Sibogella spissa Galea, Maggioni & Di Camillo, 2020

Galea, Horia R., Maggioni, Davide & Di Camillo, Cristina G., 2020, On the hydroid genus Sibogella Billard, 1911 (Cnidaria: Hydrozoa: Plumulariidae) with descriptions of two new species, Zootaxa 4860 (4), pp. 521-540 : 533-537

Sibogella spissa Galea, Maggioni & Di Camillo , sp. nov.

Figs 1G ; 2C ; 6 ; 7B ; 8E, F, K, L ; 9F, I ; 10 A′′–I′′; 11; Tables 1–2

Material examined. Holotype: MNHN-IK-2015-588, holotype, off New Caledonia, KANACONO stn DW4741, -22.86667°, 167.68333°, 210– 210 m, 23 Aug. 2016, GenBank accession numbers: MT 663356 (16S), MT 678863 (18S), MT 678861 and MT 678862 (28S), a colony composed of several up to 8 cm high, apparently sterile stems .— Paratype: MNHN-IK-2015-656, paratype, off New Caledonia, KANACONO stn DW4741, -22.86667°, 167.68333°, 210– 210 m, 23 Aug. 2016, a detached plume, ca. 5 cm high, bearing a few gonothecae (these minute structures are only seen when the specimen is mounted in toto between slide and coverslip, and examined with a compound microscope) .— Additional material: MNHN-IK-2015-586, paratype, off New Caledonia, KANACONO stn DW4768, - 23.41667°, 168.01667°, 180–210 m, 27 Aug. 2016, an apparently sterile colony composed of many sterile stems, up to 10 cm high.—MNHN-IK-2015-587, off New Caledonia, KANACONO stn DW4763, -23.28333°, 168.25000°, 192–260 m, 27 Aug. 2016, three badly-preserved, apparently sterile colonies, 6–8 cm high, each comprising several stems.—MNHN-IK-2015-589, off New Caledonia, KANACONO stn DW4764, -23.33333°, 168.25000°, 350–356 m, 27 Aug. 2016, five apparently sterile, badly-preserved colonies, 3–6 cm high, each composed of several stems .

Description. Colonies upright, up to 10 cm high, rigid, able to support themselves when out of liquid, composed of clumps of adjacent, thick, straight, unbranched, monosiphonic stems held together by an up to 28 mm long enmeshing of comparatively slender, tortuous, branching and anastomosing stolonal fibers prolonged from a rhizoid hydrorhiza embedded in sand; auxiliary tubes athecate. Stems divided into regular internodes by means of faintly-marked, transverse nodes; internodes short (giving the colony a Nemertesia -like appearance), collinear proximally, imperceptibly geniculate distally, with a distal bulge of perisarc below node, a latero-distal apophysis and several longitudinal rows of nematothecae; generally 2–3 nematothecae in each row between two successive branches on each side of the stem; perisarc very thick throughout; apophyses short, alternate, making an angle of 70–75° with the corresponding internode, flanked on each side by a nematotheca, distally with internal, perisarcal ridge, ending in transverse node; two successive apophyses (left and right) slightly shifted on to the “anterior” side of the colony alternate with the two preceding and two subsequent ones that are shifted on to the “posterior” side; each apophysis bears a cladia-bearing branch. The latter up to 9 mm long, generally unbranched, divided by means of transverse nodes, slanting in alternate directions, into up to 33 long, proximally straight and distally slightly geniculate internodes; each internode long (except for the proximal most) with a latero-distal apophysis and a nematotheca (exceptionally 2) on opposite side; apophyses set at an angle of 35–40° with the corresponding internode, alternate, long, bearing a conical mamelon shifted laterally, an unpaired nematotheca on opposite side, as well as a second nematotheca in the upper side of the axil; apophyses shifted on to one side of the cladia-bearing branch, the two rows forming an angle between them; the opening of the angle formed by the two rows of cladia belonging to a frontally shifted cladia-bearing branch faces the posterior side of the colony; conversely, the opening of the angle formed by the two rows of cladia belonging to a dorsally-shifted cladia-bearing branch faces the anterior side of the colony; each apophysis supports a cladium; exceptionally, a cladium can be replaced by a secondary cladia-bearing branch. Cladia composed of a proximal ahydrothecate internode, followed by a hydrothecate internode, ending in a whip-like appendage; first internode, when intact, long, slender, with proximal node straight and distal node oblique, with a proximal, internal perisarcal ridge, and a nematotheca (exceptionally 2) on upper side; often, proximal internode with (multiple) signs of breakage and subsequent regeneration, occasionally composed of several short segments, distalmost with one nematotheca on upper side; hydrothecate internodes short, with proximal node oblique and distal node straight, bearing a distally-placed hydrotheca flanked by a pair of lateral nematothecae; the mesial nematotheca is constantly missing although, exceptionally, it can be present here and there; distal appendage composed of up to 9 slender, articulated internodes, whose length decreases distally, each bearing a centrally-placed, trumpet-shaped nematotheca, except for the distalmost internode that can be provided with up to 3 of these, one of which is necessarily apical. Hydrothecae cup-shaped, relatively shallow, adnate for most of the length of their adaxial wall, leaving only a small portion free from the corresponding internode; abaxial wall almost straight, imperceptibly flaring distally, aperture circular, rim even, a belt of desmocytes slightly above the base. Hydranths small, able to retract almost completely into their corresponding hydrothecae, with a whorl of about 16 filiform tentacles. All nematothecae of the colony elongate, bithalamic, basal chamber deep, upper chamber shallow, the latter with adaxial wall lowered for a varied extent, except for the lateral nematothecae flanking each hydrotheca, in which the rim is even. Gonothecae, of undetermined sex and possibly immature, arising singly from axils of cladia-bearing branches; piriform, thin-walled, borne on indistinct pedicels, distally flattened, gonophore expelled through the lysis of the central portion of the apical wall. Cnidome: banana-shaped microbasic mastigophores, ca. 5.5 × 2.0 µm, in the tentacles; pseudostenoteles, 12.8 × 5.8 µm, in the nematophores, as well as disseminated in the coenosarc. Color of alcohol-preserved specimens: perisarc of stems dark-brown, that of cladia-bearing branches light brown, hydrocladia translucent.

