Sigmodota magnibacula Massin & Heterier 2004

Sigmodota magnibacula (Massin & Hétérier, 2004)

Figure 1, 5, key

Taeniogyrus contortus.— Ekman, 1927: 416 –417.— Cherbonnier, 1974: 610.— Gutt, 1988: 24.— Gutt, 1991: 324. (all or part probably non Taeniogyrus contortus (Ludwig, 1875))

Taeniogyrus cf. contortus.— O’Loughlin et al., 1994: 553, 554.

Taeniogyrus magnibaculus Massin & Hétérier, 2004: 441 –444, figs 1, 2, table 1.

Sigmodota magnibacula.— O’Loughlin & VandenSpiegel, 2010: 82, 84, figs 1e, 11, tables 2, 3.

Material examined. Antarctica, Weddell Sea, station ‘Shelf Canyon 2’, AGT 39, 76.161694– 76.16887 ºS, 27.79659– 27.79802 ºW, agassiz trawl 549.28–555.26 m, BAS cruise JR275 RSS James Clark Ross, collected by M. Mackenzie et al., 21/02/2012, NHMUK 2014.31 (1).

Description. Sigmodotid species specimen, vermiform, tapered posteriorly (posterior end missing), 15 mm long (preserved); scattered dorsal, lateral and ventral wheel-cluster papillae; 12 tentacles, seven pairs of digits, largest distally; single polian vesicle.

Ossicles in tentacles rods, some with short distally-spinous or handle-like branches along shafts, short bluntly spinous bifurcations distally, rods 205–256 µ m long; ossicles in body wall wheels and sigmoid hooks; wheels clustered into discrete papillae dorsally and laterally and ventrally, wheels rounded hexagonal with six spokes and continuous series of teeth around inner rim, diameters up to 80–117 µ m; sigmoid hooks scattered throughout body wall, fine spinelets on distal outer end, hook lengths 122–151 µ m.

Colour. Alcohol-preserved (briefly): body and tentacle and oral disc completely covered with fine “black” speckle over off-white.

Distribution. Antarctica, Weddell Sea, ‘Shelf Canyon’, 76ºS, 28ºW, 549– 555 m.

Remarks. As noted in earlier remarks, it is evident from Table 3 in O’Loughlin & VandenSpiegel (2010) that for specimens of Sigmodota magnibacula the sizes of the wheel and sigmoid hook and tentacle rods ossicles increase significantly with increasing specimen size. The size ranges of tentacle rods, wheels and sigmoid hooks for this anterior part 15 mm long specimen from the Weddell Sea are consistent with the size ranges of similarly small specimens from the South Orkney Islands and the Ross Sea.

The form of many of the tentacle rods in this Weddell Sea specimen is similar to the form illustrated by Massin & Hétérier (2004) and O’Loughlin & VandenSpiegel (2010 fig. 11), but we also observed some lateral branching along the tentacle rod shafts that we have illustrated here. This form of tentacle rod has not been reported previously for Sigmodota magnibacula. We also observed fine spinelets on the distal outer surface of the hooks. These fine spinelets may be difficult to observe in the SEM image in this publication as they appear to be slightly eroded, but they are more graphically illustrated in O’Loughlin & VandenSpiegel (2010, fig. 11). The depth of 549–555 m from which this specimen was taken is slightly deeper than the deepest previous record of 525 m in Prydz Bay in Eastern Antarctica. The significant lateral branching on the rod ossicles prompts us to refer this incomplete specimen with some reservation to Sigmodota magnibacula.