Wigwamma arctica Manton, Sutherland and Oates 1977

Thomsen, Helge A. & Østergaard and Mikal Heldal, Jette B., 2013, Coccolithophorids in Polar Waters: Wigwamma spp. Revisited, Acta Protozoologica 52, pp. 237-256 : 240-250

Wigwamma arctica Manton, Sutherland and Oates 1977

The W. arctica coccoliths are monomorphic although in some specimens examined (e.g. Figs 10, 12) it is evident that the coccoliths clustered at the flagellar pole are larger than other coccoliths (varimorphic). The most conspicuous feature of the coccolith is the tent-like superstructure composed of four converging rods which arise from the calcified scale rim. The type material from West Greenland ( Manton et al. 1977; loc. cit. Fig. 7) is characterized by rods arising from slightly enlarged rim crystallites and also by the fact that the four rods are exactly equal in length and fuse apically at the very same point. Sampling of additional material from West Greenland ( Thomsen 1981) consistently reveals coccoliths of W. arctica that exactly mirrors those of the type material. However, coccoliths like those illustrated in Fig. 4 are also frequently observed. In these the rim crystallites that support the rods are significantly enlarged giving the coccolith a pentagonal outline in side view.

Examination of material from a range of polar habitats has revealed additional variability in details of the morphology of individual calcified elements and in the assembly of these. Variation encountered in material from the Antarctic encompasses coccoliths ( Thomsen et al. 1988 and Figs 3, 5) in which the four rods meet at a single point apically, but attach to rim crystallites that do not deviate morphologically from neighbouring crystallites. A typical feature of many W. arctica cells encountered in e.g. Antarctic ‘EPOS’ samples is the extension of a single rod above the point of union of the remaining three rods ( Figs 6, 10). The overall consistency and homogeneity of this phenomenon rules out the possibility that the protruding tips can be considered preparational artefacts involving distortion of a single element of a ‘wigwam’. In the vast majority of cells examined displaying coccolith tip extensions this feature is coupled with the presence of proliferated rim crystals that support the converging rods of the superstructure. The possible taxonomic significance of the morphological variability encountered, i.e. presence or absence of enlarged supporting rim crystals and presence or absence of a protruding tip from a single rod, is eliminated when examining the rare Antarctic ( EPOS) individual ( Figs 9, 11) that combines the two basic types of coccoliths, i.e. coccoliths with struts that adjoin at a single point and without proliferation of the supporting rim crystals ( Figs 9, 11 arrows) and other coccoliths that display a protruding single rod and proliferated supporting rim crystals ( Figs 9, 11 arrowheads).

High arctic material of W. arctica from the North East Water polynya comprises cells ( Fig. 12) that are basically identical to the West Greenland type material. However, other cells from the same water mass ( Fig. 14) mirror Antarctic material with regard to the anterior protrusion of a single rod which in this particular case is morphologically distinct from the remaining three rods by having a sharp pointed tip ( Fig. 14 and inset). In these coccoliths the rods attach to scale crystallites that are not morphologically deviant from those forming the remaining part of the rim calcification.

The coccolith variability accounted for above and in Figs 3–14 is summarized in Table 1.

The overall conclusion is that W. arctica is most adequately dealt with as a taxon that is characterized by coccoliths of one type (with the occasional apicalantapical size differences), a scale rim calcification that consists of two tiers of crystallites and a tent-like superstructure with a fourfold symmetry. Superstructure attachment and termination is variable. When scrutinizing the additional morphological variability tabulated below no clear cut picture emerges that could support e.g. a separation of material from either of the poles. Wigwamma arctica must be interpreted and dealt with as a taxon characterized by extensive morphological plasticity.

W. triradiata Thomsen in Thomsen et al. 1988

The original description of this taxon ( Thomsen et al. 1988) was based on only two specimens from the Weddell Sea (AMERIEZ cruise). Subsequent collection of material from Antarctic waters has documented that this taxon is consistently present and allows for a reanalysis of details of coccolith morphology.

