THE GENUS NEOLEPTON MONTEROSATO, 1875 IN SOUTHERN SOUTH AMERICA (BIVALVIA: NEOLEPTONIDAE)

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1 THE GENUS NEOLEPTON MONTEROSATO, IN SOUTHERN SOUTH AMERICA (BIVALVIA: NEOLEPTONIDAE) DIEGO G. ZELAYA AND CRISTIÁN ITUARTE Division of Invertebrate Zoology, Museo de La Plata, 00 La Plata, Buenos Aires, Argentina (Received October 00; accepted June 00) ABSTRACT A systematic revision of the genus Neolepton in southern South America has been carried out. Neolepton concentricum, N. cobbi, N. hupei and N. falklandicum are redescribed and figured, and lectotypes for N. concentricum and N. bennetti are designated. Three new species are described, two from the Magellanic Province (N. amatoi new species and N. yagan new species) and one from the Argentine Province (N. bonaerense new species). The presence of N. umbonatum in Magellanic waters is considered doubtful. Some of the reproductive characteristics and peculiarities of the brooding behaviour of N. concentricum are described. INTRODUCTION Neolepton Monterosato, is a worldwide genus. The systematic position of the family Neoleptonidae was discussed recently by Salas & Gofas (), who considered it to be a group of paedomorphic veneroideans, instead of belonging to the Leptonoidea, Erycinoidea or Cyamioidea as had been considered earlier (Gray, ; Thiele, ; Tebble, ). Salas & Gofas () reviewed several Neolepton species (formerly described or reported under the genera Lepton, Davisia, Neodavisia and Notolepton), though they only referred to one Argentine species, Neolepton cobbi (Cooper & Preston, 0). Neolepton cobbi was the first species described from southern South America. Subsequently, three species from the Malvinas (Falkland) Islands, Neolepton bennetti (Preston, ), Neolepton concentricum (Preston, ) and Neolepton falklandicum Dell,, were described. Other species reported from the Sub- Antarctic Region are Neolepton hupei Soot-Ryen,, Neolepton cf. umbonatum (Smith, ) and Neolepton parasiticum (Dall, ). Recently, Zelaya & Ituarte (00) described Neolepton georgianum and Neolepton holmbergi from the South Georgia Islands. To date, the only neoleptonid reported from the southwestern Atlantic Ocean beyond the Magellan Region is the deep-water species Neolepton profundorum Allen, 000, described from the Argentine basin (,0, m depth). Knowledge of the Magellanic species of Neolepton is limited to old descriptions, generally lacking in precision and adequate illustration. Consequently, the species have frequently been confused. The present paper is a systematic revision of the Magellanic species of Neolepton, with the description of two new species. A third species, and the first from the Argentine Province, is described from material dredged off Buenos Aires. Data on the reproductive biology of Neolepton concentricum are also given. taken by the R. V. Eduardo L. Holmberg (Instituto Nacional de Investigación y Desarrollo Pesquero) from the Argentine continental shelf, were studied (Fig. ). All the samples were taken with a dragnet and were fixed in 0% formalin solution. The specimens were sorted from sediments under a stereoscopic microscope. Additional specimens from malacological collections housed at the following museums and governmental fishery agencies were used: Museo de La Plata, La Plata (MLP), Museo Argentino de Ciencias Naturales Bernandino Rivadavia, Buenos Aires (MACN), Royal Scottish Museum, Edinburgh (RSM) and Instituto Nacional de Investigación y Desarrollo Pesquero, Mar del Plata (INIDEP). Type specimens housed at Muséum National d Histoire Naturelle, París (MNHN), Swedish Museum of Natural History, Stockholm (SMNH), and photographs of types from the Natural History Museum, London (BMNH), were also used for comparative purposes. The species are described and illustrated using scanning electron microscope (SEM) photography. Hinge teeth terminology follows Salas & Gofas () (Fig. ). Shell measurements were recorded as follows: shell length (L), maximum anteroposterior distance; shell height (H), maximum dorsoventral distance, perpendicular to length; shell width (W), maximum distance across valves. Morphometric ratios, H/L and W/H, were calculated. The number of specimens measured (n) and values of mean and standard deviation values are given (except for Neolepton falklandicum for which only two specimens were available). Specimens for histology were decalcified by rinsing in Raillet Henry s solution for h, dehydrated and embedded in Paraplast. Serial sections (. m thick) were stained with haematoxylin/eosin, and in some cases, the alcian blue/periodic acid Shiff (AB/PAS) method for acidic and neutral mucopolysaccharides was used (Gabe, ). Downloaded from by guest on 0 October 0 MATERIAL AND METHODS This study was based on specimens dredged from Magellan Strait (at Inútil Bay, Fuerte Bulnes and Laredo Bay), Beagle Channel (at Lapataia Bay, Ushuaia Bay and Estancia Moat), southeastern Tierra del Fuego (at Sloggett Bay and Buen Suceso Bay) and Isla de los Estados (Puerto Parry) (Fig. ). In addition specimens collected by the research vessel (R. V.) Polarstern (Alfred Wegener Institute) at Burdwood Bank and from samples Correspondence: D. G. Zelaya; dzelaya@museo.fcnym.unlp.edu.ar SYSTEMATIC DESCRIPTIONS Genus: Neolepton Monterosato, Type species: Lepton sulcatulum Jeffreys, (subsequent designation by Crosse, ) Neolepton concentricum (Preston, ) (Figures ) Davisia concentrica Preston, :, fig. (Port Stanley, Falkland [Malvinas] Islands, lectotype, here designated, MNHN, Fig. A, B). J. Moll. Stud. (00) 0: The Malacological Society of London 00

