Astrotylus Plotkin & Janussen 2007

Genus Astrotylus Plotkin & Janussen, 2007

Diagnosis: Cushion­shaped encrusting sponges with hispid surface. Choanosomal skeleton consists of radial tracts of principal megascleres, between which numerous microscleres and rare small megascleres are freely scattered. Cortical skeleton formed by the main tracts arising from the choanosome, diverging into bouquets, reinforced by the irregular palisade of small megascleres and echinating the surface. Megascleres are tylostyles, microscleres are astrotylostyles.

Type species: Astrotylus astrotylus Plotkin & Janussen, 2007 (by original designation).

Astrotylus astrotylus Plotkin & Janussen, 2007 (Fig. 3)

Synonymy

Astrotylus astrotylus— Plotkin and Janussen 2007: 1395, figs. 1–3.

Material examined

Holotype: SMF 10518 (1 specimen): PS67/102­11. Paratypes: SMF 10517 (1 specimen): PS67/094­11; SMF 10519 (1 specimen): PS67/102­11; SMF 10516 (1 specimen): PS67/110­2.

Description (emended from Plotkin & Janussen 2007)

External morphology. Sponges are small cushion­shaped encrustations on pebbles measuring up to 21x18 x 4 mm (fig. 3A). Surface is uniformly or irregularly hispid, brownish or greyish in colour due to the covering sediments. A single exhalant papilla is very small or the sponges may lack a papilla. The cortex is greyish, hardly detachable and rather resilient. The choanosome is beige to greyish, soft but rather dense.

Skeleton. The main choanosomal skeleton is constituted by the radial tracts of principal tylostyles (fig. 3B). The thickness of these tracts in the basal choanosome measures 150–290 µm. The tracts branch, cross the cortex, where they diverge into bouquets, and make up the surface hispidation. The cortical bouquets of principal tylostyles are reinforced by an irregular palisade of small tylostyles. The cortex is 200–550 µm thick and the thickness of the surface hispidation is 570–2000 µm including the external area of dense sediment cover which measures 170–1200 µm. The aquiferous cavities, measuring 80–130 µm in diameter, are visible in the upper part of the cortex, among the cortical bouquets. The additional choanosomal skeleton is composed of freely scattered small tylostyles and microscleres—astrotylostyles. The tylostyles are very rare. Numerous astrotylostyles are uniformly distributed between the main tracts; their mean density is about 300–500 per 1 mm 2 of the section. We were unable to study the aquiferous system and skeleton architecture of the papillae because of their extremely weak development.

Spicules. Two size categories of tylostyles are well distinguished, and astrotylostyles constitute a third category (see frequency distributions in Plotkin & Janussen 2007). Altogether, 120 spicules of each category were measured (30 spicules in each specimen; see particular results of measurements for each specimen ibidem).

Megascleres are exclusively tylostyles with well­developed, terminally located, spherical or oval tyles. Principal tylostyles are usually straight and slender (figs. 3C–D). Their dimensions are: length 914­1814­5414 µm, tyle diameter 11­17.2­28 µm, diameter of the shaft underneath the tyle 5­11.2­20 µm, maximal diameter of the shaft 8­16.7­24 µm. Small tylostyles are straight or rarely slightly curved, fusiform, with well­developed, terminal spherical tyles (fig. 3E). They measure: length 217­396­750 µm, tyle diameter 10­13.2­18 µm, diameter of the shaft underneath the tyle 3­7.6­12 µm, maximal diameter of the shaft 5­13.3­19 µm.

Astrotylostyles are straight or curved (fig. 3F). Each spicule consists of a shaft with a small fungiform serrated tyle at one end (fig. 3G), the other end bearing a multi­rayed star­like structure resembling an asterose spicule. The latter structure varies in regularity, its one ray being always a prolongation of the main spicule shaft. The dimensions of astrotylostyles are: length 50­71.0­93 µm, diameter of tyle 1­2.8­5 µm, diameter of the shaft underneath the tyle 1­1.6­3 µm, diameter of the shaft underneath the star­like formation 2­4.7­7 µm, diameter of the star­like formation 8­20.7­35 µm.

Type locality: Antarctic: Northern Weddell Sea, ca. 4700–4900 m (known only from type locality). Remarks. The affinities of A. astrotylus with allied polymastiid species have been discussed by Plotkin and Janussen (2007).