First observation of the "double-faced X-framed cup ossicle" extracted from a deep sea holothurian in Japan

A New type ossicle form, “double-faced x-framed cup ossicle” is discovered from the undescribed deep sea holothuroid. To give a definite view on the substantial qualities of this ossicle, a SEM observation on the ossicles and a DNA barcoding analysis are conducted. Although the most internal and external morphologies of the present species agree well with the characteristics of the family Cucumariidae Ludwig, 1894, the ossicles morphologies mostly does not agree with the congeners of Cucumariidae. On the other hand, our molecular study indicates a possibility that the specimens are not cucumariids, but belong to a sister group of Cucumariidae. In our present observation, some of the peri-oral ossicles show a very similar property with the small x-framed cup-shaped structures (which sometimes occurs in cucumariids). Therefore, the double-faced x-framed cup ossicle probably could be considered as the results of derivation from the x-body: in which four extra-arms developed on the bottom face of a cup ossicle, and finally these arms equipped with an extra-rim.