2 D. G. ZELAYA & C. ITUARTE Figure. Records of Neolepton species. Downloaded from by guest on 0 October 0 Figure. Hinge teeth terminology:, a and b, cardinal teeth of right valve; a, b, cardinal teeth of left valve; PI, PIII, posterior lateral teeth of right valve; PII, posterior lateral tooth of left valve.

3 NEOLEPTON IN SOUTHERN SOUTH AMERICA Figure. Neolepton concentricum (Preston, ) from Punta Segunda, Tierra del Fuego (MLP ). A. Outer lateral view of a right valve. B. Outer lateral view of a right valve showing flattening of the anteroventral shell margin. C. Posterior view. D. Detail of protoconch. E. Detail of shell surface sculpture. F. Inner view of a right valve. G. Inner view of right valve showing anteroventral flattening of shell margin. H. Right valve, detail of hinge. I. Left valve, detail of hinge. Scale bars: A C, F H mm; D E 00 m. Downloaded from by guest on 0 October 0

4 D. G. ZELAYA & C. ITUARTE Davisia cobbi Melvill & Standen, : (not Cooper & Preston, 0; in part). Neolepton cf. cobbi Soot-Ryen, :, pl., fig., textfig. (not Cooper & Preston, 0). Neolepton cobbi Dell, :, fig., no. (not Cooper & Preston, 0; in part). Material examined: Lectotype MNHN (here designated) (Fig. A, B); specimen and valves, Scottish National Antarctic Expedition, Burdwood Bank, fathoms, //0 (RSM..); remains of specimen, Golfo de Ancud, Canal San Antonio, 0 S W, m,/ / (SMNH ); numerous specimens from Beagle Channel at: Bahía Ushuaia ( 0. S. W), m, 0// (MLP 0), Punta Segunda ( 0. S. W), m, // (MLP 0), Punta Segunda (. S W), m, // (MLP ), Punta Segunda (. S 0. W), m, 0// (MLP ), Estancia Moat (. S. W), m, //00 (MLP ); Bahía Buen Suceso (. S. W) m, //00 (MLP ); Isla de los Estados, Puerto Parry (. S. W) m, //00 (MLP ); Magallan Strait: specimens, Fuerte Bulnes, 0//00 (MLP ); specimens, Cabo Negro, 0/00 (MLP ). Distribution: Malvinas (Falkland) Islands, Burdwood Bank, Isla de los Estados, Tierra del Fuego, southern Chile. Description: Shell solid, medium sized (maximum L. mm), moderately inflated (ratio W/H , n ), shell outline orbicular (ratio H/L , n ), nearly equilateral, slightly expanded anteriorly; dorsal, ventral and posterior margins uniformly curved (Fig. A, C, F), antero-ventral margin sometimes distorted (Fig. B, G). Beaks not wide, wellvisible above dorsal margin, centrally located. Protoconch smooth, quite inflated, m long (Fig. D). Shell surface dull, sculptured with coarse, regularly spaced commarginal ridges, periostracum white-yellowish (Figs E, B). Hinge plate strong. Left valve with cardinal a moderately short, solid, slender, enlarged anteriorly; cardinal b long, forming a hook with a; posterior lateral PII low, close to dorsal margin. Right valve: cardinal triangular, very strong, short and high, cusp displaced posteriorly; a straight, low, clearly visible, b well developed, at right angles to a; PI moderately solid, relatively short, with distal cusp (Fig. H, I). Remarks: Neolepton concentricum is similar to Neolepton cobbi, from which it differs in being smaller, with a more inflated shell, having the entire shell surface sculptured with strong, coarse and regularly spaced commarginal ridges. As N. concentricum has largely been confused with Neolepton cobbi and its synonym Neolepton bennetti, the figured syntype of Davisia concentrica (MNHN), is here selected as lectotype. Neolepton concentricum has been considered synonymous with N. bennetti and Neolepton cobbi (Dell, ; Bernard, ). However, Salas & Gofas () suggested, after study of the figured syntype of Davisia concentrica (MNHN) that it seems to represent a distinct, valid species of Neolepton. Neolepton concentricum was reported by Castellanos (0a) from San Juan de Salvamento, Isla de los Estados; later, the same specimens were reported as N. cobbi (Castellanos, 0b). As these specimens are lost, we have been unable to corroborate her identification. Biological observations: Neolepton concentricum is a gonochoristic species, producing large yolky eggs (maximum observed size of ripe oocytes: m) which are retained, after spawning, attached to the outer shell surface. Several specimens showed egg masses bound by a mucous substance forming a stringshaped egg mass (Fig. A). Each egg mass was c. m long and contained embryos arranged in alternate pairs (Fig. B); in some cases up to 0 embryos were found. One or two egg masses attached to the anteroventral margin of shells were found throughout the year. Occasionally egg masses from two different reproductive events coexisted attached to the same individual; in these cases the oldest (at a more advanced developmental stage) were displaced upwards onto the shell surface. Embryos are retained until a juvenile stage with a maximum size of c. 0 m long (Fig. C). Hence, a planktonic larval stage is entirely suppressed. This had been inferred previously from the protoconch size in other Neolepton species (Salas & Gofas, ). The position of the egg masses in N. concentricum is coincident with the position of the distortion of the shell margin observed in specimens larger than. mm (Figs B, A). This suggests that distortion is likely to be caused by the presence of egg masses and not by the mechanical characteristics of the substrate as was suggested by Salas & Gofas (). Histological study reveals that all specimens that show shell distortion are females, hence the flattening of shells is considered to be a character of sexual dimorphism. Additionally, in specimens with the antero-ventral flattening of shell margin (females), the ratio W/H was consistently greater (W/H , n ) than in specimens with non-flattened shell margin (males) (W/H , n ). According to Salas & Gofas (), the sexes are separate in the species [of Neolepton] where anatomical data are available, namely Neolepton sulcatulum, Neolepton antipodum (Filhol, 0) Downloaded from by guest on 0 October 0 Figure. Neolepton concentricum, lectotype (MNHN). A. Lateral view of right valve. B. Detail of the shell surface sculpture. Scale bars: A mm; B 00 m.

