Horseshoe worms (Phylum Phoronida)
Horseshoe or phoronid worms make up a very small group of tube-dwelling marine invertebrates. Around a dozen species are known worldwide, of which 8 are found in Australia; most are widespread or cosmopolitan.
Horseshoe worms secrete and live in hardened tubes made from chitin, often embedded in the sea floor in shallow coastal waters. They may be 1-30 cm in length, but are usually less than 1 cm in diameter. Using a beautiful fan-like collection of tentacles called a lophophore, horseshoe worms draw tiny particles of food, including the larvae of other small animals, into their mouths.
The name 'horseshoe worm' is derived from an unusual feature of phoronids that distinguishes them from most other worm-like animals: their gut loops from the mouth (situated inside the base of the lophophore) down to a stomach in a bulb-shaped swelling at the base of the worm-like body, then up again to an anus situated outside and slightly below the lophophore.
Two other phyla also have a lophophore - the bivalve-like brachiopods, and the moss-animals (bryozoans). Phoronids are almost certainly very closely related to brachiopods, although the position of these two groups with respect to bryozoans is currently uncertain and contested.
When phoronids are actively feeding their most obvious feature is the lophophore, a crown of ciliated tentacles around the mouth. Source: Public Domain, https://commons.wikimedia.org/w/index.php?curid=105557
Sexual reproduction is via eggs and sperm released into the water. Fertilised eggs develop into a planktonic larva called an actinotroch, which spends some time drifting and feeding on other plankton before settling to the seafloor. Actinotroch larvae are so different in form from the adult phoronids that they were originally classified as separate organisms.
A few horseshoe worms, instead of releasing their eggs and sperm, brood their developing young alongside the body inside the tube. Many also undergo fission: a part of the adult body breaks off and becomes a new juvenile.
The microscopic, planktonic actinotroch larva of an Australian phoronid. Photo: A. Slotwinski, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=12426968
Although little is known about Australian horseshoe worms, they are common in sheltered waters such as Port Phillip Bay, Victoria, and the Brisbane River estuary, Queensland. Some species are solitary while others form colonies. One colonial species found in Port Philip Bay (Phoronis pallida) reaches abundances of 600 individuals per square metre.
Thanks to Robin Hare from the University of Western Australia for help preparing this profile
Read more about phoronids:
Emig, C. C., D. F. Boesch and S. Rainer, 1977. Phoronida from Australia. Records of the Australian Museum 30(16): 455–474. doi:10.3853/j.0067-1975.30.1977.191