Algae … cunningly disguised as frog spawn?

A couple of kilometres from Whitbarrow Quarry there is a spring that we always visit during the “Introducing Macroalgae” course because it usually yields a range of larger algae that we like to ensure that all the students can recognise. One of these forms tufts of filaments that are very slippery to the touch. There is a slight resemblance to frog spawn in both appearance and texture: under a hand lens the filaments can be seen to have a beaded appearance and this plus the texture creates a superficial resemblance to frog spawn. It is, in fact, another red alga, Batrachospermum which, like Lemanea (see “Lemanea in the River Ehen”) has an olive-green rather than red colour. I’ll explain more about that in the next post. I have also included one of Chris Carter’s photographs to show the structrure of Batrachospermum at higher magnification: the “beads” are composed of tufts of branchlets arising from a central filament.

Left hand image: Batrachospermum sp. growing at Burn Head, near Whitbarrow in Cumbria; right-hand image: filaments of Batrachospermum in the palm of my hand. Each of the “beads” is about half a millimetre across.

Batrachospermum sp. from Bodmin, Cornwall. Photograph by Chris Carter

I usually associate Batrachospermum with healthy ecological conditions: low nutrients, clear, cool water and diverse invertebrate communities. However, when I told the group on our course this, one of the participants said that he sometimes found it in quite polluted conditions. Interestingly, the same thing happened on a presentation of the course a few years ago and both the contradictory examples were from chalk streams in southern England. I went back to the published literature to reassure myself and, sure enough, these also referred to Batrachospermum as a species associated with good ecological conditions. There must be, however, some rare combination of conditions that enables Batrachospermum to occasionally proliferate in very enriched conditions. What we have, I suspect, is a common situation in ecology: we base our inferences about preferences on statistics rather than ecophysiology. This means that we assume that an association between a genus or species and a set of environmental conditions represents the realised niche of the species, without always understanding the nuances of ecology and physiology that determine these niches.

Next time: a red alga that really is red.