Guide to Zygoceros

Species of Zygoceros are marine and cosmopolitan in nearshore waters of temperate, shallow seas. Cells are benthic and typically attached to sand grains and macrophytes, although cells may detach and become part of the tychoplankton.

Valves are rhombic-lanceolate to triangular in outline, with apices that are slightly produced. The valve face curves, imperceptibly, into the mantle. The valve margin is indented, or recurved, forming a narrow flange. In girdle view, frustules are rectangular.

Striae radiate from a central annulus. The annulus may be linear, narrowly lanceolate, or sub-circular. Areolae are loculate with small, circular internal foramina. Externally, the areolae are covered by a continuous cribrum, perforated by fine pores only along the periphery of the areola. There are 2-3 raised ocelli projecting at an oblique angle from the apices. Complex, solid, densely packed spines which arise from the locular walls, cover the valve face and mantle.

Columnar projections support an ocellus, with a hyaline rim and numerous fine porelli at its apex. The external openings of the rimoportulae are found at the tips of hollow spines and may be cup-shaped. These external tubes are of variable length, but longer than the solid spines. The interior opening of the rimoportula is sessile, lip-like, and positioned in a small depression. A rimoportula located is located adjacent to each ocellus and 2-6 additional rimoportulae may be present submarginally.

The cingulum consists of 3 open, split, ligulate bands with their openings at opposite ends of the cell. The valvocopula is wider than the copulae. Each band is perforated by vertical rows of poroid areolae. The pars interior of the valvocopula is hyaline, wide, and fits into the flange of the mantle.

Living cells form zig-zag colonies with cells attached by mucilage secreted from the ocelli. The cells contain densely packed, discoid chloroplasts and the nucleus is centrally placed.

The genus Zygoceros was resurrected by Lavigne et al. (2015). Zygoceros may be considered one of the “leftover” genera within the Eupodiscales and includes species lacking a ridge at the edge of the valve face and lacking the tall processes, both characteristic of the Parodontellaceae and without the valve torsion of Cerataulus sensu stricto. As of 2022, the genus included the generitype Z. rhombus Ehrenb. 1839, Z. atlanticus (Freng.) Sar 2015, and the fossil Z. ehrenbergii Sar 2015.