Picture this?


A curious moment of serendipity saw me stuffing a new scientific paper into my bag to read on the train as I travelled to a workshop on reduction linocut printing. A second instance of serendipity occurred when I walked close to the site of Thomas Bewick’s studio in Newcastle as I contemplated the contents of this paper whilst walking from Newcastle Station to Northern Print‘s workshop in the Ouseburn Valley. I was, clearly, destined to write a post on natural history illustration, and the problems of reproducing images.

The paper I was reading related the efforts of Carlos Wetzel and colleagues to understand the taxonomy and nomenclature of a group of very small diatoms, historically placed in the genus Navicula but more recently spread between Sellaphora and Eolimna. The problem they address is essentially one of calibration: do the names we use for modern diatoms correspond to the organism to which the name was first applied, or has our understanding of that species gradually ‘drifted’ over time so that we now use it either for a different species altogether, or for a number of species that match the original description?

The plate below puts the problem into perspective. It shows a number of specimens corresponding to the description of what we thought was Eolimna minima at the time, though should now probably be called Sellaphora nigri. These cells are mostly less than a hundredth of a millimetre in length, have a linear-elliptical outline and few surface features that can be resolved easily with the light microscope.    Small variations in the valve outline and the density of striae had encouraged diatomists to establish new species and varieties until there were a large number of names in circulation and it was not always easy to separate the wheat from the chaff.   Wetzel and his colleagues had tracked down the original descriptions and the “type material” (the specimens that formed the basis of these original descriptions) in an effort to sort out the mess.


Sellaphora nigri / Eolimna minima from Menahyl River, St Mawgan Bridge, Cornwall, September 2009. The scale bar is approximately 10 micrometres (= 1/100th of a millimetre) long.   Specimens are arranged into two “morphotypes”: “narrow” and “blunt”.   Photographs: David Mann.

Our understanding of diatoms is driven in large part by the technology available to scientists at any point in history. Not surprisingly, the first diatoms to be described tended to be the larger species, relatively speaking, and it is no surprise that the earliest descriptions of the very small forms that are the subject of this post are rather basic.   These date back to Kützing in 1849, Rabenhorst in the 1850s and Grunow in the 1860s, all of whom would have had relatively basic microscopes by modern standards, and who worked without electric light with which to illuminate their specimens. They also did not have access to high resolution mountants, which only became available in the middle of the twentieth century. Nor were they able to photograph their specimens and, indeed, any drawings that they made would have had to be passed to a specialist engraver, who would have transferred the image either to a woodblock (using the ends of hard woods such as box and cherry rather than cutting into the grain) or a metal plate.   So there would have been at least two steps between the observer’s initial view of the diatom and the published illustration which, in the case of diatoms such as these, was working right at the edge of the resolution of the light microscopes of the day. It is no surprise, then, that the organism that Kützing described as “Synedra minutissima” and which later workers considered to be a small Navicula has subsequently bounced through several genera (and families), before Wetzel and colleagues decided on the basis of light and electron microscopical observations of Kützing’s original material that it probably belonged to Halamphora.

Image reproduction is, I suspect, almost as significant as optical technology in determining the rate of advances in understanding of diatoms. You only have to look back at papers in Diatom Research published only 20 years ago, and compare both the quality and quantity of images to understand this. Photo-editing packages such as Photoshop and CoralDraw are the unsung heroes of modern diatom taxonomy, enabling images to be edited and rearranged in multiple combinations.   We can now do in a couple of hours what would have taken weeks of time for an engraver in the 19th century, capturing images of a quality that would have been beyond Kützing’s wildest dreams.   Having done this, we can then discuss the results via email and Skype with people in other countries, or even different continents.

Yet there is one final twist to this tale: the plate of Sellaphora nigri / Eolimna minima is one of a series of plates that David Mann put together last year whilst he and I were pondering some RbcL sequences from field populations.   The genetic information seemed to be telling us that there were several distinct genotypes within complexes that we were identifying, with the light microscope, as Eolimna minima.   The scale of difference was such as to suggest that these genotypes may well be distinct species, albeit barely discernible even with the very best light microscopes available. We put that work aside, distracted by other, more pressing tasks, but I dug out the plates when thinking around the issues I’ve discussed in this post.   So it is quite possible that we have still not solved all the mysteries of this group of tiny, but very common, diatoms.


Wetzel, C.E., Ector, L., van de Vijver, B., Compère, P. & Mann, D.G. (2015). Morphology, typification and critical analysis of some ecologically important small naviculoid species (Bacillariophyta).   Fottea, Olomouc 15: 203-234.

More about the life and work of Thomas Bewick in:

Uglow, J. (2006). Nature’s Engraver: A Life of Thomas Bewick.   Faber and Faber, London.

The picture at the top of the post is the result of my labours at Northern Print. It is a reduction linocut (also known as a “kamikaze linocut”, as the plate is destroyed during the production of the image) showing desmids from Upper Teesdale (see “Abstraction and Reality in Upper Teesdale”)


One thought on “Picture this?

  1. Pingback: Identification by association? – microscopesandmonsters

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