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. It differs, however, by the Catalina Basin may be sufficient to explain following characters: (1) presence of only the absence of large crustaceans like G. childressi. one lateral keel on carapace instead of two (upper lateral one missing); (2) carapace The particular habitat of G. childressi comparatively longer, since it always covers now allows us to understand the scarcity of last segment of trunk; (3) pleura of sixth its allied G. affinis. The latter species is abdominal segment rounded posteriorly in- known only from 3 specimens dredged on stead of ending in spine; (4) pleura of ab- the bottom of the deep Atlantic: between Africa and Brazil at 2,744 m (Sars, 1883), dominal segments 1-3 with anterior lappet off Morocco at 2,600 m (Hansen, 1927), almost as pronounced as posterior one; and (5) channeled area on dorsal face of telson and near Azores Islands at 2,102 m (Nou- vel, 1943). It is the Atlantic twin of G. chil- narrower at proximal part. dressi, undoubtedly also living in the BBL. Remarks on Habitat.-The existence of There is another argument for the bentho- Gnathophausia childressi was first men- pelagic habitat of both G. childressi and G. tioned by Childress (1985) as a "new affinis; in spite of thousands of deep pelagic Gnathophausia species" belonging to the hauls done in the Atlantic and the Pacific, deep pelagic fauna of the San Clemente Ba- the oceans most studied, neither of these sin, restricted to the near-bottom layer species has been caught. This contrasts with where it was fished at about 0.3 m above the pelagic habitat of all other species in the the bottom. genus, which are normally found in pelagic A few years later, in the same basin and samples, their depths of occurrence depend- in the East Cortes Basin, adjacent to the for- ing on the houndaries of the oceanic water mer, Childress et al. (1989) observed the masses (Hargreaves, 1989). Both species are rare or at least more or Chorocaris chacei Williams and Rona and less localized, as revealed by the data given Alvinocaris markensis Williams, which are above. However, if they live strictly in the less linked with the Mid-Atlantic vents (Ca- BBL without contact with the sea floor, sanova et al., 1993). trawls adapted to fish in the few meters Biogeographical Remarks.-Nine species above the bottom should help to verify this belonging to the genus Gnathophausia have statement. According to Childress (personal now been described. Five of them live in communication), "They seem to be rela- the waters off California: four pelagic, pre- tively common, but are rarely collected be- viously mentioned, namely, G. ingens, G. cause they are limited to the area within 50 gigas, G. gracilis, and G. zoea (see Ort- Downloaded from https://academic.oup.com/jcb/article/16/1/192/2418789 by guest on 02 October 2021 m of the bottom at these depths [2,000 m]." mann, 1906; Banner, 1954; Clarke, 1962; They can have a distribution almost the Pequegnat, 1965), and the new benthope- same as the macroplanktonic sea cucumber lagic G. childressi. It would be interesting Peniagone diaphana (Theel) living up to 70 to know if the last is endemic in the deep m above the bottom in the San Clemente San Clemente and East Cortes Basins or if Basin, but never seen on the bottom itself it also lives outside them, at comparable (Barnes et al., 1976). depths in the BBL. Adaptations to Habitat.-One would ex- Gnathophausia childressi and G. affinis pect many adaptations in Gnathophausia probably descend from a common ancestral childressi to the benthopelagic habitat, by species having a larger distribution, which comparison with the other species of the ge- was split into smaller populations separated nus living in the open ocean. Such adapta- by geographical gaps, leading progressively tions relate to oral and locomotory append- to the separation of an Atlantic and a Pa- ages or length of gills, owing to differences cific species, perhaps after the raising of the in food supply, locomotion constraints, and Isthmus of Panama, 5 or 6 million years oxygen concentrations between these two ago. habitats. In fact, a single difference has been found in the morphology of the man- Remarks on the Mouthparts of the dibles. At comparable body size, they are Genus Gnathophausia smaller than those of the five other species The curious asymmetry of the paragnaths of Gnathophausia that I observed for com- in the family Lophogastrida has already parisons: G. ingens (Dohrn), G. zoea W.