Back in October, I showed a photograph of red algae growing in the River Ehen, naming this as Audouinella (“At last .. a red alga that is really red ...”). A friend commented in an email that I was brave to have made this identification if there were no spores or reproductive organs and that, in Germany, these filaments would simply be referred to as “chantransia stage”. Although I had seen reproductive organs earlier in the year, I worried afterwards, that maybe I had been rash, and was mightily relieved when, a couple of months later, filaments with reproductive organs re-appeared.
What you can see in the picture below is the vegetative filaments from which short branches bearing carposporangia (female reproductive organs) arise. There are over 300 species of Audouinella, mostly found on sea coasts with just a few freshwater representatives. The life cycle seems to vary considerably from species to species so it is hard to generalise but, like many algae, there are two distinct phases, a diploid phase, following fertilisation of the carpospores by a male sexual cell, and a haploid phase, which occurs after these diploid cells have undergone meiosis. In this image, you can see filaments in the haploid phase with bundles of carpogosporangia on short side branches. In Audouinella, the diploid phase looks very similar but – and here’s the problem – they also look similar to the diploid phases of many other species of red algae. Indeed, for a long time, these diploid phases were regarded as distinct species. My 1927 flora, by West and Fritsch, for example, includes these filamentous stages as a distinct genus, Chantransia, and it was only later that people realised that they were one part of a more complicated life cycle.
Filaments of Audouinella hermanii from the River Ehen, January 2014, with the carposporangia indicated by arrows. Scale bar: 10 micrometres (= 1/100th of a millimetre).
At this particular site on the River Ehen I have also found another red alga, Lemanea (see “The River Ehen in April”). Lemanea also has a chantransia stage, so it pays to be careful with my identifications. Lemanea, in my experience, tends to be found in the fastest-flowing stretches of streams attached to stable boulders, whereas Audouinella is generally more widespread across the river bed.