On the hydroid genus Sibogella Billard, 1911 (Cnidaria: Hydrozoa: Plumulariidae) with descriptions of two new species Author Galea, Horia R. Hydrozoan Research Laboratory, 405 Chemin Les Gatiers, 83170 Tourves, France Author Maggioni, Davide 0000-0003-0508-3987 Università degli Studi di Milano-Bicocca, Dipartimento di Scienze dell’Ambiente e della Terra, Piazza della Scienza 1, 20126 Milano, Italy & Università degli Studi di Milano-Bicocca, Marine and High Education (MaRHE) Center, 12030 Faafu Magoodhoo, Republic of the Maldives. davide. maggioni @ unimib. it; https: // orcid. org / 0000 - 0003 - 0508 - 3987 davide.maggioni@unimib.it Author Di Camillo, Cristina G. 0000-0002-4031-8158 Università Politecnica delle Marche, Dipartimento di Scienze della Vita et dell’ Ambiente, Via Brecce Bianche, 60131 Ancona, Italy c. dicamillo @ univpm. it; https: // orcid. org / 0000 - 0002 - 4031 - 8158 c.dicamillo@univpm.it text Zootaxa 2020 2020-10-14 4860 4 521 540 journal article 9090 10.11646/zootaxa.4860.4.3 fce2f651-c25e-4fc0-8d27-396e7047d510 1175-5326 4414289 1E0E6AC2-9740-43DD-B64E-5792E84F1D34 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. FIGURE 5. Proximal (A) and distal (B) portions of cladia-bearing branch of Sibogella flabellata sp. nov. Scale bar equals 300 µm. TABLE 1 . Comparative measurements of Sibogella spp., in µm.

S. erecta Billard, 1911		S. flabellata sp. nov.		S. spissa sp. nov.
MHNG- INVE-97930	MHNG- INVE-137161	MHNG- INVE-137160	MHNG- INVE-137166	MNHN-IK- 2015-588
Stem
- internode length	765–1335	730–800	850–1010	700–950	235–365
- length of apophyses	100-115	100–120	65–90	50–70	145–170
- diameter at node	120–250	180–250	140–160	380–400	220–265
- distance between 2 successive bran- ches on same side of stem	1665–2680	1225–2200	1730–1940	1450–2000	470–740
Cladia-bearing branches
- internode length	415–500	440–550	460–515	400–550	285–395
- diameter at node	45–80	60–150	50–70	55–60	55–110
- length of cladial apophyses	75–95	90–120	55–65	50–60	95–120
- distance between 2 successive cladia on same side of branch	845–980	750–960	815–975	860–920	580–690
Cladia
- 1st internode, length	120–200	130–200	200–305	100–300	215–260
- hydrothecate internode, length	165–230	150–220	135–145	80–110	120–140
- nematothecate internodes, length	80–270	50–260	80–270	unavailable	65–205
- diameter at node	25–35	30–35	30–35	unavailable	25–30
Hydrothecae
- abaxial wall, length	50–60	50–60	55–75	40–65	50–55
- diameter at rim	80–90	85–95	90–100	70–80	80–85
Nematothecae
- of stem internodes, length	85–95	55–100	65–85	80–95	100–115
- of stem internodes, diameter at rim	ca . 30	30–35	30–35	ca . 30	ca . 30
- axillar, length	60–70	70–75	65–75	50–60	70–75
- axillar, diameter at rim	25–30	25–35	ca . 30	ca . 25	25–30
- of branch internodes, length	75–85	60–75	70–80	50–65	70–90
- of branch internodes, diameter at rim	25–30	25–30	ca . 30	ca . 25	25–30
- of 1st cladial segments, length	60–70	55–65	60–65	unavailable	60–75
- of 1st cladial segments, diameter at rim	25–35	ca . 30	30–35	unavailable	ca . 30
- mesial, length	55–65	50–60	absent	absent	absent
- mesial, diameter at rim	25–30	25–30	absent	absent	absent
- lateral, length	60–70	55–60	70–85	50–55	65–75
- lateral, diameter at rim	30–40	25–30	40–45	35–40	30–35
- of nematothecate internodes of cladia, length	70–80	60–70	70–95	50–55	70–95
- of nematothecate internodes of cladia, diameter at rim	30–35	30–35	ca . 35	20–25	30–35
Gonotheca
- length	not observed	300–450 (♀); 370–500 (♂)	250–295 (♂?)	not observed	ca . 280
- maximum width	not observed	200–220 (♀); 100–120 (♂)	115–140 (♂?)	not observed	ca . 155

FIGURE 6. Portion of cladia-bearing branch of Sibogella spissa sp. nov. Scale bar equals 300 µm. 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 . TABLE 2 . Main distinguishing features of Sibogella spp.

S. erecta Billard, 1911	S. flabellata sp. nov.	S. spissa sp. nov.
Colony appearance	bipinnate to spiral	fan-shaped	bottle brush-shaped
Condition of stem	monosiphonic, occasionally branched, slightly genicu- late, perisarc thick	polysiphonic, branched, straight, perisarc thin	monosiphonic, unbranched, imperceptibly geniculate, perisarc thick
Stem internodes	long	long	short
Angle of apophyses for cladia-bearing branches with the stem	55–60°	90°	70–75°
Relative distance between cladia-bearing branches	fairly apart	fairly apart	closely-set
Arrangement of cladia-bea- ring branches	groups of 2 successive branches alternately in front and rear side of the colony, occasionally verticillate in distal parts of some stems	groups of 2 successive branches alternately in front and rear side of the colony, arrangement occasionally disrupted distally	groups of 2 successive branches alternately in front and rear side of the colony, throughout the stem
Angle of cladial apophyses with the corresponding internode	30–35°	55–60°	35–40°
Structure of apophysis	mamelon + 3 nematothecae	mamelon + 2 nematothecae (exceptionally 3)	mamelon + 2 nematothecae
Appearance of cladia-bea- ring branches	weakly geniculate, angle between successive interno- des 155–165°	strongly geniculate, angle between successive interno- des 115–150°	weakly geniculate, angle between successive internodes 155–160°
Position of cladia with res- pect to the longitudinal axis of the cladia-bearing branch	forming an acute angle	decidedly perpendicular	forming an acute angle
Relative position of the two rows of cladia	coplanar	coplanar	forming an angle
Number of nematothecae on the internodes of cladia-bea- ring branches	2 (occasionally 1, exceptionally 3)	2 (occasionally 1)	1 (exceptionally 2)
First cladial internode	moderately long	long	long
Hydrothecate internode	fairly long	short	short
Mesial nematotheca	present	absent	absent (exceptionally present)
Orientation of the hydrothe- cal apertures	facing away the stem	facing towards the stem	facing away the stem

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).