Antarctothoa ballia, Gordon, 2020

Gordon, Dennis P., 2020, New Hippothoidae (Bryozoa) from Australasia, Zootaxa 4750 (4), pp. 451-476 : 466-468

Antarctothoa ballia n. sp.

( Fig. 9 A–J)

Etymology. Alluding to Ballia callitricha (C.Agardh) Kütz. , the red-algal substratum on which the species grows.

Material examined. Holotype: NIWA 144833 , 40.6199° S, 176.4151° E, on beach-cast dried Ballia callitricha, Akitio , North Island , coll. D.P. Gordon, 31 December 2019 . Paratype: NIWA 144834 , same data as holotype . Other material: NIWA 144835 , same data as holotype .

Description. Colony encrusting axis of finely branched red-alga, initially biserial for 4–5 zooid generations ( Fig. 9A ), then becoming pluri-multiserial, up to 8–10 zooidal series wide ( Fig. 9B ), unilaminar, to 10 mm long, 2.5 mm maximum breadth.

Autozooids averaging more than twice as long as wide, of varying length, those at bifurcation of zooid rows being generally a little shorter. Lateral margins straight or, more generally, slightly convex, widest midlength or just proximal to orifice. Distal margin truncate or gently rounded; proximal margin truncate if budded directly from autozooid, typically tapered if budded from zooidal bifurcation. Gymnocystal shield opaque in most zooids, with occasional transverse growth lines. No median carina or series of tubercles; instead, a very stout elevated blunt umbo proximal to and generally slightly curving over the sinus; orifice flanked by 1–2 shorter such prominences ( Fig. 9C, F ). ZL 500±87, 389–702 (20); ZW 215±14, 177–243 (20).

Orifice widest about midlength; sinus deep and wide, not quite parallel-sided ( Fig. 9D ), ‘shoulders’ of orifice correspondingly short. Condyles small, pointed, variable, set on edge of sinus entrance. OL 114±13, 89–134 (20); OW 97±8, 81–109 (20).

Female cystids mostly shorter than autozooids, budded from autozooids or ooecial kenozooids directly or at bifurcation of zooid rows, with a shorter suboral umbo and no lateral prominences. Ovicell prominent, terminal, cleithral; ooecium formed by distal (ooecial) kenozooid and averaging as long as wide, with c. 7–12 scattered pseu- dopores of variable size ( Fig. 9C ). Dimorphic combined maternal aperture just over twice as wide as long, partly hidden by suboral umbo; orificial sinus a short rounded-V or U-shaped nick ( Fig. 9D ), not in same plane as rest of aperture. ♀ ZL 606±120, 448–856 (18); ♀ ZW 213±29, 168–262 (18); OoL 241±13, 214–262 (18); OoW 244±26, 195–307 (18); ♀ OrL 51±6, 43–61 (11); ♀ OrW 113±6, 103–124 (11).

Male zooids highly variable in size and shape ( Fig. 9C, F ), either resembling autozooids (though a little shorter), or short and squat, or subtubular, budded from autozooids or from other male zooids; suboral umbo generally lacking, lateral prominences absent. Orifice small with sinus generally disproportionately large ( Fig. 9G ) and not in same plane as anter; condyles on edge of sinus, tiny and pointed or rounded ( Fig. 8F ). Occasionally a male zooid regenerates in an autozooidal cystid. ♂ ZL 317±66, 202–418 (11); ♂ ZW 147±48, 93–243 (11); ♂ OL 49±5, 40–56 (11); ♂ OW 44±7, 33–55 (11).

