From June to September 2022, five colonies of the rare hydrozoan siphonophore Rhizophysa eysenhardtii were observed for the first time near the surface in False Bay, South Africa. The two colonies in June/July were small with all tentacles contracted, while the September colonies were larger, the largest up to 0.8 m long (in video), with most of its tentacles extended for feeding. In this species, tentacles are typically pink and each arises from the proximal end of a gastrozooid, which engulfs and digests the prey. Fish larvae were noted in the gastrozooids and counted and a chaetognath was observed stuck to one of the tentacles. The pneumatophore was prominent in all specimens and gonodendra bearing sexual gonophores were visible between the gastrozooids of the larger specimens.

Integrated use of molecular and morphological methods reveals unexpected diversity in the cosmopolitan siphonophore genus Nanomia. Species delimitation analyses based on COI and 16S sequences suggest up to three distinct lineages in addition to the previously accepted Nanomia bijuga (Delle Chiaje, 1844) and N. cara A. Agassiz, 1865. Here, we describe the North Pacific Nanomia septata sp. n., previously confused with both N. cara and N. bijuga, and provide improved morphological characters for the identification of these three Nanomia species. Phylogenetic analyses suggest two additional, hitherto undescribed clades from Japanese and Chinese waters, respectively, but the lack of morphological material prevents describing these putative species. The geographic distribution of molecularly and/or morphologically verified observations confirm a warm circumglobal distribution for N. bijuga and a boreal North Atlantic distribution for N. cara. Interestingly, four distinct lineages occur in the North Pacific, sometimes in close proximity. These contrasting patterns of distribution raise questions about pelagic speciation processes.Nanomia septata sp. n.: urn:lsid:zoobank.org:act:DAF15EA3-AFEA-4AE8-984F-BDFBCFE7E514urn:lsid:zoobank.org:pub:478049FC-F672-4D34-ABAE-CF4345EC64D7

A colony of the rare hydrozoan siphonophore Lilyopsis Chun, 1885, was observed for the first time in shallow water in False Bay, South Africa, swimming amongst kelp. A study of a high-quality image of this individual found it to share some characters with the prayine prayid L. fluoracantha Haddock, Dunn & Pugh, 2005, so far known only from Monterey Bay, California, in the eastern Pacific. No Lilyopsis species has previously been reliably identified from either the South Atlantic or the Indian Ocean, so this record represents an expansion of the known worldwide distribution for this genus.

The rarely observed cystonect siphonophore Bathyphysa conifera was recorded for the first time in shallow depth water (- 16 m) as a live specimen, at the entrance to the Mediterranean Sea by SCUBA divers. It is a delicate oceanic species, with earlier records coming mostly from deep water, where it readily adheres to deep sea fishing cables and nets, causing painful stings to fishermen. Deep water sightings from ROVs include in the North Atlantic, off Angola, the Gulf of Mexico and Monterey Canyon. The present specimen was swimming actively by contracting and expanding its stem, in a yo-yo movement. A review of all reliable records for this species worldwide has been carried out in order to gain a better knowledge of the present known distribution of this species, both geographical and bathymetric. Bathyphysa conifera may possibly represent an important component of the food web and be perhaps also a competitor to fish in the regions it inhabits.

The rare short-stemmed physonect Melophysa melo typically lives above the thermocline in warm waters of the world's oceans. In the past this species has been described from fragmented or distorted material, with the last two accounts being published in 1931 and 1954. A new description is given herein with pertinent figures based on nine samples recently collected in the Gulf of Mexico (GoM), including the first detailed illustrations of mature nectophores, and of a well-developed corm. The corm comprises a prominent pneumatophore, a narrow nectosome bearing small attachment lamellae for four nectophores and a much larger short, swollen spiral siphosome bearing eight cormidia. The last five cormidia of this corm are still attached to the pneumatophore, while the oldest three are free, with cormidium one being the oldest and eight the youngest. Two rings of bracteal muscular lamellae occur on the siphosome of this corm, one on the upper surface of each cormidium and a second on the lower surface. Laterally each cormidium supports an upper gonodendron, a zooid meridian bearing palpons and a lower gonodendron. Cormidium one has the most mature gonodendra, and all cormidia have palpons on the zooid meridian, which become progressively mature towards the base. Differences in corm structure from previously described corms are attributed to the younger age of the present two corms. Bracts are re-described from photographic images of both young and mature bracts.

Benthic siphonophores of the family Rhodaliidae (Cnidaria, Hydrozoa) are extremely fragile, difficult to collect, and therefore little studied. Only a few records exist worldwide, so their ecology remains largely unknown. Rhodaliids have been found at most depth horizons, but until now were believed to mainly inhabit deeper water over continental shelves, with only a few records from ca. 100 m. In this paper, a new rhodaliid is described based on an underwater photograph provided by a recreational diver via Facebook. This observation was made in Milne Bay (Papua New Guinea) at a depth of 26–27 m, and constitutes the shallowest record so far for any rhodaliid. The specimen was tentatively identified as Archangelopsis typica based on observable morphological characters and an approximate estimate of connectivity between all rhodaliid species in the Indo-Pacific region. Additionally, we highlight the scientific potential of citizen science.

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 degrees N and 26 degrees 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 individualsm-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.

