First full description of the large physonect siphonophore Halistemma amphytridis (Lesueur & Petit, 1807)

Abstract

The physonect siphonophore Halistemma amphytridis (Lesueur & Petit, 1807), previously known as Stephanomia amphytridis, was originally described in part from siphosomal material only. This paper demonstrates that the siphosome of a large mature Halistemma colony, collected during the ‘Snellius’ Expedition to Indonesia, is conspecific with two siphosomal samples of the above species taken by Bigelow on the ‘Albatross’ Expedition to the eastern tropical Pacific. Furthermore, a gap in the literature is filled herein with the first description of a H. amphytridis nectosome. The nectophores are larger and more prismatic than those of other published Halistemma species, with two vertical-lateral ridges, an incomplete lateral ridge, lateral radial canals of the nectosac more extensively looped, and a pallial canal with short diverticulum into the mesoglea. Siphosomal zooids including gastrozooids, palpons, bracts and gonophores are redescribed. The mature tentillum (on the gastrozooid tentacle) is unicornuate, with a vestigial involucrum, a cnidoband with 4–5 coils which is more extended than in other Halistemma species (except H. cupulifera Lens & van Riemsdijk, 1908), and a terminal filament without a swollen terminal process at the tip.