A new species of Cilunculus Loman, 1908 (Arthropoda: Pycnogonida: Ammotheidae) from the South-western Indian Ocean

Abstract Background A new species of Cilunculus was determined by the unique characteristic of the three distal processes present on the almost horizontal proboscis and the other differences, including a shorter and blind ocular tubercle, fewer setae on the legs and a glabrate trunk. New information New Cilunculus species from the South-western Indian Ocean


Introduction
The genus Cilunculus Loman, 1908 consists of 32 species (Bamber et al. 2018). Fry and Hedgpeth (1969) gave the diagnosis for this genus and described two species from the Antarctic. Stock (1955), Stock (1978), Stock (1997) provided keys to contemporary species and Nakamura and Child (1983) established a key to the Japanese species of Cilunculus. Five species were described since Stock's key (Bamber et al. 2018) and Bamber (2013) had preliminarily discussed all these additional species including Cilunculus misesetosus Turpaeva, 2005 from the North Atlantic and other four species, Cilunculus ateuchus Bamber, 2004, Cilunculus mergus Bamber, 2004, Cilunculus cymobostrychos Bamber, 2004 and Cilunculus roni Bamber, 2013, from Melanesia. Bamber (2013 conjectured that C. misesetosus might be a junior synonym of Cilunculus europaeus Stock, 1978, but without providing any further study. Several authors (Fry and Hedgpeth 1969, Pushkin 1973, Stock 1978 had recognised the close relationship between Cilunculus and Ammothella Verrill, 1900. However, the studies of Arango (2002), Nakamura et al. (2007) and Sabroux et al. (2017) made this outstanding problem more complicated, but up to now, still maintained the situation due to a lack of conculsive evidence one way or the other. Thus, the new species, described below, was still included under the genus Cilunculus.
Few specimens of Pycnogonida were obtained during the Chinese DY115-20 cruise which undertook the comprehensive survey on the hydrothermal vents along the Southwest Indian Ridge. Wang et al. (2013) had described one new species collected from station DY115-20VII-TVG02 which was close to the present station DY115-20VII-TVG04. Two specimens of Cilunculus were collected at this station and determined to be a new species.

Materials and methods
The specimens were sorted from benthic fauna which were collected by washing the sediment obtained by a TV-grab from Station DY115-20VII-S04-TVG04 during the Chinese DY115-20 expedition on R/V Dayangyihao in February 2009. They were preserved in 90% ethanol at the Third Institute of Oceanography, Ministry of Natural Resources, China (Nos. 20VIIS4TVG04.01, 20VIIS4TVG04.02). The specimens were drawn using a Camera Lucida and photographs were taken with an Auto-montage system on a Leica M205 FA stereomicroscope. Measurements were made axially, dorsally for the trunk, laterally for the palp, proboscis and leg and are given in millimetres.

Description
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, domeshaped, without eyes. Abdomen spindle-shape, not articulating to the body, extends horizontally to the middle of the second coxae of leg pair 4.
Proboscis barrel-shaped, 0.7 times as long as trunk, with two dorsal and one ventral triangular processes close to mouth.
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.
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.

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

Distribution
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.

Taxon discussion
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 andBranch 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.