Siphonophores are the most complex of all pelagic medusozoan hydrozoan cnidarians, bearing various types of zooids on a long stem and often termed "string jellyfish." They are extremely fragile and live almost exclusively in the open ocean. They vary in length from 50 m down to 10-20 mm. Most species bear swimming bells (nectophores) for locomotion, some have a float (pneumatophore), and all have a long stem of iterative units termed cormidia for feeding, reproduction, and also protection and buoyancy. Tentacles from the cormidia bear stinging cells (nematocysts) for prey capture, either in simple groups or lines or in more complex nematocyst batteries on side branches known as tentilla. In life, tentacles and their side branches extend into a three-dimensional net for fishing, into which prey either blunders by accident or, in a few species, is attracted by lures. Such great diversity has led to a complex systematics based on a range of morphological characters, recently enhanced by the first molecular study of the group. From this a new phylogeny has been proposed, for 17 valid families (one semi-benthic) and 177 valid species (some unassigned). Characters of these families are reviewed in two tables and 17 summaries, including diagnostic characters, number and variety of species, and, where appropriate, habitat preferences and relative success in today's seas. Figures and images showing different types of siphonophores, their morphology, stinging organs, and appearance in life accompany the main text.

Siphonophores are at times amongst the most abundant invertebrate zooplankton predators in the oceans. Historically, siphonophores have been under-sampled and of the studies conducted there has been a bias towards oceanic oligotrophic waters where they are considered to be more important. In temperate coastal regions, comparatively less is known about the diversity and abundance of siphonophores, where periodic blooms can restructure the plankton communities and have been correlated with high mortalities in the salmon aquaculture industry. To address this lack of knowledge, plankton samples were collected during two periods (March 2009-March 2011 and April 2014-November 2015) from a coastal embayment in the southwest of Ireland. In total, three siphonophore species were found, the calycophoran Muggiaea atlantica, and the physonects, Nanomia bijuga and Agalma elegans. Muggiaea atlantica was the most abundant species (250 colonies m−3), with densities an order of magnitude higher than either physonect. Muggiaea atlantica displayed a distinct seasonality, whereas the physonect species were sporadic in occurrence. Comparing siphonophores in Bantry Bay and the Western English Channel (Plymouth Marine Laboratory's L4 station) indicates both regions share a similar pattern of inter-annual occurrence and provides novel information on the seasonality and occurrence of siphonophores in Irish coastal waters.

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.

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

In this review the history of discovery of siphonophores, from the first formal description by Carl Linnaeus in 1785 to the present, is summarized, and species richness together with a summary of world-wide distribution of this pelagic group within the clade Hydrozoa discussed. Siphonophores exhibit three basic body plans which are briefly explained and figured, whilst other atypical body plans are also noted. Currently, 175 valid siphonophore species are recognized in the latest WoRMS world list, including 16 families and 65 genera. Much new information since the last review in 1987 is revealed from the first molecular analysis of the group, enabling identification of some new morphological characters diagnostic for physonect siphonophores. Ten types of nematocysts (stinging cells) are identified in siphonophores, more than in any other cnidarian; these are incorporated into batteries in the side branches of the tentacles in most species (here termed tentilla), and tentilla are reviewed in the last section of this paper. Their discharge mechanisms are explained and also how the tentilla of several physonect siphonophores are modified into lures. Of particular interest is the recent discovery of a previously unknown red fluorescent lure in the tentilla of the deep sea physonect Erenna, the first described example of emission of red light by an invertebrate to attract prey.

Dromalia alexandri is a short-stemmed physonect siphonophore found regularly in bottom trawls from the shelf waters of southern California, including biannual samples taken to monitor sewage outflow from Los Angeles and San Diego. Since the last description of the species in 1983, a greater size range of specimens is here examined, some with particularly well preserved cormidial units and detached zooids. Higher abundance and larger sizes of D. alexandri specimens collected from the Southern California Bight might be due to increased nutrient levels derived from sewage outflow. Small and large colonies are illustrated and described, and two growth zones identified in the zone of proliferation, including the first siphosomal horn to be identified in a rhodaliid. Nectophores of D. alexandri are found to lack a descending surface diverticulum, and nectophoral lamellae were identified in the naked zone below the aurophore in two specimens. In the largest colony studied, a larger number of young cormidial units are present at the beginning of the first whorl than previously reported, giving a total of c. 57 in whorl one, and in the smallest specimen whorl one has only 10 units, fewer than previously found. Much larger and more complete cormidial units of D. alexandri are figured and described than previously published, as are also a detached mature bract and mature male and female gonophores. Cormidial units are found to comprise only three full cormidia (composed of a gastrozooid, bract, and a gonodendron with gonophores and gonopalpons) when mature, and typically one or two additional bracts. Descriptions of the colony and zooids are based on axes for siphonophores standardized by recent authors and applied here for the first time to D. alexandri. A revised distributional map of the species is included covering a larger geographical area than earlier maps, and based on a greater number of records than previously available. The paper concludes with a comparison and discussion of the main characters of D. alexandri and those of five other species in the family Rhodaliidae.

BACKGROUND:The question of how many marine species exist is important because it provides a metric for how much we do and do not know about life in the oceans. We have compiled the first register of the marine species of the world and used this baseline to estimate how many more species, partitioned among all major eukaryotic groups, may be discovered. RESULTS:There are ∼226,000 eukaryotic marine species described. More species were described in the past decade (∼20,000) than in any previous one. The number of authors describing new species has been increasing at a faster rate than the number of new species described in the past six decades. We report that there are ∼170,000 synonyms, that 58,000-72,000 species are collected but not yet described, and that 482,000-741,000 more species have yet to be sampled. Molecular methods may add tens of thousands of cryptic species. Thus, there may be 0.7-1.0 million marine species. Past rates of description of new species indicate there may be 0.5 ± 0.2 million marine species. On average 37% (median 31%) of species in over 100 recent field studies around the world might be new to science. CONCLUSIONS:Currently, between one-third and two-thirds of marine species may be undescribed, and previous estimates of there being well over one million marine species appear highly unlikely. More species than ever before are being described annually by an increasing number of authors. If the current trend continues, most species will be discovered this century.

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.

This updated re-description of the praline 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.

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.

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.