Guide to Grammatophora

Valves are elongate with rounded apices. In girdle view, frustules are square to nearly rectangular. The valve margins may be smooth, slightly inflated at the center, arcuate, or undulate. A pseudoseptum is present at both valve ends. Striae are uniseriate and composed of poroid areolae. Striae are organized in transverse rows, or in a quincunx pattern. A weakly developed sternum is present. Large apical pore fields are present at both ends of the valve. One or two spines are usually present within the pore field at the upper valve mantle at one or both ends of the valve. The valve areolae may extend almost to the edge of the mantle and are continuous below the apical pore fields on the lower mantle. Two apically positioned rimoportulae are present in each valve.

The cingulum consists of a robust valvocopula and a variable number of copulae. The valvocopula is the most distinctive morphological element of the frustule. It is a closed band with an apical advalvar flange that connects tightly with the pseudoseptum of the valve. Two apically directed septa run from the apex of the valvocopula and end just short of the cell center where a small circular or elliptic space is delimited. The septa may be planar or highly undulate and may curve back towards the valve apex or end in a thickening which appears dot-like in LM (girdle view). The morphology of the septa is a useful character in differentiating between species. An important point to note is that undulate septa become less strongly undulate and eventually become planar as cell size decreases (Sato et al. 2008, 2010). A row of slit-like areolae is found on the advalvar margin of the valvocopula directly below each apical pore field. A row of plaques is found on the abvalvar margin of the valvocopula. The copulae are open, ligulate, and either plain or with single or double rows of simple poroid areolae.

Living cells typically contain 4 lobed chloroplasts which occupy the space between the valve and the septa and between septa of the two valvocopulae. The nucleus is centrally located between the edges of the septa.

The cells form colonies which are often zig-zag in shape or as simple ribbons. Individual cells are attached at their apices by mucilage produced through the simple pores of the apical pore fields. Colony shape appears to be a result of whether spines are present at only one valve apex or at both valve apices where the apical pore fields align between two cells (Sato et al. 2009).

Lobban (2015) described Grammatophora ornata as a new species from blue coral in Guam. The morphology of its valvocopula is remarkably different from that of other species in the genus in that it possesses large volate areolae along its advalvar portion which decrease in size from the ends of the frustule to its middle. In addition, the valves are asymmetric to the apical plane in that one mantle is deeper than the other at the apices. No living cells were observed so that cytological features and colony formation are unknown. Based on its morphology G. ornata may be placed in another genus in future studies.

Grammatophora is a small genus of mostly marine taxa. A total of 296 taxonomic entities are listed for the genus in DiatomBase but the number of species is undoubtedly much less than this, as a majority of these entries are of varietal or forma rank with many taxa listed as uncertain. Round et al. (1990) estimated a total of approximately 50 species for the genus. The principal habitats for Grammatophora taxa are as epiphytes on marine macroalgae and seagrasses.