Halecium bithecum Watson, 2005

Colonies consisting of several stems to 10 mm high arising from a creeping hydrorhiza; stolons tubular. Some stems lightly fascicled at base, others monosiphonic throughout; stems sparsely branched, stem and branch internodes smooth, expanding distally to hydrophore, nodes distinct, sloping obliquely away from hydrophore, a tumescence in perisarc above and below node.

Primary hydrophore adpressed to but not adnate to internode; three to five hydrophores (pedicels) in linear series, each arising from diaphragm of preceding hydrotheca, length of hydrophores variable. Secondary hydrophores given off on a short apophysis below hydrotheca of a primary hydrophore, these developing later into branches. Branching sometimes almost dichotomous, hydrophores arising one from each side of diaphragm of a hydrotheca.

Hydrotheca shallow, expanding a little from diaphragm to margin; margin circular, slightly oblique to axis of hydrophore, rim not everted; diaphragm saucer-shaped in younger hydrothecae, flatter and often obscured in older hydrothecae, a distinct ring of desmocytes above.

Male gonotheca inserted on hydrophore below hydrotheca, tongue-shaped when immature, flattened elongate oval at maturity with small apical cleft. Female gonotheca (holotype, Isles of St Francis) balloon-shaped, laterally flattened, one side convex, other side weakly concave, arising without definite pedicel from hydrophore on lower stem; aperture at about two thirds distance up body with two upwardly facing hydrothecae each with a fully formed hydranth. One gonotheca with a row of sausage-shaped gonophores around the periphery containing ova; another gonophore containing a single spherical ovum. Aperture of female gonotheca obscured, Hydranth long, robust, with 20-24 tentacles.

Perisarc of lower stems and branches moderately thick, smooth, thinning distally, rim of hydrotheca moderately thin.

Colour: Colonies pale yellow in life, white in preserved material.