Trigonaspis diskoensis Thomsen 1980a

Thomsen, Helge A. & Østergaard, Jette B., 2015, Coccolithophorids in Polar Waters: Trigonaspis spp. Revisited, Acta Protozoologica 54 (2), pp. 85-96 : 87-90

Trigonaspis diskoensis Thomsen 1980a

Trigonaspis diskoensis has so far been recorded only from the type locality in Disko Bay, West Greenland ( Thomsen 1980a; Hansen et al. 1988; Østergaard 1993; Clausen et al. 1994). Here we have the opportunity to reexamine the species based on extensive material from NE Greenland (NEW).

The general appearance of the cell including appendages and the coccolith coverage is accounted for in Figs 3, 5, 6 , while numerical facts have been assembled in Table 1. Tower-shaped flagellar pole coccoliths, 4–7 in number, form a corona at the anterior pole ( Figs 3, 6 , 11 ). The typical flagellar pole coccolith (FPC) has a base, c. 1 µm wide, that supports a tower-like structure which is fairly narrow in the middle (c. 0.5 µm), while slightly widened towards the distal end ( Fig. 8 ). In more general terms the FPC can be described as an asymmetrically double-flared tube. At the antapi- cal pole the coccolith coverage comprises disc-shaped oval organic plates ( Figs 3, 4, 6, 7 ) supporting a monolayer of triangular groups of crystallites. In between these two extremes there appears to be a more or less gradual transition in coccolith shapes, involving e.g. ‘hat-shaped’ forms ( Fig. 10 ), characterized by different degrees of elevation of the central part of the coccolith, occurring in an equatorial band around the cell. It is thus evident that T. diskoensis is not strictly dimorphic but rather varimorphic.

Triangular groups of crystallites are shown at high magnification in Figs 4, 7–10 . In the flagellar pole coccoliths the triangles appear to be organized in a singlelayered helical pattern where one turn occupies 8–10 triangles ( Fig. 9 ). The monolayer of triangles on the flattened body coccoliths (BC) also displays clear el- ements of a basic symmetrical layout involving the deposition of triangles in concentric ovals while main- taining a fairly distinct triangular matrix ( Figs 4 and 7 ). The individual triangle, irrespective of coming from a tower-shaped or a flat coccolith, is characterized by rounded to semi-pointed corners and straight or slightly concave edges. It must be emphasized that decalcifica- tion, whether natural or accidental and caused by e.g. preparational procedures, will impact on the shape and appearance of the triangular groups of crystallites. We thus interpret the tiny central hole seen in numerous triangles as a light spot (e.g. Figs 4 and 7 ) as a phenom- enon caused by dissolution. There is little variation in size among triangles from either end of a single coc- cosphere when measured as the length of the edge (see Table 1). However, slight variability does occur when comparing triangle dimensions across several speci- mens. The overall range in mean value is in T. diskoensis from 0.13–0.16 µm with a standard deviation typically one tenth of the mean value (see Table 1). We are still inclined to believe that the interpretation of a triangle being formed by three calcite rhombohedral crystallites ( Thomsen 1980a) is correct. The texture and shadowing of the surface of an individual triangle (see e.g. Figs 4 and 9 ) often indicates a tripartition of the unit.

Organic under layer scales are visible in places where coccoliths become separated ( Fig. 4 ).

A shared life cycle between the holococcolithophorid T. diskoensis and the heterococcolithophorid Pappomonas borealis ( Manton, Sutherland and McCully 1976a) Thomsen in Thomsen and Østergaard 2014b, has been reported previously ( Thomsen et al. 1991; Thomsen and Østergaard 2014b).

Biogeographical data on T. diskoensis is presented in Table 2.

Figures

Figs 3–8. Trigonaspis diskoensis. TEM whole mounts from NE Greenland (NEW). 3 – whole cell with flagella and a curled up haptonema. Tower-shaped large coccoliths encircle the flagellar pole; 4 – body coccoliths (detail from Fig. 3) with triangular crystallites arranged in a single layer on top of the visibly rimmed organic base plate. Unmineralized under layer scales are pointed out; 5 – complete cell display- ing a range of coccolith shapes, viz. tower-shaped (t), hat-shaped (h) and flat discs (d); 6 – complete cell with appendages and a distinct tuft of tower-shaped flagellar pole coccoliths; 7 – high magnification of flat body coccoliths from the cell shown in Fig. 6; 8 – detail of flagellar pole coccolith from the cell shown in Fig. 3.

Figs 9–11. Trigonaspis diskoensis. TEM whole mounts from NE Greenland (NEW) (Fig. 9), and West Greenland (Arctic Station) (Figs 10–11). 9 – high magnification of tower-shaped flagellar pole coccoliths (from Fig. 5) showing the helical placement of triangular crystallite groups; 10 – high magnification of hat-shaped coccolith detached from the cell shown in Fig. 11. Notice that the triangular crystallites are arranged in highly organized tiers; 11 – Whole cell with extended flagella and haptonema.