The beach crabs, which live in snail shells, are the most famous example: some marine animals adopt empty dwellings from others. This strategy is apparently astonishingly old, a study now shows: According to this, representatives of the priapulids, also known as penis worms, quartered themselves in foreign shells around 500 million years ago. Presumably, the bizarre-shaped beings protected themselves from enemies in this way. The findings also confirm the already considerable complexity of life in the Cambrian seas, say the researchers.
Instead of creating protective structures themselves, they take over the empty shells of mussels and snails: this strategy is now widespread, especially among representatives of marine arthropods. Hermit crabs and the like use the hard structures to protect their sensitive abdomen and to be able to withdraw quickly in case of danger. On the basis of fossil finds, a comparatively young evolutionary invention has been assumed: the crustaceans probably only developed the concept in the Jurassic Age. But as the study by an international team of researchers now shows, creatures from the Cambrian era had evidently already produced the hermit principle a few hundred million years before the crabs.
Priap worms in annexed houses
The results are based on the examination of finds that come from a fossil deposit in southern China and were dated to an age of approximately 500 million years. As the scientists report, what is special about these remains is that in addition to the shells of marine animals, traces of soft tissue have sometimes been preserved. In this way, traces of beings have been preserved that the scientists were able to assign to the Priapulida worms. Even today there are still representatives of these bizarre marine animals that live in sediments – but there are no longer species stuck in shells. Because of their appearance reminiscent of the male sexual organ, these beings were named after the Greek god of manhood – Priapus. This is where the term “penis worms” comes from.
It is already known from fossil finds that the history of the development of the priap worms goes back to the Cambrian. However, the team of scientists has now discovered four fossil specimens of a prehistoric representative of these creatures, which have a special location: These approximately two centimeters large priap worms are stuck in the conical shells of so-called hyoliths – shell-living molluscs that were apparently widespread on the sea floor at the time. “All three specimens sit in the same types of bowls and in the same position and orientation,” says co-author Martin Smith of Duham University.
Early invention after the Cambrian Explosion
The size and shape of the penis worms seem to be adapted to the respective shell and a thickening at the end of the worm-like body could have served as anchoring in the shell, the scientists write. “The only plausible explanation seems to be that the shells were home to these priap worms,” says Smith. According to this, it is a case of parallel evolution: As early as the Cambrian, living beings evidently developed a strategy that much later produced crustaceans again, the scientists state.
“That was a real surprise,” says Smith. As he explains, it is an astonishingly early development in evolutionary history. Because it comes from the time shortly after the so-called Cambrian Explosion, when more highly developed animal forms emerged for the first time. “Not long before these shell-dwellers existed, there was nothing more complex than algae or jellyfish,” says Smith. The scientists also assume that these representatives of the priap worms protected themselves through the hermit crab concept. Accordingly, there were apparently already corresponding enemies.
“It seems amazing that so soon after the first complex animals appeared, complex and dangerous ecosystems were emerging that are normally associated with much more recent periods of Earth’s history,” said Smith. This shows how surprisingly fast and flexible evolutionary processes can take place, the scientists write in conclusion.
Source: Durham University, Article: Current Biology, doi: 10.1016 / j.cub.2021.10.003