Trigonaspis melvillea Thomsen

Thomsen, Helge A. & Østergaard, Jette B., 2015, Coccolithophorids in Polar Waters: Trigonaspis spp. Revisited, Acta Protozoologica 54 (2), pp. 85-96 : 91-94

Trigonaspis melvillea Thomsen in Thomsen et al. 1988

This taxon was first described from the Weddell Sea, Antarctica, based on material from the AMERIEZ cruise ( Thomsen et al. 1988). The species is here re- examined based on supplementing Antarctic material ( EPOS and ANT X/3).

The appearance of the cell emphasizing appendages (flagella and haptonema) and the overall layout of the coccolith coverage is accounted for in Figs 17 and 18 . Numerical facts have been assembled in Table 1. Trigonaspis melvillea has dimorphic coccoliths, viz. a tuft of highly distinct spear-shaped flagellar pole coccoliths, and oval, disc-shaped body coccoliths elsewhere. Both types of coccoliths are in accordance with the Trigonaspis genus concept covered by triangular plates of crystallites. Whereas the triangles of both T. diskoensis and T. minutissima are generally found to have either a straight or a slightly concave edge, the situation is different in T. melvillea where most triangles have slightly convex edges which endows the individual triangle with a roundish appearance ( Figs 19, 20, 22 ). However, when examining e.g. a decalcified specimen, as illustrated in Fig. 21 , it is very evident from the organic matrix remaining, that the basic geometry is a distinct triangular shape. The triangle edge length in T. melvillea is markedly smaller than observed in both T. diskoensis and T. minutissima (see Table 1). The organization of triangles in the FPC is strictly pyramidal when examining the distal spear-shaped termination of the coccolith, while the shaft seems to be carrying triangles organized in the same helical pattern that has previously been described for both T. diskoensis and T. minutissima . The organization of triangles on a BC deviates in T. melvillea from the concentric and closely abutted layout that characterizes both T. diskoensis and T. minutissima . In T. melvillea the triangles are thus organized in an open mesh structure comprising interwoven pentagonal (see Thomsen et al. 1988, l.c. Fig. 29) or hexagonal rings ( Fig. 22 ). Unmineralized under layer scales are fre- quently observed (e.g. Figs 20 and 23 ).

So far there is no evidence for life history events implicating a phase shift between the holococcolithophorid T. melvillea and a species from the heterococcolithophorid genus Pappomonas . Two candidate species of Pappomonas occur ( Thomsen and Østergaard 2014b) within the geographic realm of T. melvillea , viz. P. weddellensis Thomsen in Thomsen et al. 1988 and P. garrisonii Thomsen and Østergaard 2014b . It is ar- gued below that P. garrisonii enters into a shared life history with an undescribed Trigonaspis sp. that deviates markedly from T. melvillea , thus leaving P. weddellensis as a strong candidate for sharing a life history with T. melvillea .

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

Figures

Figs 17–23. Trigonaspis melvillea. TEM whole mounts from the Weddell Sea, Antarctica. 17 – whole cell with flagella and a curled up haptonema positioned inside a corona of distinct flagellar pole coccoliths; 18 – complete cell with flagella and partly extended haptonema; 19 – detail of the distal termination of a flagellar pole coccolith (from Fig. 18); 20 – rimmed body coccoliths supporting densely packed layers of crystallite groups (from Fig. 18). Unmineralized under layer scales are pointed out; 21 – detail of unmineralized termination of flagellar pole coccolith. The still remaining organic matrix clearly reveals the triangular shape of a crystallite group; 22 – detail of body coccolith calcification. Notice that the triangles are united to form hexagonal rings; 23 – uncalcified specimen showing body coccolith base plates and under layer scales (arrows).