Diagnosis: Cortex composed of tripods with a fourth rudimentary actine. Skeleton of the tubes composed of triactines and tetractines with cylindrical actines.
Synonymies: Leucosolenia primordialis: Brøndsted 1933: 4 (see Rapp 2004), Leucascus lobatus: Rapp 2004: 119.
Type material: ZMUC POR- 245 (Holotype; Kap Farvel (6007.7 ’N, 4417 ’W), Greenland; Kap Farvel Expedition; 08/08/ 1970; depth: 230 to 250 m; substratum composed of large stones).
Type locality: Kap Farvel, Greenland.
Description: Colour is white after fixation. The cormus is lobate and the oscula are surrounded by membrane. The surface is smooth and perforated by inhalant apertures (Figure 6 A). The tubes are tightly and regularly anastomosed. The atrial surface is smooth. Reproductive elements are present.
Skeleton: The cortical membrane could be recognized only in some regions (Figure 6 B). The cortical skeleton is formed by tripods varying in size and organized in several layers. These spicules are exclusively cortical. Triactines with cylindrical actines are also present in the cortical skeleton, surrounding the inhalant apertures. The skeleton of the tubes is composed of triactines and tetractines with cylindrical actines. This last category of spicule projects the apical actine into the lumen of the tubes (Figure 6 C), but smooth tubes are also present. The atrial skeleton is composed of triactines and tetractines with cylindrical actines.
Spicules (Table 4):
(i) Tripods (Figure 6 D): Regular and variable in size. Actines are straight with blunt tips, but they are not stout as typical tripods. A fourth rudimentary actine is present;
(ii) Triactines (Figure 6 E): Regular, except around the inhalant apertures where they are sagittal. Actines are cylindrical, straight, and with blunt or sharp tips;
(iii) Tetractines (Figure 6 E): Similar to the triactines, but with an apical actine that is shorter than the basal ones and ornamented with small spines (Figure 6 F).
Remarks: According to Rapp (2004), tetractines are also present in the cortical skeleton of L. lobatus, but in the present work these spicules were found only in the skeleton of the tubes and atrium, suggesting that they are not regularly distributed in the cortex of the sponge. Moreover, the occurrence of triactines around the inhalant apertures and the presence of spines in the apical actine of the tetractines are here described for the first time. Leucascus leptoraphis is the most similar species to L. lobatus. Both of them present skeleton composed of triactines and tetractines with cylindrical actines, and tripods. Nevertheless, L. leptoraphis and L. lobatus can be easily differentiated mainly based on the shape of the cormus, spicule size, and abundance of tetractines (which is high in L. lobatus and rare in L. leptoraphis). The presence of tripods with a fourth rudimentary actine is also exclusive of L. lobatus.
Distribution: North Atlantic: Leucascus lobatus is endemic to Greenland: Julianehåb Bank (Brøndsted 1933), Kap Farvel (6007.7 ’N, 4417 ’W; 6015 ’N, 4417 ’W; 6004,5’N, 4302.7 ’W), Tjalfe station (6506 ’N, 5419 ’W), Godthaab station (6022 ’N, 4727 ’W), East of Greenland (7040 ’N, 22 W), and Thule station (7634,5’N, 6924,5’W) (Rapp 2004). Spalding et al. (2007) corresponding ecoregions: East Greenland Shelf, West Greenland Shelf, and Baffin Bay-Davis Strait.