Aliculastrum debilis ( Pease, 1860 )

Too, Chin Chin, Carlson, Clay, Hoff, Patty Jo & Malaquias, Manuel António E., 2014, Diversity and systematics of Haminoeidae gastropods (Heterobranchia: Cephalaspidea) in the tropical West Pacific Ocean: new data on the genera Aliculastrum, Atys, Diniatys and Liloa, Zootaxa 3794 (3), pp. 355-392 : 362-365

Aliculastrum debilis ( Pease, 1860)

Atys debilis Pease, 1860: 20 . Sowerby 1870: pl. 5, sp. 28. Watson 1886: 640. Pilsbry 1917: 217, fig. 7. Kay 1979: 424, fig. 137C. Gosliner et al. 2008: 23.

Atys (Alicula) debilis — Pilsbry 1895: 266, pl. 33, figs 69, 70. Kobelt 1896: 22, pl. 6, fig. 15.

Atys (Aliculastrum) debilis — Pilsbry 1896: 267.

Atys angustata — Smith 1872: 346.

Atys cortuna — Kay 1979: 424.

Type locality. Hawaii ( Sandwich Islands ).

Material examined. Tahiti, 1 shell examined, ZMB unnumbered (paratype), H = 10.13 mm ; Maui , Hawaii , ZMBN 81658 , H = 9 mm ; the Philippines, 1 shell examined, MNHN, Paris ( OT 834 ), H = 4.5 mm ; the Philippines, 2 spcs dissected, MNHN, Paris ( M 9/ OT 802 ), H = 5.3 mm, 5.9 mm; Tahiti , 1 spc. dissected, ZMBN 87083 , H= 5.2 mm ; Tahiti, 2 spcs dissected, ZMBN 87084 , H = 8.2 mm, 8.4 mm ; the Philippines, 2 spcs dissected, MNHN, Paris ( OT 834 ), H = 4.6 mm, 5.6 mm .

Animal ( Fig. 1B ): Body whitish-translucent, white dots scattered over the body, more dense on edges and mid part (between mouth and eyes) of cephalic shield, forming abundant blotches on the mantle and parapodial lobes; eyes visible, inserted in unpigmented periocular areas.

Shell ( Figs 2C , 5A, B ): Maximum height 15 mm; milky white; slightly robust, slightly thick, semi-opaque, cylindrically ovate, narrowed towards both ends, widest right below the mid-line, anterior end truncated; spire sunken, outer lip elevated above the apex, strongly sinuous at the posterior end, columella short and thick; step-like spiral grooves at both ends only (anterior = 10–18, posterior = 7–13), which become closer towards the ends, axial lines present.

Jaws ( Fig. 5C ): Present, crescent shape, jaw rodlets with 5-6 denticles.

Radula ( Figs 5D–F ): Radular formula at mid-point 20-30 x 12–8.1.1.0.8–12; median tooth with broad base, triangular central cusp with thinner membrane-like structure on both sides, pseudo lateral cusps present; inner lateral tooth presents only at right side, hook-shaped, broad base, with semi-circular projection outwardly, broader than outer lateral teeth, strong denticulation presents along outer margin; outer lateral teeth hook-shaped, slender, broad base, with semi-circular projection outwardly, groove along the outer margin, size decreases outwardly.

Gizzard plates ( Figs 5G, H , 6A, B ): Three gizzard plates; broader at anterior part, narrower posteriorly; 8–11 ridges, pseudo-rachis present; For specimens from Tahiti ( Fig. 6A ), 8 ridges, rachis absent or faint; ridges covered by rods with pointed tips along top edge, smaller rods with pointed tips densely at both anterior and posterior sides of ridges, gaps between ridges smooth, high density of rods on ridges throughout plates.

Male reproductive system ( Fig. 5I ): Total length 3.2 mm (H = 8.2 mm). Formed by three parts: prostate, seminal ducts and penial region; prostate elongated, bilobed, proximal lobe larger, opaque-yellowish, distal lobe translucent, striations on entire prostate; two seminal ducts connect prostate and penial region, seminal duct makes two coils right after prostate before branching into two, first seminal duct is narrower than second seminal duct, first seminal duct curls at the posterior end and stretches underneath of second seminal duct towards penial region, both seminal ducts yellowish but turn translucent at anterior end; penial region translucent with broad end, narrower towards genital opening.

Ecology. Sandy bottom or on Halimeda beds between depth of 6–18 m ( Gosliner et al. 2008; present study).

Geographical distribution. Tahiti, Fiji, Hawaii, the Philippines, Guam ( Kay 1979; Gosliner et al. 2008; present study).

