Scientists finally figure out evolution of bizarre ‘bristle-jaw’ worm

14 Jan 2019

Chaetognaths, or arrow worms, have a distinct jaw structure composed of a dense protein matrix and a fibrous substance called chitin. Image: Zatelmar/Wikimedia Commons

A breakthrough in evolutionary history has been made as researchers have finally found the origin of the ‘bristle-jaw’ worm.

While reminiscent of the ‘chestbuster’ in Alien, chaetognaths – which means ‘bristle-jaw’ – are a very common type of worm across the world. They are found in brackish estuaries, tropical seas and above the deep, dark ocean floor.

While these creatures – also known as arrow worms – have been around since the Cambrian period about 500m years ago, their precise evolutionary history has baffled researchers, until now.

Published in Current Biology, a study written by researchers at the Okinawa Institute of Science and Technology Graduate University attempted to verify the relationship of the predatory arrow worm with a diverse group of organisms known as spiralia, including molluscs, segmented worms and flatworms.

Approximately 200 species of arrow worms are known to exist, each resembling tiny spears that range from just 1mm in length to 12cm. They mostly feed on small crustaceans called copepods, using their keen vibratory sense to hunt prey and swallow them whole.

Incredibly, the team found that arrow worms aren’t related to spiralians, but rather a new group of animals that form a sister group to these organisms. This discovery challenges the classical view that complex organisms evolved from simple ancestors by gaining new traits over time.

Rise of the supergroups

“Arrow worms are predators; they have nervous systems, they have developed sensory organs. But the other organisms they’re grouped with are much simpler,” said Ferdinand Marlétaz, first author of the study. “If you place arrow worms here, it means there was probably a lot of independent simplification, rather than the independent emergence of complexity.”

While much different in appearance to their phylogenetic relatives, such as the microscopic rotifer, arrow worms do share the same jaw structure. Composed of a dense protein matrix and a fibrous substance called chitin, these jaws are situated near the organisms’ mouths and allow them to grasp their prey.

All of the species break down into two supergroups known as deuterostomes and protostomes, both of which have a single gut running through them from end to end. The difference is that in development, the deuterostome gut forms from the bottom up, while protostome gut formation starts at the mouth.

Though arrow worms develop bottom-up-like deuterostomes, they strongly resemble protostomes both morphologically and genetically. To clearly differentiate between the two, the researchers gathered data from 10 arrow worm species and compared it to publicly available data from other animals.

Somewhat surprised, the researchers said this placed arrow worms solidly in the protostome superclade, within a subgroup that includes microscopic organisms known as rotifers, gnathostomulids and micrognathozoans. Finally, after years, the weird creature has an evolutionary home.

Colm Gorey was a senior journalist with Silicon Republic