Eileen J. Cox

Morphological, genetic and mating diversity within the widespread bioindicator Nitzschia palea (Bacillariophyceae)

Trobajo R., Clavero E., Chepurnov V.A., Sabbe K., Mann D.G., Ishihara S. and Cox E.J. 2009. Morphological, genetic and mating diversity within the widespread bioindicator Nitzschia palea (Bacillariophyceae). Phycologia 48: 443–459. DOI: 10.2216/08-69.1. Nitzschia palea (Kützing) W. Smith is believed to be a widely distributed diatom and is common in various lotic and lentic freshwater habitats. However, it is also taxonomically problematic. As part of a multidisciplinary study of this diatom, 25 clones identified morphologically as N. palea were isolated from different freshwater habitats around the world (Belgium, Brazil, Egypt, India, Japan, Paraguay, Spain, Sri Lanka and the United Kingdom). Morphological and genetic diversity (using the hypervariable D1–D2 domains of LSU rDNA) were investigated, and an almost complete set of interclonal crossing experiments was carried out. Results indicate that N. palea is not a simple, homogeneous taxon and that N. palea will probably have to be split into three or more species. Molecular and mating groups do not separate along the traditional morphological boundaries among N. palea varieties, in particular between vars palea and debilis, two taxa that are commonly used to discriminate between different degrees of water pollution. At least two of the putative species within the N. palea complex appear to be geographically widespread. Because of the complexity of variation revealed by the LSU, mating and morphometric data, it is clear that further work, using extra genetic markers and new isolates, will be needed to determine the full extent of cryptic and pseudocryptic speciation in N. palea and to investigate whether the segregate species are ecologically differentiated and have value as indicators.

Crenic habitats, hotspots for freshwater biodiversity conservation: toward an understanding of their ecology

Abstract.  Springs are unique aquatic habitats that contribute significantly to local and regional biodiversity because of their high habitat complexity and the large number of different spring types. Many springs are small, but they are numerous and often of high water quality, and thus, provide habitats for species that are rare elsewhere because of their sensitivity to anthropogenic impacts (least-impaired habitat relicts). Springs are often species-rich and contain a larger number of Red List taxa than other aquatic habitats. Hydrological factors, particularly flow permanence, water chemistry, and temperature are important ecological factors determining species distribution and community composition. Despite their importance for biodiversity and water quality, springs are much less studied than other aquatic ecosystems. They also are insufficiently covered by protective legislation, often resulting in the destruction of their natural habitat. The authors of papers in this special issue describe specific spring biota, including multitaxon studies, and discuss the role of environmental factors, habitat variability at different spatial and temporal scales, and the importance of natural and anthropogenic disturbance in spring habitats. They suggest directions for future research, including defining reference conditions for springs and their role in long-term ecological research, the development of quality-assessment methods, and their more sustainable use as freshwater resources.

PLACONEIS MERESCHKOWSKY: THE RE-EVALUATION OF A DIATOM GENUS ORIGINALLY CHARACTERIZED BY ITS CHLOROPLAST TYPE

Several freshwater naviculoid diatoms, for which Mereschkowsky erected the genus Placoneis, have been re-examined with light and electron microscopy. The findings reveal that, in addition to their distinctive chloroplast type, their valvar structure also differs significantly from that of Navicula sensu stricto, and their separation from the latter is warranted. Placoneis is therefore re-erected to include them and an emended description and generitype are given. Some species included by Mereschkowsky have however been excluded, while other, more recently described taxa have been transferred to Placoneis. The position of a number of species remains unclear. Some of the taxonomie and nomenclatural problems associated with this group are briefly discussed.

TAXONOMY, LIFE CYCLE, AND AUXOSPORULATION OF NITZSCHIA FONTICOLA (BACILLARIOPHYTA)1

Nitzschia fonticola (Grunow) Grunow is a member of Nitzschia sect. Lanceolatae, a group of taxonomically intractable but ecologically important and widespread diatoms. We investigated the morphology and life cycle in three clones of N. fonticola and all exhibited reduced sexuality, with pedogamous production of auxospores in unpaired gametangia. The auxospores of all clones contained tangles of striplike elements that lay outside the perizonium and were distinct from it in structure and ontogeny. We introduce a new term, incunabula, to refer to such components of the auxospore wall. Semicryptic variation was detected: one clone differed from the other two in valve size and shape, stria density, and fibula density, as well as its nuclear large subunit ribosomal DNA (LSU rDNA) sequence. The implications of reduced sexuality for the taxonomy of sect. Lanceolatae are discussed. A lectotype is designated for N. fonticola from among original material of Grunow, and the application of the name is clarified further by designating illustrations and the LSU sequence AM182191 from one of our clones as epitypes.

