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).
- Boore, J. L. & Staton, J. L. (2002) The mitochondrial genome of the sipunculid Phascolopsis gouldii supports its association with Annelida rather than Mollusca. Molecular Biology and Evolution, 19, 127-137.
- Cutler, E. B. (1994) The Sipuncula. Their systematics, biology and evolution. Ithaca, N.Y.: Cornell University Press, 453 p.
- Dordel, J., Fisse, F., Purschke, G. & Struck, T. H. (2010) Phylogenetic position of Sipuncula derived from multi-gene and phylogenomic data and its implication for the evolution of segmentation. Journal of Zoological Systematics and Evolutionary Research, 48, 197-207.
- Dunn, C. W., Hejnol, A., Matus, D. Q., Pang, K., Browne, W. E., Smith, S. A., et al. (2008) Broad phylogenomic sampling improves resolution of the animal tree of life. Nature, 452, 745-749.
- Hyman, L. H. (1959) The protostomatous coelomates - Phylum Sipunculida. In: L. H. Hyman (Ed), The invertebrates: smaller coelomate groups. MacGraw-Hill Book Company, Nova Iorque, pp. 610-696.
- Saiz Salinas, J. I. (1993) Sipuncula (Vol. 4). Madrid: Museuo Nacional de Ciencias Naurales CSIC, 200 p.
- Stephen, A. C. (1964) A revision of the classification of the phylum Sipuncula. Annals and Magazine of Natural History, 7, 457-462.
- Stephen, A. C. & Edmonds, S. J. (1972) The phyla Sipuncula and Echiura. London: Trustees British Mus. (Nat. Hist.), 528 p.
- Struck, T. H., Schult, N., Kusen, T., Hickman, E., Bleidorn, C., McHugh, D., et al. (2007) Annelid phylogeny and the status of Sipuncula and Echiura. BMC Evolutionary Biology, 7.