Which Halitholus species (Cnidaria: Hydrozoa) inhabits the Arctic and high boreal Atlantic waters?

Abstract

The binomen Perigonimus yoldiaearcticae Birula was established (as Perigonimus yoldiae‐arcticae) for a hydroid species found predominantly on shells of the mollusk Yoldia arctica (now Portlandia arctica) in the White and Kara Seas. Its original description was incomplete, no medusa stage was described, no specific type locality was specified, and we know of the existence of no type specimens. Birula's nominal species cannot be distinguished from several other pandeids occurring at high latitudes of the northern hemisphere, and we regard P. yoldiaearcticae as a nomen dubium. By contrast, Halitholis cirratus Hartlaub, 1913 was founded on medusae obtained at Spitzbergen and in the Barents and Baltic Seas, and its presumed hydroid discovered in the Baltic (the latter a region with no other known species of pandeid medusae). We examined material referable to H. cirratus from the Kara Sea, including both medusae and hydroids (on Portlandia arctica), and conclude that this is the species inhabiting the Arctic Ocean and the high boreal Atlantic.