Sericosura, Fry & Hedgpeth, 1969

Staples, David A., 2019, Pycnogonids (Arthropoda, Pycnogonida) from the Southwest Indian Ridge, Zootaxa 4567 (3), pp. 401-449 : 416-419

Sericosura species indeterminate.

Figure 9 A–C; Plate 3 D–E

Material examined. One subadult specimen (NHMUK 2018.19), Southwest Indian Ocean, Coral Seamount, 41˚ 22.31'S, 42˚ 54.57'E, ROV, 732 m, specimen JC066-3440, stn 4.38, mooring site, on net containing Mango wood, 20 November 2011. One subadult specimen ( NHMUK 2018.20 ), Southwest Indian Ocean, Coral Seamount, 41˚ 22.31'S, 42˚ 54.57'E, ROV, 732 m, specimen JC 066-1298D, stn 4.38, mooring site, on net containing whale bone, 20 November 2011 .

Remarks. These specimens are closest to the male specimens from the Walvis Ridge which Child 1882a assigned unconvincingly to S. mitrata ( Gordon, 1944) however in the absence of a range of material it is difficult to compare adults and subadults of that little known species.

Sericosura mitrata ( Gordon, 1944) was described from the MacRobertson Coast Antarctica from 219 m depth and later recorded from Walvis Ridge off SW Africa at 2117–2154 m depth ( Child 1982a) and the Ross Sea at 106 m depth ( Child 1994a). Child attributed several differences between the female holotype and his two male specimens from Walvis Ridge to sexual dimorphism, but I am of the view that both are distinct species. The two SWIR specimens are subadults, perhaps females, and I suspect that they represent further examples of Child’s Walvis Ridge specimens. Fortuitously Bamber (2009, Fig. 4 ) reexamined and refigured the female holotype of S. mitrata and corrected several omissions from Gordon’s original description. In doing so Bamber made Child’s identification more tenable but though I am mindful of Bamber’s support for the attribution of the Walvis Ridge specimen to S. mitrata , there are several morphological differences that extend beyond sexual dimorphism and cause me to question Child’s determination. Unlike the holotype of S. mitrata the lateral processes of Child’s specimens are more widely separated (compare Bamber 2009, Fig. 4A ), the “clearly seen” transverse body ridges observed by Gordon ( Fig. 19b ) are not evident, the proboscis is much narrower, the auxiliary claws are longer and the abundant hair-like setae on the ventral surfaces of the tibiae are not present. Child (1982a) did not mention the shape of the abdomen but based on his Figure 6a , it is more likely arched as opposed to being almost straight in S. mitrata ( Gordon 1944, Fig. 19b ). Child’s Walvis Ridge specimens are typical deep-water blind forms recorded from 2117–2154 m depth whereas S. mitrata is one of a few blind Sericosura species recorded from relatively shallow water (106–219 m) and may indicate a further point of difference. The SWIR specimens share all significant characters with the Walvis Ridge specimens; clearly separated lateral processes, a distinctly bifurcate ocular tubercle, possession of strong spines on the distal margins of the scapes, auxiliary claws longer than half the length of the main claw and the spination pattern of the propodus. The genus Sericosura is mostly associated with hydrothermal vents and cold seeps but neither these SWIR specimens nor Child’s Walvis Ridge specimens appear to be associated with these geological features. The geographical distribution of S. mitrata is described by Child (1994a) as “wide but scattered” and “enigmatic” by Bamber (2009) but should it be established that the Walvis Ridge specimens are a distinct species then this anomaly would be largely resolved. I am disregarding the unreported ‘probable’ record of a juvenile S. mitrata from the mid-Atlantic Ridge (see Child 1994a) until such time as that particular specimen is re-examined.

The ocular tubercle in both specimens is distinctly bifurcate but that of the larger specimen ( Plate 3D, E ) was inadvertently squashed below the surface prior to being photographed and is no longer visible. All propodi are of a uniform size and shape. The chelifores of the larger specimen are extended from the cephalon with most of the scapes showing. The chela is separated from the scape by a wide (presumably extended) membrane ( Fig. 9A ). Each scape has three or four strong distal spines. The chelifores of the smaller specimen are withdrawn beneath the cephalic hood with only the chelae exposed. The tip of the oviger in the larger specimen bears two simple spines ( Fig. 9B ). The abdomen is articulated at its base and arched with several dorsal and distal spines ( Fig. 9C ). Unlike the over-reaching abdomen of the larger specimen, the abdomen of the smaller specimen is directed downwards between the fourth lateral processes but the abdomen is very flexible in the vertical plane. Both of these specimens are chelate with only partially developed ovigers. A cement gland duct is not evident in either specimen. Developing ovigers are present but segmentation is incomplete.

Wang et al. (2018) recorded a juvenile specimen which they hesitantly referred to S. heteroscela Child & Segonzac, 1996 based mainly on the forward-leaning, bifurcate ocular tubercle and compact legs. It is likely that the juvenile specimen recorded by Wang et al. and these specimens from the Coral Seamount are one and the same species and represent juveniles and subadults of Child’s Walvis Ridge specimen. Should I be correct in identifying the SWIR specimens as subadult forms of an independent species then the distribution of S. heteroscela cannot be

said to extend from the Mid-Atlantic Ridge (SW of the Azores Islands) to the southern hemisphere as proposed by Wang et al.

Figures of the subadult chela, oviger and abdomen are provided but otherwise there is nothing to add to Child’s figures of the adult specimen.

Wang et al. (2018) described S. duanqiaoensis from the Duanquiao hydrothermal field on the SWIR but unlike the present material that species has a low, non-bifurcate ocular tubercle, dimorphic legs and is far more setose.

Figures

PLATE 3. Tanystylum tiara sp. nov., holotype male. A, trunk lateral; B, trunk, dorsal; C, trunk anterior view. Sericosura species indeterminate. D, trunk dorsal; E, trunk, lateral. Austrodecus bamberi. F, trunk dorsal; G, trunk, lateral. Austrodecus taylorae sp. nov., holotype, male. H, trunk dorsal.

FIGURE 4. Nymphon dentatum sp. nov., holotype, male. A, trunk, lateral view; B, trunk, dorsal view; C, detached leg; D, tarsus and propodus; E, second coxa; F, cement gland duct; G, right oviger; H, oviger distal segments; I, palp. Paratype, female (JC066-3398). J, abdomen, lateral; K, ocular tubercle (arrow indicating anterior); L, left chelae.

FIGURE 6 Nymphon species indeterminate. A, trunk, dorsal view; B, third leg; C, palps, chela and proboscis ventral view.

FIGURE 9. Sericosura species indeterminate (larger subadult specimen). A, chelifore, scale x10; B, oviger distal segment, scale x40; C, abdomen, lateral, scale x10.

FIGURE 19. Pycnogonum cheni sp. nov., holotype. A, trunk, lateral view; B, trunk, dorsal view; C, proboscis, anterior view; D, third leg; E, tarsus & propodus leg 3; F, left oviger; G, abdomen, dorsal view.