- been discussed at length by Clarke (1961). Suhm, G. longispina Sars, G. elegans Sars, Sars (1885) was wrong when he drew both and G. fagei Casanova (e.g., the mastica- the labrum and the paragnaths as symmet- tory edge represents 80% of the length of rical (pl. VIII, figs. 2, 5), the latter being that of G. zoea); the cutting part is weaker, moreover described as membranous. Apart its prominent denticulations being more from slight differences as emphasized slender; moreover, they are less chitinous, above for the mandibles, the mouthparts are as indicated by the clear amber color of the almost the same in G. childres.si as in the masticatory edge, instead of deep brown in other species of Gnathophausia studied. all other species. It has been demonstrated A ventral view of G. longispina (Fig. that a large amount of organic matter, orig- 3A) shows that the oral aperture is closed inating from the superficial phytoplankton anteriorly by the labrum, laterally by the blooms, sinks rapidly through the water mandibles, and posteriorly by the parag- masses and covers wide areas of the deep naths. As mentioned by Nouvel (1943) for sea floor (Rice, 1983). It is probable that G. G. affinis and G. zoea, the labrum is asym- childressi feeds, at least partly, on this metrical; its right posterior edge is provided abundant flocculent material which does not with a hooked lamella. The left mandible require hard appendages for cutting or slightly overlaps the right one, its curved grinding. Although less marked, this reduc- cutting edge surrounding its narrower right tion of the mandibles is reminiscent of that counterpart; thus there is a hollow on the of the decapod shrimp Rimicaris exoculata right lateral side which is covered by the Williams and Rona, which feeds on deep small lamella of the labrum. The mastica- hydrothermal bacteria, in comparison with tory part of the left mandible is S-shaped, Downloaded from https://academic.oup.com/jcb/article/16/1/192/2418789 by guest on 02 October 2021

Fig. 3. Gnathophausia longi.spina. A, ventral view of mouth area (X 18); B, C, inner lateral views of mouthparts showing coaptation between right (B) and left (C) mandible and paragnath (X30) (arrowheads show molarlike processes of left paragnath); D, detail of lamellae constituting molar part of mandibles (X 180). L = labrum, M = mandibles, P = paragnaths. while that of the right mandible is L- agree with Clarke when he stated: "Since shaped. These parts of the mandibles lie the paragnaths apparently possess little closely against the anterior faces of the par- musculature, the incorporation of the left agnaths; this explains the asymmetry of the paragnath as an organ of mastication has latter. The coaptation between these two ap- necessitated that it be held by the mandible pendages on each side is striking (Fig. 3B, and operated by the musculature of that C). structure through the apposition of the man- The inner side of the left paragnath is dible against the paragnath.... The right partly chitinous and exhibits areas (Figs. paragnath by contrast is a thin leaflike 2B, C, 3C) consisting of tightly packed structure and is in no way coupled to the stumpy ribbon setae abraded distally (Fig. right mandible." Most of his observations 2D); one area is rounded, resembling the and interpretations seem inexact. The right molar part of the mandibles. The latter, paragnath is not thin; in ventral view, its however, is made up of parallel lamellae distal part is as thick as that of the masti- (Fig. 3D) which might be considered as catory part of the right mandible (Fig. 3A); modified setae. The innerside of the right in inner view, its base is as thick as that of paragnath is provided only with normal se- its counterpart (Fig. 3B, C). Moreover, the tae (Fig. 3B). The left paragnath certainly two paragnaths contain muscular masses as plays a role in mastication, as previously voluminous as those of the maxillipeds, un- proposed by Clarke (1961). 1 do not totally doubtedly allowing their own active move- ments. It is also difficult to accept that only thermalisme médio-Atlantique.â��Cahiers de Biolo- one paragnath has been incorporated as a gie Marine 34: 573-588. Casanova, J.-P. (In press.) Crustacea Mysidacea: les masticatory organ. This implies a reduction Lophogastrides d'Indonésie, de Nouvelle-Calédonie of the adjacent mandible and, even tightly et des îles Wallis et Futuna.â��In: A. Crosnier, ed., coupled as shown by Clarke (1961), the Resultats des campagnes MUSORSTOM, 14. Mé- paired mandible-paragnath is surely less ef- moires du Museum National d'Histoire Naturelle, (A). ficacious than a stronger mandible alone. Chaudonneret, J. 1956. Le système nerveux de la ré- On the contrary, it would be more satisfy- gion gnathale de l'écrevisse Cambarus affinis ing to consider the paragnaths of Gnatho- (Say).â��Annales des Sciences Naturelles, Zoologie, phausia as a pair of appendages in the pro- 11ème serie, 18: 33-61. cess of loss of function, leading to their Childress, J. J. 1985. Capture and live recovery of Downloaded from https://academic.oup.com/jcb/article/16/1/192/2418789 by guest on 02 October 2021 deep-sea crustaceans.-National Geographic Society membranous aspect in almost all crusta- Research Reports 21: 67-69. ceans. Such a concept would not be sur- , A. T. Barnes, L. B. Quetin, and B. H. Robison. prising, since it is acknowledged that the 1978. Thermally protecting cod ends for the recov- Lophogastrida are archaic, as summarized ery of living deep-sea animals.-Deep Sea Research 25: 419â��422. by Fage (1941). The paragnaths are not , D. L. Gluck, R. S. Carney, and M. M. Gow- generally considered as true appendages, ing. 1989. Benthopelagic biomass distribution and the most convincing argument being the oxygen consumption in a deep-sea benthic boundary fact that, in the higher decapod crustaceans, layer dominated by gelatinous organisms.