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Gnathophausia Childressi, New Species, a Mysid from Deep Near-Bottom Waters Off California, with Remarks on the Mouthparts of the Genus Gnathophausia

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Gnathophausia Childressi, New Species, a Mysid from Deep Near-Bottom Waters Off California, with Remarks on the Mouthparts of the Genus Gnathophausia GNATHOPHAUSIA CHILDRESSI, NEW SPECIES, A MYSID FROM DEEP NEAR-BOTTOM WATERS OFF CALIFORNIA, WITH REMARKS ON THE MOUTHPARTS OF THE GENUS GNATHOPHAUSIA Jean-Paul Casanova ABSTRACT Gnathophausia childressi, new species (Mysidacea: Lophogastrida), discovered by Dr. J. J. Downloaded from https://academic.oup.com/jcb/article/16/1/192/2418789 by guest on 02 October 2021 Childress in 1985, is described. It was caught in the benthic boundary layer (BBL), in the deepest parts of the San Clemente Basin (at about 2,000 m), and has also been observed in the adjacent East Cortes Basin (about 1,800 m), where it has never been found in pelagic trawls fishing to depths of 1,500 m. It is closely allied to the rarest species of the genus, G. affinis, known only from the Atlantic at depths of 2,100-2,700 m, being in a manner its Pacific twin. Gnathophausia affinis apparently has the same benthopelagic habitat, which perhaps explains why it is rarely sampled. A noticeable reduction of the mandibles of G. childressi is an adap- tation to this habitat. The paragnaths of all species of Gnathophausia are asymmetrical, lying closely against the posterior face of the mandibles. The left paragnath has molariform processes and it has been said that it is involved in mastication in cooperation with the movements of the left mandible. In fact, the two paragnaths are more probably involved in this function by their own musculature, perhaps a reminiscence of an ancestral function. They are not generally considered as appendages, but this is now questionable. Recently, Dr. J. J. Childress asked me to ton, D.C. (USNM 268483 and 268484, respectively). describe an unknown species of Gnatho- One paratype (female) is deposited in the Museum Na- tional d'Histoire Naturelle, Paris (MNHN-My 480). phausia (Mysidacea: Lophogastrida) which Their characteristics, together with the collecting sta- he had collected many years ago in the tions, are indicated in Table 1. deepest parts (about 2,000 m) of the San Clemente Basin (off California) in a few m Description.-Only characters useful for above the bottom, in the benthic boundary identification will be described, with certain layer (BBL). exceptions; others, such as most of the ap- The five specimens examined came from pendages, which are much the same as in three separate collections with two different other species, are not described. trawls (Table 1), covering distances of sev- General form rather robust for largest eral km. Two of the collections were made specimen (mature female). Body length, with an epibenthic beam trawl or shrimp measured from eye notch of carapace to end trawl (its description is in Childress, 1985). of telson, from 25.5-102 mm. The third collection was made with an Carapace more or less covering first ab- opening-closing midwater trawl (its de- dominal segment (Fig. 1 A). Dorsal spine scription is in Childress et al., 1978) fishing short. Posterior angles rounded. Supraorbi- close to the sea bed. Since the trawls were tal spines not very developed. Antennal designed to capture deep-sea crustaceans spines absent and branchiostegal spines in- alive, the specimens are in very good con- conspicuous. Rostrum of moderate size, ap- dition. A sixth specimen was lost (Table 1) parently shorter in largest specimen, al- and at least two others were captured just though its tip broken. Rostrum provided above the bottom with the submersible RV dorsally with numerous small denticles to Alvin and used for metabolic rate and C and level of eyes; armature hardly visible ven- N measurements (Childress et al., 1989). trally, present only on its distal part to level of extremity of antennular peduncles. Ros- Gnathophausia childressi, new species trum leading abruptly into rather high sharp Figs. 1, 2 dorsal crest on carapace. Dorsal keel begin- Type Series.-The holotype (mature female) and the ning not far from rostrum. Only 1 lateral allotype (male) are deposited in the National Museum keel, on lower part of carapace, disappear- of Natural History, Smithsonian Institution, Washing- ing at short distance of marginal keel run- ning along inferoposterior corner of cara- pace. Between beginning of both rostrum and lateral keel, a vertical groove. Another groove, curved and more marked, stretch- ing from anterior part of lateral keel to al- most middle of dorsal keel. Antennal scales divided into 2 parts, with apical spine on outer margin of proximal part reaching about middle of distal part (Fig. 1 B). This spine as long as distal part in small specimen. Downloaded from https://academic.oup.com/jcb/article/16/1/192/2418789 by guest on 02 October 2021 Mandibles asymmetrical, with cutting part longer and less chitinous than