Is the Order Cryptonemiales (Rhodophyta) defensible?

Abstract

The present distinction between the red algal orders Cryptonemiales and Gigartinales (based on whether or not an ‘accessory’ auxiliary-cell filament is present) appears to be logically unsatisfactory on two grounds. If an ‘accessory’ reproductive filament is defined (as it usually is) as a structure not homologous to a vegetative filament, then for a great many algae it is impossible to specify the observations that would determine correct placement in the Cryptonemiales because of inherent difficulties in establishing the existence of homologies. On the other hand, if ‘accessory’ is defined in some way that relates to observable morphological features, major anomalies ensue in which exemplars of the present Cryptonemiales (such as Dudresnaya) must be placed in the Gigartinales, whereas genera equally exemplifying the Gigartinales (such as Predaea) belong in the Cryptonemiales. From this reductio ad absurdum it is concluded that the distinction between the Cryptonemiales and Gigartinales should be abandoned and the 40 or so families of these two groups combined as the Gigartinales. The Gigartinales is re-defined as that order containing all algae with pit plugs lacking cap layers, and whose auxiliary cells form prior to fertilization and receive zygote or zygote-derived nuclei from the carpogonium via an external connection (rather than internally through enlarged pit connections). The term ‘subsidiary’, as opposed to ‘adventitious’, is proposed to describe filaments (whether vegetative or reproductive) that arise in florideophycean red algae from sub-surface cell divisions, and it is argued that auxiliary cells in both the previously-constituted Cryptonemiales and some Gigartinales are equally borne in such structures.