Hornera currieae, Batson & Tamberg & Gordon & Negrini & Smith, 2021

Batson, Peter B., Tamberg, Yuta, Gordon, Dennis P., Negrini, Marianne & Smith, Abigail M., 2021, Hornera currieae n. sp. (Cyclostomatida: Horneridae): a new bathyal cyclostome bryozoan with reproductively induced skeletal plasticity, Zootaxa 5020 (2), pp. 257-287 : 264-273

Hornera currieae n. sp.

( Figs 4–14 , 16 )

‘Homeohornera’ nom. nud.: Clark et al. 2010, p. 38. ‘Homeohornera’ n. sp. nom. nud.: Clark et al. 2019a, p. 254.

Etymology. Named for Dr Kim I. Currie in recognition of her contributions to the fields of chemical oceanography and ocean acidification, and her generous support of marine research on the Otago Shelf.

Material Examined. Holotype: NIWA 8449 from cruise TAN0104, stn 115, 42.8022 –42.8047 ° S, 179.9878‒ 179.9883° E, Ghoul Seamount , Graveyard Seamount Complex, northern Chatham Rise, 931–1013 m, 17 April 2001 . Paratype: NIWA 8443 , same locality as holotype . Other material: NIWA 3154 , stn E800, Fiordland slope, 45.3417° S, 166.6917° E, 1003 m, 20 October 1967 ; NIWA 2276 , stn TAN 0104/389, Scroll Seamount, Graveyard Seamount Complex, Chatham Rise, 42.7812° S, 179.994° E, 870–1000 m, 21 April 2001 ; NIWA 3711 stn KAH 0204/40, Cavalli Seamount, 34.1642° S, 173.964° E, 1040–1086 m, 18 April 2002 ; stn TAN 0104/399, Morgue Seamount, Chatham Rise, 42.7200° S, 179.9605° W, 890–1012, 21 April 2001; NIWA 27875 , stn TAN 0604/6, Chatham Rise, 42.7657° S, 179.9792° W, 1040–1086 m, 28 May 2006 ; NIWA 8441 ; stn TAN0104/3, Graveyard Seamount, Chatham Rise, 42.758° S, 179.9912° W, 943–1097 m, 18April 2001 ; stn TAN 0104/153, Gothic Seamount, Chatham Rise, 42.7325° S, 179.8985° W, 990–1076 m, 15 April 2001; NIWA 25439 , stn TAN0604/113, Gothic Seamount, Chatham Rise, 42.7275° S, 179.8990° W, 1000–1107 m, 7 June 2006 ; NIWA 25325 , stn TAN0604/104, Pyre Hill, Chatham Rise, 42.7162° S, 179.9057° W, 1005–1070 m, 4 June 2006 ; NIWA 25433 , stn TAN0604/112, Gothic Seamount, 42.7270° S, 179.8983° W, 990–1040 m, 7 June 2006 ; NIWA 70966 , stn TAN0803/38, Macquarie Ridge, 50.0972° S, 163.4742° E, 1070–1123 m, 1 April 2008 .

Diagnosis. Gracile, erect, open-branched hornerid with thick, porcellanous secondary calcification and sparse threadlike cancelli that propagate proximally or distally. Small, widely spaced zooidal apertures; peristomes emerge at high angles (>75°) relative to branch axis. Nervi absent or greatly reduced. Branch/peristomial anastomoses and adventitious struts absent.

Description. Colony erect, open-branching fan, flat or curled planar, up to ~ 40 mm in height and/or width. Recently living colonies strikingly porcellanous, grading from hyaline at branch tips to pearlescent or off-white hue in thicker branches ( Fig. 4 ). Colony bases often attached to coral fragments, commonly light- to mid-brown, possibly from exogenous staining ( Fig. 4B ).

Branches gracile, bifurcating (rarely trifurcating); branching generally anisotomic ( Fig. 4A–G ). Branches in small colonies generally subequal in length, but distinct ‘leader’ branches develop with increasing colony size ( Fig. 4A ). Colonial branching patterns varying from regular to very irregular. When space in growth plane limited, strictly alternating pinnae arising from leaders; if space is available, branches bifurcate more equably, and higher-order branch subsystem may develop locally (containing secondary/ tertiary branches per Horton–Strahler classification). Branch internodes varying widely in length, and often bending—but not twisting—within growth plane in response to other branches development (cf. other hornerids). Branch angles ranging across 35–100°; branches not approaching closer than 0.36 mm to each other.

