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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 ?
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.
The presence of a few sipunculans was restricted to the rainy seasons of 1984 and 1985 (Fig.
The most important of these are the evolution of segmentation at the base of the annelid-cchiuran-sipunculan line, multiplication and fusion of the ventral nerve cords in several annelid/echiuran taxa as well as in the sipunculans, the secondary loss of segmentation in Sipun-cula, and the evolution of tetraneury and a complex apical organ with eight to ten central flask cells and a number of peripheral cells in the Tetraneuralia (entoprocts and molluscs) (Fig.
For example, while molecular phylogenetics revealed a close relationship between the segmented annelids and the unseg-mented sipunculans, studies on the neurogenesis showed that the sipunculans indeed stem from a segmented ancestor and that the segmental body architecture is still rudimenta-rily present during larval development, thus providing evidence for the first time that large, free-living animals may lose a segmented body organization.
Gastrochaenid bivalves, dorvilleid, cirratullid, spionid and flabelligerid polychaetes were abundant at Punta Islotes; aspidosiphonids and phascolosomatid sipunculans, and eunicid and sabellid polychaetes were found primarily at Platanillo.
Sipunculans were dominant in coral colonies with more than 50% mortality in Punta Islotes and Platanillo (Fig.
Sipunculans were frequently found associated with boring sponges that were using the same coral substrate.
The aim of this study was to evaluate changes that could be considered compatible with an ongoing apoptotic-like process in coelomocytes of the sipunculan Themiste petricola Amor, 1964, after exposure to [H.
The normal appearance of sipunculan coelomocytes supravitally stained with acridine orange/ethidium bromide
Supravital staining with AO/EB revealed the presence of large vacuoles and granules in a large proportion of sipunculan coelomocytes.