Guide to Homoeocladia

This primarily marine taxon grows epiphytically on seaweed of coral reefs, in biofilms on sand, in mud of mangrove roots, but also in freshwater streams. While a few species are common, most appear to be very rare and have been found only by close examination with SEM.

Homoeocladia includes a group of bilaterally symmetric species that were transferred out of the genus Nitzschia. The genus is distinguished by a central keel, sparse fibulae along the keel, depressions in the valve next to the keel, and conopea covering the depressions. Species range in size and shape from small, often rostrate, elliptic-lanceolate valves to larger lanceolate or linear valves. However, the type species of the genus, H. martiana, is exceedingly long and narrow.

Edges of the conopea may be visible as parallel lines on either side of the keel (LM). The valve–mantle interface is a continuously curved surface. Striae rarely resolved in LM. Some “spathulate” species have a keel crest near the apex, visible in girdle view (LM). In SEM, areolae of many forms; three girdle bands with pores and surface features often similar to the valve (Lobban et al. 2019, 2023).

Homoeocladia is readily distinguished from most Nitzschia and other nitzschioid diatoms by the central keel, but might be confused in LM with Bacillaria. That taxon also has a central keel, but has coarse striae and dense fibulae. Homoeocladia martiana has sometimes been identified by its shape alone but a recently reported simulacrum, Nitzschia venerata, with scuta rather than conopea shows that the needle-like shape is not unique (Lobban 2023).

Homoeocladia was erected by Agardh (1827) for a species of tube-dwelling diatom, H. martiana. During the 19th century, many other species with diverse frustule morphologies were included in the genus. By the 20th century, these species had mostly been transferred into other genera based on valve characters. At that time, H. martiana and related species possessing a central keel and/or conopea were transferred to Nitzschia. For pragmatic reasons, Lobban and Ashworth (2022) reinstated and redefined Homoeocladia on valve characters.

Nitzschia sigmoidea (the type species of Nitzschia) possesses a marginal keel and may have nitzschioid or hanztschioid symmetry (position of the keel on opposite sides of each valve within a frustule vs. the keel on the same side of each valve within a frustule) (Mann and Trobajo 2014). In contrast, cell division in Homoeocladia results in bilaterally symmetric cells. Resolution of the standing of Homoeocladia may change, depending on an expected revision of Nitzschia (Mann et al. 2021).

Homoeocladia species cannot be identified by LM alone; ultrastructural characters are needed to identify them. Identification is based on features of the areolae (nine character states from transapical slits to single elongate cribra), pore pattern on the keel and girdle bands, internal and external features of the virgae and vimines, and the presence or absence of a keel crest (Lobban et al. 2019, 2023).

Homoeocladia martiana is the only species known to make and inhabit mucilage tubes, forming macroscopic colonies.