Megapogon pollicaris

Megapogon pollicaris (Jenkin, 1908)

(Figs 16 A–H; Table 12)

Original description. Jenkin 1908, p. 40, pl. XXXVII and XXXVIII, figs. 125–130.

Type locality. Winter Quarters Bay, Antarctic.

Synonym and citations. Megapogon pollicaris, Burton 1963, p. 93.

Material examined. Holotype: BMNH-1907.8.6.135 (one fragment and three slides), Winter Quarters Bay, Antarctic, National Antarctic Expedition (HMS Discovery).

Morphology. There is only one fragment of the holotype. Dark brown coloration because it was fixed in osmic acid (Jenkin 1908). Surface slightly hispid. Oscular region absent in the fragment. Size of the fragment 3.5 mm large, 2.3 mm wide and 0.6 mm thick (Fig 16A). The aquiferous system is leuconoid composed of rounded inhalant cavities under the cortex, and round choanocyte chambers which are scattered in the choanosome (Fig 16B).

Skeleton. Cortical skeleton formed by triactines tangentially placed and large projecting diactines (Fig 16C). Choanoskeleton composed of the unpaired actines of the atrial chiactines, which can project outside the surface (Figs 16 D–E). A few triactines can also be present in the choanosome, with the unpaired actines pointing towards the cortical skeleton (Fig 16E). Abundant spined microdiactines are irregularly scattered in the choanosome (Fig 16F). Atrial skeleton made up by chiactines with the paired actines placed tangentially, and the apical actines projecting into the atrial cavity (Fig 16B).

Spicules. Diactines: large and bent towards the distal end which is blunt and thicker than the proximal end (Fig 16G). Size: 506.8 ± 43.1 µm length, 20.0 ± 3.4 µm width (Table 12).

Microdiactines: small, curved and spined. One hastate end and the other sharply pointed (Figs 16F, 16H). Size: 60.1 ± 4.0 µm length, 2.8 ± 0.5 µm width (Table 12).

(*) Diactines II = Microdiactines.

Cortical triactines: sagittal with straight and long unpaired actines. Paired actines shorter, nearly straight or slightly irregular (Fig 16H). Size: unpaired actines 289.5 ± 64.0 µm length, 11.5 ± 2.1 µm width; paired actines 182.2 ± 22.7 µm length, 10.8 ± 2.7 µm width (Table 12).

Chiactines: unpaired actines straight and longer than the paired ones. Apical actines short and acerate (Figs 16B, 16G). Size: unpaired actines 338.9 ± 106.8 µm length, 12.9 ± 2.1 µm width; paired actines 204.5 ± 22.6 µm length, 12.7 ± 2.3 µm width; apical actines 81.2 ± 13.8 µm length, 10.0 ± 1.4 µm width (Table 12).

Distribution and depth. Specimens taken around Winter Quarters Bay, Antarctic shallow waters (Jenkin 1908).

Molecular identification. Not available.

Remarks. After re-examining the holotype of M. pollicaris (BMNH-1907.8.6.135), we found that this species presents similarities with M. raripilus, for example, shape of the diactines and microdiactines, presence of triactines in the atrial skeleton, and size of the spicules. Also, according to Jenkin (1908), M. pollicaris has an oscular collar built up in a similar way as in other species of Megapogon and Achramorpha, i.e. with tetractines and triactines with the unpaired actines pointing downwards, but without an oscular fringe as in M. raripilus. However, these two species show some differences; microdiactines are more abundant in M. pollicaris, diactines are slightly longer in M. raripilus (300–800 µm), and absence of trichoxeas in the cortical skeleton of M. pollicaris. However, it has been suggested that trichoxeas are not very reliable in differentiating species, e.g. in the genera Leucandra (Rapp 2015) and Clathrina (Azevedo et al. 2017). The aquiferous system in both species is leuconoid with rounded choanocyte chambers, which are bigger in M. raripilus (131.2 ± 31.7 µm diameter) µm than in M. pollicaris (109.2 ± 12.5 µm diameter). Also, the rounded inhalant cavities are also slightly bigger in M. raripilus (146.8 ± 36.4 µm diameter) µm than in M. pollicaris (141.9 ± 33.8 µm diameter), but these are more marked and numerous in M. pollicaris.

Based on our observations, we suspect that M. raripilus and M. pollicaris are different but closely related species, however, to be able to have a confident decision, it is necessary to have new and larger specimens of M. pollicaris, for a better examination and for molecular evidence.