The only species of Mohnia Friele, 1879 (Caenogastropoda, Buccinoidea, Buccinidae) in the North Pacific represents an unrecognized new genus of Newtoniellidae (Triphoroidea) Author Strong, Ellen E. https://orcid.org/0000-0001-7181-4114 National Museum of Natural History, Smithsonian Institution, PO Box 37012, MRC 163, Washington, DC 20013 - 7012, USA stronge@si.edu Author Sirenko, Boris I. Zoological Institute, Russian Academy of Sciences, St. Petersburg, 199034, Russia Author McLean, James H. Deceased; formerly of Natural History Museum of Los Angeles County, 900 Exposition Blvd, Los Angeles, CA 90007, USA text ZooKeys 2021 2021-08-05 1055 69 87 http://dx.doi.org/10.3897/zookeys.1055.68911 journal article http://dx.doi.org/10.3897/zookeys.1055.68911 1313-2970-1055-69 EDAA550DC7AB4C4D8CA91D988F4F3940 990477668F06527BB3C5B25F397886CF Pseudomohnia sp. Figs 1C, D , 5B-E , 7 Material examined. Aleutian Islands • 2 spms; Rat Islands , southwest of Amchitka Island ; 51°27.70'N , 178°35.0'E ; 384 m ; 27 July 1997 ; R/ V Dominator stn 23-971-181; RN Clark leg.; LACM 1997-156.7 (Fig. 1C, D ); • 1 spm; southwest of Buldir Island ; 52°18.50'N , 175°49.0'E ; 325 m ; 9 August 1997 ; R/ V Dominator stn 23-971-243; RN Clark leg.; LACM 1997-168.10 (Figs 5B-E , 7 ) . Description. Shell [LACM 1997-156.7]. Shell broadly turreted, spire angle ca. 41°, ~ 24 mm in adult shell length, consisting of approximately eight, thin, convex whorls, separated by deeply impressed suture (Fig. 1C, D ); growth indeterminate. Shell whitish, with thick, velvety periostracum. Larval shell non-planktotrophic, ~ 3 elevated, constricted whorls, with smooth, blunt nucleus; well-defined opisthocyrt riblets and spiral threads producing cancellate sculpture on subsequent whorls. Axial elements becoming more closely spaced toward teleoconch transition. Inferred transition to teleoconch marked by change in orientation of axial sculpture and slight expansion in whorl diameter. Teleoconch initially with six or seven thin spiral cords, somewhat irregular in width and spacing; cords becoming flatter, broader and less distinct on later whorls and on base and intercalated by additional cords. Spiral ornament crossed by variably developed, well separated, weakly prosocline axial threads and growth increments; axial threads becoming obsolete on body whorl. Aperture broad, outer lip thin, sharp. Axis weakly gyrate, pervious; columellar plait lacking. Anterior canal short, slightly recurved. Operculum [LACM 1997-156.7]. Operculum thin, corneous, honey in color, thinning toward edges; paucispiral, nucleus eccentric, occupying ca. 42% of operculum length. Radula [LACM 1997-168.10]. Radular ribbon comprising 37 rows, ~ 5.7 mm in length. Radula taenioglossate (Fig. 7A ). Rachidian small, concave, with slight constriction below broad cutting edge, tapering to flat, narrow base. Cutting edge bearing single prominent, broadly triangular, bluntly pointed, finely serrated cusp, and single smaller, irregular denticle on each side (Fig. 7B, C ). Radular membrane diagonally creased between rachidian and lateral teeth of each row. Lateral teeth robust, broad, with smooth inner edge of shaft curving posteriorly; cutting edge with prominent, dagger-like, pointed cusp and small, blunt inner cusp (Fig. 7B, D, E ). Marginal teeth long, slender, with cylindrical shafts and constriction below claw-like tips; cutting edges of inner and outer marginal teeth bearing three to five elongate, curving, pointed cusps, somewhat angular in cross section (Fig. 7E ). Figure 7. Radula morphology of Pseudomohnia sp., LACM 1997-168.10 A radular ribbon B detail of rachidian and lateral teeth C detail of rachidian D, E detail of lateral and marginal teeth F detail of inner and outer marginal teeth. Scale bars: 20 µm ( C, F ); 100 µm ( A, B, D, E ). Anatomy [LACM 1997-168.10]. Foot elongate oval. Propodium large, crescent shaped, with deep, triangular propodial groove (Fig. 5B ). Shallow furrow (= epipodial skirt) continuous with opercular lobe, evident along sides of foot, becoming obsolete in deep groove where propodium joins neck. Foot sole, particular that of metapodium, deeply wrinkled with many deep transverse grooves. Sole divided longitudinally by deep medial cleft. Rather large bilobed mesopodial pedal gland within foot below and in front of pedal ganglia on both sides of cleft, opening via large pore near center of sole. Head with short, broad, muscular snout and long, tapering cephalic tentacles (Fig. 5B ). Eyes conspicuous, on prominent ocular peduncles at outer bases of tentacles. Mantle edge smooth, with short, clearly defined siphon at left. Columellar muscle short, broad, extending roughly one-half whorl to base of mantle cavity. Ctenidium long, extending from siphon to base of mantle cavity, with long, narrowly triangular leaflets. Osphradium forming tall, narrow, undulating ridge, extending almost entire length of gill, from near anterior end almost to base. Hypobranchial gland well developed. Rectum broad, filled with sponge spicules, terminating in papillate anus near mantle edge at right. Rectum bordering pallial glandular gonoduct. Pallial gonoduct