Polymastia rara Koltun 1966

Polymastia rara Koltun, 1966 (Figure 7 (a – g))

Polymastia mamillaris rara Koltun: 1966, p. 70 – 72, fig. 41; pl. XXIX, fig 6

Description

(Figure 7 (b)). Flattened, roundish body of more or less disc-shaped form, firmly attached to substrate (represented by shells and stones) by its basal part. Dimensions of about 2 cm in height and up to 4.5 cm in diameter. Surface provided with numerous cylindrical papillae from 2 mm to 1.6 cm in length. Among them, at the centre of the body, 1 – 2 rather large (up to 2 cm in height) conical exhalant papillae with oscula on the summits occur. The body relatively dense, cork-like in consistency. Surface is even. Sometimes the sponge possesses a fringe of extra-long spicules. The colour is beige.

Skeleton

(Figure 7 (a)). Main skeleton composed of thick radial bundles of large styles that extend from the base to the surface of the sponge, but usually do not cross the upper cortex. Skeleton of the cortex consists of two layers: the upper layer of small styles is arranged in palisade manner; this layer is underlain by disorderly intermediate styles.

Spicules

(Figure 7 (c – g)). Large, straight and fusiform styles, dimensions: 860 – 1350 – 1800 (n = 30) × 17.2 – 24.4 – 30 (n = 20) µm; intermediate styles, identical to the previous, except the dimensions: 349.3 – 478 – 620 (n = 30) × 11.5 – 13.4 – 16.6 (n = 20) µm; small styles (tylo- and subtylostyles), straight or slightly curved, dimensions: 127 – 169.6 – 253.2 (n = 30) × 4.5 – 6.3 – 7.9 (n = 20) µm; slender exotyles, slightly fusiform styles of the marginal fringe, up to 3 mm in length and about 11 µm in thickness.

Distribution

Near the east coasts of Kamchatka Peninsula and Kuril Islands, East Siberian Sea [north of New Siberian islands, st. A-74), Laptev Sea (st. L-19 (two species) and L-12 (one species)]. Depth range: 36 – 126 m.

Remarks

This is the first record of P. rara after Koltun (1966) described it based on two specimens collected in the Sea of Okhotsk. In addition to the differences in spicule composition, P. rara differed from the closely related P. grimaldii (Topsent 1913) by the structure of its cortex. In the case of P. grimaldii, it encompasses an additional middle layer of collagen fibres (Plotkin et al. 2017), which is absent in P. rara.