Sibogella flabellata Di Camillo & Galea 2020, sp. nov.

Sibogella flabellata Di Camillo & Galea, sp. nov.

Figs 1 D–F; 2B; 5; 7A; 8C, D, I, J; 9E, H; 10A′–I′; 11; Tables 1–2

Sibogella sp. Di Camillo et al., 2008: 1592.

Material examined. Holotype: MHNG-INVE-137160, Indonesia, Bali I., -8.278284°, 115.597062°, 13–15 m, 31 Jan. 2017, a colony composed of 6 stems, largest 6.5 × 5 cm, some bearing gonothecae.— Paratype: MHNG-INVE- 137166, Indonesia, Siladen I., 1.626414°, 124.802058°, 16 m, 15 Feb. 2005, several fragmentary, sterile stems, tallest 3.2 cm high.

Description. Colonies fan-shaped, largest 6.5 × 5 cm, relatively flaccid when out of liquid, arising from rhizoid stolon spreading over a solid substrate. Stems moderately fascicled, irregularly branched in one plane, with a tendency to alternate; up to second order branches observed. Main tube with thin perisarc, divided by transverse, though faintly indicated nodes into long, collinear internodes, each with a latero-distal apophysis and several longitudinal rows of nematothecae; auxiliary tubes undivided, each provided with a longitudinal row of nematothecae; stem branches either mono- or polysiphonic, depending on their length, arising from bifurcations of auxiliary tubes: the divergent part (given off at an angle of 70–80°) forms a lateral branch (= lower-order stem), while a newlyformed tube is sent up along the main stem so as to maintain the polysiphony; secondary and tertiary stems with same structure as the main stem. Stem apophyses short, alternate, given off at right angles with the longitudinal axes of their corresponding internodes; with 2 axillar nematothecae, ending in slightly oblique node; two successive apophyses (left and right), slightly shifted on to the “anterior” side of the colony, alternating with the two preceding and two subsequent ones that are shifted on to the “posterior” side, giving the colony a nearly planar appearance; only distally on certain stems this arrangement can be disrupted, the cladia-bearing branches adopting a spiral arrangement. A cladia-bearing branch borne on each stem apophysis; up to 10 mm long, generally unbranched, divided into up to 25 internodes by means of transverse nodes; internodes, with the exception of the two proximal most, long, geniculate (proximally rather faintly, grading to strongly so distally), with a latero-distal apophysis supporting a cladium, and generally two nematothecae (occasionally one) in a row on opposite side; apophyses well-developed, given off at an angle of 55–60° with the long axis of the corresponding internode, with a conical mamelon, an unpaired nematotheca on opposite side, as well as a nematotheca on upper side of the axil; distally an internal, perisarcal ridge on adaxial side. Cladia set at right angles with respect to the longitudinal axis of the branch; composed of a proximal nematothecate internode, a hydrothecate internode, and a whip-like appendage distally; first internode long, with proximal node transverse and distal node oblique, with a nematotheca on upper side and a proximal, internal, adaxial perisarc ridge; hydrothecate internode short, accommodating distally a hydrotheca and its pair of lateral nematothecae; mesial nematotheca absent; distal appendage composed of a succession of up to 14 slender, articulated internodes, length decreasing distally, each bearing a nematotheca, with the exception of the distalmost that is provided with two of these, one borne in the middle part, while the second is apical; nematothecae arranged alternately left and right along the appendage. Hydrothecae cup-shaped, relatively shallow, adnate for most of their length to the corresponding internode, leaving only a small portion of their adaxial wall free; a belt of desmocytes slightly above base; abaxial wall slightly flaring below aperture; the latter rounded, rim even; apertures of all hydrothecae of the colony facing towards the corresponding (primary or lower-order) stem. Hydranths too large to be accommodated into their corresponding hydrothecae, with a whorl of 15–16 filiform tentacles. Gonothecae, possibly immature, borne singly in the axils formed by the cladial apophyses with their corresponding internodes, on side opposite to the mamelon; piriform, borne on indistinct pedicel, perisarc thin; sex probably male; gonophore liberated as a globular mass on top of gonotheca, possibly through the lysis of the central part of the apical perisarc. All nematothecae conical, movable and bithalamic, with deep basal chamber and rather shallow upper chamber, whose adaxial wall is variably lowered on adaxial side. Cnidome: banana-shaped microbasic mastigophores, (6.0– 6.2) × (2.0–2.2) µm, in the tentacles; pseudostenoteles, (11.7–12.1) × (5.5–5.9) µm, in the nematophores, as well as disseminated in the coenosarc. Color in life: hydrocauli brown, cladia transparent.

Remarks. The cladia-bearing branches are generally unbranched; branching takes place only when a cladium was broken off basally and subsequently replaced by a second order cladia-bearing branch, instead of a newlyformed cladium. The three proximal most internodes of a cladium-bearing branch can occasionally give rise to pairs of latero-distal, cladial apophyses, making a variable angle between them; unlike the subsequent cladia, the cladia they bear are not arranged in the same plane, but often point towards the frontal and dorsal sides of the colony.

Unlike S. erecta and S. spissa sp. nov. (see below), the present hydroid forms branched, fan-shaped colonies with fascicled stems. Additional distinguishing characters are summarized in Table 2.

Etymology. From Latin flābellum, meaning fan, to illustrate the appearance of its colonies.

Distribution. Only known from two Indonesian localities, viz. Bunaken (Di Camillo et al. 2008, as Sibogella sp.; present study) and Bali (present study).