molar part (Fig. 2A). Paragnaths asymmetrical, left one provided with more or less chitin- ous tubercles and ribs on inner side (Fig. 2B, C). Maxillipeds with vestigial exopod, reduced to very small triangular expansion with apical seta (Fig. 2E, F). Abdominal segments rounded dorsally, without posterior dorsal spine (Fig. 1 A). Second segment with transverse dorsal groove. Pleura not very developed, each with anterior lappet smaller than posterior one. In small specimen, only latter present and acutely pointed from segments 2-5. In 3 medium-sized specimens, only posterior lappet of segment 5 with spine. In largest specimen, this spine very reduced. Sixth segment with pleura as lamina increasing regularly in width, rounded at its ending at level of pseudoarticulation dividing seg- ment exteriorly into 2 parts; posterior part with lateral lamellar processes provided with 2 spines. Uropods as long as telson, without spines on inner edge of their basal segment. Shape and ornamentation of telson as in most spe- cies in genus; elongated hollow occupying its upper face proximally very narrow (Fig. 1 C). Etymology.-This species is named after Dr. J. J. Childress who gave me the speci- mens and who has done extensive research on the genus Gnathophausia. Comparison with Other Species.-The sev- en well-separated species of Gnathophau- sia, easily distinguishable by many evident characters, were all described before the end of the last century. However, two dis- tinct, although closely related, species have been confused under the name G. elegans; they are now separated (Casanova, in press). Downloaded from https://academic.oup.com/jcb/article/16/1/192/2418789 by guest on 02 October 2021 Fig. 1. Gnathophausia childre,s.ri, new species. A, left lateral view; B, dorsal view of right antenna] scale; C, dorsal view of telson showing channeled area (arrows). Downloaded from https://academic.oup.com/jcb/article/16/1/192/2418789 by guest on 02 October 2021 Fig. 2. Gnathophausia childressi, new species. A, inner lateral view of right mandible (X50); B, C, ventrolateral (X50) and inner lateral view (X70) of left paragnath (arrows show molarlike processes); D, detail of abraded ribbon setae constituting molarlike processes of same paragnath (x 1,200); E, right maxilliped exhibiting reduced palp (arrow) (X20); F, enlargement of palp of right maxilliped (X 150). Sars (1885) suggested the recognition of presence of three species of obvious large three groups of species and Fage (1941) de- pelagic crustaceans within the 10 m im- fined these three groups having a few char- mediately above the bottom; one of them acters in common. The new species may be was Gnathophausia childressi. They fo- placed in one of these groups: (1) Gnatho- cused attention on the fact that none of phausia gigas Willemoes-Suhm, 1873, and these species had been previously sampled G. ingens (Dohrn, 1870); (2) Gnathophau- with midwater trawls in these basins, in sia gracilis Willemoes-Suhm, 1875; and (3) spite of hundreds of hours of fishing at Gnathophausia zoea Willemoes-Suhm, depths to 1,500 m. Bottom depths ranged 1873, G. longispina G. 0. Sars, 1883, G. from 1,880-1,940 m in the San Clemente elegans G. O. Sars, 1883, G. affinis G. O. Basin and from 1,708-1,860 m in the East Downloaded from https://academic.oup.com/jcb/article/16/1/192/2418789 by guest on 02 October 2021 Sars, 1883, G. fagei Casanova, in press, and Cortes Basin. G. childressi, new species. According to Childress et al. (1989), it is Gnathophausia childressi belongs to the beyond doubt that Gnathophausia child- third group, owing to the morphology of the ressi is tightly linked with the deep benthic antennal scale, the absence of a spine on boundary layer or BBL, a water mass over- the posterior corners of the carapace, free lying the sea bed, characterized by a higher pleura on the sixth abdominal segment, the concentration of suspended particulate mat- dorsal keel of the carapace roughly contin- ter than in the water column above (Wish- uous with the upper edge of the rostrum ner and Gowing, 1987). They also noticed (there is only a small interruption), the oc- that these three species were not observed ular spines well developed, and the inner in the nearby Santa Catalina Basin and that edge of the basal segment of the uropods its shallower depth (about 1,300 m), rather smooth. Another characteristic of the spe- than the gear used, could be involved. They cies of this group was originally "devoid of emphasized that the dominant forms were palp on the maxillipeds"; now, this state- different and that, according to Smith ment must be expressed as: "no palp or (1982), the biomass in the Santa Catalina Basin was a half to a third of the values of vestigial palp on the maxillipeds." the two other basins. If the last statements Among the species of the third group, G. childressi is very closely allied to G. affinis, are the result of different hydrological con- the rarest species of the genus, known only ditions, then the shallow depth of the Santa from the Atlantic.
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