A NEW PARADIGM FOR FRESHWATER FRAGILARIOID DIATOM CLASSIFICATION? A CRITIQUE OF LANGE‐BERTALOT'S NEW SYSTEM

Abstract

Morales, E. A.1 & Trainor, F. R.2 1Phycology Section, Patrick Center for Environmental Research, The Academy of Natural Sciences of Philadelphia, PA 19103‐1195 USA; 2Department of Ecology & Evolutionary Biology, University of Connecticut, Storrs, CT 06269‐3043 USAIn a recent study of freshwater diatoms from South America (Rumrich et al. 2000), Lange‐Bertalot introduced a new paradigm for the classification of fragilarioid diatoms. This new system is antagonistic to that presented by Williams and Round (1987) because Lange‐Bertalot recognizes a marked variability in the characters chosen and a supposed overall continuity of morphological features among the genera created by his English counterparts. Lange‐Bertalot then proposes a partitioning of Fragilaria into two genera: Fragilaria and Staurosira mainly based on the presence/absence of rimoportulae and areolate girdle bands. The newly defined Fragilaria includes relatively large phytoplankters such as F. capucina and F. crotonensis. In turn, Staurosira includes, for the most part, small periphytic organisms, and contains several new species that were based on varieties of old Fragilaria taxa. This fragmentation of species and their varieties is based on a supposed morphological discontinuity. As a consequence an apparent increase in species diversity has occurred within the fragilarioid group. The present work analyzes Lange‐Bertalot's new paradigm and confronts it with recent LM and SEM evidence. The incorporation of concepts such as plasticity, polymorphism, and parallel evolution in current classification systems is also discussed. It is concluded that Lange‐Bertalot's system represents a step backward from that of Williams and Round. Some adjustments in the latter scheme could be sufficient to accommodate the diversity of fragilarioids known at present.