Hemiasterella elongata Topsent 1928

Hemiasterella elongata Topsent, 1928

(Figs. 3B, 5; Table 2)

Material examined. Four specimens collected: MNCN-Sp66-BV21 from Stn. 21; MNCN-Sp04-DR29 from Stn. 29 m; and MNCN-Sp20-BV33A & B from Stn. 33 (Table 1, Fig. 1).

Macroscopic description. Specimens with columnar shape, measuring 5–15 x 4–7 mm (Fig. 3B). The individuals are settled on rock pieces, over which slightly expand their base. The surface shows irregularly shallow folds and grooves, mostly running parallel to the longest body axis. The ectoderm is membrane-like and bears a sparse and uneven hispidation. Pore-like aquiferous openings are visible, especially in the lower half of the body. Color is bright to creamy white both in life and after preservation in ethanol.

Skeleton. Megascleres are styles, measuring 1316– 2250 x 10–30 µm. They are straight, markedly curved, or just with a slight asymmetrical curvature (Fig. 5A–B). The round end of the styles may also be in a stronglyoxea fashion; the pointing end is regularly acerate or, less frequently, stepped, not very sharp (Fig. 5A–C). Styles with both ends modified into oxea are very rare (e.g., one of 1825 x 10 µm per slide) or absent, depending on the individuals. Microscleres are abundant spherostrongylasters, with only a moderately developed centrum and 10–15 strongylote, slightly conical, spiny actines (Fig. 5A, C–E). Spines are more dense toward the end of the actines. Spherostrongylasters range from 14 to 23 µm in total diameter.

The skeletal arrangement shows no axial condensation. Ascending plumose pauci- or multispiculate tracts of styles ramify below the ectosome and may end in plumose tufts that make an hispid surface. There is scarce spongin connecting and packing the spicules in the tracts. Spherostrongylasters are very abundant overall the skeleton, but especially at the periphery, where they make a layer reinforcing the ectosome.

Distribution and ecology notes. Rare species, previously known only from its holotype collected at Cape Verde Islands, eastern North Atlantic (Topsent 1928). The herein collected individuals provide the first record of the species for the Mediterranean Sea. All the collected specimens inhabited 93 to 173 m deep, soft bottoms rich in organogenic gravel, occasionally mixed with pieces of dead rhodoliths.

Taxonomic remarks. The collected specimens bear overall similarity with the holotype described by Topsent (1928). Nevertheless, some morphological differences occur. The holotype shows two incipient branches, while the Alboranian specimens show no sign of branching. Another difference is that the Alboranian individuals have thinner styles (10–30 µm) than the holotype (25–60 µm).

Hemiasterella aristoteliana Voultsiadou-Koukoura & Van Soest, 1991, the only Hemiasterella representative recorded in the Mediterranean previously, occurs in the northern Aegean Sea. Although it has also styles and strongylasters as the only spicule types, the species is clearly distinguishable from H. elongata, because the former has much longer styles (1800–3000 x 18–37 µm) and its asters are commonly reduced to forms with only 1 to 3 actines (Voultsiadou-Koukoura & van Soest 1991).

As noted by Topsent (1928), there are some similarities between H. elongata and Hemiasterella vasiformis (Kirkpatrick, 1903) from South Africa. Nevertheless, the latter has a caliculate body shape, many styles becoming tylostyles and strongyles, and a bit larger asters (up to 30 µm of diameter) (Kirkpatrick 1903).

Together with the Antarctic Hemiasterella digitata Burton, 1929, H. elongata shows an uncommon shape within the genus, but that of H. digitata is better described as palmo—digitate, with a surface strongly hispid in small patches and neither oscules nor pores visible (Burton 1929).