Nitzschia venerata, sp. nov., a simulacrum for the conopeate, tube-dwelling diatom Nitzschia martiana, has scuta not conopea: a case of convergent evolution?

Abstract

Abstract Nitzschia martiana, recently restored to Homoeocladia (C. Agardh) emend Lobban et Ashworth, has needle-like cells and forms macroscopic colonies that are so distinctive that they have been reported on the basis of light microscopy alone. It is the type species for Homoeocladia, which have long flaps (conopea) arising from the sternum and covering a valve depression. This group has extreme diversity at the ultrastructural level and it would not be surprising to find another similar to H. martiana. However, Nitzschia venerata, sp. nov., is a surprising look-alike found in a coral reef seaweed sample from Saipan, Northern Mariana Islands (Western Pacific Ocean). It was studied with light- and scanning electron microscopy. Attempts to find colonies in live and preserved samples were unsuccessful. It is very long and narrow, with a valve depression like that in conopeate species. However, ultrastructurally it is not conopeate because the long flaps arise from the valve face and grow toward the keel and can therefore be classified as scuta. Although tube formation remains unknown, the extreme length and narrowness of the cells suggest it might occur, so that scientists identifying potential H. martiana should be alert to the possibility that they have N. venerata or some other needle-like Homoeocladia.