Guide to Microtabella

Microtabella is an araphid, benthic, marine genus that is easily recognized in girdle view by the presence of distended septa. In living cells, the septa constrain the plastids to a small portion of the frustule, a unique feature that distinguishes Microtabella from other septate genera such as Striatella, Hyalosira, Grammatophora and Tabellaria. The plastids of the other genera are not confined within the girdle.

Cells form ribbon-like colonies, with frustules attached face-to-face. The frustules are laterally compressed and have a disproportionately long pervalvar axis comprised of elongated valves connected by numerous septate girdle bands. This gives the frustules a flat profile with a prominent cingulum and inconspicuous valves. The distended septa are on the last formed copulae, separated by some pleurae without septa, so that in cells with still-developing hypocingulum the distended septa are present only in the epicingulum. The distance of the band of plastids from the hypovalve increases as the hypocingulum develops. Because the pervalvar axis is very long compared to the transapical axis, frustules and fragments are almost always observed in girdle view.

Valves of Microtabella are elongated with uniseriate striae separated by a narrow sternum. Striae are areolate and occluded by rotae. Large apical pore fields comprised of small poroids are present. Small spines near the apices may be present in some species. A rimoportula is present at each apex a short distance before the apical pore fields adjacent to the sternum. The copulae are porose, with septa extending from the closed end to the middle of the copula. In intact frustules, the ends of the septa form a zigzag appearance across the middle of the frustule along the pervalvar axis.

The genus is often epiphytic on macroalgae (Lobban 2015) and seagrasses (Frankovich and Wachnicka 2015) and is widely distributed across the warm-temperate and tropic zones, though little studied. New species are certain to be described as new areas are investigated.

The genus Microtabella was established (Round et al. 1990) to separate a group of taxa from Striatella. Others (Navarro and Williams 1991) rejected the new genus name as unnecessary and placed the taxa in Hyalosira. Lobban and Ashworth (2014) recognized that one species in the group was distinct, and it was the type species of Microtabella, which was restored as monospecific. Microtabella interrupta (Ehren.) Round 1990 was originally described as Tessella interrupta from the Baltic Sea and was lectotypified by Jahn and Kusber (2004). Lobban and Ashworth (2014) emended the description of Microtabella. A second species – M. rhombica Lobban 2015 – confirmed that distended septa constraining the plastids are synapomorphic. However, Ehrenberg’s drawing does not show distended septa and the nomenclatural consequences of that are still under study.