Leucotreton kurilense, Sanamyan & Sanamyan & Kukhlevskiy & Shilov, 2022

Leucotreton kurilense View in CoL sp. nov.

( Figs 1–4)

Holotype. MIMB 42998 View Materials , RV “Akademik Oparin” cruise 56, Station 2, Russia, Kuril Is. (southern group), Urup I. (off Pacific coast), 45°38.2′N 149°53.1′E, 253– 222 m, pebbles, 27 June 2019, coll. V. Shilov. GoogleMaps

Paratypes. MIMB 42999 View Materials , RV “Akademik Oparin” cruise 56,Station 69, Russia, Kuril Is. (northern group), Onekotan I. (off Sea of Okhotsk coast), 49°24.0′N 154°16.1′E, 146–147 m, sand, pebbles, stones, 12 Aug. 2019, 5 specimens, coll GoogleMaps . V. Shilov.

Description. Solitary specimens with tubular body. Holotype about 5.5 cm long and 1.5 cm wide in its widest part located at some distance from anterior (upper) end ( Fig. 1A). Largest paratype 16 cm long and 1.5 cm wide. Body either almost cylindrical or gradually tapering to narrow posterior end attached to substratum. All specimens laterally compressed in ethanol. Single large (4 mm in holotype) osculum leading into voluminous atrial cavity continuing to bottom of sponge. Osculum with short lamellate rim but without oscular fringe. Consistency compressible but not especially soft. Dark-brown in ethanol.

Body wall 1.2–1.5 mm thick, composed of radial tubes extending from atrium ( Fig. 2A, B, D). Some of these radial tubes free along their entire length (e.g. Fig. 2B), others in part or fully coalescent with adjacent tubes and forming characteristic “chains” of tubes (well visible in tangential sections of body wall, Fig. 3F), with large inhalant spaces (“holes” seen on surface of sponge, Fig. 1A) between them often reaching atrial wall of sponge. Coalescent distal ends of radial tubes forming very characteristic structure of external surface, allowing easy recognition of this species when collected together with other tubular calcareous sponges. External surface superficially resembling network of anastomosing tubes, as in Clathrinidae ( Fig. 1B), but actual structure very different. Radial tubes communicating with atrial cavity through either individual openings or several joined radial tubes possessing one common opening. Atrial surface visually (macroscopically) smooth and even, pierced by irregularly distributed orifices of exhalant spaces of radial tubes 0.2–0.5 mm in diameter ( Fig. 1C).

Water system intermediate between leuconoid and sylleibid. Choanocyte chambers round, (50)60–100 µm in diameter. Inner walls of radial tubes lined with choanocyte chambers usually in one layer, sometimes in two or rarely in more than two (in coalescent tubes) irregular layers, leaving, in each radial tube, empty central exhalant space ( Figs 2C, 3G: ec).

Atrial skeleton composed of single layer of tangential tetractines whose apical actines protruding into atrium ( Fig. 3E). Choanosomal skeleton articulated, composed of triactines only. Triactines supporting external walls of radial tubes, arranged in somewhat irregular rows ( Fig. 2B). Cortical skeleton composed of several layers of crowded tangential triactines cowering distal ends of free and coalescent radial tubes ( Fig. 2D: cts, arrow). Cortical skeleton not covering spaces between radial tubes and thus forming discontinuous cortex.

Spicules ( Table 1; Fig. 3A–D). Cortical triactines more or less equiactinal, almost equiangular, with slightly raised centre; actines conical, with blunt tips. Mean length of actines about 100 µm, mean width 9.4 µm (see Table 1 for details).

Choanosomal triactines more or less equiactinal, unpaired actine slightly longer than paired; actines conical, with blunt tips. Mean length of paired actines about 100 µm, that of unpaired actines 129 µm; mean width of paired and unpaired actines 9 µm (see Table 1 for details).

Atrial triactines strongly sagittal, with wide angle between paired actines; unpaired actine thinner and usually longer that paired actines; actines conical or subcylindrical, with blunt tips. Mean length of paired actines about 180 µm, that of unpaired actines 250 µm; mean width of paired actines 12 µm, that of unpaired actines 9 µm (see Table 1 for details).

Atrial tetractines equiangular and mostly equiactinal, sometimes unpaired actine slightly longer or shorter than paired. Apical actine perpendicular to basal plane, slightly thinner and about half as long as basal actines. All actines conical, present only in atrial wall. Mean length of basal actines about 160 µm, mean width 11 µm (see Table 1 for details).

Etymology. The species name is an adjective referring to the Kuril Islands , a geographical area where the type material was collected.