Newly budded branches tapering gently towards tips, roughly triangular or square in cross-section ( Fig. 4C–G ). With secondary wall thickening, branches becoming subcircular, usually within a distance of 1–3 internodes from tip ( Fig. 4G ). Basal stem up to 6× thicker than distal branches. Branch and peristomial anastomoses absent. Basal branches typically shed (probably by resorption) in medium–large colonies, leaving clean transverse stumps of varied length. Branch stumps overgrown with smooth secondary calcification, forming solid, domed tips ( Fig. 4A–C ); alternatively, branch regeneration may occur. Basal branch crown usually absent; if present it may contain 2–3 main branches. Broad attachment disc (up to 11 mm wide; Figs 4A–C , 5A ) composed of proximally growing kenozooids ( Fig. 5A ).

Autozooid openings on frontal/lateral branch surfaces only. Zooids in strictly alternating series, always comprising two longitudinal lines of frontal zooids and a single line of lateral zooids on each side ( Fig. 5B–D ). Transverse branch sections invariably containing 4–6 autozooidal chambers. Apertures round, measuring 60–87 µm in diameter (mean 78 µm). Relative to aperture diameter, autozooids are widely spaced; mean nearest-neighbor distance of frontal autozooids 407 µm (range 346–481 µm). Peristomes long and tubular, especially laterals ( Fig. 5B–D ). Mean lateral peristome length, 367 µm (range 160–690 µm). Peristomes becoming progressively thicker towards their bases, emerging from branch surface in a smooth curve ( Fig. 5B–D ). Lateral peristomes emerging at high angle to branch axis (>75°). Spacing between adjacent lateral peristomes on same side highly variable (0.42– 1.23 mm). Autozooidal chambers, estimated from micro-CT, long and narrow (~1.0– 1.3 mm); proximally they may be enlarged and/or flattened in cross-section. Lophophore has 9 tentacles (1 measurement); tentacles ~300 µm long (3 measurements). In the atrial region, membranous sac attached to zooidal chamber with ~8–9 large ligaments in circular arrangement. Colonial and zooidal morphometric ranges are summarised in Table 1.

Interzooidal communication (mural) pores are patchily distributed within the zooidal walls ( Fig. 5D ), or in lines. Pores ~5–8 µm in diameter, lined with fine inward-facing spines; viewed from within zooidal chamber, each pore lies in a shallow pit 10–12 µm wide. Mural pores sparse or absent on abfrontal surfaces of lateral autozooids not in apposition with other zooidal chambers ( Fig. 5D ); however, a single abfrontal pore invariably present at proximal tip of chamber (close to or corresponding to locus of septate budding).

Secondary calcification: branch tips pustulose. Pustules relatively large (~20–25 µm diameter), scattered across frontal and abfrontal wall surfaces, sometimes in longitudinal lines along abfrontal keels at branch tips ( Fig. 6A ). Pustules also present at sites of branch regeneration. Proximal to tips, pustules often immured by massive secondary calcification, usually by the second internode; away from tips, wall surfaces becoming smooth, porcellanous and may appear pearlescent ( Fig. 6B, C ). Occasional specimens mostly or fully pustulose, especially abfrontally. Ongoing calcification resulting in secondary walls that may be 500 µm or thicker in basal branches. Nervi (longitudinal striae) usually absent or, if present, have subdued relief; nervi more pronounced at tips and regeneration loci.

Skeletal ultrastructure of extrazooidal skeleton of laminated calcite, principally comprising hexagonal seminacre grading into pseudofoliated fabric ( Fig. 6D–H ). Individual hexagonal tablets up to ~10 µm across, 180–600 nm thick ( Fig. 6E ). Pseudofoliated fabric containing laminae tens of µm across ( Fig. 6F ). Subdominant rhomboidal and rectangular tablets may be present in crystallite seeding zones ( Fig. 6G, H ). Crystallite imbrication direction is patchy across colony: distal, proximal, lateral or near-neutral. EBSD indicates that the majority of crystallites are hexagonal calcite, with c -axes {0001} aligned perpendicular to body wall; crystalline a -axes {11-20} and m -axes {10-10} lie predominantly in the plane of the wall, but are randomly oriented in that plane ( Fig. 7 ).

