The first cold seeps biological communities were discovered in the abyssal Gulf of Mexico in 1983. Currently, hundreds of cold seeps have been discovered globally. Site F, which was reported in 2007, is one active cold seep in the South China Sea. Rather than plants or algae, animals living in the cold seeps rely on energy that generated by chemoautotrophic bacteria. The distribution of animals in site F cold seeps has a particular pattern. Biomass is greatest where seafloor methane seeps are most active and releasement of strings of bubbles can be seen. Living mussels Gigantidas platifrons and Squat lobsters Shinkaia crosnieri are dominant creatures in this area. As the distance from the most active fluid seepage increases, the lobster population is in rapid decline, and the mussels become less dense. Further away, there are just some mussel shells and bacterial mats. Besides mussels and lobsters, there are a variety of creatures living in cold seeps such as Annelida, Echinodermata, Gastropods et al. Now, over twenty species have been identified from the site F cold seep.

In 2021, I obtained the opportunity to execute scientific expedition tasks of site F cold seep on the scientific research vessel (RV) KEXUE (Institute of Oceanology, Chinese Academy of Sciences, China). In an operation of TV grab targeting the mud where far away the active fluid seepage, I was very excited to discover a new mysterious creature that was worm-shaped without any appendage. At first, I thought it should be the spoon worm. But the whole body of this creature is extremely rough, which reminded me of a creature I had been interested in since my graduate study ten years ago, Aplacophoran! Based on further next-generation sequencing by the deep-sea expert Dr. Minxiao Wang and observation by taxonomist Dr. Junlong Zhang, we identified this species as Caudofoveata, which is one of the early-branching molluscs.

 When you compare Chaetoderma sp. with other clades of Mollusca such as oysters, snails, and octopus, you will be surprised how different it is. Chaetoderma sp. is about 20 centimeters long, cylindrical, and worm-shaped without a foot in the ventral. Its elongated body is covered by cuticle and small calcareous sclerites instead of shells on the outside. The sclerites are hard to see with the naked eyes and can be distinguished through the microscope. Besides, the sclerites in different body parts showed different lengths, the tail part has the longest and densest sclerites. Most of their body burrows into the mud, only the tail of the body leaves out, and their gills are in this position, where densely sclerites provide well protection. It is also worth mentioning that the body size of Chaetoderma sp. discovered in site F cold seep is much larger than that of new Caudofoveata species reported in recent years, which is almost several centimeters or millimeters.

 After investigating the basic traits of Chaetoderma sp., we think it is a key species to study the evolution of the divergent shell and body plan of mollusc and deep-sea adaptation. Yue Wang was then interested in sequencing this creature and exploring its unique genome characters. She used PacBio and high-resolution chromosome conformation capture sequencing technology to obtain a high-quality chromosome-level genome of Chaetoderma sp. If you are also interested in this species Chaetoderma sp. or in scientific questions related to the evolution and adaptation of deep-sea molluscs, please read our article on Scientific Data.