Abstract A new deep-sea species of Ambilimbus nom. nov. (Copepoda, Cyclopoida, Erebonasteridae) is described from the Mid-Atlantic Ridge (hydrothermal vent site Rainbow, 36°13′N 33°54′W, depth 2285 m). This is the first record for the family Erebonasteridae Humes, 1987 Humes, AG. 1987. Copepoda from deep-sea hydrothermal vents. Bulletin of Marine Science, 41: 645–788. [Web of Science ®] , [Google Scholar] from the Mid-Atlantic Ridge. Ambilimbus nom. nov. is here proposed as a replacement name for Amphicrossus Huys, 1991 Huys, R. 1991. Crustacea Copepoda: Amphicrossus pacificus gen. et sp. nov., an erebonasterid copepod (Poecilostomatoida) from the New Caledonian continental shelf. Mémoires du Muséum National d. Histoire Naturelle, 152(A): 63–77. [Google Scholar], as the latter is preoccupied by Amphicrossus Erichson, 1843 Erichson, WF. 1843. Versuch einer systematischen Einteilung der Nitidularien. German Zeitschrift fuer die Entomologie, 4: 225–361. [Google Scholar] (Insecta, Coleoptera). The new species, Ambilimbus arcuscelestis, is similar to A. pacificus (Huys, 1991 Huys, R. 1991. Crustacea Copepoda: Amphicrossus pacificus gen. et sp. nov., an erebonasterid copepod (Poecilostomatoida) from the New Caledonian continental shelf. Mémoires du Muséum National d. Histoire Naturelle, 152(A): 63–77. [Google Scholar]) comb. nov. and A. tuerkayi (Martínez Arbizu, 1999 Martínez-Arbizu, P. 1999. New Erebonasteridae (Copepoda) from Vilkitzky Strait in the Arctic and from a Pacific hydrothermal vent site (northern Fiji Basin). Journal of Crustacean Biology, 19: 93–105. [Crossref], [Web of Science ®] , [Google Scholar]) comb. nov. in having the third exopodal segments of legs 1 and 2 armed with four and five inner setae, respectively (instead of three and four inner setae as in A. spinulosus (Humes, 1989 Humes, AG. 1989. A new poecilostomatoid copepod (Erebonasteridae) from deep-sea cold seeps at the West Florida Escarpment. Hydrobiologia, 175: 175–82. [Crossref], [Web of Science ®] , [Google Scholar]) comb. nov. and A. altalis (Humes & Huys, 1992 Humes, AG and Huys, R. 1992. Copepoda (Poecilostomatoida and Siphonostomatoida) from deep-sea hydrothermal vent areas off British Columbia, including Amphicrossus altalis, a new species of Erebonasteridae, with notes on the taxonomic position of the genus Tychidion Humes. Canadian Journal of Zoology, 70: 1369–80. [Crossref], [Web of Science ®] , [Google Scholar]) comb. nov.). It differs from its four congeners by the lack of the inner coxal seta on leg 3. This character and the mid-ventral processes located between the intercoxal sclerites of legs 1–4 are presumed to be apomorphies of the new species.
Background
Copepoda is one of the most prominent higher taxa with almost 80 described species at deep-sea hydrothermal vents. The unique copepod family Dirivultidae with currently 50 described species is the most species rich invertebrate family at hydrothermal vents.
Methodology/Principal Findings
We reviewed the literature of Dirivultidae and provide a complete key to species, and map geographical and habitat specific distribution. In addition we discuss the ecology and origin of this family.
