Halecium obovatum, Watson, 2018

Watson, Jeanette E., 2018, Some Hydroids (Cnidaria, Hydrozoa) from the Great Australian Bight in the collection of the South Australian Museum, Zootaxa 4410 (1), pp. 1-34 : 9-10

Halecium obovatum sp. nov.

Figure 4A–G

Material examined. SAM H2345, holotype, preserved colony; two microslides ( SAM H2577, H2478) from holotype colony. PIRSA Stn B5.

Description. A large fertile colony consisting of many stems to 60 mm high. Hydrorhiza composed of numerous stolons reptant on remnants of calcareous bryozoan. Lower part of stem and hydrorhiza heavily fascicled, polysiphonic tubes running parallel up stem, ultimate branches monosiphonic. Branches long, thin and lax, arising randomly from main stem and hydrorhiza in no particular order or direction. Branch internodes variable in length, cylindrical, smooth, nodes transverse to slightly oblique, marked by a slight indentation and thickening of perisarc.

Monosiphonic branch

length of internode 640–920 width at node 200–256 Hydrophore

length of hydrophore including hydrotheca 224–480 Hydrotheca

diameter of margin 160–180 depth, diaphragm to margin 58–60 width of diaphragm 120–144 Gonotheca

length 1160–1360 maximum width 800–1000 width at margin 600–800 Hydrophores strictly alternate, arising at an upward angle of 30–40° to branch axis, distal on internode just below node. Shorter secondary hydrophores sometimes present, lying closely parallel to primary. Hydrotheca distal on hydrophore, bowl-shaped, diameter increasing from diaphragm to margin. Diaphragm thin, transverse, without desmocytes. Margin circular, thin, not everted, without replications, ratio of depth of hydrotheca to marginal diameter 1:3.

Gonothecae borne in rows on lower fascicled regions of stem, obovate, base narrowing into stem without true pedicel, distal end a shallow thin dome of tissue, mostly collapsed to a transverse or concave sheet. Cnidome comprising microbasic mastigophores of one size: capsule large, narrowly elliptical, 42– 46 x 14–16 µm, shaft and thread> 220 µm long, shaft thickly spinous, visible in undischarged capsules, thread long and spinous.

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A. Fertile branch from holotype colony, SAM H2345. B, monosiphonic branch with alternate hydrophores. C, primary and secondary hydrophores. D, E, hydrophores and hydrothecae. F, undamaged small gonotheca. G, undischarged and discharged microbasic mastigophores.

Perisarc of polysiphonic tubes thick, thinner on monosiphonic tubes, moderately thin on gonotheca.

Colour: white (preserved material) possibly pale yellow in life.

Remarks. The thick, tangled c olony consists of many stems. Most of the hydrophores, hydrothecae and many fragile gonothecae are collapsed; the gonophores are too deteriorated to determine sex. The large discharged microbasic mastigophores (measured in glycerol mounts) are distributed throughout the coenosarc.

Determination of species of Halecium rests upon the criteria of width and depth ratio of the hydrotheca, eversion of the margin and shape and size of the mature gonotheca. No known species matches these combined criteria.

Etymology. Named for shape of the gonotheca