open for much of its length, lacking accessory pouches; thick, highly glandular tissue subdivided by deep transverse grooves (Fig. 5E ). Penis lacking. Proboscis acrembolic. Introvert rather short, muscular, oral tube not cuticularized. Jaws large, robust, surrounding anterior end of odontophore (Fig. 5C ). Buccal mass large with long radular sac (Fig. 5C, D ) emerging mid-ventrally near posterior end, continuing to right before arcing dorsally across anterior esophagus just behind buccal mass with posterior, weakly-bifid tip lying on left side of esophagus near supra-esophageal ganglion. Posterior buccal cavity with broad, deep, subtriangular, acinous salivary glands on either side of dorsal food groove. Anterior esophagus not cuticularized. Large mid-esophageal gland (Fig. 5C ) with shallow, glandular septae and voluminous lumen, narrowing to posterior esophagus near end of mantle cavity, with ca. seven low, longitudinal folds. Nervous system epiathroid. Circum-esophageal nerve ring surrounding anterior esophagus (Fig. 5C ) just behind buccal apparatus. Nerve ring highly asymmetrical, with both cerebral ganglia lying on left side of esophagus; left cerebral ganglion below and slightly in front of right ganglion, joined by very short but distinct commissure. Buccal ganglia (Fig. 5D ) joined by short commissure, lying on either side of posterior buccal mass at emergence of anterior esophagus, just below salivary glands. Small pleural ganglia lying immediately behind cerebral ganglia, separated by narrow constrictions. Long connective joining right pleural with supra-esophageal ganglion (Fig. 5C ) at left side of cephalic hemocoel near tip of radular ribbon. Sub-esophageal ganglion lying below right side of anterior esophagus, separated from left pleural ganglion by slight constriction. Long, highly asymmetric connectives joining cerebral and pleural ganglia with pedal ganglia lying within foot at short distance anterior to cerebral ganglia. Small statocysts with numerous, tiny statoconia on postero-dorsal surface of pedal ganglia. Pedal ganglia joined by short, thick commissure. Distribution and ecology. Known only from the Aleutian Islands (Fig. 2 ) in 325-384 m, feeding on sponges. Remarks. Given the absence of accessory sperm storage pouches in the pallial gonoduct ( Fretter 1951 ; Houston 1985 ), the dissected individual is inferred to be male. The disposition of the remnants of the shell from LACM 1997-168.10 on which the anatomical observations were made is unknown and there is no known photograph (RN Clark, L Groves, pers. comm.). The anatomy and radula morphology show several differences compared to Pseudomohnia kurilana and P. rogerclarki (see below). The two specimens in LACM 1997-156.7 (Fig. 1C, D ) have a broader spire angle than P. rogerclarki , but share the constricted, elevated early whorls. The spiral sculpture of the teleoconch is less distinct and more irregular than in P. kurilana and P. rogerclarki . Fragmentary soft parts from one of the two specimens in LACM 1997-156.7 (Fig. 1D ) produced a radula that we infer to be teratological, lacking a rachidian and bearing stunted marginal teeth with weakly lobed tips. The two available lots were collected in deeper waters (325-384 m) than P. rogerclarki (114-166 m). Given the fragmentary and incomplete information available, we cautiously conclude that the broad morph represented by the two specimens in LACM 1997-156.7 is conspecific with LACM 1997-168.10 and represents a third and undescribed species of Pseudomohnia . It is possible that the shells and the soft parts are not conspecific, and that the broad morph represents population variation or sexual dimorphism within the range of P. rogerclarki . Thus, we refrain from describing another species until additional comparative material becomes available. To the extent that comparisons are possible, the anatomy of Pseudomohnia sp. agrees well with that of P. kurilana . The most conspicuous differences between the two concern details of the anterior alimentary system; specifically, the salivary glands appeared more irregular, the snout longer, the radular sac shorter, the mid-esophageal gland less developed, and the length of the introvert shorter in P. kurilana . The length of the introvert is known to vary within the family ( Houbrick 1987 ; Golding et al. 2009 ), but that of P. kurilana also may have been incompletely retracted which would also explain the appearance of the snout. Pseudomohnia sp. is more similar to P. kurilana in morphology of the radula, but differs in the broader, more triangular and finely serrated central cusp of the rachidian and in the morphology of the marginal teeth which bear slightly fewer (two to four versus three to five), shorter, more smoothly conical cusps in P. kurilana . However, the range of values of marginal cusp counts overlaps in the two species and its significance could diminish with greater sampling. Like other triphoroideans, the nervous system is epiathroid with a long supra-esophageal connective, but differs in the presence of numerous, tiny statoconia in the statocysts rather than a single statolith ( Risbec 1943 ).