Molecular data. The topology of the resulting large phylogenetic tree (Electronic supplementary material 2; see Addenda) is very similar to the

K.E. Sanamyan et al. Leucotreton kurilense , a new genus and species of calcareous sponges tree published by Alvizu et al. (2018). The clades labelled by these authors as “Clade I” to “Clade X” are resolved and Leucotreton kurilense sp. nov. is resolved as a member of “Clade X” ( Fig. 4).

This clade contains two species of the genus Sycetta Haeckel, 1872 ( S. antarctica BrØndsted, 1931 and unidentified Sycetta sp. ), a species of Sycon Risso, 1827 whose morphology is similar to Sycetta ( Sycon karajakense Breitfuss, 1897 ; according to Rapp, 2015, its radial chambers are free in most of their length, only fused close to the base, almost as in Sycetta ), but also Sycon abyssale Borojevic et Graat-Kleeton, 1965 (with morphology typical for Sycon ), a species of Heteropiidae [ Sycettusa thompsoni (Lambe, 1900) ] with thin body wall and inarticulate skeleton, and two species of Grantiidae [ Sycandra utriculus (Schmidt, 1869) and Sycandra sp. ]. A sequence labelled as “ Sycetta asconoides ” is probably based on a specimen identified by Rapp (2015) as Grantia phillipsi Lambe, 1900 and may also belong to Grantiidae (see Rapp, 2015). Thus, this clade comprises species with different morphologies, belonging to three families. A large proportion of species with free ( Sycetta ) or free and irregularly coalescent ( Leucotreton gen. nov.) radial chambers in this clade is noteworthy and suggests that this feature should be treated as significant in the classification of calcareous sponges (as most authors did). On the other hand, a cladogram based on a single molecular marker (28S rRNA) can only approximate the relationships between the considered taxa. Unfortunately, neither species of Sycantha Lendenfeld, 1891 nor any species described in Dermatreton , which also have partially coalescent radial tubes, are sequenced and their relationships with Leucotreton kurilense sp. nov. cannot be revealed from the molecular data. However, they obviously are related to L. kurilense sp. nov. according to the morphological data, as discussed below.

K.E. Sanamyan et al. Leucotreton kurilense , a new genus and species of calcareous sponges

Comparison with related species and taxonomic position. The structure of the body wall of Leucotreton kurilense sp. nov., composed of linked radial tubes distributed on the central atrium, large inhalant spaces between them, the presence of distinct cortex which, however, unlike that of Grantiidae , does not delimit the inhalant water system, suggests that this species is a member of the family Sycanthidae as it was defined by Borojevic et al. (2000: 228). These authors grouped together in Sycanthidae several species “which are derived from sponges with a sycettid type of organization” and which have “a large atrial cavity that has numerous short radial tubes, which are not regularly distributed on the central atrium but form groups which communicate with the central atrial cavity by a large opening” and in which the cortex, if present, does not delimit the inhalant cavities externally.

The way how the radial tubes of L. kurilense sp. nov. are arranged is similar to that described by Jenkin (1908) for several species assigned by him to the genera Dermatreton , Tenthrenodes Jenkin, 1908 and Hypodictyon Jenkin, 1908 . Jenkin (1908) assigned these genera to three different families: Dermatreton was assigned to Grantiidae because it (as L. kurilense sp. nov.) has partial cortical skeleton, Tenthrenodes lacks cortical skeleton and was assigned to Sycettidae (syn. Syconidae ), and Hypodictyon was assigned to “ Chiphoridae ” (unavailable family name not based on available generic name) based on the presence of chiatines.

In all features, except the type of aquiferous system, Leucotreton kurilense sp. nov. fits the definition of the genus Dermatreton as given by Borojevic et al. (2000: 229): “ Sycanthidae with coalescent radial tubes whose distal parts are supported by tangential triactines that form a loose meshwork perforated by large inhalant cavities” and most closely resembles D. hodgsoni Jenkin, 1908 . The original Jenkin’s (1908) description and figures of this species are detailed and precise. In particular, the schematic drawing of the cross-section of the radial tubes of D. hodgsoni shows exactly the same pattern of “chains” of “linked” radial tubes as in L. kurilense sp. nov. (compare Fig. 3F in the present work and Fig. 70 in Jenkin, 1908). The pattern of distribution of the openings on the gastral membrane (“ports in the gastral layer” in the terminology of Jenkin, 1908) are similar in L. kurilense sp. nov. and D. hodgsoni , and quite distinct from the regular pattern seen in the sponges in which the radial tubes are not fused proximally, e.g. Sycon or Grantia Fleming, 1828 (compare Fig. 1C in the present work and Fig. 68 in Jenkin, 1908). Incomplete cortical layer, distinct on the tops of joined radial tubes but not covering inhalant spaces between them (termed “reticulated cortex” by Jenkin, 1908) is also similar in two species, although L. kurilense sp. nov. lacks diactines in it. The main difference between these species is the type of water system, clearly syconoid in D. hodgsoni (each radial tube is lined with a layer of choanocytes), but leuconoid with the traits of sylleibid arrangement in L. kurilense sp. nov. (each tube contains numerous small round choanocyte chambers).

