“Stenosemus chiversi Ferreira, spec. nov.
(Figures 1 to 4 and 5 to 9)
Diagnosis: Small. elongated chitons, strongly carinate. Tegmentum white, irregularly rugose to coarsely granulose. Valves with well defined, laterally pinched beaks. Mucro anterior. Girdle with imbricated cylindrical scales. Slit formula (holotype), 28/3-4/25. Sutural laminae continuous across shallow sinus. Gills posterior, abanal. Radula with unicuspid major lateral teeth.
Description:
Holotype - The specimen. preserved in ethyl alcohol, measures, including the girdle, 18.3 mm in length, 9.3 mm in width (at the level of valve iv), and 4.3 mm in height. Strongly carinate jugal angle about 90º. Tegmentum uniformly white, except for some black, fuliginous material adhering to the shell and girdle. Lateral areas of intermediate valves well defined but only slightly raised. The whole tegmentum sculptured with irregular rugosities (Figures 1 to 4), weaker at the jugum, bolder at the periphery; in the end valves and lateral areas these rugosities acquire a vaguely concentrical, zig-zag disposition; towards the jugum, the rugosities break up into coarse granules increasingly less defined. Valves slightly beaked, particularly in the posterior valves, iv to vii, where the beaks have a quite unique laterally pinched appearance (Figure 3). Mucro anterior, well defined, almost pointed, with the same pinched look; postmucro area straight, at a 45° slope. Articulamentum chalky white. Insertion plates short; teeth small, irregularly shaped; slit formula 28/3-4/25. Eaves moderately spongy. Sutural laminae delicate, relatively short; continuous along a very shallow, almost obsolete sinus through a pectinated sinusal plate. Girdle about 1.0 mm wide, covered with white cylindrical scales (Figure 5) strongly imbricated in a manner that only their blunt round tips show; the scales, about 300 μm in height, 70 μm in width, have a dull surface without ornamentation. Undersurface of girdle covered with rectangular, translucent scales, about 100 μm long, 16 μm wide (Figure 6). Gills abanal, in the posterior half of the foot, about 15 plumes per side. Radula 4.5 mm long (25% of the length of the specimen) comprises about 90 rows of mature teeth. Median tooth (Figure 7), oval shaped, about 35 μm wide, 80 μm long; first lateral teeth elongated; second (major) lateral teeth with slender; unicuspid head (Figure 8), about 90 μm long, 25 μm wide at the base; Seitenplatte simple in shape but very long, 250-280 μm at the main shaft (Figure 9); outer marginal teeth 55μm long, 60 μm wide.
Type Locality: North eastern Pacific, 14º52’N, 125º26’W, 4390 m, as collected by R. Dick, R/V Prospector, with a benthos free fall grab, 13 September 1975.
Type Material:
Holotype, disarticulated, with girdle and radula mounted in slides, deposited at the California Academy of Sciences, Department of Invertebrate Zoology (CASIZ, Cat. # 017683). A paratype, with the same dimensions of the holotype, part of the same lot (SIO M1188) deposited at the Benthic Invertebrate Collection of the Scripps Institute of Oceanography. Another paratype (SIO M 1419), collected “N of Equator & E of Hawaii, at (?) 4572 m, August 1977" in the course of a dredge for manganese nodules by Kennicot Corporation, measures about 12 mm in length, deposited at CASIZ (Cat. #017684).
Remarks: Stenosemus Middendorff 1847 (Type Species, Chiton albus Linnaeus, 1767, by SD, WINCKWORTH, 1926; synonyms - Lophyrus Sass, 1878 [not Poli, 1791, nomen nullum], Chondropleura Thiele, 1906, Lepidopleuroides Thiele, 1928, Lophyrochiton Jakovleva, 1952) has long been accepted as a subgenus of Ischnochiton Gray, 1847 (WINCKWORTH, 1926, 1934; A. G. SMITH, 1960; VAN BELLE, 1977; KAAS & VAN BELLE, 1980). VAN BELLE (1977) defined Stenosemus by its small posterior valve, weak tegmental sculpture, and the insertion plates of the intermediate valves which possess a variable number (1 or 2) of slits. The recognition that a number of species other than C. albus; are part of the same taxon requires a broader definition of Stenosemus and indicates its elevation to generic rank.
By their overall similarities, the following species are here considered to be members of Stenosemus: S. albus (Linnaeus, 1767), its type species; S. exaratus (Sar, 1878); S. acelidotus (Dall, 1919); S. stearnsii (Dall, 1902); S. abyssicola (Smith & Cowan, 1966); and S. chiversi Ferreira, spec. nov. Taken globally, these species lay a foundation for an understanding and definition of Stenosemus: 1) lepidopleurid-like (already noted by WINCKWORTH, 1926, and THIELE, 1928, for S. albus) its elongated shape, whitish color, simple sculpture, and posterior gills, 2) long, cylindrical, strongly imbricated girdle scales, 3) variability in the number of slits in the intermediate valves, and very numerous slits in the end valves, 4) a very shallow to obsolete sinus with almost continuous sutural laminae, and 5) a deep-water habitat. The presence of a large number (25+) of slits in the end valves, and the almost continuous sutural laminae with shallow to absent sinus in Stenosemus are features reminiscent of Callochiton Gray, 1847. The fact that the girdle scales of Callochiton--strongly imbricated, needle-like--bear some similarity in their shape and disposition to those of Stenosemus suggests a phylogenetic link between the two genera.
The species here considered under Stenosemus appear to represent two distinct groups: 1) the group of Stenosemus albus, S. exaratus and S. chiversi characterized by an elongated body, posterior gills, and unicuspid major teeth in the radula, and 2) the group of S. stearnsii, S. aceloditus, and S. abyssicola with a less elongated body, gills that extend to the anterior end of the foot, and (in S. abyssicola) bicuspid radula [the radula is not known in S. aceloditus and S. stearnsii].
Like other Stenosemus, S. chiversi appears to favor a very deep-water habitat. Its presence in depths of 4390-4572 m places it near the top of the list of deep-water chitons (see FERREIRA, 1980, Table 1), surpassed only by Lepidopleurus vitjazi Sirenko, 1977, and Leptochiton alveolus (Lovén, 1846).
The species is here named chiversi after Dustin Chivers, Department of Invertebrate Zoology, California Academy of Sciences, who was instrumental in bringing these specimens to my attention, and who, in the course of many years, has generously given me much of his time, knowledge, and friendship.”
(Ferreira, 1981: 325-328)
