Gonaxia plumularioides, Galea, 2016

Galea, Horia R., 2016, Notes on some sertulariid hydroids (Cnidaria: Hydrozoa) from the tropical western Pacific, with descriptions of nine new species, European Journal of Taxonomy 218, pp. 1-52 : 14-15

Gonaxia plumularioides sp. nov.

urn:lsid:zoobank.org:act:6A07FF84-EAA6-4AE6-8755-B450B17A28EB

Fig. 4 G–J; Table 2

Diagnosis

Gonaxia with slender, fascicled, branched or unbranched stems; cladia monosiphonic. Equivalents of internodes composed of a proximal hydrocladial apophysis and its associated axillary hydrotheca, two alternate hydrothecae above, and a second, distal hydrocladial apophysis and its associated hydrotheca. Hydrothecae tubular, slightly curved up- and outwards, comparatively slender and more immersed on stem than on cladia. Gonothecae unknown.

Etymology From the Latin plūma, meaning feather, and the Greek εἶδΟς, meaning likeness, with reference to their

plumose appearance, recalling the branching pattern displayed by some hydroids belonging to the superfamily Plumularioidea .

Material examined

Holotype

BIOCAL: Stn. CP34, a 2.8 cm high, distal fragment of a branch or a stem, itself branched twice, sterile (MNHN-IK-2012-16535).

Paratype

BIOCAL: Stn. CP34, a 7.7 cm high, unbranched colony devoid of its hydrorhiza, as well as a 1 cm high fragment corresponding to the distalmost part of a colony; both are sterile (MNHN-IK-2012-16536).

Description

Colonies attaining at least 8 cm high, composed of either simple or ramified stems of slender appearance, despite being fascicled; accessory tubes on both “frontal” and “dorsal” sides of stems; the latter uniformly grading to monosiphonic distally. Side branches, when present, arising only in “frontal” aspect of the stem or higher order branches; up to 2 nd order branching observed. Division into internodes indistinct, but equivalents of internodes composed of the regular sequence: a first, basal apophysis (60–90 µm long) and its associated axillar hydrotheca, two additional, alternate hydrothecae above, and a second axillar apophysis with its associated hydrotheca. Following internode with same sequence, but its proximal apophysis on side opposite to the distal apophysis of preceding internode. Cladia leaving axis at 75–80° from upper part of stem or branches; two successive cladia on same side of stem are distant of 2200–2310 µm; strictly monosiphonic, alternate, up to 1 cm long, comprising a succession of up to 23 alternate hydrothecae. Division into internodes indistinct; equivalent of first cladial internode comparatively longer (580–705 µm) than subsequent ones (480–515 µm); internodes short, accommodating a single hydrotheca distally. Hydrothecae widely spaced on stem (570–740 µm), less so on cladia (470–530 µm); tubular, slightly curving up- and outwards, free part angled at 50° to the axis of internode; stem hydrothecae slightly shifted on frontally; the two rows of cladial hydrothecae coplanar; less than half adnate on cladia, but considerably immersed into the stem internodes; free adaxial side nearly straight, adnate counterpart curved, ending basally in perisarc peg; abaxial side convex, with inflexion point in middle; aperture with three low cusps separated by shallow embayments; opercular apparatus lost. Gonothecae unknown.

Remarks

The branching pattern of the stems of G. plumularioides sp. nov. resembles that seen in some plumularioid hydroids that build three-dimensional colonies (e.g., Macrorhynchia philippina Kirchenpauer, 1872 , M. allmani (Nutting, 1900) , Plumularia spiralis Billard, 1911 , etc.), and is consequently different from that observed in its congeners (e.g., G. amphorifera , G. sinuosa , G. nova sp. nov., see description below), which generally build planar colonies.

The regular structure of the stem internodes is also peculiar, and is only occasionally observed in the material assigned herein to G. errans , a species known for its inconstant structure of the stem ( Vervoort 1993).

The shape of the hydrocladial hydrothecae resembles that of other congeners: 1) G. complexa Vervoort, 1993 , but in this species the hydrothecae have an exceedingly long part free from internode; 2) G. elegans , but its hydrothecae have a conspicuously straight axis instead of being convex (compare Fig. 4C and 4H ); 3) G. errans , whose hydrothecae have, however, a much longer free part (compare Fig. 4F with Fig. 4H ); 4) G. intercalata Vervoort & Watson, 2003 , but its hydrothecae are huge compared to those of the new species; 5) G. stricta Vervoort, 1993 , but here the hydrothecae have a much longer free part, and the cladial internodes are marked by internal, oblique perisarcal thickenings ( Vervoort 1993); 6) G. tasmanica Watson & Vervoort, 2001 , but its hydrothecae are comparatively larger.

Geographical distribution

New Caledonia.

Figures

Fig. 4. ― A. Gonaxia crassicaulis Vervoort, 1993 (MUSORSTOM 4, Stn. DW205), portion of cladium. ― B–D. Gonaxia elegans Vervoort, 1993 (BATHUS 3, Stn. DW809). B. Portion of stem and basal part of a cladium. C. Two consecutive internodes with their hydrothecae. D. Portion of stem with male gonothecae. ― E–F. Gonaxia cf. errans Vervoort, 1993 (BATHUS 2, Stn. CP737). E. Distalmost portion of stem with basal parts of four hydrocladia. F. Two consecutive cladial internodes with their hydrothecae. ― G–J. Gonaxia plumularioides sp. nov. (BIOCAL, Stn. CP34). G. Distalmost part of a stem, showing the structure of internodes. H. Two consecutive cladial internodes with their hydrothecae. I. Stem hydrotheca. J. Axillar hydrotheca. Scale bars: A–B, D–E, G = 1 mm; C, F, H–J = 300 µm.