Guide to Epipellis

Species in the genus Epipellis occur as cells epizoic on marine cetaceans in temperate to polar waters.

The cells are adnately attached to the skin of the animal by the raphe valve. Cells are solitary, strongly heterovalvar, heavily silicified, and with a single parietal, C-shaped chloroplast. In girdle view, one margin of the cell is straight, while the other is dome-shaped.

The valve outline is broadly oval to nearly circular. Externally, the raphe valve face is concave with biseriate striae (more evident in internal view) that are curved and radiate around a sigmoid raphe sternum. The junction of the valve face and mantle is marked by a marginal ridge (termed crista marginalis). Areolae are small, round, and closely spaced. Striae are interrupted on both sides of the raphe by unornamented linear areas that extend perpendicularly from the raphe-sternum. These lateral extensions are slightly offset from the valve middle and extend to nearly the crista marginalis. Outside of the crista marginalis, the perimeter of the valve face is delimited by 2-4 rows of irregularly arranged and closely spaced areolae. The sigmoid raphe is filiform with slightly expanded proximal raphe ends. Distal raphe ends terminate a short distance before the apices in a short transverse depression creating a T-shaped depression.

Internally, the raphe-sternum and the lateral extensions are thickened and raised above the surface of the valve interior. The sigmoid raphe branches lie within the middle of the raphe sternum toward the apices, but become strongly displaced along the outside edge of the raphe-sternum toward the central area. The proximal raphe ends are deflected in opposite directions, slightly expanded, and surround a knob-like structure at the center of the central nodule. The internal distal raphe ends are straight, simple, and lack helictoglossae. The biseriate character of the striae is more evident on the valve interior as the striae are delimited by thickened virgae. Some of the virgae in the middle part of the valve bifurcate, however, the striae here remain biseriate. The striae terminate before reaching the valve margin, where each stria transitions to a marginal alveolus formed from arched struts that connect the virgae to the unornamented valve margin. Oval foramina of the alveoli are arranged in a circumferential ring around the valve mantle.

The external surface of the sternum valve is slightly convex and slightly stepped at the valve face/mantle transition. The striae are inconspicuously biseriate, curved, and radiate around a narrow sigmoid sternum and continue to nearly the valve margin. Areolae are small, round and closely spaced.

Internally, the raphe valve is almost entirely covered by an unornamented axial plate, with only a marginal ring of ovoid foramina. The axial ribs and the elongated alveoli that extend from the sigmoid sternum are evident as alternating strips of brighter and darker areas on the interior valve surface in SEM micrographs. The rapheless valve shows traces of an infilled raphe in the central valve, as evidenced by differing levels of brightness. Alveoli are not marginal as in the raphe valve, but are continuous from the sternum to the valve margin.

The epicingulum consists of three narrow hyaline bands. The valvocopula is open and lacks fimbriae. The first copula is closed and possesses a ligula to close the opening of the valvocopula. An antiligula seems to close the opening of the second copula (next to the raphe valve).

According to AlgaeBase, there are two valid species: the generitype Epipellis oiketis Holmes 1985 and E. heptunei Denys and Van Bonn 2001 (31 Dec 2024).

Genera that Epipellis could be confused with include Cocconeis Ehrenb. 1836 and Bennettella Holmes 1985. Epipellis is distinguished by its peculiar offset lateral extensions of the raphe sternum. According to Ferrario et al. (2019), Epipellis can be separated from Bennettella by the following features: 1) the presence of ‘silica flaps’ covering the external distal raphe ends in Bennettella, 2) the raphe fissures of Epipellis do not reach the valve apices and end in a shallow, imperforated depression, transversally expanded and internally well silicified or as a half-moon embossment, and (3) the raphe fissures of Bennettella reach the valve apices and are not in a depression.