Thenea abyssorum Koltun 1964

Thenea abyssorum Koltun, 1964

(Figures 17, 18 A–B)

Synonymy.

Thenea muricata abyssorum Koltun, 1959 (nomen nudum): Koltun 1959, p. 662. Thenea abyssorum: Koltun 1964, p. 146; Koltun 1966, p. 37; Steenstrup & Tendal 1982, p. 259; Barthel & Tendal 1993, p. 83;

Witte 1996, p. 571; Weslawski et al. 2003, p. 75; Cárdenas et al. 2011, Table S1. Thenea sp.: Babiç 1915, p. 408.

Material. NTNU-VM 54948, off Vesterålen, northern Norway, 69°30.3'N, 13°55'E, 2130 m; NTNU-VM 66585, Røstbanken, northern Norway, 68°36.5'N, 11°54'E, 675- 850 m.

Comparative material.

Thenea abyssorum, ZMBN 85228, Arctic mid-ocean ridge, Greenland Sea, 73°34'N, 07°45'E, H2 DEEP cruise 2008, 2425– 2463 m, Sneli sled (Sneli 1998).

Outer morphology (Fig. 17 A). Small sub-spherical sponge (2.5 cm high, 2 cm wide) with root-like structures. Very hispid. Slightly compressible. Color alive and in ethanol is dirty brown. Choanosome is whitish to brownish (alive and in ethanol). Pores and oscule are on opposite sides (in upper part of sponge). The pore area is covered by a sieve and protected by an overhang. There is no sieve over the single oscule (ca 1 mm in diameter), it is circular and surrounded by a fringe of long oxeas.

Skeleton (Fig. 17 B–C). Radial bundles of oxeas start at the center of the specimen. Many go beyond the surface, especially on the top surface, very hispid. Triaenes are found only at the surface (Fig. 17 C), either crossing the ectosome or supporting it. Spirasters are abundant in the ectosome, but spirasters and plesiasters can be found throughout the choanosome (spirasters being much more common than plesisasters). The roots are made of bundles of oxeas and anatriaenes and they originate in the central part of the sponge. Anatriaenes are especially common in the basal part, always crossing the ectosome.

Spicules (ZMBN 85228) (Figs. 17 C–F, 18A–B). (a) oxeas, straight, length>3750 µm; width: 20–40 µm. (b) dichotriaenes (Fig. 17 C-D), rarely modified to orthotriaene or plagiotriaene, straight rhabdome or slightly bent, rhabdome length>4165 µm; rhabdome width: 20- 47.2 -70 µm; protoclad length: 60- 132.4 -210 µm; deuteroclad length: 240- 480.5 -850 µm. (c) anatriaenes I (Fig. 17 E), common, straight rhabdome with a rather blunt tip, rhabdome length: 920-1257.5-1750 µm (N=22); rhabdome width: 5- 9.0 -15 µm; clad length: 45- 87.8 -130 µm. A more or less developed swelling can be found above the cladome. (d) anatriaenes II (Fig. 17 F), common, very long and flexuous rhabdome, length between 2000 µm to more than 3600 µm; rhabdome width: 10 µm; very long clades running somewhat parallel to the rhabdome, giving to the anatriaene a closed-umbrella shape, in some cases the cladome is aborted and only an inflated tip is present, clad length: 40- 279.7 -548 µm (N=16). (e) protriaenes, absent in this specimen. (e) spirasters (Fig. 18 B), strongly spined, length: 25.0- 36.6 -57.3 µm. (f) plesiasters (Fig. 18 A), 4– 6 actines, strongly spined or more rarely smooth, actine length: 60- 84.3 -100 µm; actine width: 13–16 µm.

Reproduction. This species has been shown to be oviparous and gonochoristic, but successive hermaphroditism cannot be excluded (Witte 1996). Oogenesis takes place between June and August with spermatic cysts found only in August (Witte, 1996). Subglobular buds are relatively common in this species, they are often placed at the lower half of the specimens (Barthel & Tendal 1993; Witte 1996).

Distribution. Arctic Ocean, Norwegian Sea, Greenland Sea (Steenstrup & Tendal 1982, Fig. 1 D).

Depth. 850–3670 m (this study; Steenstrup & Tendal 1982).

Discussion. T. abyssorum is the northernmost and deepest (> 1000 m) Thenea species of the North Atlantic. On the field, this species can be easily confused with T. muricata or small T. valdiviae: a spicule preparation is therefore necessary. T. abyssorum is characterized by its large strongly spined plesiasters and streptasters. We report here for the first time a second category of anatriaene in this species, with unusually long clads (Fig. 17 F).