Form without function?

The latest copy of FBA News includes an article I wrote, along with Catherine Duigan, on the scientist Geoffrey Fryer, who celebrates 50 years as a Fellow of the Royal Society this year.   Geoffrey Fryer spent his early career working on the cichlid fish of the Great Lakes of East Africa’s rift valley.  He moved back to the UK in 1960 and spent the rest of his career working mostly on planktonic crustaceans at the Freshwater Biological Association’s laboratory at Windermere, where I first met him.    

When he arrived at Lake Malawi in 1953, the existence of “species flocks” – relatively large numbers of closely related species confined to a narrowly circumscribed region – of cichlid fish had already been recognised but the reasons that led to their evolution was barely understood.    These were piscine analogues – albeit significantly more diverse – of the finches that Darwin observed on the Galapagos Islands, and which provided crucial evidence for his theory of natural selection.  Swimming in Lake Malawi and observing cichlid fish close quarters through a face mask, then dissecting fish caught by local boys enabled Geoffrey to show how morphology was linked to function (especially feeding behaviour) and explained preferences for sandy or rocky shores.  It is one of the great examples of “adaptive radiation”.

Some of the many cichlid fish from Lake Malawi studied by Geoffrey Fryer. Top left: Pseudotropheus greshakei; top right: Ramphochroumus ferox; bottom left: Lethrinops sp.; bottom right: Copadichromus virginalis.   Photos: Ken Irvine, Institute of Hydrology, Delft.   The photographs at the top of the post show Geoffrey Fryer in the 1950s and earlier this year.

“Species flocks” have been found for other organisms including, some claim, diatoms.   The evidence is not quite so clear-cut but most of the criteria (several closely-related species within a limited geographic area) are fulfilled, and there are reasonable grounds for assuming a common ancestor.   Interestingly, too, several of the purported flocks come from areas with ancient lakes (the African Rift Valley, Lake Ohrid in North Macedonia and Lake Baikal in Siberia, for example).  What is missing from the studies on species flocks in diatoms, however, is the link between form and function. We can infer radiation of several species from a common ancestor, but we cannot explain why it is that one lake is able to support so many close relatives.  We are, once again, confronted by Hutchinson’s Paradox of the Plankton (rephrased for surface-dwelling organisms as “Baffled by the benthos”).

The plate below shows four species of the genus Epithemia from Lago di Trasimeno in Italy.   These do not represent a “species flock” because they are not confined to a limited geographical area but they serve to make a point about our understanding of diatoms in general.   Epithemia is not a particularly common genus but, when you find one species from this genus, there is a good chance that you will find another in the same sample.   I’ve found E. sorex and E adnata, in particular, together in samples from the Shetland Islands to Italy and Greece in the Mediterranean Basin.    In this sense, they are like Geoffrey Fryer’s cichlid fish: two closely related species living in close proximity.

Epithemia species from Lago di Trasimeno, Italy, September 2022.   a. E. sorex; b., c.: E. adnata (two focal planes); d. E. turgida, e. E. frickeri.   Scale bar: 10 micrometres (= 100^th of a millimetre).  

But in another sense, they are nothing like cichlid fish.   Fryer, dabbling around in the shallows of Lake Malawi, was able to explain how form linked to function, enabling each species to exploit different resources.  The literature on Epithemia mostly focuses on form, also addresses geographical differences but is almost silent on the subject of function.   We learn that Epithemia, as a genus,  is capable of nitrogen-fixation but not how individual species differ in their habitat preferences.   Clearly, if two species occur side-by-side in a sample, then the chemical composition of the water (the go-to explanation for diatom ecologists) is not going to tell us the whole story.    And it is not just Epithemia: similar patterns can be found in many other genera.

The study of diatoms is certainly not dormant.   We have learned a huge amount about their diversity and evolution over the past few decades.  We also know a lot about why apparently closely-related species live in different places (geological/chemical conditions and biogeography).   What we are less sure about is why two or more closely-related species share a habitat.    Diatom science seems to have become stuck in a cul-de-sac where diversity is celebrated but rarely explained, at least not in the functional ecology terms that motivated Geoffrey Fryer all those years ago in East Africa.   

References

Two good general introductions to “species flocks” are:

Goldschmidt, T. (1996).  Darwin’s Dreampond.  Drama in Lake Victoria (translated by S. Marx-McDonald).  MIT Press, Cambridge, MA.

Weiner, J. (1994).  The Beak of the Finch: a Story of Evolution in Our Time.   Knopf, New York.