5 NEOLEPTON IN SOUTHERN SOUTH AMERICA and Neolepton sootryeni Salas & Gofas, (new name for Neolepton atlanticum Soot-Ryen, 0). In the course of the present study, a biased sex ratio was found in Neolepton concentricum. Of adult specimens histologically studied (.. mm length), 0 were females, males and hermaphrodites. This strongly biased sex ratio (markedly different from the : ratio expected for a gonochoristic species) can be interpreted in two ways, either it is the usual sex ratio for this species or it may indicate the occurrence of protandrous hermaphroditism. However, the similar size of the examined specimens and the hermaphroditic condition found in two of them (a few previtellogenic oocytes within a fully developed male gonad) does not seem to support the latter interpretation. Hermaphrodic specimens could represent cases of abnormal sex determination. Brooding in neoleptonids has been reported only for Neolepton sootryeni by Soot-Ryen (0) who described the presence Figure. Neolepton concentricum (Preston, ) from Punta Segunda, Tierra del Fuego (MLP ). A. Specimen carrying a mass of brooded eggs. B. Detail of a mass of brooded embryos. C. Detail of developing embryos. Scale bars: A mm; B 00 m; C 00 m. of eggs and larvae in the mantle cavity, glued to the inner mantle fold by a secretion of cells from the glandular portion of the mantle edge. Taking into account the characteristics of brood protection observed in N. concentricum, Soot-Ryen s (0) description and figure seem to depict an initial stage in the process of brood protection prior to the external attachment of the egg mass. Salas & Gofas () suspected that the function of the glandular portion of the mantle might be related to brooding. However, we found in N. concentricum that these glands, which showed staining (AB/PAS) compatible with neutral mucopolysaccharides, were present and well developed in both males and females. This evidence suggests that they could function not only in the fastening of eggs to the shell surface, but also in the control of particles within the mantle cavity, as suggested by Ponder (). Brooding of larvae in N. concentricum was found to occur all year round, suggesting lack of seasonality of the reproductive cycle. This fact is in contrast with Salas & Gofas () who suggested a marked seasonality in reproduction for the species they studied. Some of the reproductive traits observed in Neolepton concentricum (sexuality, developmental pattern, egg size) were coincident with those reported by Gallardo () for other small brooding bivalves such as Gemma gemma and Gaimardia bahamondei. However, the hatching size in N. concentricum is smaller, and such an extremely reduced brood size is known in only a few species such as Dacrydium hyalinum, Cyamiocardium denticulatum, Pseudokellya cardiformis and Pseudokellya gradata (Arntz et al., ; Gallardo, ; Salas & Gofas, ). Neolepton cobbi (Cooper & Preston, 0) (Figures, ) Davisia cobbi Cooper & Preston, 0:, figs, 0 (Falkland [Malvinas] Islands; syntypes (?) BMNH 0..., 0...,... ). Neolepton cobbi Dell, :, fig., nos, ; Salas & Gofas, :, figs. Davisia bennetti Preston, :, fig. (Port Stanley, Falkland [Malvinas] Islands; lectotype, here designated, MNHN, Fig A, B; paralectotype MNHN). Examined material: Photographs of supposed types of Davisia cobbi (Fig. G, H); lectotype and paralectotype of Davisia bennetti (MNHN); specimens, Port Stanley, Falkland [Malvinas] Islands (BMNH...-) (Fig. D, F); Malvinas Islands: specimens (MACN-In 0), 0 specimens, 0 (MLP -); Santa Cruz Province: specimen, Bahía Laura ( S W), (MACN-In -), valve, Puerto Deseado ( S W) (MLP ). Distribution: Malvinas (Falkland) Islands and southern Argentina. Description: Shell rather solid, medium sized (maximum L. mm), not inflated (ratio W/H ; n ), shell outline nearly circular (ratio H/L ; n ), slightly inequilateral, anterior end slightly projecting. Ventral margin evenly curved, anterior half of dorsal margin nearly straight, forming a well-marked angle with anterior margin that is straight at dorsal half; posterior half of dorsal margin lower than anterior one (Figs A, C E, A); antero-ventral margin sometimes distorted. Beaks subcentral, pointed, not wide, clearly visible above dorsal margin. Protoconch smooth, not inflated, c. m long (Fig. B). Shell surface dull, sculptured with somewhat low, irregular commarginal ridges, not evenly spaced, coarser near ventral margin (Fig. B, C). Downloaded from by guest on 0 October 0

