Guide to Petroneis

This taxon is found in sandy sediments of temperate to tropical marine waters.

Cells are solitary, rectangular in girdle view, isovalvar, and heavily silicified. The valve outline is broadly linear to elliptic with either weakly drawn out, obtusely rounded, or rostrate apices. The valve face is flat, very gently curving at the valve margin onto a shallow mantle. The striae are distinctive; they are composed of coarse, uniseriate, punctate areolae. Striae are typically radiate and composed of large round or transapically elongate areolae. Shorter striae may be present near the valve margin between longer striae. Each areola is open externally, but closed internally by a complex velum consisting of flap-like volae which may coalesce between areolae forming continuous elongate transapical strips. Hymenes are absent in this genus. Voigt discontinuities are frequently present on the secondary side of the valve. In some species, there are no recognizable vimines separating the poroid areolae between the virgae.

The raphe is filiform and within a smooth, slightly thickened sternum that is linear to linear-lanceolate in shape. A large, rounded or rectangular central area is present at mid valve; this is thickened both externally and internally to form a central nodule. External, proximal raphe ends are usually straight and simple pores that open into a T-shaped, spathulate, or bulbous groove. Internally the proximal raphe ends take the form of a shepherd’s crook and are not expanded. Both ends curve toward the primary side of the valve. Externally, the distal raphe ends curve to the secondary side of the valve. Internally, the distal raphe ends terminate in simple helictoglossae, which may be straight or curved toward the secondary side of the valve.

The cingulum consists of three open bands in which the open end of each band is closed by the ligula of an adjacent band. Valvocopula deep with a single row of pores whereas remaining two copulae narrower with two rows of pores.

Two large, butterfly-shaped plastids are appressed against opposing valves. Each plastid consists of four deeply indented arms and their margins are usually lobed. Four elongate pyrenoids are present in each plastid. The nucleus lies in a cytoplasmic bridge surrounded by a large central vacuole and two conspicuous volutin granules (polyphosphate bodies) are associated with each plastid.

Petroneis was erected by Stickle and Mann in Round et al. (1990) to accommodate heavily silicified and coarsely striate taxa formerly placed in the Section Punctatae of Navicula. To the best of our knowledge, no new species of Petroneis have been described since its creation by Stickle and Mann; the genus has been enlarged only by transfers from the Section Punctatae. The only other study of this genus was by Jones et al. (2005), who provided details on its siliceous morphology, cytology, and asexual and sexual reproduction. The description of Petroneis presented above represents a synthesis of these two studies.

Petroneis might be confused with two other coarsely punctate, naviculoid genera from sandy marine sediments: Lyrella Karayeva and Moreneis J.Park, Koh and Witkowski. It is easily distinguished from the first of these by the presence of a thickened, solid lyre-shaped area interrupting the transapical striae in Lyrella that is absent in Petroneis. Moreneis cells possess a single large plastid appressed to the girdle of the secondary side of the valve with lobes that extend toward the primary side of the valve, whereas in Petroneis there are two butterfly-shaped plastids appressed to the valve face surfaces. The external proximal and distal raphe ends in Moreneis are both curved toward opposite sides of the valve, while in Petroneis both are curved toward the same side of the valve. These differences in plastid and raphe structure are clearly illustrated in Fig. 7 of Park et al. (2012).