Other papers:

DeYoe, H. R., Lowe, R. L., & Marks, J. C. (1992). Effects of nitrogen and phosphorus on the endosymbiont load of Rhopalodia gibba and Epithemia turgida (Bacillariophyceae) 1. Journal of Phycology, 28(6), 773-777.

Kociolek, J. P., Hamsher, S. E., Kulikovskiy, M., & Bramburger, A. J. (2017). Are there species flocks in freshwater diatoms? A review of past reports and a look to the future. Hydrobiologia, 792: 17-35.

Stelbrink, B., Jovanovska, E., Levkov, Z., Ognjanova‐Rumenova, N., Wilke, T., & Albrecht, C. (2018). Diatoms do radiate: evidence for a freshwater species flock. Journal of Evolutionary Biology 31:1969-1975.

Some other highlights from this week: 

Wrote this whilst listening to:   And in the Darkness, Hearts Aglow, new album by Weyes Blood

Currently reading:   Nadime Gordimer’s A World of Strangers

Cultural highlight:   The Wonder,  adaptation of a 2016 novel which I have not read, featuring Florence Pugh.   The lighting palette often recalls Vermeer’s interiors.  –

Culinary highlight:  roast broccoli and plum salad.  Would not have thought of combining these two ingredients before trying it in the café at the Royal Botanic Gardens in Edinburgh.  The secret lies in the dressing, which should evoke hoisin sauce.

Cassop Pond in November

My latest trip to Cassop Vale was, by some margin, also the shortest.   The very worst of Storm Arwen had blown through overnight but the winds were still considerable, and I had to steer the car around trees that had fallen across the roads.   I parked in Cassop village, then darted down the hill to the pond amidst the sleet.   All I did when I got to the edge of the pond was dip my hand in to grab some of the flocs of Riccia fluitans, and stuff this into a bottle, before dashing back up the hill to the warmth of the car.   Don’t judge me: the thermometer was hovering around zero and the considerable wind chill made it feel much colder.   You would have done the same.

Flocs of Riccia fluitans overlain by Lemna minor at the margins of Cassop Pond, November 2021.

These flocs of Riccia fluitans have been present around the edge of the pond all year, typically floating just under the surface.  I’ve written about them on several occasions already (most recently in “Microscopic mysteries in Cassop Pond …”).  The main difference apparent with the naked eye today was that there was more duckweed on top of them than on previous visits. This, however, could be due to nothing more than wind action on the surface of the pond.   Duckweeds float at the water surface rather than just underneath, as Riccia fluitans doe, so are more susceptible to be moved around by the wind.   

Under the microscope, the main difference was that the fronds seem to have lost some of their vitality.  There are more dead or dying cells, it seems, and these impart a brownish, rather than green, hue to many of the fronds.  In addition, the Nostoc that had smothered the Riccia fronds during the summer had disappeared, leaving just a few cells of Epithemia as epiphytes.  They are quite hard to see against the brown backdrop of the Riccia fronds, so I have arrowed them in the photograph below.   One way or another, nitrogen-fixing organisms have been an ever-present in Cassop Pond over the course of the year, which must be telling us something about the pond’s supply of nutrients. 

Microscopic view of a frond of Riccia fluitans from Cassop Pond, November 2021.  The position of Epithemia cells are indicated by arrows.  Scale bar: 20 micrometres (= 1/50^th of a millimetre).

We recently bought a copy of Gordon Graham’s Flora and Vegetation of County Durham, first published in 1988 when we were too impoverished to afford a copy.   I remember Gordon Graham expressing great scepticism of my first report of Riccia fluitans from Cassop, but I must have convinced him to look because he records it in his book as “in abundance” here, along with just two other records from the county.   This is about as far north in the country as you will find this species.  Quite why it flourishes here but in so few other places, I do not know.  Just ahead of Riccia fluitans in the flora is Ricciocarpos natans, the only other aquatic liverwort in the UK.   Again, there are just a few records in the county, one from Cassop Pond and the others from ponds nearby.   I remember Ricciocarpos natans being quite abundant in the pond in the past but I have not seen it this year.   The Atlas of British and Irish Bryophytes mentions that there has been a decline in in R. natans which began in the 1950s and has continued to the present day, with a brief resurgence in the 1980s (when I first found it here).  That gives me confidence that its absence from Cassop is not simply due to my overlooking it, but it does not really offer any cogent explanation.  A species at the northern limit of its range and said to favour eutrophic water would seem to be encouraged, rather than threatened, by recent changes across the country.

