Guide to Bennettella

Species in the genus Bennettella are epizoic on cetaceans in temperate to polar marine waters.

The cells are attached to the skin of the animal by the raphe valve. Cells are solitary, heterovalvar, heavily silicified, and possess a single parietal, C-shaped chloroplast. In girdle view, the frustule is asymmetric; the raphe valve side is straight while the rapheless valve side is dome-shaped.

The valve outline is elliptic to rhombic-elliptic. Externally, the raphe valve face is slightly concave with biseriate striae adjacent to the raphe becoming triseriate and finally tetraseriate or pentaseriate on the valve mantle. The junction of the valve face and mantle is marked by a marginal ridge called the crista marginalis. The striae are almost straight and parallel to slightly radiate in the valve middle, becoming curved and radiate towards the apices. Areolae are small, round and closely spaced. They are internally occluded by hymenes with circularly arranged pores along the periphery of the hymen. The external raphe fissure is sigmoid with slightly displaced proximal ends that are slightly expanded. The distal raphe endings are obscured by triangular flaps extending from the valve apices giving the false impression of a bifurcated raphe (Ferrario et al. 2019). The axial area is sigmoidal with distinctive offset lateral extensions that narrow towards the valve margin where they merge with the crista marginalis.

Internally, the raphe sternum and its lateral extensions (similar in shape and position to the central and axial areas on the external side of the valve but wider) are thickened and raised above the valve face, creating an asymmetrical canopy along the raphe. The sigmoid raphe branches lie within the middle of the raphe sternum but appear displaced due to the asymmetrical shape of the canopy. The transapical extensions of the sternum extend to the edge of the valve face and expand into flattened unornamented areas. The proximal raphe endings are elongated, expanded towards the thickened and elevated central nodule and slightly deflected in opposite directions. The internal distal raphe endings are deflected in opposite directions and terminate in helictoglossae raised slightly above the raphe sternum. The raphe valve possesses alveoli that are restricted to the valve margins. Oval foramina of the alveoli are arranged in a circumferential ring around the valve mantle.

Externally, the sternum valve is slightly convex with a shallow mantle. Its central area is small, asymmetric, slightly transapically and diagonally expanded, and nearly indistinguishable from the axial area. The axial area is very narrow, slightly sigmoid, and narrows to a very thin line towards the apices. The striae are triseriate with small, nearly circular, similarly sized, and closely spaced areolae that are somewhat irregularly arranged. Areolae are internally occluded by hymenes with circularly arranged pores.

Internally, the sternum valve is almost entirely covered by an unornamented axial plate with a marginal ring of alveolar foramina. The alveoli are not marginal as in the raphid valve but are continuous from the sternum to the valve margin.

The epicingulum consists of three hyaline, open bands. The valvocopula is widest and is next to the sternum valve. The second copula possesses a wide ligula to close the opening of the valvocopula and an antiligula to close the opening of a narrower third copula. Observations of a hypocingulum have not been produced.

According to AlgaeBase there are 3 valid species – Bennettella ceticola, B. constricta, and B. wheeleri (30 Nov 2024). Note, however, Ferrario et al. (2019 ) consider B. wheeleri to be synonymous with B. ceticola.

Bennettella could be confused with Cocconeis Ehrenb. and Epipellis Holmes. Bennettella is distinguished from Cocconeis by the peculiar offset lateral extensions of the raphe sternum of Bennettella. According to Ferrario et al. (2019) Bennettella is distinguished from Epipellis 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 positioned within a depression.