6 D. G. ZELAYA & C. ITUARTE Hinge plate strong. Left valve with cardinal a long, slender, straight, enlarged at anterior end; cardinal b very short, blunt. Right valve: cardinal strong, high, with long base, cusp displaced backward; a well developed, long and slender, b small; PI a long slender lame, with cusp displaced distally, PIII inconspicuous (Fig. F, G). Remarks: Neolepton cobbi is most similar to N. concentricum, but it differs in having a larger and less solid shell, a more definite circular shell outline with a marked angle at the junction of dorsal and anterior margins, and hinge line more widely curved. The protoconch is larger in N. cobbi than in N. concentricum, and the commarginal ridges in N. cobbi are lower, irregular and not Downloaded from by guest on 0 October 0 Figure. Neolepton cobbi (Cooper & Preston, 0) from Malvinas (Falkland) Islands (MLP -). A. Outer lateral view of left valve. B. Detail of protoconch. C. Posterior view. D. Left valve, inner view. E. Right valve, inner view. F. Left valve, detail of hinge. G. Right valve, detail of hinge. Scale bars: A, C G mm; B 00 m.

7 NEOLEPTON IN SOUTHERN SOUTH AMERICA evenly spaced. Neolepton cobbi also differs from N. concentricum in having the left cardinal b consistently shorter and stronger, and the right cardinal with a larger base. Neolepton bennetti is here considered a junior synonym of N. cobbi on the basis of a similar shell outline and shell surface sculptured with uneven and irregularly spaced commarginal striae. Davisia cobbi was described from the Malvinas (Falkland) Islands (Cooper & Preston, 0). Despite the original description being rather poor, it clearly states as diagnostic the irregularly concentrically striated shell surface. The BMNH houses specimens of Davisia cobbi that according to Dell () were received originally from Preston and, consequently, have some status as type material. Despite Dell s assumption, the real status of these specimens as type material remains uncertain. The specimens (two broken loose valves and a poorly preserved whole specimen) are presently labelled as lectotype and paralectotype (Fig. G, H), although this designation was never published (A. Campbell, personal communication). Another large series of specimens from the Malvinas (Falkland) Islands (BMNH...-) is labelled as: Davisia cobbi, possible type material. Being uncertain of their real status as types and according to the International Code of Zoological Nomenclature (), Art., none is eligible as a lectotype. As Neolepton bennetti has been confused with N. concentricum, it is justified to designate the syntype of Davisia bennetti in the collection of MNHN as lectotype. Biological observations: A few specimens (those larger than. mm long) from the Malvinas (Falkland) Islands (MLP -, MACN 0) showed a shell margin distortion, consisting of some flattening in the anteroventral margin. These specimens were, as in Neolepton concentricum, more inflated (ratio W/H , n ) than those without flattening (ratio W/H , n 0). If the shell margin flattening was related to brood protection (see biological observations on N. concentricum), brooding behaviour of Neolepton bennetti may be similar to that found in N. concentricum. Neolepton hupei Soot-Ryen, (Figure ) Neolepton hupei Soot-Ryen, : (between Isla Quenu and Isla Chidguapi, 0 S 0 W, m, Golfo de Ancud, Chile); Soot-Ryen, :, pl., fig., textfig. (redescription); Dell, :, fig., no.. Figure. Neolepton cobbi (Cooper & Preston, 0). A, B. Lectotype of Davisia bennetti (MNHN). A. Outer lateral view of a left valve. B. Detail of shell surface sculpture. C. Detail of shell sculpture of specimen from Malvinas (Falkland) Islands (MLP -). D F. Three possible types of Davisia bennetti (BMNH...-). G, H. Syntypes (?) of Davisia cobbi (BMNH 0..., 0...). Scale bars: A, D H mm; B, C 00 m. Downloaded from by guest on 0 October 0

