Guide to Lyrella

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-lanceolate to linear-elliptic, with either obtusely rounded or broadly rostrate apices. A flat to undulate valve face gently curves into a shallow mantle. The uniseriate striae are distinctive, giving the valve a strongly punctate appearance. Striae are typically radiate, consisting of large round poroid areolae. Shorter striae may be present near the valve margin between the longer striae. Each poroid areola is open externally but closed internally by complex flap-like volae. In some species, the individual poroid areolae are discernible as single units whereas in others the flap-like volae coalesce forming a continuous strip running the entire length of the stria. In the latter case there are no discernible vimines separating the poroid areolae between the virgae. Hymenes are absent in this genus.

Perhaps the most recognizable character of Lyrella is the presence of lyre-shaped sterna which divide the transapical striae into an inner zone adjacent to the valve midline and a much wider outer zone. These lateral sterna are slightly depressed into the exterior valve face and protrude above the prominent virgae of the interior valve face. The lyrate sterna are non-porous but in some species may be ornamented with ribs or granules. Voigt discontinuities are frequently present within the inner striate zone on the secondary (last formed) side of the valve.

The raphe is filiform and straight and lies within a narrow axial area delimited by the two zones of interior striae. The four lateral sterna exhibit various degrees of curvature toward the poles from the central area. External, proximal raphe ends are usually straight, simple pores that open into a narrow groove which widens toward the central nodule. Internally, the proximalraphe ends take the shape of a T, anchor, or shepherd’s crook and are not expanded. In the latter case, the two ends point toward the primary side of the valve. The external distal raphe ends are well-developed terminal fissures which both curve to the secondary side of the valve. Internally, the distal raphe ends terminate as simple helictoglossae, which may be straight or curved toward the secondary side of the valve.

The cingulum consists of three open bands where the open end of each band is closed by the ligula of an adjacent band. Valvocopula is wider than both copulae; all with a single row of pores immediately advalvar to junction of pars interior and pars exterior.

There are either two or four plastids per cell. In the former case, one plastid occurs on one side of the transapical plane of the cell and the second on the other side of this plane. Furthermore, each plastid consists of two large, lobed plates - one appressed to the epivalve and the other to the hypovalve. The two plates of each plastid are connected to one another by a pervalvarly oriented isthmus where both isthmuses lie immediately adjacent to and on the same side of a central nucleus. Each of these four plates within the cell is deeply incised along its apical axis, superficially resembling the isthmus of a desmid cell. Finally, each of the four plates of the two plastids contains a bilobed pyrenoid and is associated with a volutin granule (polyphosphate granule). In cells with four plastids, the pervalvarly oriented isthmuses are lacking between the two plates; however, all other structural features are identical to those in a cell with two plastids (i.e., four pyrenoids and four volutin granules per cell). For additional details on the complexity of plastid structure and its division in Lyrella, see Mann and Stickle (1993, 1997).

Lyrella was erected by Karayeva (1978) with L. lyra as its type to accommodate heavily silicified and coarsely striate taxa formerly placed in the Section Lyratae of Navicula. Later, Mann in Round et al. (1990) transferred a large number of taxa from this section into Lyrella. Virtually all Lyrella taxa listed as valid in AlgaeBase represent transfers from Navicula, but recently some new species have been described (e.g., Nevrova et al. 2013). The width, ornamentation, degree of curvature, and polar extent of the lyre are important characters in identification of Lyrella taxa.

The only genus that Lyrella could conceivably be confused with is Fallacia Stickle and D.G. Mann, which possesses lyre-shaped sterna on its valve face and is also motile in marine sandy sediments. However, Fallacia possesses conopea, hymenate pore occlusions, and has a very different structure and arrangement of its plastid(s). In addition, species of Fallacia tend to be much smaller and more finely structured that those of Lyrella.