Sipunculans are soft-bodied, unsegmented coelomate worm-like animals, commonly called peanut worms (Fig. 22.4G). They consist of a muscular trunk or body and an anteriorly placed, more slender introvert that bears the mouth at the anterior extremity. The introvert is highly elastic, capable of considerable extension; at other times it is partially or completely retracted within the body cavity. The mouth leads to a long, recurved, spirally wound alimentary canal that lies within the coelom. Tentacles either surround the mouth or are associated with it. Chaetae are absent. One or two pairs of nephridia are present. The body wall is usually thick and often rubbery and has well developed circular and longitudinal muscles and occasionally a thin layer of oblique or diagonal muscle is also present. The skin often contains pigmented papillae and the surface of the introvert may be covered in hooks. Around the anus a caudal shield or calcareous knob or cone is often present. Sexes are separate, fertilisation is external, and a free swimming trochophore larva is produced.

While sipunculans are typically regarded as sedentary, some species, especially those that burrow in sand or silt, show considerable activity when they dig. When threatened, most species are able to rapidly retract their sensitive and probing introvert. Species respire through their body wall and tentacles, and cells in both the vascular and coelomic systems contain a red respiratory pigment, haemerythrin, which only gives up its oxygen at a very low partial pressure—much lower than that commonly found in sea water. It has been suggested that it functions only in adverse situations, such as may occur on intertidal reef flats on the reef especially during low tides in the middle of the day during summer.

Sipunculans are abundant on the reef, although not often seen unless sediment samples are collected or reef rock broken off (Fig. 22.4H). They live in characteristic shaped burrows in reefal substrate or in tubes in the sediment; others bore into solitary corals or inhabit empty gastropod shells. The solitary coral, Heteropsam-mia michelini, commonly contains the sipunculan Aspi-dosiphon muelleri. Rock boring species secrete an acid-like substance and after settling as larvae metamorphose and bore into the substrate, forming a flask-shaped burrow where they are effectively entombed whilst retaining a narrow passage to the exterior through which they obtain their oxygen and release their gametes. They are detritus feeders, using their tentacles and extensible introverts to collect algae, sediment and detritus. Sipunculans themselves are selectively fed upon by some molluscs and humans eat them in selected localities in the Indo-Pacific (although whether they were eaten by Australian Aborigines is not known).

Six families of sipunculans are known and numerous genera and species have been recorded from the GBR but many remain to be described. They appear to have phylogeographic affinities with other Indo-Pacific species. Juveniles are difficult to identify as they lack many of the adult characteristics.

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