( Fig 6, Table 6)
Etymology. From the Latim “digitus” (finger), for the finger-shaped cormus of this species.
Type locality. Tekeho (Nuku Hiva), Marquesas Islands. French Polynesia.
Material examined. Holotype: UFRJPOR 6460 = MNHN-IP- 2018-31 — Nuku Hiva, Marquesas Islands, Station MNH04 ( 08° 57.661’ S– 140° 10.149’ W), depth: 16 m, coll. C. Debitus, 30/VIII/2009, P114.
Diagnosis. White Leucascus with digitiform cormus and large oscula. Skeleton composed of tripods, triactines and tetractines with large spines on the apical actine.
Colour. White alive and in ethanol ( Fig 6A).
Morphology and anatomy. Sponge digitiform, massive but delicate ( Figs 6A, B). The cormus is formed by thin, regular and tightly anastomosed tubes forming a continuous delicate cortex ( Fig 6C). Each protuberance (digitus) present a terminal osculum surrounded by membrane. Below each osculum there is an atrial cavity supported by tetractines. Aquiferous system solenoid. The specimen is full of embryos. The skeleton is composed of tripods and triactines on the cortex ( Fig 6C), triactines and tetractines in the choanosome ( Fig 6D), and tetractines in the atrial wall.
Spicules ( Table 6)
Tripods. Regular or sagittal. Similar to large triactines. Actines are conical with blunt tips ( Fig 6E). Size: 88.0/ 9.3 µm.
Triactines. Regular or sagittal. Actines are conical with blunt tips ( Fig 6F). Size: 54.7/ 5.3 µm.
Tetractines. Regular or sagittal. Actines are conical with blunt tips ( Fig 6G). The apical actine is very long, thick, conical and sharp, covered by spines ( Fig 6H). Size: 57.4/ 5.5 µm (basal actine); 35.4/ 5.0 µm (apical actine).
Geographical distribution. Nuku Hiva, Marquesas Islands (present work).
Remarks. Currently 10 species of Leucascus are recognised: L. simplex Dendy, 1892 ( type species of the genus) from South Australia; L. albus Cavalcanti, Rapp & Klautau, 2013 from southeastern Brazil; L. clavatus Dendy, 1892 from South Australia; L. flavus Cavalcanti, Rapp & Klautau, 2013 from Indonesia; L. leptoraphis ( Jenkin, 1908) from Antarctica; L. lobatus Rapp, 2004 from Greenland; L. neocaledonicus Borojević & Klautau, 2000 from New Caledonia; L. protogenes ( Haeckel, 1872) from South Australia; L. roseus Lanna, Rossi, Cavalcanti, Hajdu & Klautau, 2007 from southeastern Brazil; and L. schleyeri Van Soest & De Voogd, 2018 from South Africa.
Considering skeleton composition, L. digitiformis sp. nov. is similar to L. leptoraphis and L. lobatus, as all of them have tripods, triactines and tetractines. The new species, however, differs from them by several characteristics. For example, L. leptoraphis and L. lobatus have triactines and tetractines with cylindrical actines, while our new species has spicules with conical actines. Besides, in L. leptoraphis tetractines are very rare and in L. lobatus the tripods have a kind of rudimentary fourth actine in the tripods. Moreover, the spines of the apical actine of the tetractines of L. digitiformis sp. nov. differ from those of all the other species of Leucascus, as they are large in the new species.
These large spines resemble those of Borojevia, however, the new species clearly has an atrial membrane, a characteristic of Leucascus and absent in Borojevia. Nonetheless, looking at Borojevia spp., we found that the habitus of Borojevia tubulata Van Soest & De Voogd, 2018, a species from Maldives, is very similar to ours. Besides, a picture showing the atrium of the specimen ZMA Por. 12435, from the Seychelles, suggests that that specimen has a true atrium, although this specimen had grouped molecularly inside Borojevia ( Van Soest & De Voogd, 2018). We unfortunately did not succeed to get a DNA sequence of our specimen to compare it with B. tubulata. Therefore, we compared both species only morphologically and we found differences in the size of the spicules [ B. tubulata— holotype—Tripods: 92˗133˗189/ 11˗13.4˗16; Triactines: 54˗63˗111/ 5.0˗6.3˗7.5; Tetractines: 51˗68˗96/ 6.0˗6.6˗9.0 (basal), 24˗37˗48/ 3.5˗4.4˗5.0 (apical)] ( Table 6). As we found these differences in the size of the spicules and as Van Soest & De Voogd (2018) stated that their species do not have a true atrium, we decided to consider our species as a new one, still, it is desirable in the future to compare them molecularly.