DISCUSSION

Our work on the present group of species contributes to a growing opinion that endemism in freshwater diatoms, particularly those based in Southern Hemisphere locations, may be much more common than was previously thought (e.g., Mann & Droop 1996, Mann 1999, Kociolek & Spaulding 2000, Kociolek & Stoermer 2001, Kilroy et al. 2003, Vanormelingen et al. 2008). However, recent taxonomic work on selected diatom taxa from Himalayas (Jüttner et al. 2004) and the current study has led to the recognition of an increasing number of endemic taxa in the freshwater diatom flora of the Indian subcontinent, particularly biodiversity hotspots like Western Ghats and Eastern Himalayas. Based on unpublished work by Karthick (Ph.D. Dissertation on Ecology of Stream Diatom Community in Central Western Ghats, to be submitted to Mysore University), some of the Gomphonema species found in the Western Ghats are widespread on other continents, but a few others, particularly those in streams of Western Ghats, seem to have limited geographical distributions. India has a diverse flora, but there is still a lot to do to document and create a more substantial understanding of this large and complex country. The three new species presented here occur in several environments, and are the dominants or represent a large proportion of the diatoms present in the collections. While there have been many (over 100) Gomphonema taxa reported from India, it still does not approach the number recorded from the intensely studied continent of Europe (e.g. Krammer & Lange-Bertalot 1986, e.g. Reichardt 1999) or the little studied country of the USA (Kociolek 2005, lists 237 Gomphonema taxa reported in the literature).

Of the three new species, G. difformum is quite different from almost all other Gomphonema species. Of particular note is the presence of what appears to be apical pore fields at both the headpole and footpole. Our observations illustrate at the headpole, groups of pores on the mantle at either side of the external distal raphe end that are separate from and quite dissimilar to the areolae. Their oblong to rounded appearance is more similar to the porelli of the apical pore fields at the footpole than the slit-like areolae found in G. difformum. Structures similar to apical pore fields at the headpole are also seen in G. kaznakowi, described from high mountain sites from China (Mereschkowsky 1906). Kociolek (1992) showed with electron microscopy that hyaline areas at the headpole of this species were composed of densely arranged areolae, that were physically separate from valve face areoale, but not structurally differentiated from nor more compact (at least in terms of the porelli found at the footpole) than the valve areolae. Gomphonema difformum differs from G. kaznakowi by a number of features, most notably by possessing a stigma and having external proximal raphe ends that are quite dilated. Gomphonema gandhii has a unique feature, namely the presence of a hood or siliceous fold over the central nodule, the edge of which is suggestive of the internal proximal raphe ends. In this feature it looks very similar to Gomphocymbella species from the East African Rift Valley lakes (Kociolek & Stoermer 1993); the feature is found in no other freshwater gomphonemoid diatoms. Gomphonema diminutum seems to be closely allied with species described from the Himalayas, though more work is necessary to affirm their relationships. For example, though not described nor illustrated in the original work, it appears that the Himalayan species do have both septa and pseudosepta. These features have been overlooked by 231 many students of the genus Gomphonema (e.g. Patrick in Patrick & Reimer 1966, Reichardt 2005, 2007). These similarities with species from a variety of areas support the idea of biogeographic distributions that have a phylogenetic basis. Williams & Reid (2006) have addressed this issue amongst the Eunotioid diatoms. These three new species occur in oligotrophic, low conductivity, pH neutral water, whereas the commonly reported Gomphonema species in Southern India, such as G. parvulum, G. gracile, G.affine, and G. pseudoaugur, occur in eutrophic, alkaline and high conductivity, waters. The distribution ranges of all three species were restricted to Western Ghats streams; hence these three species appear to be endemic to Western Ghats. However studies on diatoms in peninsular India are meager and it is too early to comment on the distribution of these species.

A phylogenetic analysis based on morphological data is necessary to further confirm the relationships of these three taxa with African and Himalayan taxa. The data from fossil and contemporary faunas indicate that, throughout the late Cretaceous, India maintained biological exchanges with adjacent lands (Briggs 2003). This could be a reason for these species connection with the African and Himalayan species. The biotic components of Africa, Madagascar and Western Ghats have inspired centuries of speculation relating to the mechanisms by which these biotas came to reside in these regions, and regarding their commonalities. Most of the authors claim that the most probable causal factors are Gondwanan vicariance and/or Cenozoic dispersal (Yoder & Nowak 2006). It would be interesting to study further on diatom flora of Western Ghats in detail and compare them with Indian Ocean islands and African species to elucidate their biogeographic history.

While recent researches on diatom taxonomy from tropical regions are challenging the ubiquity hypothesis for diatoms, they also seem to confirm that diatom communities are controlled by the same processes affecting macro-organisms in a different scale (Vanormelingen et al. 2008). These studies therefore also highlight the need for conservation and the protection of unique and isolated areas, such as Western Ghats, against habitat alterations and introduction of exotic species. Thus, it is important for future studies of diatom biodiversity to include the mechanisms generating diatom species diversity and distributions. Previous reports of Gomphonema taxa from Western Ghats are from light microscope observations and therefore are subject to further verification. The current report improves our knowledge of status, and phylogentic relation of Gomphonema and the biodiversity of freshwater diatoms of Western Ghats. These current results underscore the pressing need to continue research into diatom taxonomy and ecology in least explored geographical zone on earth particularly southern hemisphere.