Halecium macrocaulus Watson 2008, sp. nov.

Halecium macrocaulus sp. nov.

Figure 9A-F

Holotype NMV F147467, Station 107, one microslide displaying a branch; also a large preserved fragmented colony. Paratypes NMV F147468, Station 107, one microslide displaying two branch fragments. NMV F147469, Station 107, two microslides displaying branch fragments.

Diagnosis. Thickest fragments (stem or major branches) 6 mm wide at base and 200 mm long; branching profuse and irregular, originally all around stem. Stem and branches heavily fascicled by numerous thin, almost parallel tubes; tubes becoming fewer on younger branches, ultimate branches (hydrocladia) monosiphonic; several monosiphonic branches also given off from lower fascicled stem region. Branching sub-dichotomous, forks arising from within a hydrotheca; base of branch typically heavily corrugated, walls becoming smooth distally.

Ultimate branches straight, flaccid, internodes fairly short, walls smooth, nodes oblique, tilted away from hydrotheca, wall of internode expanding distally to accommodate hydrophore; hydrophore short, cylindrical, very close but not adnate to internode.

Hydrotheca shallow, adcauline wall closely adpressed to internode; walls expanding only slightly from diaphragm to margin; rim circular, tilted at 50 – 55° to axis of internode, very weakly everted. Diaphragm distinct, a row of minute thorn-shaped desmocytes closely above; a septum (false diaphragm) of perisarc passing diagonally through hydrophore from near junction of diaphragm with abcauline wall to adcauline wall.

Hydranths large with c. 24 tentacles, poorly preserved.

Secondary hydrophores rare but when present, arising from diaphragm ofhydrotheca; hydrophore typically becoming a new branch beginning with two to four deep irregular twists, increasing in number with age to 10 deep corrugations.

Gonothecae abundant on monosiphonic branches, inserted without distinct pedicel on hydrophore or in hydrotheca; gonotheca sub-ovoid to almost triangular, strongly compressed, apex with minute twin peaks and no apparent aperture; gonophore oval, with clearly defined central spadix and apical pad of tissue armed with an array of large nematocysts.

Nematocysts of two morphological categories, none discharged:

- large bean–shaped isorhizas, 19-24 x 7-9 μm, abundant in apical pad of gonophore and throughout coenosarc of branches.

- leaf-shaped nematocysts (category indeterminate) in tentacles, 8 x 2 μm.

Perisarc of hydrocladia, hydrothecae and gonothecae very thin; thicker and robust in older parts of colony.

Colour of thickest branches (preserved material) dark brown fading to honey brown in younger parts, monosiphonic branches pale creamy white to transparent, gonophores brown.

Measurements (μm)

Monosiphonic branch

length of apophysis, abcauline side 237 – 395

length of proximal (corrugated) 882 – 1,568 internode

length of succeeding (hydrophorate) 703 – 798 internodes

width at node 182 – 237

Hydrotheca

diameter at diaphragm 198 – 221

depth, margin to diaphragm 59 – 66

diameter at margin 215 – 254

Gonotheca

length overall 882 – 1,431

maximum width 588 – 1,078

Remarks. It is uncertain whether the material originally consisted of a large colony, now broken, or several smaller colonies. Despite the apparent robustness of the colony the hydrocladial perisarc is very thin, possibly having deteriorated during preservation.

The adcaulinewall of the hydrotheca is so closely adpressed to the internode that the gap is only obvious on close examination. Adiagonal septum (false diaphragm) below the transverse diaphragm occursinmanyhydrothecae; it isusually obscured by tissue and visible only in cleared mounts. The desmocytes are also difficult to see, being very small and set close to the diaphragm. No replication of hydrothecae was found.

The numerous gonothecae are quite small compared with the size of the colonies. The gonophore almost fills the gonotheca, suggesting near-maturity. As there is no evidence of an aperture, the contents are probably released by rupture of the very thin perisarc. The nematocysts are abundant in the apical pad above the gonophore and in the coenosarc of the colony; they are very large and visible even at low magnification.

Large arborescent species of Halecium with closely adpressed hydrophores considered were: Halecium beanii (Johnston, 1838), Halecium halecinum (Linnaeus, 1758), Halecium sessile Norman, 1867 and Halecium jaederholmi Vervoort, 1972. H. beanii was rejected because the colonies and hydrothecal dimensionsare smaller than thoseof Halecium macrocaulus; H. halecinum and H. sessile were discounted because of differences in colony morphology. Vervoort’s (1972) description of H. jaederholmi is of a much smaller species with completely adnate and deeper hydrothecae.

Etymology. The species is named for the large size of the colony.