Phorbas megasigma Rios & Cristobo 2007

Phorbas megasigma Ríos & Cristobo, 2007

(Fig. 9, Tab. 6)

Phorbas megasigma Ríos & Cristobo, 2007a: 1486 –1488, figs. 1–5, tab. 1.

Material. Several fragments of 1 (?) specimen from station 048-1 (SMF 11817, 11833), 602.1 m, 70° 23.94' S, 8° 19.14' W, 12.01.2008.

Description. Sponge grows upright reticular, composed of 2 to 5 mm thick fibres, rarely forming wider, flattened structures (Fig. 9 A). Fibres regularly intercrossing, often in rectangular pattern. Surface bristled, knotty, porous, often with small openings (0.5 mm) piercing through entire sponge body. Consistency rather soft, spongy. Color in ethanol brownish beige.

Skeleton: Made up of plumose tracts of acanthostyles, interconnected by irregularly scattered acanthostyles (Fig. 9 B). In epidermis, echinating bundles of tornotes present. Chelae regularly distributed throughout the tissue.

Spiculation (Tab. 6): Main spicules acanthostyles (Fig. 9 C), 520–645 µm in length and 15–25 µm in diameter. Additionally, smaller acanthostyles occur (Fig. 9 D), 175–410 µm in length with diameter 5–20 µm. Further megascleres tornotes (Fig. 9 E), 305–420 µm in length and 5 µm in diameter. Main microscleres unguiferate chelae (Fig. 9 F-H), 15–20 µm in length. The characteristic large sigmas (Fig. 9 G) extremely rare, but clearly present within the tissue,about 100–270 µm x 46.5–105 µm.

parameter SMF 11817 Ríos & Crostobo (2007a) P. nexus Koltun (1964) Acanthostyle I

length 520–578.8–645 (30) 580–687–780 520–720

diameter 15–19.1–25 (30) 10–18–20 15 –25 Acanthostyle II

length 175–244–410 (30) 200–263.25–295

diameter 5–13.3–20 (30) 10–17.5–25 Tornote

length 305–378–420 (30) 300–383.92–445 370–480

diameter 5 (30) 2.5–4.75–5 5–7 Sigma

length 100–192–270 (30) 25–116.1–230

max. width 46.5–75.9–105 (30)

Arcuate Isochelae

length 20–21.33–25

max. width 5–7.35–10 Unguiferate Isochelae

length 15–16.5–20 (30) 15–17.58–20 16–20

Remarks. There are several similarities between the species P. m e g a s i g m a and Phorbas nexus (Koltun, 1964): Both possess large acanthostyles, tornotes and unguiferate isochelae very similar in size and shape (compare Ríos & Cristobo 2007a; Koltun 1964; see also Tab. 6). Main differences are the occurrence of small acanthostyles, arcuate isochelae and sigmas in P. megasigma, which are absent in P. nexus. Still it has to be noted, that in Koltun's (1964) figure of P. nexus smaller acanthostyles are shown, which are not mentioned in the text. Our specimen thus combines characters of both species: First, it has small acanthostyles, which as mentioned above are of little taxonomic value. Also it has large sigmas, similar to those characterizing P. megasigma. Arcuate chelae on the other hand are absent in our specimen. Although the sigmas are very rare, we chose to count them as evident for the identification and thus assigned our specimen to P. m e ga s ig m a. Still, our sponge seems to be a transitional form between both species. The distribution of the species is remarkable: whereas P. megasigma was reported from the Bellingshausen Sea (Ríos & Cristobo 2007a), P. nexus originates from Wilhelm II coast (Koltun, 1964). Our specimen from the Weddell Sea is positioned between those two, not only in characters, but also geographically.