Amicula guide
  1. Valves small (<10 µm)
  2. Marginal ring of areolae
  3. Raphe branches terminate before apices
  4. Central nodule distinct
  5. Central area thickened

The genus Amicula was originally a monospecific genus created by Witkowski, Lange-Bertalot and Metzeltin in 2000. At that time, the genus consisted only of Amicula speculum, which Witkowski described in 1994 as Navicula speculum from Puck Bay, Poland in the Baltic Sea (Witkowski 1994).

Amicula is a small (<10 um), raphid, marine diatom inhabiting sediments (Witkowski 1994, Witkowski et al. 2000; Gastineau et al. 2022) and the prop roots of mangrove (Navarro 1982, as Navicula amphipleuroides).

Valves are nearly flat and elliptic to linear-elliptic, with a distinct central nodule. The axial area is thickened. Striae are restricted to the valve margin.

The two plastids are of the naviculoid type – plate-like, and appressed to the girdle.

Scanning electron microscopy reveals short raphe branches that terminate below the apices. A single circumvalvar row of radially-elongated areolae, with external cribrate pore occlusions and internal hymenes is present.

The genus is widely distributed across the temperate and tropic zones, although it is rarely reported which is likely due to its small size. Gastineau et al. (2022) described new Amicula species and ascribed the genus to the family Diploneidaceae based on phylogenetic analyses.