Stegolaria laevigata, Watson, Jeanette E., 2017

Watson, Jeanette E., 2017, Two bathyal hydroids (Hydrozoa: Leptothecata) from the Southwest Indian Ocean, Zootaxa 4247 (3), pp. 336-340 : 336-338

Stegolaria laevigata sp. nov. ( Figure 1A–G )

Material examined. Holotype, NHMUK 2016.1, infertile branched colony 2 cm long without hydrorhiza; two microslides from holotype colony. Coral Seamount, Southwest Indian Ocean Ridge (41° 22.31’S, 54° 57’E), depth 732 m. Paratype, NMV F231666, infertile branched colony 2.5 cm long without hydrorhiza; NMV F231666.1, NMV F231666.2, microslides, from paratype colony. Coral Seamount, Southwest Indian Ocean Ridge (41° 22.31’S, 54° 57’E), depth 732 m.

Description. Hydrocaulus thick, heavily fascicled, longest branch 35 mm long and 0.1–0.12 mm wide at base, polysiphonic tubes running parallel up branches. Branching irregularly alternate, planar, directed obliquely upwards from below a hydrotheca.

Hydrothecae alternate, in older stem region immersed in polysiphonic tubes, in younger monosiphonic region adherent to single axial tube. Hydrotheca long, tubular, curving outwards at approximately 60° from axial tube, proximally narrow, gradually increasing in diameter to outward bend, thereafter cylindrical, adnate adcauline wall parallel to axial tube, free wall curving convexly outwards, a small loop in perisarc at base of adnate adcauline wall; abcauline wall smoothly concave. Ratio of free to adnate adcauline wall 1:1.6. Hydranth without caecum and with c. 10 tentacles, a very thin transverse perisarcal lamella anchoring base of hydranth to hydrotheca by thin strands of tissue. Margin of hydrotheca fragile, closed by a gable-shaped operculum of two delicate valves, usually shredded when open, shredding often accompanied by a depression in adcauline wall.

Cnidome comprising heteronemes of two sizes, none discharged:

- capsule lozenge-shaped, 22–25 x 6–7µm, moderately common,

- capsule oval, 11 x 4 µm, very rare.

Colour of preserved colony white. Perisarc of accessory tubes moderately thick, hydrothecal perisarc thin.

Measurements (µm) of Stegolaria laevigata sp. nov.

Branch

distance between alternate hydrothecae 760–800 Hydrotheca (on monosiphonic axial tubes)

overall length to margin 700–1160 length of adnate adcauline wall 656–880 length of free adcauline wall 416–560 length of abcauline wall 860–1280 width at margin 224–240 width at floor 120–160 depth of opercular valve 200–220 Remarks. The genus Stegolaria was instituted by Stechow (1913) to accommodate two species, Cryptolaria geniculata Allman, 1888 and Cryptolaria operculata Nutting, 1905 . A third species, Stegolaria irregularis , was described from New Zealand by Totton (1930). Stegolaria geniculata is a widely distributed deep water species recorded from world oceans ( Vervoort 1946; Ramil & Vervoort 1992; Calder & Vervoort 1998) and S. operculata is recorded from the Hawaiian Islands and the New Zealand region ( Vervoort & Watson 2003). Allman’s description and figure of S. geniculata are of a distinctly geniculate hydrocaulus, a hydrotheca with a gable-shaped operculum, (probably readily shredded), and a smooth flask-shaped gonotheca with circular orifice. Totton (1930) described but did not figure S. irregularis . The hydrocaulus was figured by Ralph (1957) from the type specimen and Vervoort & Watson (2003) described and figured a tubular gonotheca with a plicate conical operculum from New Zealand. None of these descriptions match the present specimen.

Watson & Vervoort (2001) doubtfully assigned a species from south-east of Tasmania (46° S, depth 800 m) to S. geniculata ( Allman, 1888) . Their species had a smooth gable-shaped hydrothecal operculum, a gonotheca with a scoopshaped aperture and operculum of a single flap. The match of colony morphology and hydrothecal dimensions leaves little doubt that the Tasmanian species is S. laevigata .

Etymology. The specific name derived from the Latin word laevis refers to the smooth hydrothecal that distinguishes the species.

Figures

FIGURE 1 A – G Stegolaria laevigata sp. nov. 1 A, branch of holotype colony. 1 B, polysiphonic tubes of branch and monosiphonic hydrocladium. 1 C, hydrotheca immersed in axial tubes, two intact opercular valves. 1 D, hydrotheca on monosiphonic axial tube with shredded operculum and collapsed free adcauline wall. 1 E, basal perisarcal lamella with anchoring spokes of tissue in hydrotheca. 1 F, small and large heteroneme nematocysts. Scale bar: A, 10 mm. B, 2 mm. C, D, 0.5 mm. E, 0.2 mm. F, 20 µm.