I was thinking about my recent encounters with epiphytic algae (see “Cladophora and friends“) during my most recent visit to the River Ehen, when I noticed algae attached to some of the pearl mussels on the stream bed. We’ve seen this a few times before but today I had my Olympus TG2 camera at the ready and was able to take a few photographs. There have been occasions when the quantity of algae on the shells has been greater than this, and there has been concern about how this algae may affect the mussel’s ability to feed. It is one of several possible stresses on the pearl mussel population that we are investigating at the moment.
When I put a small piece of algae that I removed from the mussel shell onto a slide and had a look at it through my microscope, I saw that it was Oedogonium, which we have already met in some posts from earlier this year (see “More about Oedogonium” and links therein). My guess is that this is not a sophisticated relationship between host and alga, but simply that the mussel shells represent a convenient and relatively stable substratum for opportunistic filamentous algae, which seem to thrive in the River Ehen at this time of year.
Pearl mussels (Margaritifera margaritifera) on the bed of the River Ehen, several with bright-green growths of Oedogonium attached to the shells. Note, too, the moss Fontinalis antiypretica in the upper part of the picture.
The description of an alga growing on an animal shell makes a nice counterpoint to the recent posts on algae growing on other algae. There are other accounts of epizoic algae, though nowhere near as many as for epiphytic algae. This might simply mean that we have not looked in the right places. The most spectacular of the few records that I do know about comes from my Belgian friend, Luc Denys, who scraped the backs of beached whales and found two previously unknown genera of diatoms that seem to grow exclusively on this rather unusual habitat.
Epizoic Oedogonium from the River Ehen, September 2014 at low (x100, left) and medium (x400, right) magnification. Scale bar: 10 micrometres (1/100th of a millimetre).
Denys, L. (1997). Morphology and taxonomy of epizoic diatoms (Epiphalaina and Tursiocola) on a sperm whale (Physeter macrocephalus) stranded on the coast of Belgium. Diatom Research 12: 1-18.