Cancelli ~15–35 µm at outer entrance, threadlike and sparsely distributed, especially in older branches ( Fig. 8A–D ); cancelli are connected with some, but not all, hypostegal pores. Ratio of ~3 visible cancelli per autozooid aperture typical in mid-colony branches when viewed frontally (ratio changes with branch age due to cancellus coalescence). Away from branch tips, cancellus openings typically hooded, resembling lunaria, causing opening to face proximally or distally when viewed under SEM ( Fig. 8B, C ). Adjacent cancelli may coalesce into single cancellus with ongoing calcification ( Fig. 8D ). Owing to semi-transparent skeleton, passage of cancelli through body wall usually visible by light microscopy ( Figs 8A , 9A ). Most cancelli trending proximally, but outer regions of cancelli within 4–7 branch internodes of distalward gonozooid usually change direction and propagate towards gonozooid. Towards colony base, cancelli intersecting with wall surfaces at increasingly shallow angles, coalescing into shallow longitudinal grooves or parallel lines; on basal stem of larger colonies, cancelli grading into proximally oriented kenozooids with low, arcuate openings up to 70 µm wide, and sparse scattered mural pores ( Fig. 8E, F ). Atypical cancellus-like structures also common; these tubes or cavities are not connected with hypostegal pores, and commonly become obscured by ongoing wall calcification. These may be microborings or resorption traces.

Seven fertile H. currieae n. sp. colonies examined, each with 1–5 gonozooids, all located on distalmost 1– 3 branch internodes ( Figs 9A–F , 10A–C ). Colony-base-to-gonozooid internode count 8‒20 (see Fig. 2 ). Inflated incubation chamber of gonozooid positioned on abfrontal wall, but arising from rapidly immured tube growing from frontal aperture of developmentally proximal portion of gonozooid ( Figs 9D, F , 10C ). Cryptic tube wrapping around outside of branch (cf. Schäfer, 1991), expanding into bulbous flask-shaped or irregular incubating portion 0.94–1.38 mm long ( Fig. 9A–F ). Unlike other Hornera species , the inflated chamber of gonozooid is wrapped most of the way around the autozooidal bundle ( Fig. 10A, B ). Gonozooid thickly calcified (walls/roof 100 µm+), with scattered large cancelli (~40–80 µm) ( Figs 9 , 10A, B ). Ooeciostome distal, medial or laterally offset; ooeciostome varying from short spout to simple rimmed or rimless opening. Terminal ooeciopore large (~245 µm, 1 measurement; Fig. 9D ), a flattened elliptical shape, opening in distal or distofrontal direction. Removal of gonozooid roof reveals numerous pores and holes (6–17 µm in diameter) lining interior, resembling skeletal resorption structures ( Figs 9E , 11A ). Interzooidal walls near gonozooids often heavily resorbed. Gonozooidal wall cancelli may coalesce, forming complex morphologies ( Fig. 11B ). One or more larvae may settle on or near gonoozooid and metamorphose; such ancestrulae typically expire at early stage (or potentially become fused with the parent colony) and become immured seamlessly into skeleton, creating distinctive, truncated, chimney-like structures ( Fig. 11C, D ). Gonozooid shedding scars absent (cf. other hornerids).

Several ancestrulae observed growing on abfrontal branches and gonozooids of several H. currieae n. sp. colonies ( Fig. 11E, F ). Ancestrula broad and pustulose, with centered, vertical ancestrular tube. Protoecium ~410 µm in diameter, zooidal aperture ~100 µm. One or more periancestrular daughter zooids budding vertically from protoecium roof, sharing wall with ancestrular tube ( Fig. 11F ).

Remarks. Despite its atypical morphology, Hornera currieae n. sp. meets the diagnostic criteria for the genus Hornera set out in Mongereau’s (1972, p. 316) redescription. These include: (1) vertical growth of the ancestrular tube; (2) distinct frontal and abfrontal surfaces, with autozooids emerging from the frontal/lateral surfaces only; (3) abfrontal position of the inflated region of the gonozooid; and (4) a ‘vacuole’-bearing (= cancellate) body wall contoured by nervi. In addition, we found probable evidence of skeletal-resorption-mediated branch shedding and regrowth of basal branches in older colonies. Although not specified in Mongereau’s description, shedding of parts is ubiquitous in Hornera ( Batson et al. 2020) . Finally, H. currieae n. sp. polypides bear nine tentacles, as in other Hornera species ( Tamberg & Smith 2020).

Nevertheless, several characters make H. currieae n. sp. readily distinguishable from all other Hornera species. Although nervi can occur, they are weakly developed and are usually immured by continued secondary calcification. Most of the colony outer wall comprises smooth porcellanous skeleton—the polar opposite of the ropey, pustulose, anastomosing nervi that exemplify other hornerids (e.g., Hornera robusta MacGillivray, 1883 ). Cancelli of H. currieae are thin, threadlike and sparse in number. The reproductively induced variability in cancellus growth direction is unique to this species. Colonies always have two longitudinal rows of alternating frontal autozooids, and their apertures are unusually widely spaced.

