Constancea 83, 2002
University and Jepson Herbaria
P.C. Silva Festschrift

Nulla Vestigia Retrorsum

The Case of Pleurosigma aequatoriale Cleve

F.A.S. Sterrenburg
Westerstraat 47
1655LC Sijbekarspel
The Netherlands

Fig. 11


By an unfortunate combination of circumstances, “Pleurosigma aequatoriale" Cleve failed to be validly described, although it could have become a good species. The much later name P. diverse-striatum F. Meister (1934) must, therefore, be used for this neglected but widespread marine diatom. It has been mistaken for the different species P. inflatum Shadbolt—which has P. naviculaceum Brébisson for a later synonym.


The inexorable progress of Science sometimes embarrasses its diligent student. After completion of a paper on the type of Pleurosigma inflatum Shadbolt and related taxa (Sterrenburg 2001) new findings created a complex situation, just in time to be mentioned in an addendum. It was hoped that this would forestall my having to eat crow (Corvus edulis Sterrenburg) later on—in vain, as it now turns out. As Paul Silva was instrumental in this gustatory digression, it seems fitting to describe the puzzle here, and because this “Festschrift” reflects his wide-ranging interests and expertise, a brief introduction may serve to refamiliarize readers with the diatom genera Gyrosigma and Pleurosigma.

The genus Pleurosigma was created by W. Smith (1852) and extended by W. Smith (1853) to include biraphid diatoms whose valve contour and/or raphe sternum are more or less sigmoid and where the areolae (the “puncta” of Victorian microscopists) are arranged in regular decussate or perpendicular rows (“striae”). In continuation of Smith's excellent work, major contributions were subsequently made by Grunow in Cleve and Grunow (1880) and when H. Peragallo (1891) wrote a monograph to create some order in what he calls “le chaos des espèces dans ce genre difficile”, he referred to circa 150 taxa.

Cleve (1894) divided the genus Pleurosigma sensu lato W. Smith into the genus Pleurosigma W. Smith sensu stricto (with decussate areolae leading to 3-system striation, Fig. 1) and Gyrosigma Hassall sensu stricto Cleve (with 2-system perpendicular striation, Fig. 2). Both generic names have been conserved.

Fig. 1

Fig. 1. Typical Pleurosigma morphology, decussate (3-system) striae. Red bar = 10 µm.

Fig. 2

Fig. 2. Typical Gyrosigma morphology, with perpendicular (2-system) striae. Red bar = 5 µm.

Since the end of the 19th century, new taxa have been described but no later synthesis had been published. In fact, it appears that these genera—which occur in freshwater but are particularly well-represented in coastal wetlands and the marine littoral—have scared people away. It is an ominous sign, for instance, that in the course of almost a century of publication the Atlas of A. Schmidt (1874-1959) did not include a single species of these rich genera among its many thousands of illustrations!


These organisms are particularly plentiful in the area where I live and in the course of time, I came to suspect that the difficulties in identification were not so much caused by an inherent lack of morphological individuality of these diatoms as rather by shortcomings in the literature, such as the following:

Therefore it appeared attractive to examine species of these genera:


Morphological criteria for taxonomy were investigated in Sterrenburg (1991) and their applicability has been verified by others (Stidolph 1992, 1993, 1994, Hellum von Quillfeldt 2000) and in subsequent personal studies. Three such aids to identification—the first two not previously applied—of Pleurosigma species are of particular relevance here:

Colour in darkfield
By simple standardization of darkfield illumination, diatoms in these genera display a remarkably stable colour at low power — e.g., with a 10x to 20x objective. Main groups can be distinguished: golden, silvery, bright blue, peacock, faint violet or none, see Figs. 3 (peacock) and 4 (bright blue) for two examples.
Fig. 3

Fig. 3. In standardised darkfield, Pleurosigma (marinum var.?) barbadense Grunow shows a deep peacock colour. Red bar = 10 µm.

Fig. 4

Fig. 4. In standardised darkfield, Pleurosigma subrigidum Grunow is bright blue. Red bar = 20 µm.

