Asymmetron inferum Nishikawa, 2004
The body is up to 14.7 mm long and up to 1.5 mm high. It is elongated longitudinally and much flattened laterally, tapering gradually towards both ends. The preatrioporal region measures 10.9 mm, the atriopore to anus distance is 1.0 mm, postanal region is 1.8mm, and the urostyloid process is 1.0 mm. The rostral fin is extensive and semi-circular, and is clearly separable from the other part by dorsal and ventral notches. The dorsal fin is very low in the anterior two-thirds of the body, and becomes much higher posterially. The left metapleuron terminates abruptly and markedly just behind the atriopore, while the right is confluent with the well-developed ventral fin, followed by a thin and high caudal fin that is somewhat higher than the dorsal fin. The urostyloid process is separable from the dorsal and caudal fins by its own lower dorsal and ventral fins. There are 270 dorsal fin-chambers, with the relative length to width for posterior tall ones up to 3.3. Preanal fin-chambers are indiscernible, but instead, an elongated vacuole can be detected along the ventral side of the rectum between the atriopore and the anus. There are 83 myotomes in total, consisting of 58 in the preatrioporal region, 9 in the region from the atriopore to the anus, and 16 in the postanal region, thus giving the myotome formula of 58+9+16=83. A black pigment spot is detectable in a neural tube just anterior to the first myotome. A tentacular ring composed of ca. 30 tentacles (including a few of the dorsal-most minute ones), with its anterior margin situated at the level of the axial (=proximal) part of the 5th myotome. The intertentacular membrane is extremely higher in the span of the 6 tentacles next to the much shorter midventral tentacle than elsewhere. The mid-gut coecum is elongated, and ranges at a level from the 20th to the 48th myotomes. There is no gonad.
Ecology and Distribution
From bottom sand near the bones of the sperm whale Physeter macrocephalus at a depth of 229 m, off Cape Nomamisaki, southwestern end of Kyushu Island, Japan. (Nishikawa, 2004).