â��Limnol- the integral proprioceptors of these organs ogy and Oceanography 34: 913â��930. are different from those of other append- Clarke, W. D. 1961. A giant specimen of Gnatho- phausia ingens (Dohrn, 1870) (Mysidacea) and re- ages, leading one to consider the paragnaths marks on the asymmetry of the paragnaths in the as extensions of the posterior wall of the suborder Lophogastrida.â��Crustaceana 2: 313-324. mouth rather than appendages (Laverack, . 1962. The genus Gnathophausia (Mysidacea, 1987). This should now be reviewed, how- Crustacea), its systematics and distribution in the Pa- ever, taking up again certain arguments al- cific Ocean.-Ph.D. thesis, University of California, San Diego, California. Pp. 1-251. ready advanced by Chaudonneret (1956), Fage, L. 1941. Mysidacea Lophogastridaâ��I.â��Dana who supported their appendicular nature, Reports 19: 1-52. owing to, on the one hand, their own in- Hansen, H. J. 1927. Malacostracés I.â��Sergestidés et nervation comparable to that of the gnathal schizopodes.â��Expéditions Scientifiques du "Tra- appendages, and, on the other hand, the vailleur" et du "Talisman" pendant les années 1880, 1881, 1882, 1883 9: 1-26. presence of a mobile palp on the paragnaths Hargreaves, P. M. 1989. The vertical and horizontal of Tanaidacea, strongly suggesting that distribution of four species of the genus Gnatho- these are appendages. It would thus not be phausia (Crustacea: Mysidacea) in the eastern North surprising that the Tanaidacea and Lopho- Atlantic Ocean.â��Journal of Plankton Research 11: 687-702. gastrida might have kept traces of the ap- Laverack, M. S. 1987. The nervous system of the pendicular origin of paragnaths. Crustacea, with special reference to the organisation of the sensory system.-Nato Advanced Study In- ACKNOWLEDGEMENTS stitute, series A: Life Sciences 323â��351. Nouvel, H. 1943. Mysidacés provenant des campag- I thank Dr. James J. Childress (Oceanic Biology nes du Prince Albert ler de Monaco.â Résultats des Group, Department of Biological Sciences and Marine �� Science Institute, University of California, Santa Bar- Campagnes Scientifiques Accomplies sur son Yacht bara, California, U.S.A.) for asking me to describe this par Albert ler Prince Souverain de Monaco 105: 1â�� 128. new species. Ortmann, A. E. 1906. Schizopod Crustaceans in the U.S. National Museum. The families Lophogastridae LITERATURE CITED and Eucopiidae.-Proceedings of the United states National Museum 31: 23â��54. Banner, A. H. 1954. Some "schizopod" crustaceans from the deeper water off California.-Occasional Pequegnat, L. H. 1965. The bathypelagic mysid Papers of the Allan Hancock Foundation 13: 1â��49. Gnathophausia (Crustacea) and its distribution in Barnes, A. T., L. B. Quetin, J. J. Childress, and D. L. the eastern Pacific Ocean.-Pacific Science 19: 399â�� Pawson. 1976. Deep-sea macroplanktonic sea cu- 421. cumbers: suspended sediment feeders captured from Rice, A. L. 1983. Thomas Henry Huxley and the deep submergence vehicle.-Science 194: 1083â�� strange case of Bathybius haeckelii; a possible al- 1085. ternative explanation.-Archives of Natural History Casanova, B., M. Brunet, and M. Segonzac. 1993. 11: 169-180. L'impact d'une epibiose bactérienne sur la morphol- Sars, G. O. 1883. Preliminary notices on the Schizo- ogie fonctionnelle de crevettes associées Ã� l'hydro- poda of H. M. S. "Challenger" Expedition.â��For- handlinger det Kongelige Norske Videnskabers Sel- Wishner, K. F., and M. M. Gowing. 1987. In situ fil- skab 7: 1â��43. tering and ingestion rates of deep-sea benthic bound- ����â ��â â . 1885. Report on the Schizopoda collected by ary-layer zooplankton in the Santa Catalina Basin.â�� H. M. S. Challenger during the years 1873â��76.â�� Marine Biology 94: 357-366. Report on the scientific results of the voyage of H. M. S. Challenger during the years 1873-1876, Zo- RECEIVED: 20 January 1995. ology 13(37): 1-228. ACCEPTED: 26 April 1995. Smith, K. L., Jr. 1982. Zooplankton of a bathyal ben- thic boundary layer: in situ rates of oxygen con- Address: Laboratoire de Biologie Animale (Planc- sumption and ammonium excretion.-Limnology ton), Universite de Provence, 13331 Marseille cedex and Oceanography 27: 461â��471. 3, France. Downloaded from https://academic.oup.com/jcb/article/16/1/192/2418789 by guest on 02 October 2021

ANNOUNCEMENT

The Crustacean Society Summer Meeting and the Third International Large Branchi- opod Symposium (ILBS3) will be held jointly at the University of San Diego, San Diego, California, U.S.A., 14-18 July 1996. A peracarid Symposium will be organized within the TCS schedule. The ILBS3 proceedings will be published in a special volume of Hydrobiologia. The deadline for registration, abstracts, and housing is 1 March 1996. For further information contact: TCS/ILBS33 % Marie A. Simovich Biology Department University of San Diego San Diego, California 92110, U.S.A.

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