Gymnangium pusillum, 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 : 22-23

Gymnangium pusillum sp. nov.

Figure 9A–F

Material examined. SAM H2538, holotype, preserved material; one microslide ( SAM H2593) from holotype colony. Near South Australia-Western Australia border, depth 180 m, trawl; coll: R. Southcott 1/07/1988.

Description. Fragile infertile stem 65 mm long broken off at top. Hydrorhiza a cluster of thin wrinkled tubular stolons. Stem lightly fascicled from base by several straight tubes, becoming monosiphonic distally, lower stem region unbranched for about one third distance up from base, branches thereafter alternate, widely spaced, each frontal on a wide apophysis with indistinct transverse distal node.

Hydrocladia with up to 10 hydrothecae, first three internodes ahydrocladiate, each with a central nematotheca, nodes weakly oblique.

Hydrotheca narrowly saccate, adnate and occupying almost entire internode; internode narrow beneath hydrotheca, with two strong complete vertical septa, one below posterior of hydrotheca and one adjacent or below lateral nematotheca. Hydrothecal abcauline wall convex posteriorly, becoming shallowly concave then rising abruptly with distinct thickening to margin, adcauline wall adnate, flat, a short thick posterior septum passing vertically less than halfway into hydrotheca. Hydrotheca widening to margin with a small anterior cusp and two pairs along rim, the first pair anterior, very weakly lobate, the second pair behind lateral nematothecae (best seen in anterior view).

Median nematotheca slender pencil-shaped, about one third length of hydrotheca, adnate to abcauline wall with short free part, orifice (lateral view) terminal, circular, small, a secondary circular orifice just above hydrothecal wall. Lateral nematotheca flask-shaped, directed outwards beyond hydrothecal margin (anterior view) with a circular terminal orifice and wide foramen connecting with internode. Two nematothecae one above the other on stem, the lower opposite apophysis, and a third on apophysis, all similar to laterals with a terminal orifice and basal swelling.

FIGURE 9A–J.

9A–F. Gymnangium pusillum sp. nov. A, holotype colony SAM H2538. B, part of stem with proximal branch. C, hydrothecae. D, hydrotheca, anterior view. E, median nematotheca. F, lateral nematotheca. 9G–J. Gymnangium vegae ( Jäderholm, 1903) . G, hydrothecae. H, bifurcate orifices of median nematotheca. I, lateral nematotheca. G, gonotheca.

Stem and branches (preserved material) pale honey brown; hydrorhiza, hydrocladia and hydrothecae colourless, hydrocladial perisarc thin.

Remarks. Two microslides of Halicornaria tubulifera (NMV F58408 ) in the Bale collection of Museum Victoria are designated syntypes by Stranks (1993). The slides are labelled in Bale’s handwriting “210, Great Australian Bight, 1914” and “211; Great Australian Bight, 1914”. As specimen 211 is the better displayed I select it as lectotype of Gymnangium tubulifer (Bale, 1914) .

Gymnangium pusillum closely resembles Gymnangium tubulifer in habit and overall morphology but measurements of the lectotype show that the hydrothecae of G. tubulifer are much larger than those of G. pusillum . As consistency of hydrothecal dimensions are usually diagnostic of a species, I therefore erect Gymnangium pusillum it as a new species.

The calcite grains attached to the filamentous stolons of G. pusillum indicate growth on a calcareous sedimentary substrate.

Etymology. Named for the small size of the colony and hydrothecae.