8 D. G. ZELAYA & C. ITUARTE Downloaded from by guest on 0 October 0 Figure. Neolepton hupei Soot-Ryen,. A. Paratype (SMNH 0), outer view of left valve. B I. Specimens from Estancia Moat, Beagle Channel (MLP ). B. Outer view of left valve. C. Posterior view. D. Detail of protoconch. E. Detail of shell surface sculpture. F. Left valve, inner view. G. Right valve, inner view. H. Left valve, detail of hinge. I. Right valve, detail of hinge. Scale bars: A C, F I mm; D, E 00 m. 0

9 NEOLEPTON IN SOUTHERN SOUTH AMERICA Examined material: Holotype (SMNH ) and paratypes (SMNH 0) (the type material is not well preserved); specimens, Estancia Moat, Beagle Channel (. S. W), m (MLP ); specimens, Burdwood Bank ( S W), m, //00 (MLP ); 0 specimens, Burdwood Bank ( 0 S W), m, //00 (MLP 0). Distribution: Tierra del Fuego, southern Chile and Malvinas (Falkland) Islands. Description: Shell solid, medium to large sized (maximum L. mm), not inflated (ratio W/H , n ), shell outline elliptic, moderately high (ratio H/L , n ), equilateral; ventral margin widely and evenly curved, anterior and posterior margins nearly equally arcuate (Fig. A C, F, G). Beaks small, low and central. Protoconch smooth, not particularly marked, c. 0 m long (Fig. D). Shell surface whitish, shiny, sculptured with very low, irregularly spaced commarginal ridges (Fig. E). Hinge plate somewhat solid, narrowing markedly below the beaks; teeth placed very close to the inner hinge plate margin, nearly hanging from it. Left valve with cardinal a short, straight, triangular, greatly enlarged at anterior end; cardinal b strong, slightly enlarged at the tip, at right angles to a; posterior lateral PII short, solid, well outstanding from dorsal margin, with distal cusp. Right valve: cardinal strong, high, sharply triangular, with relatively short base and cusp displaced posteriorly; a slender and low, b short, slender, forming a wide angle with a; PI well developed, with low distal cusp (Fig. H, I). Remarks: Neolepton hupei is defined by its large size, ellipsoid shell outline and the peculiar position of the strong cardinals ( in the right valve, a and b in the left one) close to the ventral margin of the hinge plate. Neolepton hupei is most similar to Neolepton georgianum Zelaya & Ituarte, 00, from which it differs in having a larger, higher and more equilateral shell. Neolepton holmbergi Zelaya & Ituarte, 00, another similar species, differs in having a more inflated shell, sculptured with coarse and regularly spaced commarginal ridges. Neolepton hupei differs from both N. georgianum and N. holmbergi in having a whitish instead of pinkish periostracum. Neolepton falklandicum Dell, (Figure ) Neolepton falklandicum Dell, : 0, fig., no, (0 S 0 0 W, m, Falkland [Malvinas] Islands; holotype BMNH /). Examined material: Holotype (photographs); specimens, Argentine continental shelf, between Malvinas Islands and Tierra del Fuego (MLP ). Distribution: Malvinas (Falkland) Islands and Southern Argentina. Description: Shell solid, small (maximum L. mm), somewhat inflated (ratio W/H 0.), shell outline triangular, high (ratio H/L 0. and 0.), nearly equilateral, posterior end expanded, rounded, anterior end shorter than posterior one, rounded; anterior and posterior half of dorsal margin equally arcuate, sloping steeply to posterior and anterior ends. Ventral margin widely and evenly arcuate (Fig. A E). Beaks central, narrow, inflated, projecting above dorsal margin. Protoconch 0 m long. Shell surface shiny, with very low, evenly spaced commarginal ridges; periostracum yellowish straw colour. Hinge plate not solid, only enlarged and well defined below cardinals. Left valve with cardinal a long, slender, high, enlarged at the anterior tip; cardinal b very small, slender, slightly curved, forming an acute angle with a; posterior lateral PII strong, short, not well separated from dorsal margin. Right valve: cardinal strong, high, with distal cusp and short base; a well developed and discernible from the anterior dorsal margin; b minute and very low; PI short and low, with rounded and distally displaced cusp (Fig. F, G). Remarks: Neolepton falklandicum can be easily identified by its triangular, nearly equilateral shell shape and full prominent beaks. These characters clearly distinguish this species from the other Neolepton species present in Malvinas (Falkland) Islands. Neolepton falklandicum differs from Neolepton hupei in being much smaller, with fuller beaks, and higher, more equilateral and triangular shell, and with more delicate cardinal teeth. Neolepton amatoi new species (Figure 0) Type material: Holotype (MLP ) and 0 paratypes from the type locality (0 specimens MLP -; specimens MACN-In 0; specimens MNHN). Type locality: Estancia Moat, Beagle Channel (. S. W), m. Other material examined: numerous specimens from Estancia Moat, Beagle Channel (. S. W), m (MLP -) and Sloggett Bay ( 00 S 0. W), m, Tierra del Fuego (MLP ). Distribution: Tierra del Fuego. Description: Shell solid, small (maximum L. mm), not inflated (ratio W/H ; n ), shell outline somewhat triangular, high (ratio H/L ; n ), inequilateral, posteriorly truncate, anteriorly produced; dorsal margin very short, ventral margin evenly curved, anterior margin sharply sloping, moderately arcuate and posterior margin widely and evenly curved. Beaks displaced backwards, triangular, clearly-visible above dorsal margin (Fig. 0A, C E). Protoconch smooth, well marked, forming a cap, c. m long (Fig. 0B). Shell surface shiny, yellowish straw colour, nearly smooth, only sculptured with faint commarginal striae. Hinge plate strong. Left valve with cardinal a long, straight, slender, slightly enlarged at anterior end; cardinal b very short, not slender, forming an acute angle with a; posterior lateral PII short, very close to dorsal margin. Right valve: cardinal delicate, sharply triangular, high with relatively short, low base and pointed distal cusp bent upwards; a slender and low, b relatively short and low, PI well developed, peg-like, moderately solid, short, with distal cusp (Fig. 0F, G). Remarks: Neolepton amatoi is clearly defined by its small size and nuculoid-like shape. Neolepton amatoi is similar to N. yagan new species, from which it differs in having a much higher profile with more prominent beaks, less convex and more inequilateral shell, and a smooth shell surface. Moreover the hinge plate and teeth in N. amatoi are stronger than those in Neolepton yagan. Etymology: The species is named after Jorge Amato, Commandant of Argentine Marine Cruiser (A. R. A.) Alférez Sobral during the field trip on which the species was collected. Downloaded from by guest on 0 October 0

