Pearl mussels with some unexpected bedfellows …

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.


Nitzschia and a friend …

I drove from Bogle Bridge, my sample site on Stockerley Burn, up a low hill and down the other side into the next valley where another tributary stream of the River Browney flowed. I scrambled down to the stream and hunted amongst the stones for any algae. There were fewer green filaments here but I did find a small area of a stone covered with a dark brown mat that intrigued me.   My initial suspicion was that it was a mat of the filamentous blue-green alga Phormidium, but it turned out to be composed almost entirely of Nitzschia sigmoidea, a diatom that we last encountered in the brackish waters around Jarrow (see “All things bright and beautiful?”).


Nitzschia sigmoidea from Smallhope Burn, Knitsley Bridge, August 2014.   The upper image shows a single cell, the lower image two cells shortly after division.   Scale bar: 20 micrometres (= 1/50th of a millimetre).

If you look back at the post on Nitzschia sigmoidea at Jarrow, you’ll see that it was smothered in epiphytes.   Some of the cells here at Smallhope Burn also had attached epiphytes, though this time it belonged to a different genus.

I have called this epiphyte “Synedrella subconstricta”, though it has had several different names during the course of my career.   When I started, it was Synedra parasitica var. subconstricta but it has also been moved to the genus Fragilaria and, from there, to Pseudostaurosira.   Some have regarded it as a variety, others as a distinct species.   What is generally agreed is that it is usually found attached to Nitzschia sigmoidea, and occasionally to other large motile diatoms.   Frank Round regarded this as a major reason for creating a new genus, Synedrella, for this and a related taxon, S. parasitica.   Don’t let this species name misguide you: there is no evidence that it is a parasite; it is more likely to be a commensal relationship of some kind.   I am less convinced by more recent efforts to move it to the genus Pseudostaurosira, which generally forms long chains rather than sharing the habit of single cells with a short but flexible stalk.   This move reflects the tendency of my fellow diatomists to spend too long staring at the empty cells of diatoms and not enough time watching them in their live state. I have hunted around to see if there is any molecular evidence to support any of these genus names but, so far, have not had any luck.   How, I wonder, would we get the pure culture that we need as our source of DNA when the diatom seems to be an obligate epiphyte?

And, one more question that puzzles me: what advantage does a sigmoid outline confer onto a diatom?   You’ll see from the second of my two images that there is also a cell of another sigmoid genus, Gyrosigma, in the same habit. This is a relative of Pleurosigma, which we at Whitburn last year (“Microscopic monsters in mud”) and there are also sigmoid species in the genus Stenopterobia.   The sigmoid habit must have evolved several times though it is interesting that all of these are motile genera and two (Nitzschia and Stenopterobia) also include straight species.  Any ideas?


Nitzschia sigmoidea with an epiphytic cell of Synedrella subconstricta and, just above them, a cell of Gyrosigma (probably G. acuminatum).   Scale bar: 10 micrometres (1/100th of a millimetre).


Medlin, L., Jung, I., Bahulikar, R., Mendgen, K., Kroth, P., Kooistra, W.H.C.F. (2008). Evolution of the diatoms. VI. Assessment of the new genera in the araphids using molecular data.   Nova Hedwigia Beiheft 133: 81-100.

Medlin, L., Yang, I. & Sato, S. (2012). Evolution of the diatoms. VII. Four gene phylogeny assesses the validity of selected araphid genera.   Nova Hedwigia Beiheft 141: 505-514.

Round, F.E. & Maidana, N.I. (2001). Two problematic freshwater araphid taxa re-classified in new genera. Diatom Research 17: 21-28.

Cladophora and friends

I was back at Stockerley Burn last week (see “A case of mistaken identity?”) and was surprised to find that the Oedogonium that was so prolific back in June has now almost completely disappeared and the stream bed is now dominated by Cladophora glomerata. How do I know?   Look at the illustration below: even with the relatively weak magnification offered by a hand lens you can see the characteristic branched structure that distinguishes it from an unbranched species such as Oegogonium.   There is almost nothing else that could be confused with Cladophora in our freshwaters, so long as you perform this simple test.

I saw Cladophora glomerata at almost all of the sites I visited during this trip, which was not a great surprise as it is a very common species in enriched lowland rivers and streams.   However, the Cladophora filaments did not have their characteristic green colour at many of these sites, mostly tending to a brownish hue due to the large numbers of diatoms which were growing on and around these filaments.


Left: Filaments of Cladophora glomerata on my fingertip, showing the characteristic branched structure; right: a wet mount of Cladophora photographed with a macro lens. The entire structure is about four millimetres long.

Two of these diatom-smothered filaments are illustrated below.   You can see the upright, curved cells of Rhoicosphenia abbreviata, each attached to the filament by a small mucilage pad, along with some lower-profile cells, with an oval outline. These belong to the genus Cocconeis and I think I could see at least two species (C. pediculus and C. euglypta) though it is hard to be sure when examining them in the live state.   The cells on the lower of the two images do look more like C. pediculus to me.   There are also a few cells of Fragilaria, again standing erect on the filament.

The second image shows another filament from the same location but this one is covered almost entirely with Cocconeis cells (mostly C. pediculus, I think). The lower, more streamlined, profile of these makes them more suited to situations where they are more exposed to the current.   Indeed, I suspect that there are subtle differences in the composition of this epiphyte flora throughout the tufts of filaments though it is hard to do more speculate when these differences are at a scale that is difficult to either see or handle whilst standing in a stream.

What effect do all these epiphytes have on the growing Cladophora?   The diatoms are trapping sunlight that would otherwise have been used by the Cladophora so the net effect of all of these is akin to moving a houseplant from a well-lit window ledge to a dark corner.   On the other hand, Cladophora grows very quickly so an probably stay one step ahead of the epiphytes most of the time. There is evidence that prolific epiphyte growth leads to the eventual death of some aquatic plant.   This has been suggested in the Norfolk Broads where gradual increases in nutrients favoured epiphytes which, in turn, smothered their host plants (slower growing than Cladophora), leading to the long-term deterioration of the unique habitats of the Broads.


Epiphytes on a Cladophora filament from Stockerley Burn, August 2014; a. Rhoicosphenia abbreviata; b. Cocconeis sp (from above: valve view); c. Cocconeis sp (from the side: girdle view); d. Fragilaria sp. Scale bar: 10 micrometres (1/100th of a millimetre).


Cocconeis cells epiphytic on a filament of Cladophora glomerata.   Scale bar: 10 micrometres (1/100th of a millimetre).