Guide to Ardissonea

Ardissonea is a marine genus commonly found as an epiphyte on seagrasses and macroalgae. Living cells contain numerous discoid chloroplasts, which are golden-brown in color. The cells may form branching colonies via the secretion of mucilaginous stalks from the cell apex or rosettes of numerous cells attached to a substrate by the valve apices.

Frustules are heavily silicified and rectangular in girdle view. Valves greatly elongated with generally linear margins and rounded to slightly tapered apices. Transapical striae are uniseriate, consisting of poroid, dash-like areolae, slightly elongated in the apical direction. The transapical striae continue without interruption on to a more or less shallow mantle.

There are 2 longitudinal ribs running parallel to one another for nearly the entire length of the valve. The ribs fuse at the apices to form a greatly elongated ring (i.e. a bifacial annulus according to Mann 1984). Each rib is midway between the valve midline and margin. The transapical striae may form uninterrupted rows of poroid areolae across the entire valve face or appear slightly offset at the valve midline where the valve is slightly depressed or grooved.

A true sternum is lacking and careful focusing along the valve midline (Mann 1984 has suggested the term “fault line”) will reveal that the transapical ribs branching in a centripetal direction from the 2 longitudinal ribs are not continuous but merely abut one another at the valve midline.

At each valve apex, the regular transapical ribbing is absent where the radiation of striae from the bifacial annulus continues as small, circular, scattered areolae. An apical pore field is absent as are rimoportulae and spines.

The most distinctive morphological character of the genus is that the valve is chambered and consists of the areolate valve face and a thick, internal plate. These 2 structural elements are separated by the 2 longitudinal ribs to form chambers. There are either 3 or 4 longitudinal rows of small, circular apertures (referred to as “pearls” by the earlier light microscopists) in the internal plate which run nearly the entire length of the valve. Two of these rows are marginal and occur where the valve face curves to form the mantle; careful focusing is necessary to observe these. If there are a total of 3 longitudinal rows the remaining row is central, whereas in the case of 4 longitudinal rows the remaining 2 rows are equidistant between the valve margin and midline.

The copulae are relatively large and either 2 or 3 closed bands are typically present. The valvocopula possesses a notch at each end and has a row of pores along the junction of the pars interior and the wider pars exterior. These pores are circular on the band outside but slightly elongated in the pervalvar axis on the band inside, particularly at the valve apex. The pars interior of the valvocopula forms a large flange which fits inside the valve mantle. The second copula lacks the apical notch of the valvocopula but has the same poroid pattern as the latter and possesses large fimbriae at the valve apex.

Ardissonea frequently co-occurs with other greatly elongated diatom taxa including Toxarium undulatum Bailey, T. hennedyanum (Greg.) Pelletan, and “Synedra” bacillaris (Grunow) Hust.

The number of Ardissonea taxa appears to be small, but the genus is largley unknown. Hustedt (1932) included 8 species of heavily silicified, greatly elongated diatoms in the subgenus Ardissonea within the genus Synedra Ehrenberg, clearly recognizing the distinctness of the group. One of the dichotomies within his key to the subgenus Ardissonea was to separate the chambered species (i.e. those with an internal plate and longitudinal rows of apertures) from the remaining taxa. Round et al. (1990) took this one step further and recognized these chambered species as the only ones constituting Ardissonea sensu stricto; this is the concept of the genus employed here. The remaining taxa appear to be best placed in up to 3 genera (see Sullivan and Wear 1995). Finally, Hustedt (1932) described and beautifully illustrated 3 species of chambered Ardissonea. The structure of the species was stated to be identical except for differences in valve shape and stria density. The most common species in our experience is A. formosa (Hantzsch) Grunow.