10 D. G. ZELAYA & C. ITUARTE Neolepton yagan new species (Figure ) Type material: Holotype (MLP -) and paratypes from the type locality (0 specimens MLP -; specimens MACN-In 0; specimens MNHN). Type locality: Inútil Bay, West Tierra del Fuego, Chile. Other material examined: Numerous specimens from Beagle Channel at: Ushuaia, (MLP ), Punta Segunda ( 0. S 0. W), m, //, (MLP ), Punta Segunda (. S. W), m, 0// (MLP ); numerous specimens from Magellan Strait at: Inútil Bay (MLP -) and Laredo Bay, Chile, //000 (MLP ). Distribution: Tierra del Fuego. Description: Shell delicate, small (maximum L. mm), somewhat inflated (ratio W/H ; n ), shell outline obliquely ovate, high (ratio H/L ; n ), slightly inequilateral; posterior rounded, expanded, larger than anterior; posterior half of dorsal margin gently curved, higher than anterior one; anterior half of dorsal margin slopes markedly to anterior margin. Ventral margin unevenly arcuate, more acutely rounded posteriorly, anterior margin short and curved (Fig. A, B, D, E). Beaks subcentral, slightly displaced and directed anteriorly. Protoconch smooth, well marked, inflated, 0 m long (Fig. C). Shell surface shiny, sculptured with irregular and fine commarginal ridges, periostracum yellowish straw colour (Fig. F). Downloaded from by guest on 0 October 0 Figure. Neolepton falklandicum Dell,. A. Holotype (BMNH /), outer view of left valve. B G. Specimens from Argentine continental shelf between S and S (MLP ). B. Left valve, outer view. C. Posterior view. D. Left valve, inner view. E. Right valve, inner view. F. Left valve, detail of hinge. G. Right valve, detail of hinge. Scale bars: A G mm.

