Plesiothoa densa Gordon 2020, n. sp.

Plesiothoa densa n. sp.

(Figs 2C, D; 3A, B)

Etymology. Latin densus, dense, alluding to the densely crowded nature of zooids in mature colonies.

Material examined. Holotype: NIWA 122660, on low-tidal rock, 37.0448° S, 174.5102° E, 0 m, Wonga Wonga Bay, Whatipu, Manukau Harbour entrance, Auckland, New Zealand, 27 October 2017, collected by D. Gordon. Paratypes: NIWA 132848, 132849, same data as for holotype.

Description. Colony moderately large, ramifying, zooids uniserial and widely separated to pluriserial and densely crowded,> 50 mm maximum diameter (boundaries between adjacent dense or overlapping colonies difficult to determine).

Autozooids in early and uncrowded later parts of colony bilaterally symmetrical and clavate with long caudae, especially in axial runners (Fig. 2C); autozooids budded laterally from such zooids have shorter caudae but their daughters restore longer caudal length. Autozooids in crowded parts of colony very variable (Fig. 2D), depending on available space—bilaterally symmetrical or asymmetrical and cauda reduced or even absent. Gymnocystal shield smooth with very fine longitudinal striae indicating crystallite orientation, as well as sporadic transverse growth lines. ZL 470±135, 312–827 (23); ZW 178±32, 119–258 (23).

Orifice a little raised, generally slightly inclined downwards distad, longer than wide viewed en face (Fig. 3A); anter widest in distal third, narrowing proximad toward stout angular condyles set below short shoulders of orifice; sinus almost U-shaped but sides slightly diverging; generally a tiny pore proximal to sinus. OL 69±9, 54–81 (15); OW 59±8, 47–72 (15).

Distal wall of autozooid a broad inverse V as seen in transparency, with tiny communication pore on either side. Lateral autozooidal communications typically 2–4 along each wall, comprising small, rounded-triangular porechambers in base of wall, but this arrangement can be modified in crowded parts of colony. Budding can occur from all or none of the pore-chambers in open parts of colony but all appear to be involved in interzooidal communication in dense parts of colony where zooids overlap. Typically the distolateral pair give rise to autozooids starting lateral branch series in less-crowded parts of colony, and a proximolateral pair bud zooeciules; or 1–4 of the pore-chambers bud zooeciules. Female zooids are also typically budded from the distolateral pair. In crowded parts of colony where almost no substratum space is visible, other budding arrangements are seen, e.g. autozooids from zooeciules and ooecial kenozooids, autozooids and female zooids from two originating zooids. Abundant interzooidal communication occurs in crowded colony parts among all the zooid morphs.

Female cystids typically with slightly wider dilatation than parent autozooids, suboval to almost parallelsided, produced in crowded parts of colony; cauda moderately long to non-existent. Ovicell prominent, terminal, cleithral; ooecium formed from flattened distal kenozooid, ooecial surface smooth, typically with small pair of apical pseudopores or these fused. Dimorphic combined maternal aperture wider than in autozooids, with similar sinus and condyles. Ooecial kenozooid with distal pore-chamber opening. Lateral pore-chambers of female zooid as in autozooids. ♀ ZL 433±85, 323–587 (15); ♀ ZW 196±26, 155–244 (15); OoL 154±23, 131–214 (15); OoW 162±24, 132–218 (15); ♀ OrL 61±6, 54–78 (15); ♀ OrW 64±7, 55–79 (15).

Zooeciules typically short parallel-sided structures (Figs 2C,D, 3A) with subterminal orifice and tiny distal pore-chamber, sometimes with 1–2 smaller additional frontolateral pore-chamber openings. Shape highly variable in crowded parts of colonies, from small to large, elongate to squat. Orifice subcircular, or somewhat hoof-shaped and narrower proximally with a hint of pivot structures. Operculum present. ZoL 95±49, 54–243 (15); ZoW 75±48, 41–218 (15); ZoOL 18±1, 16–20 (9), ZoOW 17±2, 15–20 (9).

Ancestrula longitudinally oval (Fig. 3B), margin non-crenulated, resembling autozooids but without cauda and no apparent lateral pore-chambers. Three daughter autozooids, one mid-distal, two proximolateral, and a proximal zooeciule. AncL 205±9, 198–214 (3), AncW 125±19, 111–147 (3).

Remarks. Plesiothoa densa n. sp. is remarkable for its within-colony variability. Young openly ramifying colonies are very Hippothoa -like (Fig. 2C), the autozooids in axial runners having moderately long caudae, whereas zooids branched laterally typically have shorter caudae. In densely crowded colony parts zooids can abut so closely as to leave no substratum visible. Both open and crowded parts of large colonies, however, have zooids with identical orifices, pore-chamber openings and zooeciules, and transitions from open to crowded are obvious. Frontolateral pore-chamber openings in bleached zooids resemble pseudopores, 2–4 of which typically occur along the lateral margins in species of Plesiothoa. They are lacking in more-open parts of the colony in P. densa n. sp., but are common in crowded parts of the colony, often only on one side.

In some characters, the species resembles Plesiothoa trigemma (Ryland & Gordon, 1977), also from New Zealand, but P. trigemma autozooids are never Hippothoa -like with long caudae and the ancestrula is crenulated with a different budding pattern. Plesiothoa dorbignyana (Viviani, 1977) from southern Peru to central Chile can produce hippothoiform zooids with long caudae, but is distinguished by having more frontolateral communication pores along each zooid margin (at least five in hippothoiform autozooids, eight or more in more-compact autozooids, and six or seven in female zooids) (Moyano 1986). Plesiothoa coquimbana (Moyano & Gordon, 1980) from northern Chile is much less similar in colonial and zooidal morphology (no hippothoiform zooids) and also lacks zooeciules. Plesiothoa calculosa Hayward, 1993, from the South Shetland Islands and Ross Sea, has a cruciform branching pattern and moderately elongate autozooids but these are not hippothoiform. The “sporadically present” uncalcified windows described by Hayward look more like bore-holes, and the bifid orificial condyles are typical of Antarctothoa, as are the ovicell with multiple pseudopores and broad female orifice.

Plesiothoa densa n. sp. is so far known only from the type locality on the North Island west coast, the intertidal and shallow waters of which have scarcely been explored bryozoologically.

Distribution. Endemic; known only from the type locality, Wonga Wonga Bay, Whatipu, Manukau Harbour entrance, New Zealand, lower shore under rocks.