Gillian M. Mapstone

Trends in the Diversity, Distribution and Life History Strategy of Arctic Hydrozoa (Cnidaria)

This is the first attempt to compile a comprehensive and updated species list for Hydrozoa in the Arctic, encompassing both hydroid and medusa stages and including Siphonophorae. We address the hypothesis that the presence of a pelagic stage (holo- or meroplanktonic) was not necessary to successfully recolonize the Arctic by Hydrozoa after the Last Glacial Maximum. Presence-absence data of Hydrozoa in the Arctic were prepared on the basis of historical and present-day literature. The Arctic was divided into ecoregions. Species were grouped into distributional categories according to their worldwide occurrences. Each species was classified according to life history strategy. The similarity of species composition among regions was calculated with the Bray-Curtis index. Average and variation in taxonomic distinctness were used to measure diversity at the taxonomic level. A total of 268 species were recorded. Arctic-boreal species were the most common and dominated each studied region. Nineteen percent of species were restricted to the Arctic. There was a predominance of benthic species over holo- and meroplanktonic species. Arctic, Arctic-Boreal and Boreal species were mostly benthic, while widely distributed species more frequently possessed a pelagic stage. Our results support hypothesis that the presence of a pelagic stage (holo- or meroplanktonic) was not necessary to successfully recolonize the Arctic. The predominance of benthic Hydrozoa suggests that the Arctic could have been colonised after the Last Glacial Maximum by hydroids rafting on floating substrata or recolonising from glacial refugia.

Redescriptions of two physonect siphonophores, Apolemia uvaria (Lesueur, 1815) and Tottonia contorta Margulis, 1976, with comments on a third species Ramosia vitiazi Stepanjants, 1967 (Cnidaria: Hydrozoa: Apolemiidae)

Abstract Two species referable to the physonect siphonophore family Apolemiidae are re‐described: Apolemia uvaria (Lesueur, 1815), the type species, and Tottonia contorta Margulis, 1976. Descriptions, based mainly on three colonies from the Mediterranean (A. uvaria) and three taken off California (T. contorta) contain new information on some zooids and update previous information on others. Figures show the arrangement of nectophores on the nectosomal stem, budding zones, possible degenerative zones and all zooid types (except gonophores in A. uvaria). The morphology of these two species is discussed in relation to a third species Ramosia vitiazi Stepanjants, 1967. These descriptions provide a benchmark for up to 10 putative new species of apolemiids believed to exist but not yet described.

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

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.

Correction: Global Diversity and Review of Siphonophorae (Cnidaria: Hydrozoa)

[This corrects the article DOI: 10.1371/journal.pone.0087737.].

Two deep-living rhodaliids (Cnidaria, Siphonophora) from the Mid-Atlantic Ridge

ABSTRACT Rhodaliids are a semi-benthic family of 14 physonect siphonophore species found in all oceans, except the Mediterranean and Arctic. They inhabit species-specific depth ranges in isolated locations and records are mostly sparse. Here, the first ever observations of rhodaliids from the Mid-Atlantic Ridge are given, from three images of two putative species. They come from depths of 3482 and 3667–3670 metres, on the Mid-Atlantic Ridge (MAR) at 23°N and 26°N, near two hydrothermal vents. Rhodaliids are very delicate animals and extremely hard to sample, particularly from such great depths, and the only comparable deep-living species so far described is the Galapagos Dandelion (Thermopalia taraxaca) from 2480–2938 metres on the Galapagos Rift and East Pacific Rise. This species inhabits the outer zone of certain hydrothermal vents where there is no hydrogen sulphide in the water, but connectivity with the MAR species is unlikely, since the latter inhabit a different ocean basin. Two, or possibly three, rhodaliid species have so far been collected in the North Atlantic, and all occurred near continental margins. These new observations are therefore important, and the images included here will hopefully alert future expeditions to hunt for, and perhaps even collect, more specimens. Rhodaliids are mostly observed individually adhering to a variety of substrata with their long tentacles, but very occasionally abundances of from 1 to 11 individuals m−2 have been noted. Rhodaliids feed on copepods, other small pelagic crustaceans and fish larvae, and can thus represent important members of deep-sea ecosystems. This paper provides a distribution map of all species with an accompanying table showing coordinates, depths and number of specimens collected, and a second table of comparative diagnostic rhodaliid characters, which is used to suggest possible identities for the two putative new MAR species.

Occurrence of the physonect siphonophore Apolemia uvaria off Plymouth and in south-west England

In September 2007, observations were made of a siphonophore in surface waters and near to the seabed by sea users off south Devon and south-east Cornwall. The same siphonophore was also recorded from regular samples collected offshore of Plymouth. The species is identified as Apolemia uvaria, which had not previously been recorded off Plymouth. It was sampled until March 2008 and re-appeared, in smaller numbers, in autumn 2008 until February 2009 but was not reliably reported in autumn 2009 (to end of October). The occurrence is unlikely to be due to sea warming, but more likely some variation in oceanic currents, possibly influxes of Atlantic water

Re-description of Rosacea cymbiformis, a prayine siphonophore (from the Mediterranean Sea), with comments on nectophore designation and bract orientation

This updated re-description of the prayine siphonophore Rosacea cymbiformis includes figures of all zooids (except larval nectophores) and is based on material held in the collections of the Natural History Museum (NHM), London. Rosacea cymbiformis was originally described in 1830 under the name Physalia cymbiformis,, and subsequently reported many times during the 19th Century. However, during the 20th Century it was confused with the closely related species R. plicata, and the two species are still not clearly differentiated. Previous descriptions are reviewed herein, including conflicting interpretations of nectophore designation in R. plicata, and bract orientation in R. cymbiformis and R. plicata. To identify these siphonophores to species level and separate them from other closely related prayines, it is essential to distinguish the first definitive nectophore from the second, and the right paired bracteal canals from the left canals. This becomes critical when only detached siphonophore zooids are available, as for example, in plankton samples collected with nets. A summary of the differences between R. cymbiformis and the five other currently recognized Rosacea species, R. plicata, R. repanda, R. limbata, R. flaccida and R. arabiana, is presented. The full synonymy of R. cymbiformis is too long for inclusion here and is deferred to a later paper.