Raphe angle
This is measured as indicated in Fig. 5 and was found to be constant within a few degrees for a species. The raphe angle is a quantitative measure of the degree of curvature of the raphe sternum, the prime element involved in the frustule morphogenesis of raphid diatoms.
Fig. 5
Fig. 5. The curvature of the raphe sternum can be quantified as the “raphe angle”, the angle subtended by the two red lines drawn in this schematic figure.
Stria intersection angle
This is measured as in Fig. 6 and was also found to be constant within a few degrees for a species. In one “section” of the genus Pleurosigma, the Affines sensu H. Peragallo (1891), the intersection angle is markedly greater centrally than near the apices. There is a corresponding change in colour in darkfield, typically from peacock (apices) to silvery or golden (center). The diatoms discussed here all belong to this “Affines” section.
Fig. 6
Fig. 6. The “stria angle” is the intersection angle between the two systems of obliquely crossing lines .

In addition, of course, several other criteria come into play for separation of species in these genera, such as fineness of structure, shape of valve or particular structures that may be visible only in SEM. An example of the latter will be given here.


After examination of classic species described between circa 1835 and 1860 in the original materials (full list of references available on request by E-mail), I was left with an increasing number of “obscure” species—species that have vanished from the records during the past century. Nevertheless observations on my own samples from widely different areas suggested that they might actually be abundant and widespread. However, attempts at identification were particularly hampered here by the lack of data—especially, reliable illustrations.

Literature research revealed that many of these “obscure” taxa were recorded in the catalogues of the series of slides Cleve and Möller (1877-1882, 324 slides) and Tempère and Peragallo Ed. 1 (1889-1895, 625 slides) and Ed. 2 (1907-1915, 1000 slides). An investigation of these slides was therefore undertaken and this on-going project is expected to result in the first photographic documentation, differentiation and typification of several dozen “lost species”. This paper describes the first such conundrum tackled.


In Sterrenburg (2001) a taxonomic study was made of Pleurosigma inflatum Shadbolt in the original material, with photographic documentation and typification of this species (Fig. 7). An investigation of the taxon P. naviculaceum Brébisson in the original material (Fig. 8) did not reveal any morphological differences (e.g., in stria fineness, valve shape, colour in darkfield, raphe angle etc.) and both taxa also belong to the section “Affines” sensu H. Peragallo. According to the publication dates, P. naviculaceum (now typified) is a later heterotypic synonym of P. inflatum. The present paper will, therefore, refer to P. inflatum with the inference that the reference also includes the type of P. naviculaceum.
Fig. 7

Fig. 7. Pleurosigma inflatum Shadbolt, from the type material. Raphe angle drawn in red = circa 10°. Red bar = 10 µm.

Fig. 8

Fig. 8. F. Meister's illustration of his P. diverse-striatum. Raphe angle drawn in red = circa 16°. Red bar = 10 µm.

Literature research then revealed that some 70 years later, F. Meister (1934) described P. diverse-striatum. According to its protologue, this taxon does not show a single morphological discontinuity in comparison with P. inflatum, which F. Meister did not consider in a differential diagnosis. It was evident, however, that P. diverse-striatum had a clearly greater raphe angle, a parameter not considered by F. Meister—compare Figs. 7 and 8. Specimens conforming to the types of P. inflatum and P. diverse-striatum were then examined in SEM. This showed that the external areolae of P. diverse-striatum (Fig. 9) have a morphology different from those of P. inflatum (Fig. 10).

Whilst according to Meister's diagnosis P. diverse-striatum could only be regarded as a synonym of P. inflatum, the new findings indicated that these organisms are not identical. P. diverse-striatum is not at all rare in several of my own materials from warmer seas, e.g., the Indian Ocean and Caribbean.

Fig. 9

Fig. 9. In P. inflatum, the external areolar foramina are in the form of tiny slits. Red bar = 1 µm.

Fig. 10

Fig. 10. In P. diverse-striatum, the external areolar foramina are much wider and oval. Red bar = 1 µm.

To summarise the species descriptions:

Both P. inflatum and P. diverse-striatum are about 60–120 µm long, about 16–22 µm wide, have approximately equally fine “Affines”-type striation and have the same general valve shape. The raphe sternum of P. inflatum is much less strongly curved than that of P. diverse-striatum, however, with representative raphe angles of 10º and 18º respectively.