We examined described and undescribed hornerids from Australasia and beyond. Morphologically, the mostsimilar taxon to H. currieae n. sp. is an undescribed New Zealand hornerid (‘ Hornera cf. caespitosa’, sequenced by Waeschenbach et al. 2009; GenBank accession numbers FJ409614 , FJ409590 ). This small hornerid has nervi, cancelli and autozooidal parameters intermediate between those of H. currieae n. sp. and coastal Australasian hornerids (Smith et al. in prep.). It also occurs in offshore oceanic settings around southern New Zealand, although at shallower depths than H. currieae n. sp.

While the assignment of H. currieae n. sp. to Hornera is justifiable in our view, we recognise that considerable differences exist between this species and other described Hornera species. A revision of the austral Horneridae , including molecular sequencing, which is currently underway (Smith et al. in prep.) may result in genus reassignment.

The porcellanous secondary calcification of H. currieae n. sp. colonies is striking under a light microscope. The pearlescent or hyaline appearance probably arises from the laminated, semi-nacreous ultrastructure and the relative lack of light-refractive surface texturing (e.g., pustules, nervi). In some specimens, faint patches of varying colours are visible within the skeleton; these may mark horizons where the drying skeleton has become delaminated, forming very thin slit-like air spaces that cause structural iridescence.

Given the unusual structure of the skeleton of H. currieae n. sp., its crystallographic attributes are of interest. EBSD mapping revealed the crystalline c -axes of the constituent crystallites to be roughly perpendicular to the branch surface, indicating that the {0001} crystalline planes lie in the same plane as the flat crystallites laminating the branch wall ( Fig. 7 ). This arrangement is broadly similar to that of other Hornera species documented by Taylor & Weedon (2000), and is consistent with the observed skeletal ultrastructure in H. currieae —i.e., mostly hexagonal calcite semi-nacre and/or pseudofoliated laminae with roughly hexagonal (~120°) interfacial angles along their exposed faces. One difference, however, is that relative to other hornerids, the absence of topography-altering nervi and pustules in mature H. currieae branches greatly reduces scattering of the c -axes at a fine scale of ~100 µm (Batson et al. unpublished data).

The crystalline a -axes and m -axes represent the orientation of the three {11–20} planes and three {10-10} planes, respectively, and describe the rotation of the hexagonal calcite lattice around the c -axis. In H. currieae n. sp., the mapped a-/m- axes were widely scattered and are probably randomly distributed around the corresponding c- axes ( Fig. 7 ). This observation, along with the alignment of crystalline c- axes, may have biomechanical implications, given the strong radial anisotropy of the hexagonal calcite crystal.

In addition, based on observed crystallite morphology, other configurations of calcite are also likely to be present in the skeletal wall of H. currieae n. sp. Flat rectangular and rhomboidal tablets were common, especially in seeding zones ( Figs 6G–H ). Such crystallites might correspond to c -axis angles of ≤45° and ~22° relative to the wall surface, respectively (A. Checa, pers. comm.). Focused EBSD indexing of individual crystallites will be needed to determine whether these morphologies truly reflect different crystallographic orientations, and it should be noted that many crystallites appear only roughly crystalline in form.

The characteristic ‘triple spikes’ reported from the centres of newly seeded crystallites in other Hornera species ( Taylor & Jones 1993) were not seen in H. currieae ; in their place, single raised bumps ~ 200–500 nm across were common. Compared to its use in other biomineralizing phyla, EBSD has been underutilised as a tool to study crystallographic properties of bryozoan skeletons (a notable exception is Jacob et al. 2019).

Hornera currieae n. sp. may be among the most-calcified of living stenolaemates—which is surprising for such a gracile species. This characteristic results from extensive secondary-wall thickening combined with an unusually low density of autozooids and cancelli ( Fig. 11G, H ). Transverse branch sections can have an internal skeletal- to non-skeletal-volume ratio of up to ~93% ( Fig. 11H ). This value is comparable to that of other very heavily calcified unilaminate cyclostomes (e.g., crisinids), and palaeostomates such as fenestellids—e.g., see Fig. 7C of Penniretepora pseudotrilineata Ceretti, 1963 , in Ernst & Minwegen (2006). In terms of absolute calcimass, however, the thickened colony margins of the fenestrates such as Lyropora Hall, 1857 can contain solid skeletal walls up to 5 mm thick (A. Ernst, pers. comm.). While the Bryozoan Skeletal Index proposed for radial-exozone-bearing taxa by Wyse Jackson et al. 2020, is not directly comparable, owing to methodological differences, H. currieae n. sp. is more calcified than any of the trepostomes and cystoporates assessed by this metric.

