Antarctothoa pansa Gordon 2020, n. sp.

Antarctothoa pansa n. sp.

(Fig. 8 A–G)

Etymology. Latin pansus, spread out, alluding to the diverging of the proximal bi/triserial form of the colony into a broad expanse.

Material examined. Holotype: NIWA 92635, NIWA Stn TAN1105/69, 34.1843° S, 172.1382° E, south of Great Island (Manawatawhi), Three Kings Islands, 73–77 m, 30 March 2011, epialgal. Paratype: NIWA 12678, same data as for holotype. Other material: NIWA 144800, non-type material from same locality.

Description. Colony encrusting thin red-algal laminae, broadly multiserial, usually with lobes, from short bise-rial–oligoserial runner phase (Fig. 8A), unilaminar, 10–12 mm long, 6–8 mm wide.

Autozooids elongate, variable in relation to width, such that length can be 2.4–3.8 times width. Lateral margins straight or, more generally, slightly convex; distal margin truncate; proximal margin truncate if budded directly from autozooid, typically tapered if budded from zooidal bifurcation. Gymnocystal shield shiny, hyaline, with weak transverse striations (Fig. 8B) and median subcarinate row of very short tubercles; these can be a little more prominent in post-ancestrular zooids. ZL 572±99, 450–769 (12); ZW 208±19, 166–239 (12).

Orifice widest about midlength; sinus variable, being strictly U-shaped with parallel sides (Fig. 8E) or margins of U diverging somewhat. Condyles L-shaped, occupying entire width of orificial shoulders. OL 81±8, 71–93 (9); OW 70±6, 62–79 (9).

Female cystids of similar size to autozooids or, more usually, shorter; budded from autozooids or male zooids directly or at bifurcation of zooid rows and having identical gymnocystal characters. Ovicell prominent, terminal, cleithral; ooecium formed by distal (ooecial) kenozooid and about as long as wide, with c. 3–5 small scattered pseudopores mostly in distal half. Dimorphic combined maternal aperture wide and very narrow, partly concealed frontally by short, broadly truncate projection (Fig. 8B) in front of proximal margin; orificial sinus a short, narrow drop-shaped slit (Fig. 8G). ♀ ZL 616±118, 482–818 (9); ♀ ZW 203±20, 158–227 (9); OoL 219±7, 203–226 (11); OoW 196±17, 172–223 (11); ♀ OrL 38±3, 36–41 (3); ♀ OrW 100±10, 91–111 (3).

Male zooids resembling autozooids in size or narrower, budded from autozooids or from other male zooids; with identical gymnocystal characters or umbones lacking. Orifice tiny, almost one-third the width of an autozooidal orifice with proportionately narrower sinus (Fig. 8F). ♂ ZL 549±113, 358–714 (11); ♂ ZW 185±19, 167–230 (10); ♂ OL 33±3, 28–36 (8); ♂ OW 25±1, 24–27 (8).

Ancestrula broadly ovoid, widest in distal half, with gymnocystal frontal wall and orifice resembling that of autozooids. On each side distolaterally is a large basal pore-chamber from which arise a pair of daughter zooids, non-simultaneously. These meet distally around the ancestrula, leaving no intervening space (Fig. 8D). The daughter zooids in turn bud further zooids biserially for 4–6 zooidal generations before rapidly diverging into a multiserial fan-like expansion that continues unbroken or as 2–3 lobes. Sometimes a third autozooid can be interpolated between the post-ancestrular zooids to form a partly bi/triserial linear series before the oligo–multiserial expansion. In one colony a pair of male zooids was produced, one distal to the other, on the side of an otherwise biserial chain of autozooids, about four zooidal generations from the missing ancestrula. AnL 268±24, 222–296 (7); AnW 205±9, 198–222 (7).

Remarks. Antarctothoa pansa n. sp. is distinguished from almost all other southern-hemisphere species (cf. Ryland & Gordon 1977; Moyano & Gordon 1980; Hayward 1995; Wright et al. 2007; Kuklinski & Barnes 2009) by its initially biserial colony that expands rapidly into a broad fan-like sheet, in combination with its early astogeny in which the two distolateral daughters abut laterally without an intervening third zooid. Antarctothoa antarctica (Moyano & Gordon, 1980) has a similar biserial early colony form but the colony never widens beyond a pluriserial state; also the autozooidal orifice is narrower and the ancestrula is lageniform.

Distribution. Endemic; known only from the type locality south of Great Island, (Manawatawhi), Three Kings Islands, off Northland, New Zealand, 73–77 m, on small thin-bladed red algae.