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Visual comparison for three genera of “little red jellies” provided for assistance with identification of genera, each image is a unique specimen: Crossota (A1-3), Pectis (B1-3), and Benthocodon (C1-3). Top row, left to right: in situ images for C. rufobrunnea (A1—DR457 @ 968 m in Monterey Bay), P. profundicola (B1—T622 @ 777 m in Monterey Bay), and B. pedunculatus (C1—T1113 @ 3,483 m in Monterey Bay, California) taken by MBARI ROVs. Of the three genera, only Pectis (B1) has centripetal canals and this individual has 8 in each quadrant, irregular in size and just above the canal ring. Row 2 (A2,B2,C2) provides closeup and partial dissections taken in lab for different individuals of C. rufobrunnea (A2—T1023SS1 @ 955 m in Astoria Canyon, Oregon), P. profundicola (B2—T1023SS2 @ 1,117 m in Astoria Canyon, Oregon), and B. pedunculatus (C2—T623b @ 3,607 m in Monterey Bay, California). Note the pendant gonads and absent peduncle of Crossota (A2), and the semi-pendant gonads and short peduncle for Pectis (B2). Benthocodon (C2) shows the folded, per-radial lips, tentacular arrangement, and well-developed velum. See Figure 4 for a better view of the peduncle for which Benthocodon pedunculatus is named. Row 3 (A3,B3,C3) provides a dorsal view of the subumbrella region. The subumbrella for C. rufobrunnea (A3—T766 @ 1,375 m in Monterey Bay, California), unlike both P. profundicola (B3—T834SS9 @ 1,384 m in Monterey Bay, California), and B. pedunculatus (C3—T835SS5 @ 3,285 m in Monterey Bay, California), appears closed and does not allow a view through clear mesoglea into the peduncle.
Source publication
Several species of small, red, deep-sea Trachymedusae have been described and then re-described over the past 20 years, leading to some confusion in the scientific literature. This paper provides an overview of three genera (Benthocodon, Crossota, and Pectis) in the family Rhopalonematidae (Cnidaria: Hydrozoa) that have been observed and examined b...
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Citations
Medusozoans are diverse in species number (4100) and life stages and are of ecological and social importance worldwide. However, studies of medusozoans in the Mexican Pacific (MP) are limited and scattered. Given that its maritime (2,320,380 km2) and coastal (7828 km) regions are the most extensive in Mexico, as well as its geomorphological and ecosystem complexity, the medusozoan fauna of the MP needs better documentation. To that end, this work summarises medusozoan diversity data of the MP based on a review of refereed publications (1897-2023) and the examination of recently collected specimens, field observations, and photographs (2015-2022). Information gathered from the literature search and the collections was compiled into an annotated list of species. As a result, 423 accepted medusozoan species were found in the MP, corresponding to 10% of the 4100 medusozoan species known worldwide. This study highlights three important decades of published work: taxonomic works in the 1930s, ecological works in the 1980s, and recognition of biodiversity under multidisciplinary works in the 2010s. Of the taxa collected in the present work, Cirrholovenia sp., Linuche sp., and Monotheca flexuosa are new records for the MP. Furthermore, Hydrocoryne sp. and Coryne pusilla are new records for the Gulf of California.
The observation of singleton or rare species in the deep sea is extremely valuable for gaining a census of biodiversity. At hadal depths (> 6000 m), these records provide a more complete picture of the vertical distribution of fauna. In this study, we present new in situ video records for Trachymedusae (Hydrozoa), Ascidiacea (Tunicata), and Tentaculata (Ctenophora) taken by submersible and supporting landers in the western Pacific Ocean. Together, these three taxonomic groups are present at depths far deeper than previously known. Observations of the rhopalonematid trachymedusa ( Pectis cf. profundicola ) from both lander and submersible dives at 10,063 and 10,040 m in the Philippine Trench (NW Pacific Ocean) extend the maximum depth of the Hydrozoa by a further 997 m and is the first record of Hydrozoa > 10,000 m. The predatory tunicate Octacnemidae sp. was observed nine times at 7799 m in the Mariana Trench and once at 8077 m in the Izu-Ogasawara Trench (NW Pacific Ocean), therefore extending its maximum depth by 1726 m, and 1002 m to historical accounts of what might also be in the Phlebobranchia order of Ascidiacea. Several observations of large, globular ctenophores with long filamentous tentacles and lacking oral lobes from 10,040 m in the Kermadec Trench (SW Pacific Ocean) increase the maximum depth of the Ctenophora by 2823 m and is the first record of Ctenophora > 10,000 m. Benthic ctenophores were also recorded from the Japan Trench at 8001 m, extending their known depth range by a further 2750 m.
Introduction
Botrynema, a genus of medusozoans in the trachyline family Halicreatidae, currently contains two species: B. brucei and B. ellinorae, distinguished by the presence or absence, respectively, of an apical knob as a diagnostic character. However, no study has corroborated if these taxonomic diagnoses have a biological and evolutionary basis. Therefore, in this study we attempted to address the question “do the two nominal species in the genus Botrynema represent independent phylogenetic lineages, or two phenotypic variants of a single species?
Methods
In this study we took advantage of legacy collections from different research expeditions across the globe from 2000 to 2021 to study the phylogenetics and taxonomy of the genus Botrynema.
Results
B. brucei and B. ellinorae present partially overlapping vertical distributions in the Arctic and as a whole in the Arctic the genus seems to be limited to the Atlantic water masses. The phylogenetic reconstruction based on the concatenated alignment corroborates the validity of the family Halicreatidae and of genus Botrynema as monophyletic groups. However no clear differentiation was found between the two presently accepted species, B. ellinorae and B. brucei.