Noticing an absence is harder than recording a presence.  It needs a long-term perspective, and we will only ever have the personal memory that is required for a few locations.   In other cases, we may have the benefit of other people’s records (see Heather’s parallel account of the poor condition of the terrestrial flora at Cassop, but having that personal link over time adds a personal, poignant element to the experience.   There does not seem to be an obvious human-induced cause of its decline nationwide, so I am not led to reflections of the follies of mankind.  But it does reinforce in me an awareness of change: that things that were are no longer.  Not knowing why only adds further layers of reflection.   

Enough for now. I am getting maudlin when there is no real need.   Eleven months into my investigations into Cassop Pond have reignited memories that have pulled me 35 years back into the past.  One month left and my year of visits will be complete.   Then I will need to step back and see if I can put all the pieces of the jigsaw together again. 

Wrote this whilst listening to:    Christmas in Puebla, 17^th century Christmas music from Siglo de Ora.

Cultural highlights:   Petit Maman, latest film from Céline Sciamma, best known for Portrait of a Lady on Fire.   Wonderful film, well worth seeking out.

Currently reading:  The Thursday Murder Club by Richard Osman.

Culinary highlight:  A chocolate and almond cake from Claudia Roden’s The Food of Spain.

A return to Cassop

Cassop Pond, and Cassop Vale, looking towards Durham, April 2015.

One of my earliest posts described the algae that I found in Cassop Pond, which lies at the foot of the Permian escarpment close to my house in County Durham (see “Cassop”).   I returned there a couple of days ago to grab a couple more samples and see what had changed since my last visit.   The first of these samples was a handful of submerged plant stems, which I crammed into a sample bottle and shook to dislodge the algae.   I pipetted a couple of drops of the brown suspension that this produced onto a microscope slide and put it under my microscope.   Prominent amongst the diatoms that I could see in this sample was a nice colony of Gomphonema truncatum var. capitatum and also a cell of the large diatom Cymbella lanceolata, displaying its characteristic lobed chloroplast. I also saw a few cells of another diatom, Epithemia adnata. This diatom is relatively uncommon in the UK, but it gives us some interesting insights into the ecology of Cassop Pond.

Diatoms from Cassop Pond, April 2015.   a. Gomphonema truncatum var. capitatum; b. Cymbella lanceolata; c. Epithemia adnata, valve view; d. Epithemia adnata, girdle view of two recently divided cells.   Scale bar: 25 micrometres (= 1/40^th of a millimetre).

Cells of the genus Epithemia contain small cyanobacteria-type cells that are capable of nitrogen-fixation (the reality is a little more complicated – see the reference by Prechtl et al. below for more information). This means that it can thrive in situations where nitrogen is relatively scarce compared to other nutrients.   I came across it in a small stream in Northumberland, downstream of a forestry plantation. I was interested in the stream because the plantation was being fertilised with phosphorus at the time and I wanted to see what this would do to the stream.   Every time it rained (which was quite often in the Northumberland hills), some of the phosphorus was washed into the stream and, gradually, over a period of about a month after the fertilisation, the proportion of Epithemia in my samples increased from undetectable at the start to over forty percent of all the diatoms a couple of months later.   What I suspect was happening was that most of the algae in this remote stream could not use this extra phosphorus because nitrogen was naturally very scarce. However, Epithemia is one of a very small number of diatoms that can overcome nitrogen limitation and so was able to thrive. Finding it in Cassop Pond is, therefore, a clue that this pond is, periodically at least, limited by nitrogen rather than by other nutrients.

The other photograph I’ve included in this post was taken by Chris Carter and shows Epithemia growing on the surface of Chara virgata. Epithemia is a genus that does often seem to be associated with plants, although I have also seen it growing on rocks. Chris’ photograph also shows the lobed chloroplast very clearly.

Epithemia sp. growing on a stem of Chara virgata.   Photograph by Chris Carter.

Reference

DeYoe, H.R., Lowe, R.L. & Marks, J.C. (1992). The effect of nitrogen and phosphorus on the endosymbiont load of Rhopalodia gibba and Epithemia turgida (Bacillariophyceae). Journal of Phycology 23: 773-777.

Kelly, M.G. (2003). Short term dynamics of diatoms in an upland stream and implications for monitoring eutrophication.   Environmental Pollution 125: 117-122.

Prechtl, J., Kneip, C., Lockhart, P., Wenderoth, K. & Maier, U.G. (2004). Intracellular spheroid bodies of Rhopalodia gibba have nitrogen-fixing apparatus of cyanobacterial origin. Molecular Biology and Evolution 21: 1477-1481.