Saturday, 13 January 2018

Xenoturbella japonica: A new species of Xenoturbellid Flatworm from the Pacific coast of Honshū Island, Japan.

Flatworms are simple animals that lack through guts or a body cavity in which the organs are held. They have long been thought to be most primitive group of Bilaterians (all animals other than Sponges, Cnidarians and Ctenaphores), though they have been shown by genetic studies to be a polyphyletic group (similar animals with different origins), with several types of Flatworms apparently having evolved from more complex animals. However two groups of Flatworms do appear to have diversified from the Bilatarians before the development of through guts and body cavities, the Xenoturbellids and the Acoelomorphs (together the Xenacoelomorphs) which have been shown by genetic analysis to be related to one-another and to have a sister-group relationship to all other Bilaterians. There are currently five described species of Xenoturbellids, all placed in a single genus, Xenoturbella, with one species known from the Baltic coast of Sweden and four from the Pacific coasts of Mexico and the United States.

In a paper published in the journal BMC Evoltionary Biology on 18 December 2017, Hiroaki Nakano and Hideyuki Miyazawa of the Shimoda Marine Research Center at the University of Tsukuba, Akiteru Maeno and Toshihiko Shiroishi of the Mammalian Genetics Laboratory at the National Institute of Genetics, Keiichi Kakui of the Faculty of Science at Hokkaido University, Ryo Koyanagi and Miyuki Kanda of the DNA Sequencing Section at the Okinawa Institute of Science and Technology Graduate University, Noriyuki Satoh of the Marine Genomics Unit at the Okinawa Institute of Science and Technology Graduate University, Akihito Omori of the Misaki Marine Biological Station of The University of Tokyo, and the Sado Marine Biological Station of Niigata University, and Hisanori Kohtsuka, also of the Misaki Marine Biological Station of The University of Tokyo, describe a new species of Xenoturbella from the Pacific coast of Honshū Island, Japan.

The new species is named Xenoturbella japonica, in reference to the fact that it was discovered in Japan. The species is described from two specimens, a female 5.3 cm in length from a depth of between 380 and 554 mm off the coast of Jogashima Island in Sagama Bay, and a juvenile 1.1 cm in length from a depth of between 517 and 560 m off the Sanriku coast of Iwate Prefecture. Both are orange in colour and have smooth surfaces.

Live specimen of Xenoturbella japonica with anterior to the left. White arrowhead, ring furrow. Scale Bar is 2 cm. Nakano et al. (2017).

Like other Xenoturbellids, Xenoturbella japonica, lacks any true organs such as nerve cords, respiratory or even gonads, but Nakano et al. were able to find a single pore towards the front of the animal, though the purpose of this is unclear. A genetic analysis suggests that this new species is closely related to the Baltic Xenoturbella bocki and the East Pacific Xenoturbella hollandorum, all relatively shallow species, found at less than 650 m, and less than 6 cm in length, with the remaining three Pacific species forming a second group, all of which are found at depths of between 1700 and 3700 m, and reach lengths of 10-20 cm.

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