11 NEOLEPTON IN SOUTHERN SOUTH AMERICA Hinge plate not solid, narrowing below the beaks. Left valve with cardinal a short, rather strong, enlarged at anterior half; cardinal b small, robust, forming a hook with a; posterior lateral PII relatively elongate, discernible from dorsal margin. Right valve: cardinal delicate, high, with sharply triangular distal cusp and moderately short base; a weak, merged with the anterior half of dorsal margin, b minute, very low, forming a wide angle with a; PI well developed, slender, elongate, with low distal cusp (Fig. G, H). Remarks: Neolepton yagan is similar to N. hupei, differing in being much smaller, with a higher profile, more inflated and inequilateral shell. The shell sculpture is more accentuated in N. yagan. Neolepton yagan also differs from another small Magellanic species, N. amatoi, by its lower and more elongate shell outline, more inflated protoconch and strongly marked shell sculpture; in N. yagan, the left cardinal a is shorter, the right cardinal is smaller with a shorter base, and the posterior lateral PI long and slender, not peg-like. Neolepton yagan shares with N. georgianum a similar shell outline, differing in having a smaller and wider shell, smaller protoconch and coarser shell surface sculpture. Some specimens from Inútil Bay have a more equilateral shell, with a more triangular and higher shell outline. These differences in shell morphology are accompanied by differences in hinge teeth morphology: cardinal with a more perpendicular base and stippled cusp, cardinals a and b forming an acute angle, and PI stout and higher. However, these differences do not seem to be enough to warrant species segregation. Etymology: The species is named after the Yagan people, one of the aboriginal populations in Tierra del Fuego Island. Figure 0. Neolepton amatoi from Estancia Moat, Beagle Channel. A, B. Holotype (MLP -): A. outer view of right valve. B. Detail of protoconch. C G. Paratypes (MLP -). C. Posterior view. D. Left valve, inner view. E. Right valve, inner view. F. Left valve, detail of hinge. G. Right valve, detail of hinge. Scale bars: A, C F mm; B 00 m. Downloaded from by guest on 0 October 0

12 D. G. ZELAYA & C. ITUARTE Neolepton bonaerense new species (Figure ) Type material: Holotype (MLP -) and 0 paratypes from the type locality, R. V. Eduardo L. Holmberg (0 specimens MLP - ; specimens MACN-In 0; specimens MNHN). Type locality: S W, Argentine continental shelf off Buenos Aires, m depth. Distribution: Only known from the type locality. Description: Shell small (maximum L. mm), not inflated (ratio W/H ; n 0), shell outline triangular, high (ratio H/L ; n 0), somewhat inequilateral; posteriorly rounded, slightly produced anteriorly; dorsal margin short, ventral margin widely curved, anterior margin short, strikingly curved (Fig. A, C E). Beaks subcentral, somewhat displaced posteriorly. Protoconch smooth, well marked, forming a cap 0 m long (Fig. B). Shell surface shiny, whitish, nearly smooth, only sculptured with faintly marked commarginal striae. Hinge plate strong. Left valve with cardinal a rather long, straight, slender, anterior end expanded; cardinal b very short, forming an acute angle with a; posterior lateral PII short, very close to dorsal margin. Right valve: cardinal delicate with acute distal cusp; a very slender and low, b quite short, PI reduced in size, low, with subcentral cusp (Fig. F, G). Remarks: Neolepton bonaerense is similar to N. amatoi but differs in having a more equilateral shell and triangular shell outline, lower beaks and larger protoconch. The left lateral PI in N. bonaerense has a more centrally located cusp, cardinal a is slightly shorter and cardinal with less stippled cusp than Downloaded from by guest on 0 October 0 Figure. Neolepton yagan from Inútil Bay, Tierra del Fuego, Chile. A. Holotype (MLP -), outer view of right valve. B H Paratypes (MLP -). B. Posterior view. C. Detail of protoconch. D. Left valve, inner view. E. Right valve, inner view. F. Detail of shell sculpture. G. Left valve, detail of hinge. H. Right valve, detail of hinge. Scale bars: A, B, D E, G H mm; C, F 00 m.

13 NEOLEPTON IN SOUTHERN SOUTH AMERICA in N. amatoi. Another similar species, N. falklandicum, differs from N. bonaerense in having a more equilateral shell, and more inflated and projecting beaks. The general shell shape and the anteriorly displaced beaks separate N. bonaerense from the remaining Magellanic Neolepton species. Etymology: The species is named after Buenos Aires Province. Morphology DISCUSSION Regarding shell morphology, three groups of species are recognized. One, including Neolepton concentricum and N. cobbi is characterized by a rounded shell outline, subcentrally located beaks and shell surface with marked commarginal ridges; a second, including the large N. hupei, N. georgianum and N. holmbergi (the last two treated in Zelaya & Ituarte, 00), with an ovate shell outline and anteriorly displaced beaks; and a third group, with beaks located in variable positions, comprising the small species N. falklandicum, N. bonaerense, N. amatoi and N. yagan, with a triangular shell outline and nearly smooth shell surface. All the species studied here, as well as those reported for South Georgia Islands (Zelaya & Ituarte, 00), have a smooth protoconch, differing in this respect from several western Atlantic and New Caledonian species which have pustulose or granulated protoconch (Salas & Gofas, ). Figure. Neolepton bonaerense from off Buenos Aires ( S W). A. Holotype (MLP -), outer view of right valve. B G. Paratypes (MLP -): B. Detail of protoconch. C. Posterior view. D. Left valve, inner view. E. Right valve, inner view. F. Left valve, detail of hinge. G. Right valve, detail of hinge. Scale bars: A, C G mm; B 00 m. Downloaded from by guest on 0 October 0