Conclusions/Significance
Dirivultidae are only present at deep-sea hydrothermal vents and along the axial summit trough of midocean ridges, with the exception of Dirivultus dentaneus found associated with Lamellibrachia species at 1125 m depth off southern California. To our current knowledge Dirivultidae are unknown from shallow-water vents, seeps, whale falls, and wood falls. They are a prominent part of all communities at vents and in certain habitat types (like sulfide chimneys colonized by pompei worms) they are the most abundant animals. They are free-living on hard substrate, mostly found in aggregations of various foundation species (e.g. alvinellids, vestimentiferans, and bivalves). Most dirivultid species colonize more than one habitat type. Dirivultids have a world-wide distribution, but most genera and species are endemic to a single biogeographic region. Their origin is unclear yet, but immigration from other deep-sea chemosynthetic habitats (stepping stone hypothesis) or from the deep-sea sediments seems unlikely, since Dirivultidae are unknown from these environments. Dirivultidae is the most species rich family and thus can be considered the most successful taxon at deep-sea vents.
Crinoids (Echinodermata) exhibit unique morphological and behavioral characteristics that facilitate a wide range of symbiotic relationships with diverse organisms. Our comprehension of their interactions with microscopic copepod crustaceans is, however, still in a nascent and fragmented state. Here, we review and discuss the 166 literature records to date in which a total of 39 copepod species in 6 families have been reported in association with 33 species of the crinoid order Comatulida. Many of these associations have been reported just once. The respective localities cover 5 of the World Ocean’s 12 ecoregions, with a notable concentration of both host and symbiont diversity in the Central and Western Indo-Pacific. In contrast, the documentation of copepod–crinoid associations in the Atlantic appears markedly limited. Copepods have been found predominantly in ectosymbiotic relationships with crinoids, with a lower incidence of endosymbiosis. Copepods of the genera Collocheres Canu, 1893 and Pseudanthessius Claus, 1889 are particularly prominent in the list, and the comatulid family Comatulidae displays the most diverse assortment of copepod associations. The current scope of knowledge encompasses a mere 5% of the potential crinoid host diversity, underscoring the need for more extensive research in this area.
Females of the new species Exrima walteri sp. nov. were found in sediment trap samples deployed over different sites of the East Pacific Rise (13°N) at 2600 m depth. Four traps were deposited during the HOPE99 cruise (1999) and recovered during the AMISTAD (1999) cruise on the research vessel ‘L'Atalante’. The new species is distinguished from congeners, E. singula Humes, 1987 and E. dolichopus Humes, 1987, by the following derived characters: first somite of the urosome with 3 (one dorsal and two lateral) stout conical extensions; distal endopodal segment on the swimming leg 4 broad. In addition, many specimens of copepodids I and lecithotrophic nauplii, identified as belonging to Dirivultidae gen. sp., were found in the samples of all sediment traps. This is the first record of copepodids I and of a nauplius of dirivultids from the Pacific Ocean. Study of type and additional material collected during different Ifremer cruises at different vent sites (HERO91, EXOMAR, PHARE and MoMARETO) required synonymy of four species of Aphotopontius Humes, 1987, Stygiopontius Humes, 1987 and Rhogobius Humes, 1987. Aphotopontius rapunculus Humes and Segonzac, 1998 is transferred to the genus Rhogobius because it possesses all presumed derived attributes of this genus: last abdominal somite with lobes at sides of anal operculum; second segment of antennal endopod elongate and slender. A new study of the type material suggests that: Aphotopontius temperatus Humes, 1997 is a synonym of A. atlanteus Humes, 1996; Stygiopontius lumiger Humes, 1989 is a synonym of S. sentifer Humes, 1987 while S. bulbisetiger Humes, 1996 is a synonym of S. pectinatus Humes, 1987. Females of the three synonymized species were found to be sub-adult females at copepodid V. Leg 6 on these specimens is one seta located dorsolaterally on the posterior part of the genital somite. This position for leg 6 is unknown for copepodid V of other siphonostomatoids whose leg 6 is located ventrally at copepodid V; the dorsolateral position is presumed derived and shared by the dirivultid genera Aphotopontius Humes, 1987 and Stygiopontius Humes, 1987. A new key to the Dirivultidae genera is presented.
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