Guide to Thalassiophysa

Thalassiophysa is a monotypic genus represented by Thalassiophysa hyalina (Greville 1865) Paddock and Sims 1981. Thalassiophysa hyalina is a common and easily recognizable member of the flora growing epiphytically on seagrasses and macroalgae in the tropical Pacific (Lobban et al. 2012) and Atlantic (Frankovich and Wachnicka 2015).

Frustules are lightly silicified, amphoroid in general appearance, with strongly dorsiventral valves, usually lying in girdle view. Valves are canoe-shaped and deeply V-shaped in cross-section with a prominent keel, which runs the length of the valve. The canal raphe is strongly biarcuate, fibulate, and runs along the keel from the apices toward the central valve. Before reaching the central nodule, the raphe branches curve sharply toward the valve margin traversing the ventral valve faces and becoming nearly parallel to each other at the central nodule where they terminate very close to each other. The paths of the raphe branches create a deeply incised raphe outline in girdle view at mid-valve. Internally, the central nodule extends into frustule cavity as a distinctive mucronate (spur-like) structure termed a mucro by Paddock and Sims (1980). The distal raphe ends are positioned near the valve apices. Helictoglossae are absent. Small, arched, staple-like fibulae are distributed along the length of the canal raphe from near the mucro to the valve apices. Rows of straight, strut-like fibulae (termed “adventitious” by Paddock and Sims 1980) connect the internal dorsal and ventral valve faces below the keel. The “true” and “adventitious” fibulae appear as puncta in LM and in drawings from the older literature. Striae are uniseriate and composed of small, round areolae occluded by hymenes.

The girdle consists of numerous open bands with the same striation as the valve. Valvocopula are not differentiated from the copulae. The advalvar margin of bands lack striae.

Living cells have a single large plastid divided into two lobes by a narrow constriction of the plastid at the median. The two lobes of the plastid are symmetric on each side of the median apical plane (i.e., the full length and width of the plastid is evident in girdle view).

The unique morphology of Thalassiophysa has been described as a transitional form between Amphora, specifically members of the Oxyamphora clade, and members of the Rhopalodiales and Surirellales (Stepanek and Kociolek 2014). Thalassiophysa is similar to several Amphora species in having strong dorso-ventral valves, multiple girdle bands, and a single large plate-like plastid. It is distinguished from the Oxyamphora group by having a fibulate canal raphe. Thalassiophysa is most similar to Auricula of the Surirellales. Auricula can be distinguished by having transapical costae between the striae.

The generitype, Thalassiophysa hyalina, has several synonyms, which have been variously placed in the genera Amphiprora, Amphora, Auricula, Proboscidea, and Proboscineis (see Appendix in Paddock and Sims 1981 for more details). Paddock and Sims (1980) placed one of these synonyms as the type of their new genus Proboscidea, but this name is superfluous as it is a later homonym of the angiosperm genus Proboscidea Schmidle 1763.  Hence, the proper generic name is Thalassiophysa.