Hornera currieae n. sp. branches grow thicker on the abfrontal side, forming a zone of near-solid skeleton up to 500 µm thick. Heavy calcification is not restricted to the basal regions of the colony—tips of fertile and/or load-bearing leader branches are often thickly calcified within two or three internodes of the branch tip (e.g., Fig. 4A ). This observation is intriguing with respect to life history, the energetics of calcification and the deep-sea environment in which this species occurs. It suggests a long-lived, slow-growing life strategy, with multi-annual to multi-decadal lifespans. Energy-dispersive X-ray spectroscopy of H. currieae n. sp. shows anticorrelated banding of sulphur and magnesium in the secondary skeleton (P. Batson et al., unpublished data). Similar S/Mg oscillations are reported in European Corallium rubrum (Linnaeus, 1758) , with a periodicity corresponding to validated annual growth rings ( Vielzeuf et al. 2013). Elemental variation may prove instructive for investigation in future growth and ageing studies of deep-sea cyclostomes that cannot be reared in the laboratory or marked in situ.

Observations of fertile colonies support an interpretation of a slow-paced life strategy. Only seven of the 43 specimens of H. currieae n. sp. examined had developed gonozooids. This is similar to some Antarctic tubuliporids collected from shallower depths, which also have a low proportion of fertile colonies ( Ostrovsky & Taylor 1996). Low metabolic rates or/and rare fertilization events could be potential reasons (discussed in Nekliudova et al. 2021), although gonozooid shedding is a possibility, as it is common in other hornerids; see Batson et al. 2020). Five colonies possessed one or two gonozooids, whereas the two largest specimens had three and five gonozooids. Micro-CT of one gonozooid revealed that two fertile zooids grew tubes leading into the same abfrontal incubation chamber ( Fig. 9F ) suggesting fusion reminiscent of lichenoporids possessing a ‘colonial’ incubation chamber ( Borg 1926). Fertile colonies with intact bases had 16–21 branch nodes between the base and the tip of the fertile branch (including pinnae). None of the broken branches bearing gonozooids had fewer than 10 nodes. Reproduction may by contingent on exceeding a certain colony-size threshold relating to resource allocation (e.g., Nekliudova et al. 2021). Indeed, some gonozooids apparently fail to develop fully, and become aborted and overgrown, as revealed by micro-CT ( Fig. 10C ). Similarly failed gonozooids were observed in Hornera mediterranea by Harmelin (2020). We did not see obvious gonozooid shedding scars, but the absence of gonozooids away from the distal three internodes suggests that shedding might occur as it does in other hornerids.

None of the H. currieae n. sp. colonies examined were small enough to reveal early colony development— which, in hornerids, is invariably immured in secondary calcification ( Borg 1926). However, a number of putative ancestrulae were found growing on fertile colonies ( Figs 9F , 10A , 11C ). These were concentrated around gonozooids, suggesting settlement of larvae on the parent colony (an alternative possibility is that these structures are pseudoancestrulae generated by the parent colony). Inferred ancestrula development is similar to that documented in Hornera cf. robusta , with a fully interior-walled protoecium, vertical ancestrular tube and peri-ancestrular budding ( Batson et al. 2019). The protoecium is large (410 µm diameter), equating to an approximate larval diameter of ~275 µm using the 1:1.5 cyclostome larvae-to-protoecium size relationship reported by Taylor & Jenkins (2017). The large (~245 µm) gonozooidal ooeciopore is also consistent with a large larval size. Assuming larvae are this large, H. currieae n. sp. is probably less fecund than continental-shelf hornerids, since the gonozooids are 2–5 times smaller.

Distribution. Most records of Hornera currieae n. sp. are from depths of ~ 700 to 1100 m across the Zealandia undersea continent ( Fig. 1 ). A single anomalous record from 260 to 280 m depth was collected from Mahina Knoll, northwest of Whakaari/White Island, Bay of Plenty (NIWA 2534). Hornera currieae n. sp. occurs in tectonically passive and active continental margins and on the subduction-associated Macquarie Ridge, with a latitudinal range of 34‒ 51° S. It is locally common on seamounts of the Graveyard Seamount Complex on the northern flank of the Chatham Rise. There it appears to be closely associated with the scleractinian coral Solenosmilia variabilis , often growing on its degraded fragments. H. currieae n. sp. is not recorded beyond the New Zealand Exclusive Economic Zone, so is potentially endemic.

Figures

FIGURE 1. Known distribution of Hornera currieae n. sp. (left); inset of region indicated in Box 1, at left, shows seamount features on the northern Chatham Rise from which this species was collected (right).

FIGURE 2. A subset of morphometric data collectedfrom Hornera currieae n. sp. Parameters 1–12 and 15 were measured from 32 colonies; parameters 13–14 were measured from SEMs of three colonies. One additional parameter, number of gonozooids per colony (GN), not shown.

