Atopos laidlawi Collinge, 1902

Atopos laidlawi Collinge, 1902 View in CoL

( Figs. 2, 4A, B, 5A–C)

Atopos laidlawi Collinge, 1902: 90 View in CoL , 91, pl. 5, figs. 53–55, type locality: Ban Kong Rah , District of Gaboing [in the area of Kabang District, Yala Province, Thailand].

Material examined. One ethanol-preserved specimen ( Fig. 2A, B; length 21 mm, width and height 5 mm) ( CUMZ 160201 View Materials ), Phra Cave (Site III), Ban Nam Yen, Tanintharyi Region, Myanmar, 11°14′01.5″N, 99°10′42.8″E. One ethanol-preserved specimen ( Fig. 2C, D; length 25 mm, width and height 5 mm) ( CUMZ 160205 View Materials ), same locality as specimen CUMZ 160201 GoogleMaps .

Diagnosis. Upper part of mid-lateral line pale yellow; lower part pale brown and separated by irregular brown band. Mid-dorsal keel rounded and fairly prominent. Clear white mid-dorsal line flanked by brown stripes. Perinotum and foot off-white. Anterior and posterior ends of notum white.

External characters. Preserved specimen ( Fig. 2). Animal elongated elliptical, tapering at both ends, and anterior part thicker than posterior part. Dorsal side with fairly prominent mid-dorsal keel along its entire length. Notum rather smooth, and uniformly covered with small- to medium-sized and randomly scattered papillae. Papillae generally elliptical shaped and interconnected with rows of ridge ( Fig. 4A, B). Anterior and posterior ends of notum white. Notum pale yellowish, off-white to pale brown; mid-dorsal line white and flanked by irregular brownish stripes; mid-lateral line with a broad dark brown band and irregular margins. Upper and lower parts of mid-lateral line yellowish, and sometimes lower part with darker colour than upper part. Perinotum narrow with off-white sharp keel; foot (f) narrow and about half of body width ( Figs. 2, 5B). Anterior notum covers head region. Head and tentacles off-white; upper tentacles slightly long; lower tentacles short, stout, and bifid.

Genital organs. Male genital pore located at base of lower right tentacle. Atrium short; penis (p) long cylindrical shape and situated at base of right body cavity. Vas deferens (vd) thin tube, originating at base of hermaphroditic-prostate gland complex, bound with oviduct before penetrating body wall on right side of foot, and then emerging from body wall near male genital pore and inserting near distal end of penis ( Fig. 5A, C). Penial retractor muscle (pr) inserted at distal end of penis. Simroth glands present on both sides: left Simroth gland (lsg) much smaller and less developed. Right Simroth gland (rsg) well-developed and consists of three parts: (i) evaginable part (es) is cylindrical and almost same length as penis; (ii) interconnected duct long, curled, and thin tube; and (iii) secretory part (sp) enlarged and long bulbous shape. Simroth gland retractor muscle (sr) attached at junction of interconnected duct and evaginable part ( Fig. 5C).

Female genital pore (fg), anal pore (an), and pulmonary orifice (po) located around 4 to 6 mm from male genital pore in right pedal groove ( Figs. 2A, C, 5B). Vagina (vg) short tubular duct. Receptaculum seminis (rs) club-shaped and situated at distal end of vagina. Oviduct (ov) long cylindrical tube. Albumen gland (ag), lobulated hermaphrodite gland (hg), and prostate gland (pg) bound together and undifferentiated ( Fig. 5A).

Distribution. Atopos laidlawi is known from the type locality in southern Thailand ( Collinge, 1902); the new record reported herein is from Tanintharyi Region, southern Myanmar.

Comparative diagnosis. Atopos laidlawi can be distinguished from Atopos rugosus Collinge, 1902 and A. gravelyi by its fairly prominent mid-dorsal keel, mid-dorsal line white and flanked by irregular dark brown stripes, pale yellow and pale brown colours at upper and lower parts of mid-lateral line, respectively, and whitish anterior and posterior ends of notum. In comparison, A. rugosus possesses a well-developed mid-dorsal keel, yellowish-brown mid-dorsal line, notum dark green decorated with yellowish-brown spots, mid-lateral line absent, and anterior end of notum almost white ( Collinge, 1902: pl. 5, figs. 46–48). In contrast, A. gravelyi possesses a prominent mid-dorsal keel, dark clove-brown mid-dorsal line, mid-lateral line with pale buff blotches above and clove-brown band below, upper part of mid-lateral line sepia with dark brown blotches and small spots, and lower part of mid-lateral line blue with slaty-black blotches and spots ( Ghosh, 1915: pl. 16, fig. 1). Regarding the genitalia, A. laidlawi has both left and right Simroth glands and an enlarged bulbous secretory part ( Fig. 5C), while A. gravelyi possesses only the right Simroth gland and a very slender secretory part ( Ghosh, 1915: pl. 19, fig. 27).

Remarks. Atopos laidlawi was described based on specimens collected during the ‘Skeat Expedition’ in southern Thailand with only a brief description of external morphology ( Collinge, 1902). However, the record from Luang Phrabang Province, Laos (see Inkhavilay et al., 2019: figs. 19a, 55a) seemed to differ from the typical by having no off-white colour at the anterior and posterior ends of the notum. The Laotian specimen has a sharp mid-dorsal keel, light brown mid-dorsal line, yellowish-brown notum with tiny black spots, irregular yellowish-brown mid-lateral line, and brown mottling on upper mid-lateral line, which suggest it is more likely to be A. maximus than A. laidlawi .