Body length 1.79 mm. Trunk glabrous (Fig. 1a-e), completely segmented, dorsal segmentation lines raised and swollen. Lateral processes slender and smooth, well separated by a little more than their own diameter. Cephalon extends anteriorly into a hood, over 1/5 of the length of the chelifores. Ocular tubercle inconspicuous, dome-shaped, without eyes. Abdomen spindle-shape, not articulating to the body, extends horizontally to the middle of the second coxae of leg pair 4.

Figure 1.  

Cilunculus tricuspis sp. n., male; holotype: a. trunk, dorsal view; c. trunk, lateral view; e. proboscis, frontal view; f. palp; g. oviger; h. leg 3; i. tarsus, propodus and claws of leg 3, enlarged. Male; paratype: b. trunk, dorsal view; d. trunk, lateral view. Scale bars a-d = 0.5 mm; e, f, g, i = 0.2 mm; h = 1 mm.

Proboscis barrel-shaped, 0.7 times as long as trunk, with two dorsal and one ventral triangular processes close to mouth.

Chelifores stout, with one-articled scape; chela atrophied, without fingers.

Palp of nine articles (Fig. 1f); article 2 longest, almost 1.5 times as long as article 4; distal five articles very short, each with row of ventral setae.

Oviger glabrous (Fig. 1g); article 2 longest, slightly longer than article 4; distal five articles short, of decreasing length, article 7 to article 10 with compound spines, formula 1:1:1:2, spines on article 10 thicker.

Third leg (h-i) slender, longest articles with long setae. First coxa short, with few setae; second coxa 1.5 times as long as first or third coxa, with ventrodistal and dorsal protuberances; femur 2.6 times as long as second coxa, with dorsal and distal long setae, tall cement gland tube dorso-distally; first tibia slightly longer than second tibia and 1.4 times longer than femur, with dorsal, lateral and ventral rows of setae; second tibia with ventral and lateral rows of setae and sparse dorsal setae; tarsus small, subtriangular, with one protuberance dorsally, one spine and few setae ventrally; propodus without heel, sole with seven spines and two distal setae, with two long setae dorsally and some short setae dorsally and distally; main claw slender, gently curved, 0.6 times as long as propodus; auxiliary claws half the length of main claw.

Female unknown.

Measurements of holotype in mm: trunk length from the anterior margin of the cephalon to the tip of 4th lateral processes 1.79; width across second lateral processes 1.0; proboscis length 1.27; abdomen length 0.55. Chelifore scape length 0.24. Palp article 1 (Pa1) 0.07; 2 (Pa2) 0.51; Pa3 0.08; Pa4 0.33; Pa5 0.08; Pa6 0.09; Pa7 0.08; Pa8 0.07; Pa9 0.06. Oviger article 1 (O1) 0.11; O2 0.39; O3 0.16; O4 0.35; O5 0.21; O6 0.10; O7 0.08; O8 0.08; O9 0.07; O10 0.02. Third leg: coxa-1 0.26, coxa-2 0.40, coxa-3 0.24, femur 1.06, tibia-1 1.49, tibia-2 1.41, tarsus 0.11, propodus 0.54, main claw 0.32, auxiliary claw 0.16.

This specific name is from the Latin tricuspis (three-pointed), referring to the three processes on the proboscis.

This new species was found only at the type locality, the substrate of which consisted of white and yellow foraminiferan oozing along with dead coral and shells and a small amount of black basalt.

According to the key given by Stock (1997), this new species would identify as Cilunculus haradai Nakamura & Child, 1983 and the common characteristics between these two species impelled us to consider this new species as a deep-sea form of C. haradai. After further examination, several differences with C. haradai convinced us to establish this new species, as it presented shorter and blind ocular tubercle, glabrous lateral processes, fewer setae on legs, absence of heel spines and almost horizontal proboscis with three distal processes.

Cilunculus tricuspis n. sp., keying to couplet 15 of Stock’s key, was distinct from C. cymobostrychos and C. roni in glabrous trunk, without wavy barbed setae or tubercles and distinguished from C. mergus and C. roni by absence of a dorsal hump on the propodus and is also different from C. misesetosus which has long auxiliary claws almost equal to the main claw.

Amongst the 32 species, only Cilunculus australiensis Clark, 1963, Cilunculus galeritus Nakamura & Child, 1991 and C. roni showed the proboscis adorned with processes, but the new species could be easily distinguished from C. australiensis which presented tall spinose tubercles on the trunk and C. galeritus which presented the unique larger cephalic segment hood and could be distinguished from C. roni by the tubercles on the trunk and legs.

There are few species of Cilunculus reported from the Indian Ocean. Amongst these are Cilunculus bifidus (Stock, 1968) only found off False Bay (South Africa) (1361 m) (Stock 1968), Cilunculus kravcovi Pushkin, 1973 which is found in the Crozet Islands (255-309 m) and the Prince Edward Islands (South Indian Ocean) (360-376 m) (Pushkin 1973, Arnaud and Branch 1991) and Cilunculus sewelli Calman, 1938 which was found off Zanzibar (1789 m), Natal (South Africa) (440 m), Kenya (1520 m) and the Mozambique Channel (1628-1600 m) (Calman 1938, Fage 1956, Stock 1968). The new species was relatively close to C. kravcovi (Crozet Islands), although they occur several hundred kilometres apart (Fig. 2). The three recorded species were differentiated from C. tricuspis n. sp. based on their distinctive characteristics, such as C. bifidus having a tall and bifid ocular tubercle, C. kravcovi and C. sewelli having an acute ocular tubercle and noticeably long setae on legs.

Figure 2.  

The collecting station for station of Cilunculus tricuspis n. sp. and the distribution of four species of Cilunculus ▼-C. tricuspis n. sp.; ▲-C. bifidus; ■-C. kravcovi; ◆-C. sewelli.