FIGURE 4. Hornera currieae n. sp. colony and branch morphology.A. Abfrontal view of fan-like colony bearing five gonozooids (TAN0104 stn 153). B. Proximal portion of large colony, showing basal disc attached to a scleractinan coral fragment; note rounded, immured basal branch stumps in Figs A–C (TAN0104 stn 153). C. frontal view of colony with large attachment disc and main stem stained brown, as is common in this species (holotype, NIWA 8449). D. Frontal view of distal branches showing hyaline skeleton and long lateral peristomes emerging at high angles from branch axis (TAN0104 stn 153). E. Developing branch tip showing budding of 2 x 2 frontal/lateral autozooid arrangement, frontal view (NIWA 8449). F. SEM of distal branch showing ongoing secondary calcification leading to tapered form (NIWA 8449). G. Abfrontal surface of developing branch tip, with transition from pustulose to smooth walls with secondary thickening (NIWA 8449). Scale bars: A, C, 5 mm; B, 3 mm; D, 2 mm; E–G, 500 µm.

FIGURE 5. Hornera currieae n. sp. colony morphology.A. Colony base and stem comprising proximally growing kenozooids, base overgrowing encrusting cheilostome (NIWA 8449). B. Close-up frontal view of branch in Fig. 4D; at this stage of development the interzooidal walls of the frontal autozooids are visible as a longitudinal partition in the hyaline skeleton (TAN0104 stn 153). C. Frontal view of internode immediately proximal to the distal internode; note the relatively even spacing of lateral cancelli, which have not yet coalesced (NIWA 8449). D. Young branch ground to expose frontal and lateral autozooidal chambers, interzooidal pores arrowed; note additional secondary thickening around the peristome at right (NIWA 8443). Scale bars: A, 1 mm; B, 500 µm; C, D, 200 µm.

FIGURE 6. Hornera currieae n. sp. branch development and ultrastructure. A. Abfrontal surface of developing branch tip with prominent pustulose keels and scattered pustules (NIWA 8443). B–C. Comparison of branches under SEM and light microscopy highlighting smooth walls, widely spaced autozooidal openings and sparse cancelli (NIWA 8443). D–F. Representative skeletal ultrastructural fabrics: D. Seeding zone with directionally neutral crystallite imbrication; quadrilateral tablets are possibly rhombic semi-nacre (TAN0104 stn 153). E. Fabric dominated by proximally imbricated hexagonal semi-nacre (TAN0104 stn 115). F. Mature wall comprising pseudofoliated fabric arising from fused or expanded hexagonal semi-nacreous tablets (TAN0104 stn 153). G–H. High magnification views showing variability of surface crystallites in seeding zones: G. Large rhombic crystallite undergoing screw dislocation (left); hexagonal tablet with distinct sectoring and trilete arrangement (right) (TAN0104 stn 153). H. Large rectangular crystallite at left, with central mound at point of origin; various other crystallites with hexagonal or rhomboidal interfacial angles (TAN0104 stn 153). Scale bars: A, 100 µm; B, C, 200 µm; D, F, 10 µm; E, 5 µm; G, H, 1 µm.

FIGURE 7. Crystallographic orientation of Hornera currieae n. sp. skeletal walls. A. EBSD map of calcite from a polished longitudinal-tangential section of a mid-colony branch bifurcation, grain boundaries demarcated by black lines (inset, imaged section relative to branch axis). B. Crystallite orientation frequency distribution for entire map. C. Pole figures for c-, a- and m-axes; color ramp in all cases indicates the orientation of the crystalline c-axes relative to the sectional plane. Crystallites at edges of EBSD map lie at approximately 60°–70° to the sectional plane, grading to ~0°–15° inclination (i.e., roughly wallparallel) along midline region; note, the plane of the polished section is probably inclined ~10° from the wall surface in the longitudinal axis, hence rarity of crystallites indexed at 0° inclination. The pole figures are equal area projections; each shows orientations of 29,274 indexed crystallites displayed at 1 point per grain (upper hemisphere projection). The broad swathe of c-axes in the pole figure reflects the cylinder-wrapping orientation of indexed crystallites. The map and pole figures indicate c-axes are consistently near-perpendicular to skeletal wall surfaces, whereas a-axes and m-axes are scattered randomly around the corresponding c-axes. The crystallite indexing rate drops rapidly towards the edges of the section due to the increasingly thin (~200 nm) profiles of exposed laminae oriented at low angles to the EBSD detector. Scale bar: 100 µm.

