THE MARINE, PLANKTONIC DIATOM FAMILY THALASSIONEMATACEAE: MORPHOLOGY, TAXONOMY AND DISTRIBUTION

Abstract

The morphology, taxonomy and distribution of Thalassionema (six species), Thalassiothrix (five species), Trichotoxon (one species) and Lioloma (three species) are examined. They are needle-shaped diatoms ranging in size from a few μm to several millimetres. Colony formation is represented in all genera but apparently not in all species. Heteropolarity is present in all genera. The two cell ends are different in outline, and one end has apical protrusion(s) or spine(s). Exceptions are Thalassionema nitzschioides, in the sense that the two ends have the same shape, and Thalassiothrix longissima by having protrusions at both ends. Confluent areolae or a marginal ridge at one valve end, a pore in the valve mantle below the apical protrusions, and basically Y-shaped external areolar occlusions are morphological features setting Thalassionema apart from other genera. Marginal pointed spines and winged apical spines are the main morphologically distinctive features of Thalassiothrix. Trichotoxon is morphologically close to Thalassiothrix antarctica but forms ovoid instead of radiating colonies and has no marginal and only minute apical spines. Lioloma has poroid areolae occluded by cribrum or rota type vela, in contrast to the loculate areolae of the three other genera, and furthermore, it possesses bubble-shaped structures on the valve inside. Girdle bands with slit-like perforations were seen in Lioloma whereas bands in the other genera were unperforated. The labiate processes (one at each valve end) of Thalassionema spp., Thalassiothrix longissima, T. antarctica and Trichotoxon have lips of the same size and shape, whereas the other Thalassiothrix spp. and Lioloma have labiate processes shaped like a parrot-beak. A comparison between morphological features of the families Thalassionemataceae, Fragilariaceae and Rhaphoneidaceae shows that prominent diagnostic features of Thalassionemataceae, like the absence of apical pore fields, one labiate process at each valve end and areolation restricted to the valve border, are represented in Rhaphoneidaceae and to some extent in Fragilariaceae. If we accept these characteristics as diagnostically important, then a closer relationship of Thalassionemataceae to Rhaphoneidaceae than to Fragilariaceae is indicated. The characterisation in the literature of Thalassionema nitzschioides as cosmopolitan in distribution, Thalassiothrix longissima as restricted to the northern and temperate parts of the northern hemisphere and T. antarctica and Trichotoxon reinboldii to the Southern Ocean is supported by the present investigation. The remaining species are recorded from warmer waters.