Halecium elegantulum Watson 2008, sp. nov.

Halecium elegantulum sp. nov.

Figure 6A-C

Holotype NMV F147463, Station 103, two microslides displaying several infertile stems. Paratypes NMV F147464, Station 103, two microslides displaying stems. NMV F147476, Station 103, one microslide displaying a small colony. All material probably from same colony on sertulariid hydroid.

Diagnosis. Stems arising from a creeping hydrorhiza; stolons tubular, smooth to crumpled. Hydrocaulus minute, variable in length, simple, unbranched, walls cylindrical, narrow, smooth, without nodes but with occasional shallow constrictions marking zones of regrowth after breakage. Hydrocaulus with either a single terminal hydrothecaor a linear series of up to six hydrophores each arising from diaphragm of preceding hydrotheca; base of hydophore swollen above supporting diaphragm.

Hydrotheca moderately deep, trumpet-shaped, expanding strongly from diaphragm to margin; rim circular, weakly outrolled; no replication of margin. Diaphragm delicate, transverse to concave, no desmocytes visible.

Perisarc thickest in proximal stem region, thinning distally; very thin on hydrotheca.

Gonotheca absent.

Measurements (μm) Remarks. As there are few clearly visible diaphragms and concave pseudo-diaphragms are visible in some hydrothecae. In most instances the depth from rim to diaphragm has been estimated as the distance from the base of secondary hydrophores to the rim of the supporting hydrotheca.

The few remaining hydranths are too decomposed for description but seem to have been very long and extensile. No desmocytes were seen but the interior of most hydrothecae is obscured by adventitious matter.

Two small, closely related species were considered: Halecium tenellum Hincks, 1861 and Halecium fragile Hodgson, 1950. Although most measurements of Halecium elegantulum fall within the range given by Cornelius (1995) for H. tenellum from the North Atlantic and by Watson (2003) for H. tenellum from the subantarctic, both of these species are branched whereas all material of H. elegantulum is unbranched showing no evidence of branching. Furthermore, the hydrotheca of H. elegantulum is elongated, not shallow as in H. fragile.

Etymology. The name refers to the elegantly simple hydrocaulus.