Strange worm-like creatures live deep on the ocean floor. These creatures, Xenoturbella, have a very simple body plan. They have mouths but no eyes, brains, stomachs or anuses. They also have bilateral symmetry, meaning that the right and left sides are more or less symmetrical. This body plan has led to confusion about exactly where these beings fit into the tree of life.

Now, in a recently published study, researchers from Japan have observed an exciting mode of reproduction that could shed light on the life history of these species. Scientists from the University of Tsukuba reported that Xenoturbella bocki releases gametes (reproductive cells) through cracks in its body wall.

Lead author of the study, Associate Professor Hiroaki Nakan, said: “The phylogenetic position (evolutionary history) of these species is still unclear. Because the development of a species can often give clues to how that species has evolved, we wanted to investigate the reproduction and development of these species in the lab.”

Keeping the adult Xenoturbella alive in a lab environment was a major challenge for scientists during the research.

The researchers were able to keep adult worms alive because Xenoturbella bocki is slightly more robust because it lives in shallower water than the other species in this genus. Adult Xenoturbella were obtained from Sweden’s Gullmarsfjord and once in the lab they were made to reproduce once a month. Our studies not only showed that this species breeds in winter, but also revealed an intriguing mode of reproduction.

Associate Professor Nakano said: “As the animals spawned, we noticed that the eggs and sperm were released from new openings in the body wall. These openings seemed to rupture into the body which appeared only to spawn. Based on our observations, we think that gametes probably begin to mature on the surface of the gut, are released into the body cavity when mature, and are then released into the water through these cracks.”

The phylogeny of these peculiar marine worms will eventually be made clearer by an understanding of Xenoturbella’s reproductive characteristics. The basis for investigating larval development and, perhaps, providing more answers to this evolutionary puzzle comes from successful laboratory reproduction.

Magazine reference:

  1. Nakano, H., Nakano, A., Maeno, A. et al. Induced spawning with gamete release through body tears during reproduction of Xenoturbella bocki. Commune Biol 6, 172 (2023). DOI: 10.1038/s42003-023-04549-z