Synedrosphenia bikarensis Lobban sp. nov.

Figs 7, 8

Diagnosis.

Valve spathulate, heteropolar, differing from S. parva and S. gomphonema in valve shape and in the fimbriate pars exterior of the copula.

Description.

Cells heteropolar, probably attached by basal pole to seaweeds. Valves spathulate, 214-277 µm long with linear, wider apical portion 12-14 µm wide, narrowing abruptly about halfway down the valve and then more gradually to 6 µm near the basal pole, which is weakly inflated to 7 µm (Fig. 7A-C). Both poles bluntly rounded. Striae parallel except weakly radiating at apical pole, 21 in 10 µm; areolae circular, 25 in 10 µm (Fig. 7D-F). Internally costae on all virgae except at poles (Figs 7G, 8D, E). Bifacial annulus indicated only by an offset between striae on valve face and mantle (Fig. 7D, oval). There was a junction down the midline of the valve where the inward-growing striae met but no sternum (Fig. 7B-E).

Girdle bands: Cingulum comprising two large, closed bands and a large apical cap (pleura) (Figs 7I, J, 8). Valvocopula porate (Figs 7I, J, 8A-E); the pars interior forming a sturdy comb (Fig. 8A-C) that matched up with the internal costae of the valve (Fig. 8D), though it was not clear whether they aligned or interdigitated with the costae. This was followed by an internal ridge and an adjacent series of pores, often hidden under the valve (Figs 7I, J, 8B, C). The pars interior was modified asymmetrically at each pole (Fig. 8D, E), with a shelf supporting a prominent asymmetrical notch (Fig. 7I, J and Fig. 8A, C). Apical and basal asymmetries were on opposite sides of the valvocopula (Fig. 8D, E, arrows) and the symmetry on the hypotheca was the mirror image of the epitheca (Fig. 7I, J, arrows). Pars exterior with three irregular rows of pores, 30 in 10 µm, reducing to two and then one at the basal pole. The copula also had a comb along inner edge of pars interior and a row of pores just under the overlapping valvocopula (Fig. 8F, G). Copula internal comb continued around the apical pole; pars exterior dominated by a series of long slits (rimae) that often extended to abvalvar margin (Figs 7I, J, 8G), resulting in a fringe along the edge of the cingulum except at poles. Both valvocopula and copula narrowed at the poles, leaving a space filled by the pleura, which is possibly a continuous band (Fig. 7I, J). We did not see the pars interior of the pleura but it is likely to be fimbriate (see S. parva below).

Holotype

(designated here). Specimen at 14.9 mm E and 9.6 mm S of the mark on slide 2920, deposited at ANSP, accession # ANSP-GC20090. Fig. 7A-C.

Registration.

Phycobank http://phycobank.org/103236.

Type locality.

Bikar Atoll, Marshall Islands, 12.223°N, 170.096°E, on seaweed filaments attached to coral from a farmer fish territory in the lagoon. Collection number BA-5, October 2019. Andrew McInnis leg.

Etymology.

Named for Bikar Atoll, Republic of the Marshall Islands, where it was collected.

Taxonomic comments.

The apparent absence of a bifacial annulus is unique among the Synedrosphenia species described here, where there is usually at least a hyaline line visible on the valve face or at the face-mantle margin. There is, however, a misalignment of striae evident in places along the valve-mantle junction (Fig. 7D, E), as observed in the other bifacial-annulus taxa described here, and hence no reason to suppose the growth of the valve proceeds differently. We hypothesize that there is an effective annulus present.