Remarks. This species comes close to S. erecta , but is more robust, has comparatively more approximated cladia-bearing branches, and its hydrothecae are borne on short internodes almost constantly devoid of a mesial nematotheca. The presence of the latter was, however, observed in a couple of instances, but it does not appear to be the normal condition met with in this hydroid. Additional distinguishing features with respect to S. erecta and S. flabellata sp. nov. are summarized in Table 2 .

Etymology. From Latin spissus, -a, -um, meaning tight, dense, compact, to illustrate the appearance of its colonies.

Distribution. Only known from off New Caledonia.

Figures

FIGURE 1. Six colonies of Sibogella erecta Billard, 1911 (A–C). Three colonies of S. flabellata sp. nov. from the holotype (D–F). Holotype colony of S. spissa sp. nov. (G). Scale bar equals 2 cm.

FIGURE 2. Comparison of portions of stems of Sibogella erecta Billard, 1911 (A), S. flabellata sp. nov. (B), and S. spissa sp. nov. (C). Scale bar equals 400 µm.

FIGURE 6. Portion of cladia-bearing branch of Sibogella spissa sp. nov. Scale bar equals 300 µm.

FIGURE 7. Enlarged portions of cladia-bearing branches of Sibogella flabellata sp. nov. (A) and S. spissa sp. nov. (B), showing an internode and its corresponding cladium. Scale bar equals 200 µm.

FIGURE 8. Details of the cladial apophyses of Sibogella erecta Billard, 1911 (A, B), S. flabellata sp. nov. (C, D), and S. spissa sp. nov. (E, F) seen from above (A, C, E) and laterally (B, D, F). Hydrothecae of S. erecta (G, H), S. flabellata (I, J), and S. spissa (K, L) seen frontally (G, I, K) and laterally (H, J, L). Scale bar equals 50 µm.

FIGURE 9. Male (A, B) and female (C, D) gonothecae of Sibogella erecta Billard, 1911, either empty (A, C) or showing their gonophores (B, D). Gonothecae of S. flabellata sp. nov. (E) and S. spissa sp. nov. (F). Cnidomes of S. erecta (G), S. flabellata (H), and S. spissa (I). Scale bars: 100 µm (A–F), 10 µm (G–I).

FIGURE 10. Nematothecae of Sibogella erecta Billard, 1911 (A–I), S. flabellata sp. nov. (A′–I′), and S. spissa sp. nov. (A′′–I′′): from the stem apophyses supporting the cladia-bearing branches (A, A′, A′′), from the stem internodes (B, B′, B′′), axillar borne on the cladial apophyses of cladia-bearing branches (C, C′, C′′), unpaired thecae of the cladial apophyses of cladia-bearing branches (D, D′, D′′), from the internodes of cladia-bearing branches (E, E′, E′′), from the first cladial internode (F, F′, F′′), laterals associated to the hydrothecae (G, G′, G′′) and mesial (H), of the whip-like, distal appendages (I, I′, I′′). Scale bar equals 50 µm.