A new species ofExrima,synonymy of four species ofAphotopontius, StygiopontiusandRhogobius, and record of first copepodid stage of Dirivultidae (Copepoda: Siphonostomatoida) from deep-sea hydrothermal vents of the East Pacific Rise (13°N)

Abstract

Females of the new speciesExrima walterisp. 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. singulaHumes, 1987 andE. dolichopusHumes, 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 ofAphotopontiusHumes, 1987,StygiopontiusHumes, 1987 andRhogobiusHumes, 1987.Aphotopontius rapunculusHumes and Segonzac, 1998 is transferred to the genusRhogobiusbecause 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 temperatusHumes, 1997 is a synonym ofA. atlanteusHumes, 1996;Stygiopontius lumigerHumes, 1989 is a synonym ofS. sentiferHumes, 1987 whileS. bulbisetigerHumes, 1996 is a synonym ofS. pectinatusHumes, 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 generaAphotopontiusHumes, 1987 andStygiopontiusHumes, 1987. A new key to the Dirivultidae genera is presented.