Sipuncula are marine invertebrate worms commonly known as peanut worms (or star worms), with approximately 150 recognized species (Cutler 1994). They are widely distributed throughout the world's oceans from the tropical intertidal to cold deep-water habitats. These little-known marine invertebrates are often confused with holothurians, echiurans or nemerteans and are easily overlooked by inexperienced observers. However sipunculans have several characteristics that separate them easily from these other groups. The body consists of a cylindrical trunk and an introvert that invaginates completely inside the trunk. The mouth is located at the tip of the introvert and may be surrounded by digitiform tentacles. The peculiar position of the anus in the antero-dorsal region of the trunk is an easily seen external character that distinguishes the sipunculans from other worm-like invertebrates. Sipunculans are dioecious but sexes are not distinguishable externally. Fertilization is external and development may be either direct with no larval form or indirect, usually with a trochophore larva followed by a pelagosphera, a larval type unique to sipunculans. Pelagosphera larvae of many species can spend long periods of time in the water column; consequently they are capable of long distance dispersal. At least one species (Aspidosiphon elegans) is able to reproduce asexually by fission of small pieces from the posterior. As infaunal animals, sipunculans burrow into the substrate or they are cryptic inhabitants of coral rubble or empty gastropod shells and are therefore not readily observed or collected.
The phylogenetic position of sipunculans has been contentious. This group has been ranked at differing taxonomic levels such as family, order, class or phylum (Saiz Salinas, 1993; Cutler, 1994). Phylum status for this group was established only in the middle of the 20th century (Hyman, 1959) and the current name, Sipuncula, was proposed by Stephen (1964) and restated by Stephen and Edmonds (1972). More recently, molecular phylogenetic studies have provided strong evidence that sipunculans are either within, or closely related to, annelids (e.g. Boore & Staton, 2002; Struck et al. 2007; Dunn et al. 2008; Dordel et al. 2010).
The name Sipunculida has been used as an alternative name for the phylum Sipuncula, as well as for a class (Brusca and Brusca 2003) and possibly an order within the Sipuncula. General information on sipunculans can be found on the Sipuncula page.
The sipunculans ("peanut worms") are an exclusively marine group of about 250 known species of round, unsegmented worms with a body consisting of a thick trunk and a slender rectractile proboscis. Sipunculans can be found from the intertidal zone to depths of more than 5,000 meters. Sipunculans range from less than a centimeter to around 50 cm in length, but most are in the range of 3 to 10 cm long. The mouth opens at the front of the proboscis and is surrounded by lobes or tentacles. The gut is U-shaped and highly coiled. The inconspicuous anus opens on the upperside of the body near the front of the trunk. In nearly all species, sexes are separate but not distinguishable externally. Males spawn first and the presence of sperm in the water subsequently stimulates the release of eggs by females. Fertilization is external and development may be direct or indirect (through metamorphosis of a swimming trochophore larva, and in some species involving a second larval stage as well). At least some species are capable of reproducing asexually by dividing into a small posterior fragment and a larger anterior portion, both of which then regrow the missing parts. Most sipunculans are burrowers in substrates that range from mud to coral-rock, although some may seek shelter in other refuges such as snail shells. Under normal conditions, the fluid in the body cavity (coelom) is nearly isotonic to the surrounding seawater (Gosner 1978; Brusca and Brusca 2003 and references therein)
The exact phylogenetic position of the sipunculans has been elusive. Hyman (1959) elevated the group to phylum status, a treatment that stood for many decades, but in recent years molecular phylogenetic analyses have provides strong evidence that the sipunculans may actually be nested within the phylum Annelida (e.g., Struck et al. 2007; Mwinyi et al. 2009; Dordel et al. 2010).
Evolution and Systematics
Discussion of Phylogenetic Relationships
Sipunculidea (Sipunculidae , Golfingiiformes), Sipunculidae (Sipunculus , Xenosiphon , Siphonosoma , Siphonomecus , Phascolopsis), and Phascolosomatidae (Phascolosoma , Apionsoma , Antillesoma) of Cutler (1994) are most likely not monophyletic (Maxmena et al. 2003, Schulze et al. 2007).
Molecular Biology and Genetics
Statistics of barcoding coverage
Specimens with Sequences:388
Specimens with Barcodes:361
Species With Barcodes:65
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