Peanut Worms (Phylum Annelida)
For many years peanut worms were classified in their own phylum, the Sipuncula, separated from the more common and diverse annelid worms because of their unsegmented bodies. However, recent molecular analyses indicate that peanut worms are in fact unsegmented annelids.
Peanut worms are soft-bodied, worm-like marine organisms that live in soft sediments on the seafloor as deep as 6000 m, and on coral reefs and kelp beds. Their name alludes to the fact that they retract into a peanut shape when disturbed.
Peanut worms are unsegmented, grow 3-10 cm long, and are characterised by a highly flexible introvert or proboscis forming a spoon-like projection from the main body. Hooks and spines may be present inside the introvert, which are used to feed on detritus, or to filter-feed from passing currents.
Like many aquatic invertebrates, peanut worms reproduce by releasing sperm or eggs into the water column. Some species are known to undergo asexual fission, with up to one-fifth of the parent's body shearing off to form a new, genetically identical individual.
Peanut worms use a rare purple pigment, haemorythrin, to help transport oxygen in the blood, rather than the red haemoglobin found in humans and other vertebrates or the blue hemocyanin found in arthropods and segmented worms. Only a handful of other animals (in the phyla Priapulida and Brachiopoda employ this rare purple pigment.
Peanut worm fossils are rare, but have been found in 500 million year old Cambrian sediments. This indicates that both peanut worms and annelid worms are extremely old. The worm-like body plan seems to be an evolutionary winner - if you're a worm, there doesn't seem to be much need to evolve to be anything other than a worm.
A peanut worm, Phascolosoma arcuatum, from Wadeye, Northern Territory. Photo: (c) Tony Griffiths
There are around 48 species of peanut worms in Australia, and given the low global diversity of the group, it is unlikely that many more will be discovered. Nevertheless, this is one of the least-studied groups in the world, and new species are possible.