A group of small black eggs discovered by a remotely operated vehicle in the Pacific Ocean’s abyss is the first tangible evidence that deep-sea flatworms exist below 6,000 meters (3.7 miles) deep. Initially, the researchers at the University of Tokyo in Japan were unsure of what they saw when the undersea vehicle shone its light on the mysterious black spheres. Yasunori Kano, a marine researcher who was operating the ROV that day, was intrigued and decided to retrieve the capsules from their resting place around 6,200 meters (20,341 feet or 3.85 miles) deep in a trench of the northwest Pacific. Kano noticed that most of the black spheres were attached to rock and torn and empty. He sent four intact ones to Hokkaido University invertebrate biologists, Keiichi Kakui and Aoi Tsuyuki. Upon examination, the duo discovered that each leathery casing, or ‘cocoon,’ was around 3 millimeters wide and contained three to seven developing flatworms.
“When we opened the egg capsules, a milky liquid … that might have been yolk was observed along with the flatworms,” write Kakui and Tsuyuki in their paper.
The duo sampled the DNA of worm embryos and discovered that they belonged to a previously unknown species of platyhelminth that was most closely related to two suborders that exist in shallower waters. Kakui, one of the researchers, mentioned that he didn’t realize the rarity of the eggs when he received them from Kano. The embryos looked the same as shallow-water flatworms.
It’s worth noting that all flatworms are hermaphrodites, which means they can produce both male and female gametes. Shallow water species reproduce sexually by laying eggs in leathery cocoons, which are usually attached to a substrate.
Scientists have little knowledge about deep-sea, free-living flatworms, including how they reproduce. Prior to this discovery, the deepest evidence of a ‘potential platyhelminth’ was found on a piece of sunken wood just over 5,200 meters deep.
It was unclear whether the wood sank with a shallow-water species or whether a deep-sea flatworm discovered the wood after it sank. Not only was this creature not confirmed as a flatworm, but the deepest ocean flatworm confirmed before this discovery was found at a depth of 3,232 meters.
“This study provides the deepest record for free-living flatworms and the first information on their early life stages in the abyssal zone, which were very similar to those in shallow-water forms,” conclude Kakui and Tsuyuki.
It appears that the eggs found were laid by at least two adults of the same species. The embryos were retrieved from the deep seabed at different stages of development, suggesting that the cocoons were laid over time, rather than all at once. This discovery suggests that shallow-water flatworms may have gradually moved into deeper habitats over time. However, the reason for this migration and the method they used to do so still remains a mystery.
This news is a creative derivative product from articles published in famous peer-reviewed journals and Govt reports:
References:
1. Kakui, K., & Tsuyuki, A. (2024). Flatworm cocoons in the abyss: same plan under pressure. Biology Letters, 20(1), 20230506.
2. Rasanga, G. V., Hiraishi, K., Hodoshima, R., & Kotosaka, S. (2023). Sidewinding locomotion of flatworm-like mesh robot WORMESH-II. ROBOMECH Journal, 10(1), 26.
3. Bouchet P, Warén A. 1994 Ontogenetic migration and dispersal of deep-sea gastropod larvae. In Reproduction, larval biology and recruitment of the deep-sea benthos (eds Young CM, Eckelbarger KJ), pp. 98-117.
4. Johnson GD, Paxton JR, Sutton TT, Satoh TP, Sado T, Nishida M, Miya M. 2009 Deep-sea mystery solved: astonishing larval transformations and extreme sexual dimorphism unite three fish families. Biol. Lett. 5, 235-239.
5. Hestetun JT, Rapp HT, Pomponi S. 2019 Deep-sea carnivorous sponges from the Mariana Islands. Front. Mar. Sci. 6, 371.