Australia's Biodiversity

Priapulids (Phylum Priapulida)

The phylum Priapulida is named after the minor Greek god Priapus (protector of livestock, fruit plants, gardens and male genitalia) due to the resemblance of some species to a penis (hence they are sometimes called penis worms). Various other worm-like creatures such as some peanut worms (sipunculans) are also sometimes vaguely penis-shaped, but the priapulids are the real thing.

 

Priapulida is the smallest phylum in Australia, likely containing only two species, both of which have been named. It is also one of the world's smallest phyla, with only around 18 known species. However, these are very poorly studied in most parts of the world, and there may well be unrecognised species.

About half of the known priapulids are small (~1 mm long) and live between grains of sand, while the other half are much larger (12-15 cm long). They are unsegmented, with an eversible feeding organ called an introvert, a trunk, and a tree-like tail appendage.

A priapulid or 'penis worm'

Image © Sars-senteret. Source: https://www.uib.no/en/news/36517/penis-worms-uncover-evolution-digestive-system

 

The suggestive shape, however, is probably the least interesting and important thing about priapulids. Despite their few species, priapulids have a very long and important evolutionary history. Exquisitely preserved fossils of a priapulid are abundant in the famous, 500 million year old Cambrian Burgess Shale. In fact, the Global Boundary Stratotype Section and Point (GSSP) marking the base of the Cambrian at Fortune Head, Newfoundland – essentially the type locality for the Ediacaran-Cambrian boundary – is defined by the appearance of complex trace fossils, particularly the canonical Treptichnus pedum, which is likely to be a priapulid.

 

Given that the Ediacaran-Cambrian boundary is arguably one of the most important in the history of life on Earth, the time when complex life as we know it today first evolved, priapulids are evolutionarily very significant organisms. Studies of the development and developmental genetics of priapulids also shows that they develop their intestines just like humans, fish or starfish do, an important result in understanding the evolution of the intestine, a key innovation in animals.

 

An exquisitely preserved specimen, the holotype of Ottoia tricuspida, from the Burgess Shale: Royal Ontario Museum 63057. 

 

And consider this. Of all the weird and wonderful creatures of the Burgess Shale – the wonderfully named Hallucigenia and Wiwaxia, the scary predator Anomalocaris – all are very extinct and nothing like them are alive today. They are quintessentially prehistoric.

 

But through all of the 500 million years that separates use from these Cambrian creatures, priapulids have been essentially unchanged, slowly ploughing their way through the sludges and sediments of time without any great need to change. Gives you pause for thought in this fast-moving world.

 

The 500 million year old Cambrian priapulid Ottoia is very similar to priapulids that can be found in soft sediments today.. 

Image source: Wikipedia, by Smokeybjb (CC-BY-SA 3.0)

 

Priapulids are unsegmented, worm-like creatures ranging from a few mm long to 40 cm. They have a characteristic spiny, toothed 'introvert' at one end, which can be retracted into the body or extended as necessary. The introvert is used both to anchor one end as the animal moves through soft sediments, and to grasp prey. Some species are known to feed on slow-moving worms, while others may consume and digest sediment.

 

Priapulids are rarely collected in Australia, but have been found at scattered locations around the coast in inshore waters and estuaries. Some species occur in Antarctica. There are no specialist priapulid taxonomists in Australia. 

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