14 D. G. ZELAYA & C. ITUARTE Diversity and distribution The present study revealed a highly diversified neoleptonid fauna in southern South America, where nine species are recognized (present study; Zelaya & Ituarte, 00). In the Southern Ocean the genus is represented in the Argentine Province by only one species (N. bonaerense). The Magellan Region has the most diverse fauna, with eight species. In spite of many studies carried out in the Antarctic Region, the genus Neolepton has never been found there. Its southernmost record is the South Georgia Islands (Zelaya & Ituarte, 00). The genus Neolepton has never been found either in the Brazilian Province or in the Magellan Region northward of Ancud Gulf, Chile. The genus Neolepton exhibits a bathymetric range wider than that given by Salas & Gofas (), who reported it as living in algal mats on intertidal or shallow subtidal rocky shores. In the present study most of the species were found in shallow waters up to 0 m depth (N. concentricum, N. cobbi, N. amatoi and N. yagan); however, the genus was also found on sandy bottoms up to m depth (e.g. N. hupei). Furthermore, Allen (000) reported N. profundorum at,0, m depth. Species doubtfully present in southern South America Linse (, ) reported Neolepton umbonatum (Smith, ) from the Chilean Beagle Channel and Malvinas (Falkland) Islands. This species, described from the Kerguelen Islands, was not found during the present study, despite the huge collection examined that covers a wide Sub-Antarctic area, and we decided to exclude it from this revision until further evidence is available. Species formerly erroneously quoted as Neolepton species Waldo parasiticus (Dall, ) a circumantarctic species, which lives as an epibiont on irregular echinoids, has been referred to as a neoleptonid species under the genera Lepton, Notolepton, Neolepton and Neodavisia (Dall, ; Arnaud, ; Osorio & Bahamonde, 0; Bernard,, respectively). However, Salas & Gofas () and Zelaya & Ituarte (00) excluded this species from the family Neoleptonidae. Furthermore, Rios () reported Lepton cema (Narchi, ) and Lepton lepidum Say,, from Brazilian waters, two species that undoubtedly are not neoleptonids. ACKNOWLEDGEMENTS The authors wish to acknowledge A. Tablado (MACN), P. Bouchet, V. Herós (MNHN), A. Warén, K. Sindemarck (SMNH) and S. Pye (RSM) who kindly provided us with specimens for comparative purposes; A. Campbell for sending photographs and additional information on types lodged at BMNH; A. Roux (INIDEP) who allowed us to study samples from R. V. Capitán Cánepa; C. Romero, E. Morrison and Y. Cariceo who provided us with samples from Beagle Channel and Magellan Strait, respectively. C. Gallardo who kindly contributed with comments on reproduction in brooding bivalves. The technical assistance of R. Urréjola (MLP Scanning Electron Microscope Unit) is greatly appreciated. The authors especially thank the logistic support received from the Base Naval Ushuaia, and particularly M. Matesa and J. Amato, commanders of A. R. A. Alférez Sobral. This study was partially funded by a grant from Fundación Antorchas (D. Zelaya). The authors are members of the Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), Argentina. REFERENCES ALLEN, J.A A new deep-sea species of the genus Neolepton (Bivalvia: Cyamioidea; Neoleptonidae) from the Argentine basin. Malacologia, :. ARNAUD, P.. Echinodermes littoraux de Terre Adélie (Holothuries exceptées) et Pélécypodes commensaux d echinides antarctiques. Expeditions Polaires Françaises (Mission Paul-Emile Victor), :. ARNTZ, W. 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15 NEOLEPTON IN SOUTHERN SOUTH AMERICA TEBBLE, N.. British bivalve seashells. Trustees of the British Museum (Natural History), London. THIELE, J.. Handbuch der systematischen Weichtierkunde.. Gustav Fischer, Jena. ZELAYA, D.G. & ITUARTE, C. 00. On the identity of Waldo parasiticus with the description of a new species (Bivalvia: Montacutidae). Nautilus, : 0. ZELAYA, D.G. & ITUARTE, C. 00. New species of Neolepton Monterosato, from South Georgia Islands (Bivalvia: Neoleptonidae). Nautilus, :. Downloaded from by guest on 0 October 0

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