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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).

References in periodicals archive ?
nudus and other sipunculans were reported by Spongberg (2006).
No recent published data is available on the abundances of brachiopods and sipunculans from the Gulf of Nicoya estuary.
Thus, the objectives of this study were to make accessible data on the abundances of sipunculans, brachiopods and hemichordates (1984-1987, 2013) at a sand-mud flat; and on trace metals (1996, 2000) and abundances (2015) of sipunculans and brachiopods at a sand flat in the upper Gulf of Nicoya estuary.
Sediment clumps were removed with a shovel to a depth of 20 cm, broken up by hand, and brachiopods and sipunculans placed in acid-washed polyester bags.
In our study, in vitro exposure of sipunculan coelomocytes to 100 mM [H.
Our initial characterization of apoptosis in the sipunculan Themiste petricola may contribute to the future assessment of potential disruptors of apoptosis regulation in this small but geographically widespread phylum of marine worms.
Phagocytosis of yeast by coelomocytes of the sipunculan worm Themiste petricola: opsonization by plasma components.
LPS-induced stimulation of phagocytosis in the sipunculan worm Themiste petricola: possible involvement of human CD14, CD11b and CD11c cross-reactive molecules.
Total bivalve relative abundance tended to increase while total sipunculan relative abundance tended to decrease with increasing level of site degradation.
At all sites, total density, especially sipunculan density and relative abundance, was higher in coral more than 50% dead than in coral less than 50% dead.
On the other hand, the sipunculan Phascolosoma elegans was present at Platanillo but was not found in Golfo Dulce.
Polychaete densities in corals with greater than 50% dead substrate were lower and sipunculan densities were similar to densities reported from other dead substrate reefs in the central Pacific and Indian Ocean (Kohn and Lloyd 1973, Kohn and White 1977, Pari et al.