Ancestrula ovoid ( Fig. 9A, E, H, J ), widest midlength, with gymnocystal frontal wall, overhanging blunt suboral umbo ( Fig. 9I ) and orifice resembling that of autozooids. On each side internally is a large basal pore-chamber occupying 2/3 ancestrular length. These pore-chambers unequally and non-simultaneously bud a daughter zooid, but the outcome and appearance are variable. If unimpeded by processes from the algal surface, the two daughters are budded distolaterally, each on one side of midline, leaving a small gap between them mid-distal of ancestrular orifice ( Fig. 9J ). Flanking the orifice on each side is a conspicuous prominence; either both are produced from the ancestrula just above internal opening of pore-chamber, or one is more associated with proximal end of first daughter zooid ( Fig. 9I, J ). Algal surface topography affecting budding and relationships of first pair of daughter zooids, such that early astogeny can be variable ( Fig. 9E, H, J ), but typically a biserial chain of zooids develops distal to ancestrula. AnL 279±25, 248–323 (10); AnW 176±32, 148–262 (10).

Remarks. The algal substratum, Ballia callitricha , produces feathery branches in three vertical planes from its central axis. When viewing an algal stem from an apical view, there are three concave lateral surfaces that the bryozoan can occupy, each surface forming an angle of c. 120°. A larva that settles against the axial stem on one of the three sides metamorphoses into an ancestrula. Its two daughters, each produced from a lateral pore-chamber, can bud in a distal direction (i.e. adapically) along the axis, producing a biserial early colony, or, owing to the surface topography of the alga, one or both daughters can be deflected, such that a branch can be produced along one of the two other algal surfaces from one daughter. Each of the daughters then buds two others that result in initially biserial subcolonies that later broaden.

Antarctothoa ballia n. sp. is not the only hippothoid to settle on a species of Ballia . In Chile, Plesiothoa australis was recorded by Moyano & Gordon, 1980 on an unnamed Ballia from Isla Mocha. Its colonies are uni–plurise-rial.

Antarctothoa ballia n. sp. is easily distinguished from all other southern-hemisphere species of the genus (cf. Ryland & Gordon 1977; Moyano & Gordon 1980; Moyano 1986; Hayward 1995; Wright et al. 2007; Kuklinski & Barnes 2009) in its combination of characters. No other species combines the same relative sizes of zooids (and features like suboral umbones and orificial tubercles) and orificial characters (sinus and condyle shapes), coupled with the variable form of early astogeny. Significant features include the unusually deep and wide autozooidal sinus, relatively large male orificial sinus and highly variable size and shape of male zooids.

Distribution. Endemic; known only from the type locality at Akitio, SE North Island (Tararua District), encrusting beach-cast Ballia callitricha (Rhodophyta) . At that locality the beach is long and sandy with a variety of macroalgae growing on rocks beyond the wave-break zone.

Figures

FIGURE 8. Antarctothoa pansa n. sp., A (NIWA 144800), B–G (NIWA 92636, paratype): A, ancestrulate linear colony (damaged) prior to distal expansion; B, autozooids, ovicellate zooids and two male zooids (far right) in expanded part of mature colony; C, operculate orifices of ovicellate zooid, autozooid and male zooid; D, ancestrula with early astogeny; E–G, polymorphic autozooidal, male and ovicellate orifices, respectively (the latter surrounded by diatoms). Scalebars: A, 2 mm; B, 500 μm; C, 200 μm; D, 100 mm; E, 30 μm; F, 10 μm; G, 50 μm.

FIGURE 9. Antarctothoa ballia n. sp., NIWA 144835: A, biserial phase of ancestrulate colony along red-algal axis; B, mature phase showing near-maximum spread; C, F, autozooids, ovicellate zooids and male zooids (♂), showing the varied size of males, the left-hand one in F also showing rare frontolateral adventitious buds; D, tilted view of dimorphic combined maternal aperture and autozooidal aperture en face; E, H, a form of early astogeny, with budding of two daughter zooids displaced leftwards, the one asterisked curving round the algal stem axis; G, tilted (en face) view of male orifices showing the relatively large sinus; I, lateral view of ancestrula and daughter zooids; note how ancestrular right-lateral orificial tubercle pertains to daughter zooid; J, conventional astogeny, with neither daughter displaced, and with small gap between them mid-distal of ancestrular orifice; right-lateral tubercle pertains to ancestrula, the left-lateral one to the left daughter zooid. Scalebars: A, B, 1 mm; C, E, F, H–J, 200 μm; D, G, 100 μm.