W. triradiata is a dimorphic species with a ring of larger circum-flagellar coccoliths that differ markedly from those that cover the remaining part of the cell body ( Figs 15, 19). All coccoliths display a base plate calcification that consists of two tiers of crystallites ( Fig. 17), with narrow rods forming a proximal layer which is superimposed by a distal ring comprised of larger rectangular crystallites. All coccolith base plates appear to be of approximately the same size, ca. 0.7 × 1 µm. The flagellar pole coccoliths range in height from 1.3 to 2.0 µm. Despite the very large number of specimens available for scrutinizing the exact configuration of the superstructure is not fully elucidated. The superstructure appears to consist of four rods arranged in pairs that attach individually to much enlarged neighbouring crystals from the distal scale rim tier ( Fig. 17). One rod from each pair is fairly narrow, whereas accompanying rods are wider and one of them with a characteristic attenuated tip that projects above the point of union of the remaining rods ( Figs 18, 19). Whereas the difference in appearance of neighbouring rods is fairly conspicuous in Fig. 17, this is by no means always the case, see e.g. Fig. 19 where all rods appear to be quasisimilar. Body coccoliths of W. triradiata are characterized by a three-armed superstructure ( Fig. 16) that is only slightly raised above the base plate, ca. 0.7 µm ( Fig. 19). Each rod attaches to enlarged rim crystallites. The linkage between rim crystallite and rod is when considering both circum-flagellar and body coccoliths facilitated by a distal notch in the subtending crystallite ( Fig. 17, arrows). Non-mineralized underlayer scales (ca. 0.2 × 0.3 µm) are present and sometimes visible between coccoliths (e.g. Fig. 16, arrow).

The analysis and documentation of a large number of EPOS and ANT X/ 3 W. triradiata specimens has provided material ( Figs 15–19) that constitutes an improved basis for future recognition of this taxon. However, it is also obvious that the abundance of cells available for study has not substantially altered the original interpretation of this taxon. Thomsen et al. (1988) states that ‘it deserves mentioning that in W. triradiata the two parallel rows of rod-shaped crystallites, which make up the base-plate rim calcification, appear to be almost of the same size (like in W. arctica )’. However, this is not convincingly documented in Thomsen et al. 1988 (l.c. Fig. 20) and the access to additional material has in fact unequivocally documented that W. triradiata is in fact similar to e.g. W. annulifera in terms of displaying a noticeable size difference between elements of the two tiers (e.g. Fig. 17). It is further evident from scrutinizing Fig. 17 that the elements in the upper tier are offset relative to the ones in the lower tier so that they overlap like bricks. If this proves to be a general feature for species of Wigwamma it will support their affinity with genera currently allocated to the Papposphaeraceae (see Andruleit and Young 2010).

The threefold symmetry of body coccolith superstructures remains a unique feature among species of Wigwamma . The circum-flagellar coccolith superstructures display a functional bilateral symmetry which is achieved through a grouping of four rods in neighbouring pairs. In this sense and also with reference to the ‘wing-like’ appearance of one rod within each pair, the W. triradiata circum-flagellar coccolith superstructure can be interpreted as morphologically bridging the gap between W. arctica with a distinct fourfold symmetry and single rod distal projections, and W. annulifera which is characterized by a full-fledged bilateral symmetry and a dramatic hypertrophy of the wing-like element of each pair of rods ( Figs 20, 25).

W. annulifera Manton, Sutherland and Oates 1977

Coccoliths of W. annulifera are dimorphic. Whereas coccoliths covering the major part of the cell body are of a fairly standard Wigwamma design, with the usual two tiers of crystallites forming an upright rim along the periphery of the oval, organic baseplate ( Figs 22, 23), those that form a circum-flagellar rosette are very distinct in this species ( Figs 20, 21). The superstructures here consist of two narrow rods that converge to a point distally ( Fig. 20). Each rod emanates from an enlarged upper rim crystallite and is associated with a wing-like calcified lamina. One of the wings is typically twice as wide as the other. The narrow wing has an obliquely truncated distal end ( Figs 20, 21). Material of W. annulifera from either polar region appear to be morphologically very similar (see Figs 20 and 24) when examining details of the superstructures. However, a Svalbard cell examined using a scanning electron microscope reveals ( Fig. 25) slightly deviating features by possessing superstructure lamina plates that are unusually large and closely appended for their entire length. It is similarly evident that the W. annulifera body coccoliths are virtually identical in material from both hemispheres, and also that all cells displayed ( Figs 20, 24 and 25) share a distinct variability in overall size among body coccoliths from the same periplast. Smaller sized coccoliths (0.8–1.0 µm) occupy the middle region of the cell, while larger coccoliths (1.2–1.5 µm) occur at either end of the cell and most conspicuously so towards the anterior end of the cell. A final feature that deserves mentioning is the fact that superstructures of markedly reduced dimensions may occur at the antapical cell end ( Fig. 25). This feature is to our knowledge so far only observed in a single cell from Svalbard. However, a similar condition is observed in e.g. W. antarctica (see Thomsen et al. 1988, l.c. Fig. 9) and is also known from a range of taxa belonging to other genera of weakly calcified polar coccolithophorids. It thus seems that the occurrence of miniaturized ‘apical coccoliths’ at the antapical cell end is a sporadic morphological manifestation that can hardly be assigned taxonomic significance.