VARIATION IN PATTERNS OF VALVE MORPHOGENESIS BETWEEN REPRESENTATIVES OF SIX BIRAPHID DIATOM GENERA (BACILLARIOPHYCEAE)

Scanning electron microscopic studies of silica valve formation in naviculoid diatoms representing six different genera revealed that the precise sequence of depositional events varied among genera. Valve deposition begins with the formation of the raphe sternum, from which virgae (lateral outgrowths) extend. Areolae (pores) are formed between the virgae by the fusion of cross‐extensions (vimines). In most of the species studied (Craticula ambigua (Kützing) D. G. Mann, Frustulia vulgaris (Thwaites) De Toni, Craspedostauros australis E. J. Cox, and Gomphonema truncatum Ehrenberg), areola (pore) formation began near the raphe sternum before completion of the valve margin, but in Pinnularia gibba Ehrenberg the valve margin fused before the areolae were formed. Silica deposition in all these taxa was mainly distal to proximal (with respect to the cytoplasm), but in Haslea sp. it was mainly proximal to distal. Haslea also differed in that areolae were defined as the valve margin was completed. These data have also contributed to the interpretation of taxonomically important features, such as raphe endings. In P. gibba the internal central raphe fissures were laterally deflected but subsequently obscured by additional silicification of the valve, whereas in G. truncatum they were initially straight, becoming laterally deflected as valves mature. External raphe fissures in Frustulia became Y‐shaped only just before maturity; in immature valves they were dotlike, as in Amphipleura Kützing. The comparison of developmental pathways in diatoms is a useful adjunct to morphological and other approaches in diatom systematics and warrants renewed attention.

TAXONOMIC STUDIES ON THE DIATOM GENUS NAVICULA V. THE ESTABLISHMENT OF PARLIBELLUS GEN. NOV. FOR SOME MEMBERS OF NAVICULA SECT. MICROSTIGMATICAE

The diatom genus Libellus Cleve was erected for naviculoid species with numerous girdle bands, L. grevillii (CA. Ag.) Cleve being the type species of the genus. The structure of this species has been compared with that of Navicula tripunctata (Mull.) Bory, the type of Navicula Bory. There are considerable structural differences between these two, on the basis of which N. delognei Van Heurck (L. grevillii) cannot remain in a genus typified by N. tripunctata. For nomenclatural reasons Libellus Cleve cannot simply be re-erected to accommodate N. delognei and other species of similar structure, and so the new genus Parlibellus is established for them. The genus is described, as is a new term for a structure at present peculiar to Parlibellus. Members of Navicula sect. Microstigmaticae sensu Hustedt were also investigated. Some are placed in Parlibellus, others are excluded. Species previously included in Libellus have been examined and their taxonomie position considered. The species now placed in Parlibellus...

MUCILAGE TUBE MORPHOLOGY OF THREE TUBE-DWELLING DIATOMS AND ITS DIAGNOSTIC VALUE1

Colonies of Berkeleya rutilans (Trent.) Grun., Navicula pseudocomoides Hendey and N. ramosissima (C. A. Ag.) Cleve were examined using light and electron microscopy to elucidate the structure of their mucilage tubes. Each species was found to have a characteristic tube construction which could be used for identification. A summary of features on which these species can be recognized was compiled.

A reappraisal of the diatom genus Amphipleura Kütz. Using light and electron microscopy

Light and electron microscopic examinations have been made on a number of species previously attributed to the genus Amphipleura Kutz. As a result, two groups of species can be delimited, both of which warrant generic status. The genus Berkeleya Greville is therefore extended to include the marine tube-dwelling species while Amphipleura is confined to the freshwater free-living species. Comparison is made with species of the closely related genus Frustulia C. Ag. and emended generic descriptions are given for Amphipleura and Berkeleya.

ONTOGENY, HOMOLOGY, AND TERMINOLOGY—WALL MORPHOGENESIS AS AN AID TO CHARACTER RECOGNITION AND CHARACTER STATE DEFINITION FOR PENNATE DIATOM SYSTEMATICS1

This article reviews current knowledge of wall morphogenesis in pennate diatoms in relation to the way characters are defined and described for taxonomic and systematic analyses. It argues that an understanding of ontogeny is essential for the accurate identification of character homologies, which in turn must underpin all phylogenetic and systematic analyses. Terminology should reflect character homology, but most diatom terminology fails to do this, with concomitant confusion and potential taxonomic mistakes. Identifying where information is lacking or misinterpreted are first steps toward improving our understanding of diatom structure and relationships. After reviewing the current knowledge on pennate diatom structure and its development, this article briefly discusses the significance of morphological variation, character polarity, and the vital importance of applying diatom terminology correctly.

Effects of salinity on growth and on valve morphology of five estuarine diatoms

The effects of salinity on the growth and valve morphology of five benthic estuarine diatoms (Nitzschia pusilla, N. frustulum, N. palea, N. filiformis var. conferta and Eolimna subminuscula), isolated from both freshwater and brackish/marine habitats, were investigated. The four Nitzschia strains grew well over a broad salinity range, though some (N. pusilla, N. frustulum) showed a broader salinity range tolerance (from fully saline down to at least 9.5 ppt) than others (N. palea, N. filiformis var. conferta had reduced growth at salinities of 16 ppt and above). Salinity significantly affected the valve morphology of the five strains studied. However, there was no consistent pattern in either the morphological characters affected or the direction of the effects. Although significant, the effects of salinity on valve morphology were very small and therefore it seems that the taxonomic usefulness of some of the classical taxonomical characters is not undermined.