Gonaxia constricta ( Totton, 1930 )

Galea, Horia R., 2021, Second records and redescriptions of two rare thecate hydroids (Cnidaria: Hydrozoa) from the southwestern Pacific, Zootaxa 5082 (4), pp. 373-383 : 379-382

Gonaxia constricta ( Totton, 1930)

Figs 7–8 ; Table 2

Symplectoscyphus constrictus Totton, 1930: 181 , text-fig. 31, pl. 1 fig. 3.— Ralph, 1961: 800, fig. 14F. — Stepanjants, 1979: 69, 70.— Dawson, 1992: 20.

Gonaxia constricta — Vervoort, 1993: 140, figs 16D, 17A.— Vervoort & Watson, 2003: 139.

Material examined. MNHN-IK-2019-2090, KANADEEP 2, Stn. CP 5095, off southern New Caledonia, Capucine Bassin, 23°38’ S, 167°53.1’ E, 29 Sep 2019, 1087– 1081 m, a ca. 9 cm high, sterile colony .

Description. Colony erect, ca. 9 cm high, of flaccid appearance, unable to support itself when out of liquid ( Fig. 7 ); basally, there are remains of a branched, rhizoid stolon apparently firmly attached to a hard substrate. Stem unbranched, weakly fascicled in its proximal 3 cm, then monosiphonic towards the distal tip. Division into internodes almost indistinct, but equivalents of internodes long, slightly geniculate, bearing distally a hydrotheca. Although minor irregularities occur here and there, the stem is composed of successive modules comprising four alternating hydrothecae of which the proximal- and the distalmost ones become axillar through the insertion of two cladial apophyses given off on opposite sides ( Fig. 8A ). Cladia borne on short lateral apophyses of the stem, given off from below the axillar hydrothecae ( Fig. 8B ); distally, an oblique node; cladia up to 2 cm long, composed of up to 20 hydrothecate internodes; nodes almost indistinct, except for an inconspicuous notch on side where the hydrotheca becomes free from the corresponding internode, a situation also noted in the stem; first cladial internode comparatively longer than subsequent ones, and delimited by a distinct adaxial notch from its corresponding stem apophysis ( Fig. 8A, B ); regular internodes moderately long, distinctly geniculate, each bearing a hydrotheca distally ( Fig. 8A ). Hydrothecae similar on stem and cladia; tubular, with straight axes ( Fig. 8C ), except for the axillar hydrothecae of the stem that are distinctly curved upwards ( Fig. 8B ); regular hydrothecae with longitudinal axes forming angles of 80–90° with the internodes above; hydrothecae adnate for about 1/4 their length to the corresponding internodes; ab- and adaxial walls straight; hydrothecal base not closed laterally, but composed of two intrathecal projections of the perisarc: one given off from the end of adnate adaxial wall, while the other projects some distance above into the hydrothecal lumen from the proximal end of abaxial wall; second ridge projects distinctly upwards in regular hydrothecae ( Fig. 8C ), and downwards in their axillar counterparts ( Fig. 8B ); hydrothecal aperture distal, set transversely; margin composed of three triangular cusps (one adaxial, two latero-abaxial) separated by broad, rounded embayments; aperture broadly triangular in frontal view, closed by three triangular flaps (two latero-adaxial, one abaxial) forming a low roof ( Fig. 8D ). Hydranths preserved in some hydrothecae, but very contracted, provided with 14–16 filiform tentacles. Gonothecae absent. Perisarc straw colored, hydranths white in this ethanol-preserved material.

Remarks. The branching pattern of the stem was inaccurately described in earlier accounts. On one hand, Totton (1930: 181) stated that G. constricta is a “branched species […] with regularly alternate pinnae. Separating each alternating pair of pinnae is a pair of alternating hydrothecae, and in the axil of each pinna a hydrotheca”. On the other hand, Vervoort (1993: 140) noted that “between two successive hydrocladia there are two free hydrothecae, in addition there is an axillary hydrotheca at base of each hydrocladium”. Nevertheless, the present material shows a combination of both accounts: although the nodes are indistinct, the stem is composed of successive modules of four alternating hydrothecae, of which the proximal- and distalmost ones become axillar upon the insertion of two cladial apophyses given off on opposite sides; with a few minor exceptions, the arrangement of cladia in conserved at each following module ( Fig. 8A ). A similar branching pattern is only met with in G. errans Vervoort, 1993 [ Galea (2016: 14, fig. 4E, as G. cf. errans ; 2016: 15, fig. 4G, as G. plumularioides Galea, 2016 ); Galea & Maggioni (2021: 411, fig. 8A)].

The gonothecae of this species still have to be discovered.

Distribution. Off Three Kings Island [ Totton (1930, as Symplectoscyphus constrictus )], off New Caledonia (present study). Occurs at depths between 549–1087 m [ Totton (1930) and present study, respectively].

Figures

FIGURE 7. Gonaxia constricta (Totton, 1930). Preserved colony (many cladia were lost). Scale bar: 1 cm.

FIGURE 8. Gonaxia constricta (Totton, 1930). A. Portion of stem showing delimited structural module. B. Axillar hydrotheca of the stem. C. Cladial hydrotheca. D. Hydrothecal aperture in frontal view, showing opercular apparatus. E. Cross section through a cladial internode at the level of the hydrothecal base, showing (differently-colored) internal septa. Scale bars: D, E = 100 µm; B, C = 500 µm; A = 1 mm.