FIGURE 8. Cancelli and kenozooids of Hornera currieae n. sp. A. Abfrontal surface of fertile branch; threadlike cancelli visible through porcellanous skeleton (TAN0104 stn 153). B. SEM of proximal-facing cancellus opening, with raised ‘hood’ composed of smaller crystallites than surrounding skeleton (colony base at left) (TAN0104 stn 115). C. Micro-CT render showing directional aspect and trailing ridges of cancellus hoods emerging at low angles (TAN0104 stn 153). D. Micro-CT isosurface back-face render of fertile branch interior with frontal wall facing downward: paths of multiple cancelli visible, as well as possible aborted cancelli or resorption features (arrowed) in secondary wall (TAN0104 stn 153). E. SEM of colony base imaged in Fig. 5A: basal stem thickened by proximally directed kenozooids, basal lamina overgrowing cheilostome at right (NIWA 8449). F. Magnified view of Fig. 8E: kenozooids have distinctive arcuate openings and also develop hypostegal pores from which new kenozooids can bud (NIWA 8449). Scale bars: A, E, 500 µm; B, C, 10 µm; D, F, 100 µm.

FIGURE 9. Gonozooid morphology of Hornera currieae n. sp. A. Lateral view of branch showing abfrontal inflated portion of gonozooid; proximal to gonozooid are distally directed cancelli (TAN0104 stn 153). B, C. Abfrontal and distal views of gonozooids, showing distally positioned, spout-like ooeciostome and large cancelli (TAN0104 stn 153). D. Frontal view of gonozooid with closed zooid aperture (arrow), marking the origin of the tube that grows across the branch exterior to the abfrontal inflated portion of the gonozooid; ooeciostome faces frontally in this example (NIWA 8443). E. Gonozooid with mechanically removed roof: the thick gonozooid walls contain large cancelli with broad, densely porous chambers that connect to the gonozooid interior; pustulose branch exterior and perforate gonozooid floor (NIWA 8443). F. Back-face isosurface render showing interior architecture of gonozooid and surrounding autozooids, viewed from the front: this gonozooid is supplied by tubes (highlighted orange) originating from two fertile zooids; also present is an immured ancestrula, highlighted green (TAN0104 stn 153). Scale bars: A, 1 mm; B, C, 500 µm; D, E, 200 µm; F 100 µm.

FIGURE 10. Back-face isosurface renders of H. currieae n. sp. gonozooids. A. Dorsal view of gonozooid: complex interior architecture includes various thin and thick cancelli, some anastomosing, and two immured ancestrulae marked by asterisks (TAN0104 stn 153). B. Lateral view of the same gonozooid, partly obscured by autozooids of side branch (top left): wide antechambers are present at the base of the large cancelli and cover most of the inner wall of the proximal region of the gonozooid roof (three of these chambers are highlighted orange); the cancellus chamber floors contain hypostegal pores. This view also reveals that the main gonozooid chamber is wrapped most of the way around the autozooidal bundle.The two immured ancestrulae shown in B are arrowed (TAN0104 stn 153). C. Three isosurface views of early (probably aborted) gonozooid development: left, exterior frontal view showing bulge (arrow) at site of ‘missing’ aperture of fertile zooid; middle, same view, but using back-face isosurface rendering, revealing the tube connecting the distal and proximal regions of the gonozooid; right, view of abfrontal surface, showing roughly triangular gonozooidal chamber wrapped around lateral autozooid (TAN0104 stn 153). All scale bars: 200 µm.

FIGURE 11. A. Hornera currieae n. sp.: magnified view of floor (left) and walls (right) of inflated gonozooid incubation chamber in 9E after removal of the roof: the broken wall reveals several of the broad chambers shown in 10C at the base of the cancelli (arrow); the gonozooid chamber floor is densely pitted with resorption structures of unknown origin (NIWA 8443). B. Partial backface isosurface render showing a cross-section of gonozooid roof with thick anastomosing cancelli (exterior at bottom) (TAN0104 stn 153). C. Inferred H. currieae n. sp. ancestrula, seamlessly immured, growing on the abfrontal surface of a colony of the same species (NIWA 8443). D. Backface isosurface render of the two ancestrulae in Figs 9F, 10A, B: ancestrular tube, white arrow; periancestrular daughter zooid, black arrow (TAN0104 stn 153). E. Early-stage H. currieae n. sp. ancestrula on branch of same species (TAN0104 stn 399). F. Magnified view of ancestrula in E, showing orifices of primary and daughter zooid, and pustule-bearing wall (TAN0104 stn 399). G–H. Heavy secondary calcification in H. currieae n. sp. G. Frontal view of branch from mid-region of colony; in older branches autozooidal orifices became increasingly spaced and cancelli are reduced in number due to coalescence (NHMUK genetic voucher specimen AW999, courtesy of Dr. Andrea Waeschenbach). H. BSE-SEM of polished branch section, showing extensive secondary thickening. Scale bars: A–E, H, 100 µm; F, 50 µm; G, 200 µm.