Remarks. This species is often misidentified with Aliculastrum cylindricum due to their extreme similarity in shell morphology. Pease (1860) described Aliculastrum debilis as having a pellucid and fragile shell. The specimens examined during this study agree in part with this description; they have semi-translucent shells, which are, however, relatively robust. A paratype of this species (see examined material; Fig. 7 ) from Tahiti has a solid opaque shell, which might indicate that Pease used shells from more than one species to describe Aliculastrum debilis . Confusion around the characters to distinguish this species is well patent in the literature. For example, Pilsbry (1917) mentioned that the illustration of Aliculastrum debilis in Sowerby (1870: pl. 5, sp. 28a, b) show a projecting point at the posterior end of the columella, although such feature is not referred by Pease (1860). Carpenter (1865) mentioned that Aliculastrum debilis is identical to Atys succisa , and simply a slender version of Atys alicula . Pilsbry (1895) mentioned that Atys angustata is a synonym of Aliculastrum cylindricum but, his description of the species: “a very narrow shell with both ends thinner and strongly sinuous at the posterior ends”; resembles Aliculastrum debilis rather than Aliculastrum cylindricum . Similarly, Kay (1979) expressed doubts about a species described by Pilsbry (1917), which he named Atys cortuna that only differs from Aliculastrum debilis by possessing a wider posterior end; Kay (1979) synonymized both species under the name Atys debilis .

Aliculastrum debilis has a more elongated semi-translucent shell with deep spiral grooves at both ends, when compared with Aliculastrum cylindricum , which has a more inflated opaque shell with shallower spiral grooves at both ends. Besides, the former usually has a narrower shell towards both ends, whereas the latter has a broader anterior end compared to its posterior end. Moreover, these two species can be distinguished easily based on their anatomical features as documented above.

This species has been placed under the genus Atys by most authors although it possesses features that are different from the type species Atys naucum ; for example, a cylindrical shell, gizzard plates with large ridges and complex male reproductive system with two seminal ducts. Aliculastrum debilis has an asymmetric radula; the inner lateral tooth with strong denticulation along the outer margin appears only at the right side. This character is found also in another Aliculastrum species studied in this work (e.g. A. parallelum ). Preliminary phylogenetic data ( Too 2011) suggest the inclusion of this species in the genus Aliculastrum .

Figures

FIGURE 1. Live images. A. Aliculastrum cylindricum (Koumac, northern New Caledonia; H = 28 mm, MNHN, Paris), B. Aliculastrum debilis (Maui, Hawaii; ZMBN 81658; H = 9 mm), C. Aliculastrum parallelum (Guam; UF 374138; H = 19 mm), D. Atys multistriatus (Guam; UF 374136; H = 7 mm), E. Atys pittmani nov. sp. (Maui, Hawaii; ZMBN 81673; H = 10 mm), F. Atys semistriatus (Maui, Hawaii; ZMBN 81656; H = 6 mm), G. Atys ukulele nov. sp. (Maui, Hawaii; ZMBN 89710; H = 5.4 mm), H. Diniatys dentifer (Guam; UF 374130; H = 9 mm), I. Diniatys dubia (Guam; UF 299907; H = 12.7 mm), J. Liloa curta (Guam; UF 374131; H = 15 mm), K. Liloa porcellana (Maui, Hawaii; ZMBN 89712; H = 4.2 mm); L. Liloa porcellana (Maui, Hawaii; ZMBN 89712; H = 2.8 mm). Image A, courtesy of W. B. Rudman; Images B, E–G, K, L, courtesy of C. Pittman.

FIGURE 2. Shells. A. Aliculastrum cylindricum (the Philippines; MNHN, Paris (S3/ OT236); H = 22 mm), B. Aliculastrum cylindricum (New Caledonia; MNHN, Paris; H = 13.7 mm), C. Aliculastrum debilis (the Philippines; MNHN, Paris (OT834); H = 4.5 mm), D. Aliculastrum parallelum (Guam; UF 374138; H = 10 mm), E. Atys multistriatus (Guam; UF 374136; H = 5.3 mm), F. Atys pittmani nov. sp. (Maui, Hawaii; ZMBN 81673; H = 10 mm); G. Atys semistriatus (Tahiti; ZMBN 87082; H = 4.1 mm). Scale bar: 1 mm.

FIGURE 5. Shell and anatomy of Aliculastrum debilis. A. Detail of anterior part of shell. B. Detail of posterior part of shell. C. Detail of jaw. D. Detail of median tooth at the middle of radula. E. Inner lateral tooth with denticulation at the right side of radula. F. Detail of outer lateral teeth at right side of radula. G. Top view of gizzard plate of specimen from the Philippines. H. Rods on ridges at posterior edge of gizzard plate. I. Male reproductive system. go, genital opening; pr, prostate; ps, penial sheath; sd 1, first seminal duct; sd 2, second seminal duct. Scale bars: A, B = 200 µm; C = 2 µm; D, E = 10 µm; F, H = 20 µm; I = 1 mm. A, C, D: ZMBN 87084, H = 8.4 mm; B= MNHN, Paris (M9/ OT802), H= 5.3 mm; E–H: MNHN, Paris (M9/ OT802), H = 5.9 mm; I: ZMBN 87084, H = 8.2 mm.

FIGURE 6. Gizzard plate anatomy of specimen Aliculastrum debilis from Tahiti. A. Top view of gizzard plate. B. Rods on middle ridges of gizzard plate. Scale bars: A = 100 µm; B = 20 µm. A, B: ZMBN 87084, H = 8.4 mm.

FIGURE 7. Paratype of Aliculastrum debilis in Museum für Naturkunde (Berlin, Germany). (H = 10.13 mm).