Zygophylax crozetensis Millard, 1977

Watson, Jeanette E., 2017, Two bathyal hydroids (Hydrozoa: Leptothecata) from the Southwest Indian Ocean, Zootaxa 4247 (3), pp. 336-340 : 338-340

Zygophylax crozetensis Millard, 1977 ( Figure 2 A–D )

Zygophylax crozetensis Millard, 1977: 15 , fig. 4.— Millard, 1979: 140.

Material examined. NHM UK 2016.2, microslide, Coral Seamount, Southwest Indian Ocean Ridge (41° 22.31’S, 54° 57’E), depth 732 m.

Description. An infertile fragment 10 mm long, pinnate, lax, planar, alternately branched, a few parallel polysiphonic tubes running for a short distance up stem.

Hydrocladia alternate, diverging at an angle of 45–90° to stem. Hydrocladium (broken off at tip) 4 mm long with 10 hydrothecae, usually an axillar hydrotheca at base of hydrocladium. Apophysis of stem large, blister-like, a slight twist or partial node between apophysis and hydrotheca. Hydrothecae alternate, diverging at an angle of 30–50° from hydrocladium, body long, campanulate, some hydrothecae borne on accessory stem tubes. Abcauline wall straight to weakly concave, adcauline wall slightly convex, margin circular, oblique to axial tube, some everted and some with one or two replications. Diaphragm distinct, transverse to oblique, with central foramen and a few thorn–shaped desmocytes just above. Nematotheca deep tubular, expanding a little to margin, seated on a very short pedicel, margin circular, slightly everted.

Perisarc of stem glassy, transparent (preserved material), moderately thick, thinner on hydrothecae, very thin on nematothecae.

Remarks. Zygophylax crozetensis is recorded from 180 m deep at the Crozet Islands ( Millard 1977; 1979). The present specimen agrees in most respects with Millard’s description and dimensions, in particular the twisted and swollen hydrothecal pedicel. Comparison of the single complete nematotheca in the present specimen with Millard’s description strongly indicates the species are identical.

The present specimen is lax and colourless, in contrast to the usual horny texture and brown colour of Zygophylax .

The small round holes in the hydrocaulus of Zygophylax cyathifera ( Allman, 1888) reported by Rees & Vervoort (1987) also occur in the present specimen; they are hydrostatic pores for equilibration of internal cauline with external water pressure.

Measurements (µm) of Zygophylax crozetensis

Distance between alternate hydrocladia 800–900 Stem and hydrocladia, width 88–96 distance between alternate hydrocladial hydrothecae 440–496 Apophysis, depth 72–96 Hydrotheca

length, diaphragm to margin 328–400 width of margin 160–184 width at diaphragm 72–88 Nematotheca

length 68–84 width at margin 40–48 Distribution. This is the second record of Zygophylax crozetensis ; previously known only from the type locality of the Crozet Islands.

Figures

FIGURE 2. A – D Zygophylax crozetensis Millard, 1977. 2 A, part of branch. 2 B, hydrocladium. 2 C, hydrotheca. 2 D, nematotheca. Scale bar: A, 1.0 mm. B, 0.5 mm. C, 0.2 mm. D, 0.1 mm.