We consider the differences between the syconoid and the sylleibid/leuconoid organisations in the two species to be significant enough to assign them to different genera (by the analogy with the genera Achramorpha Jenkin, 1908 and Megapogon Jenkin, 1908 , the former of which has the syconoid organisation, while the latter has the sylleibid or leuconoid one; this is the only difference between them, see Borojevic et al., 2000: 241, and Alvizu et al., 2019). Therefore, the new genus Leucotreton gen. nov. is proposed to accommodate the new species.

Nomenclature and taxonomy of nominal genera related to Leucotreton gen. nov.

Jenkin (1908) described three genera, Dermatreton , Tenthrenodes and Hypodictyon , which contain species with similar arrangement of radial tubes. The nomenclature and taxonomic status of these genera, however, are confused and need to be discussed.

1. Hypodictyon Jenkin, 1908 . This name is a junior homonym of Hypodictyon Cope, 1885 ( Amphibia) and is permanently invalid. The genus Hypodictyon originally included a single species, H. longstaffi Jenkin, 1908 . Borojevic et al. (2000: 229) transferred H. longstaffi to the genus Sycantha stating that this species “is apparently one of the typical representatives” of this genus (despite the fact that according to Jenkin, 1908: 28, in this species “each chamber opens directly into the gastral cavity through an irregular apopyle”, while in Sycantha , as it was understood by Borojevic et al. (2000: 228), “radial tubes are grouped and fused proximally, each group communicating through a wide opening with the atrial cavity”). Hence, Borojevic et al. synonymised Hypodictyon with Sycantha .

2. Tenthrenodes Jenkin, 1908 . Two species were originally included in this genus, T. antarcticus Jenkin, 1908 and T. scotti Jenkin, 1908 , and one more species, T. primitivus BrØndsted, 1931 was added subsequently. The type species is T. antarcticus Jenkin, 1908 , subsequently designated by Dendy & Row (1913: 796).

Borojevic et al. (2000: 218, 228) stated that they synonymised Tenthrenodes with Sycantha . This statement is erroneous because in the same paper these authors transferred all three species of Tenthrenodes to three different genera and none of them to Sycantha : T. scotti was transferred to Dermatreton ( Borojevic et al., 2000: 228–229) , T. antarcticus to Sycon ( Borojevic et al., 2000: 228) , and T. primitivus to Sycetta ( Borojevic et al., 2000: 218) . Since T. antarcticus (type species) is assigned to Sycon , Tenthrenodes is a junior synonym of Sycon , syn. nov.

3. Dermatreton Jenkin, 1908 . Originally, Jenkin (1908) included in Dermatreton two species: D. chartaceum Jenkin, 1908 and D. hodgsoni . Dendy & Row (1913: 789) designated D. chartaceum as a type species of Dermatreton . Borojevic et al. (2000: 229) designated another species, D. hodgsoni , as a type species of Dermatreton . This subsequent designation is invalid (Code, Article 69.1). In the same paper, Borojevic et al. established a new genus Breitfussia Borojevic et al., 2000 in the family Jenkinidae , with the type species Ebnerella schulzei Breitfuss, 1896 , and included in it D. chartaceum and Grantia vitiosa BrØndsted, 1931 . The type species of Breitfussia and D. chartaceum were recently redescribed by Rapp (2015) and Rapp et al. (2011) as Breitfussia schulzei and B. chartacea , respectively. Rapp et al. (2011) confirmed the congenerity of these two species. Therefore, since D. chartaceum is the type species of Dermatreton Jenkin, 1908 , Breitfussia is invalid being a junior subjective synonym of Dermatreton , syn. nov., and the valid generic name for all three species currently assigned to Breitfussia is Dermatreton : D. chartaceum Jenkin, 1908 , D. schulzei (Breitfuss, 1896) , comb. nov., and D. vitiosum (BrØndsted, 1931) , comb. nov. We consider that the diagnosis of Dermatreton is the same as the diagnosis of Breitfussia given by Borojevic et al. (2000: 230): “ Jenkinidae with a simple tubular body and syconoid organization. The choanoskeleton is reduced to the unpaired actines of the subatrial triactines, and occasionally contains the proximal part of radial diactines”.

The remaining species originally included in Dermatreton , D. hodgsoni , is taxonomically distinct and not congeneric with the type species of Dermatreton (see Borojevic et al., 2000) and therefore it requires a new generic name. We propose the name Scytotreton gen. nov. for it.