into a trunk and retractable introvert, typically with tentacles on the tip of the introvert. Known as peanut worms, sipunculid worms, or sipunculans, members of this phylum are divided into two classes, Sipunculidea and Phascolosomatidea, with the Sipunculidea characterized by an array of tentacles surrounding the mouth at the anterior end of the introvert, and the Phascolosomatidea characterized by tentacles arranged in an arc around the nuchal organ (a chemoreceptor) on the tip of the introvert (Schulze 2004). There are about 150 species of Sipuncula.
Sipunculans play an important ecological role in marine food chains, consuming small organisms collected by deposit or filter feeding and being consumed by fishes and other predators. They have a planktonic larval stage in which they can be quite numerous in the water column, with one species (Themiste lageniformes) recorded with a density of up to 2,000 individuals in square meter (Schulze 2004). They also are important in the recycling of detritus. Even humans have eaten these worms, which, although many are small, down to a few millimeters in length, can reach over half a meter in some larger specimens. Sipunculid worm jelly (土笋冻) is a delicacy in the town of Xiamen in Fujian province of China.
Their body consists of an introvert and a trunk, the introvert being retractable into the trunk. There is no segmentation or septa present in peanut worms. The ratio between the trunk length and introvert length varies among species (Schulze 2004). The trunk length of adult sipunculans ranges from two millimeters to over 500 millimeters (0.08 to 20 inches) in length (McGraw-Hill 2005). Sipunculus nudus, for example, commonly reaches up to 15 centimeters (six inches), but can reach 25 centimeters (ten inches), and its introvert can be about one-third of the trunk length (Schulze 2004).
The most recognizable part of peanut worms is their anterior section, which may have a mass of 18 to 24 tentacles, all of which may be inverted into the body. In the class Siplunculidae, the mouth at the anterior end of the introvert is surrounded by an array of tentacles; in the class Phascolosomatidea, the chemoreceptor, nuchal organ, also located on the tip of the introvert, is surrounded by an array of tentacles (Schulze 2004).
Papillae may be present on the trunk and introvert, and proteinaceous, nonchitinous hooks are often found on the distal part of the introvert, sometimes arranged in rings and sometimes scattered (Schulze 2004). The animals may be cylindrical or spherical, and may lack color or be various shades of yellow, gray, or brown (McGraw-Hill 2005), with sometimes reddish, purple, or green pigment in the tentacles or papillae (Schulze 2004).
The digestive tract of peanut worms 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. That is, there is a U-shaped intestine, with the ascending and descending branches coiled around each other (Schulze 2004). The anus is often not visible when the introvert is retracted into the trunk. While the anus typically is at the anterior end of the trunk, in some species it opens onto the introvert (Schulze 2004). 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 nervous system has a cerebral ganglion and a ventral nerve cord, with one or two nephridia (Schulze 2004). The body wall is strong and muscular; when threatened, sipunculans can retract their body into a shape resembling a peanut kernel. This is where the name "Australian peanut worm" comes from.
Sipunculans are found in marine benthic habitats in tropical, temperate, and cold environments. They have been found from the intertidal zone to a depth of 6,860 meters (22,510 feet) (Schulze 2004). They are relatively common overall, with the density of Themiste lageniformes reaching more than 2000 individuals per square meter (Schulze 2004).
Sipunculans live in such habitats as burrows, discarded shells (like hermit crabs do), crevices under rocks, or bore into soft rock, live coral, or, in the case of one species, even a whale skull (Schulze 2004). They also are found in algal mats, root mats of mangroves or sea grass, byssal threads of bivalves, and large sponges (Schulze 2004).
Behavior of sipunculans is poorly understood. Most are deposit feeders, although members of the genus Themiste used their elaborate, branched tentacles for filter feeding (Schulze 2004). Some ingest sediment and associated biomass collected with their tentacles and rock-dwelling species use their introvert hooks to scrape organisms and sediment from the rock surface (Schulze 2004). Sipunculus nudus lives in semi-permanent sand borrows, and during the daytime hides in the burrow, but at night may extend its tentacles for feeding (Schulze 2004).
Asexual and sexual reproduction can be found in sipunculans, although asexual reproduction is uncommon. Most species are dioecious, with only one species, Nephasaoma minutum, known to be hermaphroditic (Schulze 2004). There is no sexual dimorphism known.
Sipunculans reproduce asexually via transverse fission followed by regeneration of vital body components. In sexual reproduction, the gametes are produced in the coelomic lining, where they are released into the coelom to mature. These gametes are then picked up by the metanephridia system and released into the aquatic environment. Fertilization in sipunculans is external. Once male and female matured gametes meet, a trochopore larva develops, followed by a pelagosphera larva, which then develops into a juvenile and finally into an adult sipunculan. The pelagosphera, which is planktonic, stays up to six months in the water column before settling (Schulze 2004).
The phylogenetic placement of this phylum 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 Molluska, mostly from developmental and larval characters. Nowadays, these two phyla are generally included in a larger group, the Lophotrochozoa, that also includes annelids, ribbon worms, and four other phyla.
Sipuncula typically is divided into two classes, four orders, six families, seventeen genera, and 147 species (Schulz 2004; Schulze et al. 2004; ITIS 1999).
The fossil record of the Sipuncula is, not surprisingly for soft-bodied animals, sparse. Definitive fossil Sipuncula are unknown (Schulze 2004). Some scientists, however, consider that hyoliths, operculate shells from the Paleozoic, may be related to the sipunculids, with the only remnant of the shell in extant forms being the anal plate. Ottoia prolifica from the Burgess Shale is proposed as a fossil sipunculan, but may be an aschelminth or Priapulida (Schulze 2004).
Fossils burrows possibly created by sipunculans are known from the Paleozoic (Schulze 2004). Huang et al. (2004) report on some fossils from China believed to be sipunculans that extend back to the Cambrian. They exhibit the general morphology of peanut worms with a sausage-shaped body with a slender retractable introvert and a wider trunk; a crown of tentacles; hooks, papillae, and wrinkle rings on the body surface; and a U-shaped gut with the anus opening near the introvert-trunk junction. Fossils of the genera Archaeogolfingia and Cambrosipunculus from China are not drastically different from members of the Sipunculidea living today (Huang et al. 2004).
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