Gromia melinus Rothe, Gooday, Cedhagen, Fahrni, Hughes, Page, Pearce & Pawlowski, 2009, SP. NOV.

GROMIA MELINUS SP. NOV. (FIGS 5, 6)

Diagnosis: Small species of Gromia with a fairly robust test, which is spherical, droplet-shaped, subtriangular, or asymmetrically irregular in shape; length 0.6–1.5 mm; length: width ratio 1.0–1.5. One, occasionally two, small oral capsules. Test wall fairly stiff, with distinctive network of fine ridges and surface coating of clay particles.

Type material and locality: The holotype and paratypes are from an EBS deployment at station 80#9, 70°39.07 ′ S, 14°43.36 ′ W, 3108-m depth, collected on 23rd February 2005 (Table 1). They are deposited at the Research Institute and Natural History Museum Senckenberg, Frankfurt am Main. The holotype is catalogued under reg. no. SMF XXVII 7400. The paratype is catalogued under reg. no. SMF XXVII 7401. The type specimens were extracted from the > 500-Mm sediment residue, and are preserved in buffered 4% formaldehyde solution.

Additional material: Stations 80#9 and 81#8; 16 specimens.

Derivation of name: From the Latin melinus, meaning ‘honey’, alluding to the honeycomb pattern of the test wall.

Overall appearance: The 16 specimens range in length from 0.6 to 1.5 mm (mean 0.9 ± 0.2 mm), and in width from 0.4 to 1.4 mm (mean 0.8 ± 0.2 mm). The length: width ratio varies from 1.0 to 1.5 (mean 1.2 ± 0.1). The test morphology varies from subtriangular, to droplet-shaped, to spherical, or asymmetrically irregular (Figs 5A–D, 6A). Specimens are brown to yellowish in colour after formalin fixation.

Oral capsule: Most specimens have a single oral capsule (Fig. 5A–C), but two are present in one individual (Fig. 5D). The oral capsule is roughly circular in plan view, and is a distinct feature. In some specimens it protrudes slightly from the test surface and gives rise to a long, flaccid, translucent extension, composed of organic material (Fig. 5B). In other individuals it is much flatter, circular, and hardly protrudes from the surface (Fig. 5A, D).

Test wall: The wall is fairly rigid and characterized by a very distinctive pattern of ridges, which are clearly visible under the binocular microscope. The ridges form a network, the meshes of which vary in shape but tend to be more or less polygonal (Fig. 5E). The diameter of the polygonal elements ranges from ~30 to ~50 Mm. The surface of the test between the ridges is covered with tiny, plate-like particles (Fig. 5F), which were identified as clay minerals, based on X-ray elemental microanalyses that indicated the presence of K, Mg, and Fe, in addition to Al and Si. The inner side of the wall is composed of multiple honeycomb membranes, which appear as a series of regular lines in the TEM images (Fig. 6B–D).

Distribution: Off Kapp Norvegia, eastern Weddell Sea, 3103- and 4392-m depth (Fig. 1).

Remarks: Gromia melinus sp. nov. is distinguished from most other Gromia species by the prominent, polygonal pattern of ridges raised from the general test surface. They are not to be confused with the layer of ‘honeycomb’ membranes described by Hedley & Wakefield (1969) and Bowser et al. (1996). This unusual feature has also been described by Aranda da Silva (2005) in Gromia sp. 2 from the Oman margin of the Arabian Sea. In both cases, the polygons that make up the pattern are formed from the outermost test layer. In G. melinus sp. nov. the relationship of the polygonal ridges to the inner layers of the wall is obscured by the layer of clay particles, which is not present in the Arabian Sea species. The Arabian Sea species also has a more consistent shape (Aranda da Silva, 2005) than G. melinus sp. nov. The two species are clearly separated by the molecular analysis (see below).

Like G. oviformis, G. melinus sp. nov. displays a variety of morphologies. It is a small species, being approximately 0.8-mm wide and 0.9-mm long, and is therefore similar in size to G. pyriformis (< 1-mm long), but is smaller than G. oviformis (up to 5-mm long), G. sphaerica (up to 38 mm), and G. schulzei (8–9-mm long) (Schulze, 1875; Hedley & Bertaud, 1962; Gooday et al., 2000; Gooday & Bowser, 2005). Whereas G. melinus sp. nov. typically has one oral capsule, like most other gromiids, one spherical specimen had two capsules. According to Jepps (1926), occasional specimens of G. oviformis also have more than one aperture. In contrast to the distinct moundlike oral capsule in G. marmorea sp. nov., which clearly projects from the test surface, G. melinus sp. nov. has a relatively flat capsule, which hardly protrudes. The long organic extension that arises from the central canal of the oral capsule in some specimens of G. melinus sp. nov. also occurs in G. marmorea sp. nov., as well as in G. oviformis (Jepps, 1926).