Microxina sarai Calcinai & Pansini 2000, n. sp.

Microxina sarai n. sp.

(Figs 8; 9)

MATERIAL EXAMINED. — Holotype: stn MM106, 1995-1996 campaign, 95-110 m, POR226 (MNA No. 4); paratypes: haul 24, 20.I.1988, POR271 (MSNG). — Stn Faraglione, 100 m, 19.II.1998, POR265; 1993-1994, POR169, POR172, POR174; 13.I.1994, POR278, POR279. — Stn Adelie Cove, 10.I.1994, POR276, POR277; 1989-1990 campaign, POR280.

ETYMOLOGY. — The new species is dedicated to Prof. Michele Sarà who has been heading a benthic research unit of the Italian National Program of Antarctic Research for ten years.

TYPE LOCALITY. — Terra Nova Bay, 74°42’20S, 164°10’90E.

DESCRIPTION

From the broken specimens available (the biggest is 10 cm high and 5 cm across), the sponge is massive amorphous with large lobate processes and may reach a large size (Fig. 8A.) The colour is pale yellow or beige. The sponge is inelastic, tough but very fragile, both wet and dry. The surface is bristly from the protruding spicule brushes. A thin ectosomal membrane is lost in most of the dredged specimens. Oscules are round, slightly raised (1-2 mm), with a diameter of 6-8 mm (Fig. 8A).

Skeleton

The ectosome is a thin layer of disorderly arranged tangential oxeas with abundant microscleres (Fig. 8C). The choanosomal skeleton is formed by multispicular (six to ten) tracts of oxeas which dichotomically divide or merge forming a regular ascending pattern (Fig. 8D). Primary tracts are connected by oxeas (one to three) thus forming a loose reticulation with scarce spongin. In proximity of the sponge surface, the ascending tracts diverge to form tufts of oxeas (Fig. 8B) which penetrate the dermal membrane. Interstitial oxeas and microscleres are abundant, also in the choanosome.

Spicules

Oxeas straight or slightly curved (Fig. 9A), measuring 209-382 (278.9) × 14-20 (17.3) µm. Slender forms, which are always straight and generally longer than average are rather frequent (Fig. 9A).

Sigmas occur in two categories. Small sigmas are thin and more closed: 16-24 (25.3) × 1-2 µm (Figs 8F; 9B). Large sigmas are slightly thicker and more open in shape (Fig. 9C): 36-62 (45.9) × 1-3 µm.

Microxeas straight or very slightly curved, sometimes with the tips bent back (Figs 8E; 9D). They measure 50-80 (62.8) × 1-2 µm.

REMARKS

The new species is placed in Microxina Topsent, 1916 based on its choanosomal skeleton with characteristic polyspicular tracts ending in superficial tufts and presence of microxeas and sigmas as microscleres. The presence of a thin ectosomal membrane with tangential oxeas is an important character of M. sarai n. sp. that is not shared by the other species in this genus. Two other species of Microxina with microxeas as microscleres are known from the Antarctic: M. benedeni (Topsent, 1901) and M. simplex (Topsent, 1916). They share with the new species the structure of the choanosomal skeleton, but the polyspicular tracts (15-20 oxeas) are much stouter and the oxea size is at least twice as big. Also their external morphology is different because they have an uneven surface covered by long slender processes (especially in M. benedeni) that gives them a peculiar spiny appearance. Several specimens of M. benedeni were recorded from Terra Nova Bay.

ECOLOGY

The species is abundant on the continental shelf of Terra Nova Bay between 95 and 110 m depth on hard bottoms with a clear sponge predominance (Cattaneo-Vietti et al. 1999b). It may attain a remarkable size (at least 30 cm in height) as seen on the R.O.V. videos but since it looks very similar to Microxina rudis (Topsent, 1901) in the same area, the image interpretation requires caution.