Aliculastrum parallelum (Gould, 1846)

Bulla parallela Gould, 1846: 98 . Gould 1847: 251. Gould 1852: 220, pl. 15, figs 267, 267a (type lost; USNM 19091).

Cylichna parallela — Adams 1862: 152.

Atys parallela — Sowerby 1870: pl. 4, sp. 21a–c. Watson 1886: 640.

Atys parallelus — Martens & Langkavel 1871: 53.

Atys (Alicula) parallela — Pilsbry 1895: 266, pl.28, figs 21, 22. Kobelt 1896: 21, pl. 6, figs 13, 14.

Atys (Aliculastrum) parallela — Pilsbry 1896: 237.

Aliculastrum parallela — Carlson & Hoff 2003: 281.

Type locality. West Pacific (Not specified in the original description).

Material examined. Mariana Islands, Guam, 4 spcs dissected, UF 374138, H = 6.8–9.6 mm .

Animal (Fig. 1C): Body whitish-translucent, white dots scattered over the body, more dense at pallial lobe edge and ventral areas of parapodial lobes, white blotches scattered over cephalic shield; between eyes, edges of cephalic and parapodial lobes with darker pigmentation; red blotches scattered on the mantle; eyes visible.

Shell (Figs 2D, 8A, B): Maximum height 12.5 mm; milky white; thick, robust, semi-opaque, cylindrical, both sides of body whorl nearly parallel, rounded anteriorly and conical posteriorly, apex imperforate; spire sunken, aperture narrow, widening towards posterior edge, outer lip elevated above the apex; spiral grooves at both ends only (anterior = 17–18, posterior = 14–20), fine longitudinal growth lines present throughout shell; orangecoloured periostracum observed.

Jaws: Present, crescent shape.

Radula (Figs 8C, D): Radular formula at mid-point 27 x 12.1.1.0.12; median tooth with broad base, triangular central cusp with thinner membrane-like structure on both sides, central cusp becomes more pointed posteriorly, pseudo-triangular cusp on both sides; inner lateral tooth presents only at right side, hook-shaped, broader base compared to outer lateral teeth, with semi-circular projection outwardly, broader than outer lateral teeth, strong denticulation along outer margin; outer lateral teeth hook-shaped, slender, with semi-circular projection outwardly, groove along the outer margin, size decreases outwardly.

Gizzard plates (Figs 8E, F): Three gizzard plates; broader at anterior part, narrower towards posterior end; 8–9 ridges, covered by tiny rods with pointed tips at both anterior and posterior sides, density of rods higher at anterior half of gizzard plate; posterior half of gizzard plate lacks rods on posterior side of ridges.

Male reproductive system (Fig. 8G): Total length 3.4 mm (H = 8.54 mm). Formed by three parts: prostate, seminal ducts and penial region; prostate elongated, semi-translucent, faint striations at distal end, division into lobes not evident; two seminal ducts, first seminal duct whitish, narrower than second seminal duct, connects distally at a pouch close to prostate and proximally close to penial sheath, coils around second seminal duct, second seminal duct yellowish, broad, proximal part of both ducts semi-translucent; penial region elongated, semitranslucent.

Ecology. There is hardly any record on the habitat for this species. Materials examined were sampled on shallow areas up to 18 m depth.

Geographical distribution. Japan, Guam, Fiji, Tahiti (Adams, 1862; Martens & Langkavel, 1871; Watson, 1886; present study).

Remarks. Aliculastrum parallelum was originally ascribed to the genus Bulla, but from very early on, authors like Sowerby (1870), Martens & Langkavel (1871), and Watson (1886) consider this species to belong on the genus Atys; later Pilsbry (1921) and Habe (1952) included it in the genus Aliculastrum .

Adams (1862), apparently unaware of Gould (1846) ’s work, described a similar species as Cylichna parallela, which was collected in Tsushima, Japan (Strait of Korea; as Tsu-Sima). Comparison of the original descriptions revealed that these two species are identical and likely synonyms (Gould, 1846; Adams, 1862).

Aliculastrum parallelum does not have an ovoid inflated shell as the type species of the genus Atys ( Atys naucum); moreover, the gizzard plates are also different from those of A. naucum with presence of few broad ridges (Carlson & Hoff 1999). On the other hand, Aliculastrum parallelum shares similarities with the other studied species of Aliculastrum by possessing a cylindrical robust shell, and, as in the species Aliculastrum debilis, an asymmetric radula with strongly denticulated inner lateral teeth and a male reproductive system with two seminal ducts. Moreover, preliminary phylogenetic data (Too 2011) suggest the inclusion of this species in a clade together with the other two species of Aliculastrum .