Discussion
Based on the evidence we gathered, we conclude that while the genus Botrynema does contain at least two species lineages, these lineages are not concordant with current species definitions. The species B. ellinorae is reassigned as a subspecies of B. brucei and diagnostic characters are provided.
A list of genera represented in the hadal zone (>6000 m), 22 trenches, two trench faults, and two basins was compiled for Tanaidacea, Isopoda, Bivalvia and five classes of Echinodermata. Among these 161 genera, hadal endemics comprised only 4%, which is lower than the previous estimate. Geographic and bathymetric distributions were examined in four groups of genera based on their lower bathymetric distribution limits: i) 6001–7000 m, ii) 7001–8000 m, iii) 8001–9000 m and iv) greater than 9000 m. These groups were compared by the percentage of genera with a narrow and wide geographical distribution in the hadal zone and in the world ocean from intertidal to hadal depths. Most of the hadal genera were characterized by a wide geographic range at shallower depths and a narrow geographic range in the hadal zone. Almost half of the 161 genera considered in this study had lower limits at 6001–7000 m. The percentage of narrowly distributed in the hadal genera found only in one trench decreases from 72% to 4% from the first to fourth group, whereas the proportion of widely distributed genera known from four or more trenches increases from 3% to 70%. Lower limit of the vertical distribution is positively correlated with the number of trenches and other hadal areas where the genera occurred. The wide distribution of the deepest-dwelling genera at hadal depths suggests that the most successful colonization of the hadal zone results from the high tolerance of some genera to multiple factors, including hydrostatic pressure, food distribution, topography, and potential ecological interactions. The lower hadal zone seems to be inhabited mainly by ubiquists.
Although members of the Rhopalonematidae family (Cnidaria, Hydrozoa, Trachymedusae) are known to exhibit unusually powerful jet swimming in addition to their more normal slow swimming behaviour, for the most part, reports are rare and anecdotal. Many species are found globally at depths of 600–2000 m, and so observation and collection depend on using remotely operated submersible vehicles. With a combination of in situ video footage and laboratory measurements, we have quantified kinematic aspects of this dual swimming motion and its electrophysiology. The species included are from two Rhopalonematidae clades; they are Colobonema sericeum, Pantachogon haeckeli, Crossota millsae and two species of Benthocodon. Comparison is made with Aglantha digitale, a species from a third Rhopalonematidae clade brought to the surface by natural water movement. We find that although all Rhopalonematidae appear to have two swimming modes, there are marked differences in their neural anatomy, kinematics and physiology. Giant motor axons, known to conduct impulses during fast swimming in A. digitale, are absent from C. sericeum and P. haeckeli. Slow swimming is also different; in C. sericeum and its relatives it is driven by contractions restricted to the base of the bell, whereas in A. digitale it is driven by contractions in the mid-bell region. These behavioural differences are related to the position of the different clades on a ribosomal DNA-based phylogenetic tree. This finding allows us to pinpoint the phylogenetic branch point leading to the appearance of giant motor axons and escape swimming. They place the remarkable dual swimming behaviour of members of the Rhopalonematidae family into an evolutionary context.
Hydrozoa, Scyphozoa, Larvacea (Appendicularia) and Ctenophora are not typically associated with hadal communities. Here, we report observations of these groups based on 136 benthic camera lander deployments that spanned all five oceans, encompassing 14 deep sites, culminating in >1000 h of video in the near-bottom waters between 5000 and 10 925 m. Of the Hydrozoa, trachymedusae had a maximum depth of 9066 m in the Mariana Trench, narcomedusae were recorded to a maximum depth of 7220 m in the San Cristobal Trench and a single siphonophore was seen at 7888 m in the Mariana Trench. Scyphozoans were seen as deep as 6898 m in the New Hebrides Trench. The deepest ctenophore was seen at 6037 m in the Kermadec Trench. Larvaceans were seen in the Agulhas Fracture Zone and the Puerto Rico, Kermadec, South Shetland and Java trenches, with the deepest being 7176 m in the Java Trench. None of these groups were seen in the deep Arctic or Antarctic deeper than 6000 m. Narcomedusae, siphonophorae, Scyphozoa and Ctenophora appear very rare at hadal depths, while the larvaceans and trachymedusae appear to be relatively conspicuous in the benthopelagic at hadal depths.
Gelatinous zooplankton are prominent fauna in the deep ocean, especially in the water column associated with the sea floor. This interface zone, the benthic boundary layer, is critical to biogeochemical exchanges between pelagic and benthic communities. A conspicuous member of this benthic boundary layer community in the abyssal Northeast Pacific (Sta. M; 4000 m depth) is the hydrozoan medusa, Benthocodon pedunculata. A 3-year time-series study was conducted of B. pedunculata from October 2014 to November 2017 using an autonomous time-lapse camera system and acoustic current meter deployed on the sea floor. Remotely operated vehicle benthic video transects and collections were also conducted during six servicing cruises for the long-term instruments. Over three years, a total of 123,887 B. pedunculata were recorded in hourly images with densities ranging from 0 to a high of 33 individuals m⁻³ in March 2016. Density of B. pedunculata was highest when current speeds were low, and declined precipitously as speed increased beyond ~1 cm s⁻¹. Movement of B. pedunculata was parallel to the continental margin in northwest and southeast directions. The large numbers of B. pedunculata observed over this 3-year study suggests their ecological importance in the benthic boundary layer food web at Sta. M.
