Move over, hermit crabs! A 2021 paper published in the journal Current Biology suggests similar shell-occupying behavior in an unexpected source: Cambrian priapulid (priapulan) worms, also known as “penis worms” due to their gross morphology (overall shape of the organism) bearing a remarkable similarity to the human penis.
World class quality “penis worm” fossils can be found at the Walcott Quarry, Burgess Shale.
These fierce predators show up in the fossil record after the Cambrian explosion – roughly 530 million years ago. Researchers have found fossil evidence suggesting that these worms nestled inside the shells of dead hyolithids – a type of shelled animal. This “hermitting” behavior is the earliest example ever found, predating classic hermit crabs by hundreds of millions of years.
Why would a worm want a second-hand home? The working hypothesis is that these shells likely offered protection from predators and helped the worms secure food in the Cambrian seafloor’s soft sediment. This discovery adds to our understanding of early animal behavior, providing further evidence that complex ecological strategies emerged around the time of the Cambrian explosion.
A longstanding hypothesis states that an increase in ecological complexity during the Cambrian Explosion accelerated the rate of diversification of major animal lineages. This paper adds to the evidence that increasing ecological complexity was concomitant with the Cambrian Explosion and supports the hypothesis.
So next time you see a hermit crab, remember – they’re just the latest in a long line of resourceful squatters.
Xiao-yu Yang et al. A ‘hermit’ shell-dwelling lifestyle in a Cambrian priapulan worm. Current Biology, published online Nov 8, 2021; DOI: https://doi.org/10.1016/j.cub.2021.10.003