by Dhugal Lindsay, Japan Agency for Marine-Earth Science and Technology
June 29, 2018
This beautiful, big medusa, seen during Dive 15 of the expedition, belongs to the genus Cyanea, which is a group of scyphozoan jellyfishes well-known for their tendency to "bloom" and occur in large numbers in surface waters. They can also be a major food item for leatherback turtles. For many years it was thought that there were only a few, mostly cosmopolitan species, but we now know that many morphotypes are actually valid species in their own right.
In order to be sure of the species, one characteristic to look for is the presence or absence of circular muscle folds with many intrusions arising from the radial gastric canals. However, with the underside of the bell obscured by the many tentacles (counting these is also a good character but one would really have a hard time...), this characteristic is unable to be observed. The bluish coloration in this large individual suggests that it could well be Cyanea lamarckii (Péron and Lesueur, 1809).
In any case, this observation of the genus in deep waters near the seafloor is very valuable. In recent years, there have been several records of various large scyphomedusan species that are known to bloom in surface waters being found in the near-bottom layers in deep waters. Some of these reported that the medusae were gravid, or laden with babies, and the polyp stages of these medusae have also been found on substrates such as plastic, lying on the deep-sea mud. Shallow and deep waters may not be as separate as we have traditionally considered.