FIGURE 12. Cancelli in Hornera currieae n. sp. A. Longitudinally ground branch; abfrontal cancelli originate from hypostegal pores (arrows) located in the grooves between lateral zooids (NIWA 8443). B. Cancelli originate at hypostegal pores; pore detail showing fine inward facing spines (NIWA 8443). C. BSE-SEM of ground, diamond-polished branch; cross-section of a cancellus showing preserved skeletal asymmetry, a consequence of the sustained presence of raised cancellus hoods during growth. D. Transverse semithin section of several H. currieae n. sp. cancelli showing epithelial linings (arrowed); note delamination of decalcified skeleton. E. Detailed view of two abfrontal cancelli (micro-CT back-surface render): adjacent cancelli with similar fine-scale fluctuations in direction and thickness; cancellus on right has coalesced with a third cancellus (TAN0104 stn 153). F. Magnified view of 8A showing nodular appearance of cancelli under light microscope (TAN0104 stn 153). G. Openings of proximally bent cancelli with trailing ridges (NIWA 8449). H. Coalescence of five cancelli (highlighted yellow) during secondary calcification; single-channel extract of anaglyphic 3D image, hence slight ghosting (TAN0104 stn 153). Scale bars: A, F–H, 100 µm; B, 10 µm; C, 40 µm; D, 30 µm; E, 50 µm.

FIGURE 13. Micro-CT imaging of abfrontal cancellus bending in Hornera currieae n. sp. (TAN0104/153): whole colony with CT-scanned branch region indicated by red box (top left); X-ray of scanned branch, with subregions A–D indicated (bottom left); centre: simplified schematic of cancelli along scanned section showing transition from distal to proximal bending, note the changing relative positions of bends along branch. Branch subregions A–D (right), as follows:A, cancelli immediately proximal to gonozooid bend proximally, then turn distally; B, internode directly below gonozooid with cancelli initially growing outward, then bending strongly in distal direction; C, cancelli propagating proximally, then bending distally; D, cancelli only bending proximally. From internode D to the colony base all abfrontal cancelli bend proximally.

FIGURE 14. Cancellus bending and crystallite imbrication in Hornera currieae n. sp. A. SEM montage of gonozooid-bearing branch tip of fertile colony, abfrontal surface; arrows show the growth direction of the terminal parts of cancelli, determined from the orientation of their hooded openings. In addition to distal and proximal bending, cancelli at branch axils can be seen to bend laterally toward the fertile branch; (gz, gonozooid) (TAN0105 stn 115). B. Example of the relationship between terminal cancellus growth direction and mean surface crystallite imbrication score at the internode scale: arrows indicate dominant cancellus growth direction, red/blue branch shading indicates positive or negative mean crystallite imbrication scores for each region (blue, net proximal; red, net distal) (TAN0104 stn 153). C. Mixed front-face/back-face isosurface composite image of a branch bifurcation showing cancellus hood orientation and underlying cancellus bending; left branch is a lateral branch with proximal cancellus growth, right branch is the main support branch with a distalward gonozooid, towards which the initially proximally growing cancelli have turned (TAN0104 stn 153). Scale bars: A, 200 µm; B, 5 mm; C, 100 µm.

FIGURE 16. A. Crystallite seeding zone in Hornera currieae n. sp.: a zone of newly seeded ~hexagonal crystallites runs from bottom left to top right of the image, overgrowing a region of distally imbricated semi-nacre. B–D. Gonozooid morphology in the fenestrate hornerid, Hornera foliacea MacGillivray (coll. 92 m depth, Otago continental shelf, New Zealand). B. SEM of a mature H. foliacea gonozooid on the abfrontal wall of frond (distal direction towards top-right); the multi-lobed gonozooidal chamber is partly surrounded by radiating kenozooidal ridges approaching from distal and lateral directions; ridges absent or poorly developed on proximal side. C. Enlarged view of gonozooid lobe and approaching ridges developed from the branch body wall; the ridges do not fully contact the gonozooid, and a few hypostegal pores are visible, suggesting a kenozooidal affinity. D. Freshly collected colony with gonozooid. The deep orange colour of the gonozooid is derived from masses of presumed cells/ bodies concentrated in the hypostegal cavity, inferred to have a nutritive role. The radiating orange streaks around the gonozooid are concentrations of the same material within the inter-ridge spaces and at the openings of nearby cancelli. Scale bars: A, 10 µm; B–D, 300 µm.