W. antarctica Thomsen in Thomsen et al. 1988

The material from the EPOS and ANTX 3 cruises to Antarctica ( Figs 26–31 ) corroborates in all details the observations provided in the original species description ( Thomsen et al. 1988). The W. antarctica coccoliths are of two discrete types. The scale rim calcification consists in all coccoliths of an upright rim formed by two tiers of crystallites (rod shaped proximally versus distally more rectangular, see Figs 29, 30 ). A coccolith superstructure is a characteristic feature of a ring of circum-flagellar coccoliths ( Figs 28, 29 ). Rods arise pairwise from neighbouring widened scale rim crystallites. It is evident from Fig. 29 that both the rod and the subtending enlarged crystallite are apically notched to improve contact. One rod protrudes distally as a claw-like structure above the others ( Fig. 29 ). Rods within each pair are differentiated by size (width). Wigwamma antarctica most closely resembles W. annulifera . Both species have dimorphic coccoliths, basically identical scale rim calcification, simple body coccoliths with only scale rim calcification, and flagellar pole superstructures that consist of pairs of rods arising from opposite points along the scale periphery where each pair consists of a slender rod and a wing-like lamina (which is little pronounced in W. antarctica while profusely developed in W. annulifera ), and a claw-like anterior projection terminating the superstructure. The claw arises from a wing-like element. The stunning similarity between the flagellar pole coccoliths of W. triradiata and W. antarctica must also be emphasized.

W. armatura Thomsen sp. nov.

Diagnosis: Cells spherical, ca. 5 µm in diameter with two flagella and a somewhat shorter haptonema. Cells are devoid of chloroplasts. Cell surface covered by an underlayer of organic scales and coccoliths of two types, i.e. body coccoliths with a simple rim calcification and circum-flagellar coccoliths with an additional superstructure. Organic under layer scales measure 1.0–1.5 µm and display fairly regular radiating threads that are crossed by irregular arcs of eccentric fibrils. Oval body coccoliths (1.8–2.2 µm) form a complete armour that surrounds the cell at some distance (ca. 1 µm) from the plasmalemma. The rim calcification consists of a ring of rod-shaped crystallites encircling the organic base plate and superimposed on this a ring of larger, rectangular crystallites. The organic base plate displays radiating microfibrils and a pattern of irregular threads crossing these. The superstructure of a circum-flagellar coccolith is composed of four fairly narrow rods (ca. 1.5 µm long) that attach to enlarged crystallites from the rim calcification. All rods cross apically to form a cluster of short, projecting claw-like spines.

Ethymology: The specific epithet ‘armatura’ (L) = ‘armour’ denotes the protective layer of coccoliths encasing the cell body.

Type specimen: Holotype: Fig. 35.

Type locality: ANT X/3 # 21/355, Antarctica, Weddell Sea at 7.20 W and 70.22 S. Sampling depth 2 m and water temperature –0.6°C .

Wigwamma armatura resembles W. triradiata ( Fig. 17), W. annulifera ( Figs 22, 23) and W. antarctica ( Fig. 30 ) with regard to details of body coccolith calcification. Also the flagellar pole coccoliths that occur interspersed among body coccoliths ( Figs 35, 37) are ‘wigwams’ reminiscent of those found in other species of Wigwamma . It is difficult due to the lack of proper three-dimensional images and the frequent dislocation of coccolith elements when cells are dried down on a grid surface, to verify whether rods are equidistantly arranged as in W. arctica ( Fig. 4) or grouped in two quasi-opposing pairs as in W. riradiata , W. annulifera and W. antarctica circum-flagellar coccoliths. However, most evidence (e.g. Fig. 36 showing two superstructures in side view) seems to support that the geometry of W. armatura superstructures is basically similar to that of e.g. W. arctica and characterized by rods that distribute themselves fairly regularly along the scale periphery.

A feature which unequivocally sets W. armatura apart from other species of Wigwamma is the highly distinct gap between the cell surface and the coccolith armour ( Figs 32–34), a feature which also manifests itself by the voluminous appearance of the coccolith investment compared to the protoplast itself in TEM whole mounts ( Figs. 35, 37). The tuft of projecting spines terminating the W. armatura superstructure is another unique feature for this taxon.

The description of W. armatura is based solely on material from ANT X/3. However, when re-examining material from the AMERIEZ cruise it was evident, that a few somewhat damaged cells of W. armatura were also recorded here.

Figures

Figs 26–31. Wigwamma antarctica TEM (Figs 26–30) and LM (Fig. 31) whole mounts (Antarctica / EPOS). 26 – complete cell with coiled up haptonema; 27 – complete cell; notice that the entire cell almost equals an Emiliana huxleyi coccolith in overall size; 28 – detail from Fig. 26 showing the two basic types of coccoliths; 29 – detail of flagellar pole coccolith; notice the protruding finger-like projection of one rod, the notched interlocking between a rod and the subtending enlarged crystallite; 30 – reversed printing of body coccolith showing scale rim calcification and base plate patterning (radial fibres); 31 – light micrograph (Nomarski contrast) of whole cell.