When these taxonomic investigations had been completed, I had begun to examine the “obscure” taxa in the Cleve and Möller slides. These slides are accompanied by notes specifying that “all slides were examined by M. (NOTE: this means ‘Mr.’) Grunow”. In Van Heurck (1899), page 110, it is stated that the slides are “accompanied by a detailed analysis by A. Grunow”. As Van Heurck was a close associate of Grunow's, this information can be regarded as conclusive. I will therefore refer to “Grunow's notes”.

In the notes for box #3 of the Cleve and Möller collection, Grunow records “Pleurosigma naviculaceum var.? aequatoriale Cleve” for slides #145/#146, “Java”. Such slides from the Naturhistoriska Riksmuseet, Stockholm, were examined and slide #145 yielded a specimen (Fig. 11) that superficially resembled P. inflatum and could therefore be compatible with Cleve's concept of a “variety” of the latter taxon. It displays a peacock colour in darkfield, which grades into silvery-golden in the centre—a sure sign that it belongs to Peragallo's section Affines, like P. inflatum. However, the raphe angle is circa 20°, clearly greater than for P. inflatum but corresponding to that of P. diverse-striatum. Other data, like size and stria density, also match P. diverse-striatum.

I hoped that this slide could become the type of the name P. aequatoriale Cleve, F. Meister's named diatom becoming a later synonym of this taxon. Tracing the records in their historical context led to a nomenclatural puzzle and therefore I called on Paul Silva for help.

Fig. 11

Fig. 11. “P. aequatoriale” in standardised darkfield, from the type material. Note strongly curved raphe sternum incompatible with P. inflatum, but similar to P. diverse-striatum. Red bar = 10 µm


In Grunow's notes for the Cleve and Möller slides, no diagnosis or illustration was supplied. The taxon was, therefore, a nomen nudum when first published in these notes.

In Cleve and Grunow (1880) Grunow refers to a new taxon, “P. aequatoriale” Cleve, infers that it is identical to P. australe Grunow (1867) and finally suggests that the latter (one more “obscure” species which shall have to be dealt with separately in the future) is a variety of “P. naviculaceum”, which is now known to be a heterotypic synonym of P. inflatum. What wonderful complications for a minimum number of records! Grunow gave neither description nor illustration of “P. aequatoriale”, however, so that his new taxon was again a nomen nudum—and a mooted synonym into the bargain.

H. Peragallo (1891) supplied descriptions and illustrations of both “P. naviculaceum” and P. aequatoriale. He illustrated “P. naviculaceum” from authenticated Brébisson material and P. aequatoriale from “Java”, an obvious reference to the Cleve and Möller material. Although his illustrations show a marked difference in the raphe angle for “P. naviculaceum” and P. aequatoriale (Fig. 12), he was not aware of its significance and concluded that “P. naviculaceum” and P. aequatoriale are “wholly related”.

Fig. 12

Fig. 12. From H. Peragallo (1891): “P. naviculaceum” (left) and “P . aequatoriale”, Java (right). Note marked difference in the raphe sternum curvature.

As regards nomenclature: although Peragallo's data (i.e., his drawings showing the difference in raphe angle) were correct, he did not notice the implications. His erroneous relegation of P. aequatoriale to synonymy meant that this taxon for the third time failed to be validly described and become the good species it deserved to be. Cleve (1894) had apparently given up by then and for the fourth time deprived his diatom of a radiant future by saying that it was a synonym (of P. australe)...

The Latin quote in the title originally referred to the den of one of the Fiercer Animals like the cave-bear (Ursus spelaeus) but nicely characterizes the situation for P. aequatoriale: several tracks of this taxon entered the lair of the ICBN, but none came out... The end result is that the first valid description of this diatom was as P. diverse-striatum F. Meister. Rather undeservedly so, because his diagnosis was insufficient justification for describing this diatom as a new species! In Sterrenburg (2001) the identical morphology of P. diverse-striatum and P. aequatoriale could be reported just in time for publication as an addendum, but the synonymy puzzle took longer to solve correctly!


The practical significance of differentiation between P. inflatum and P. diverse-striatum lies in the fact that the former occurs in temperate and tropical regions, whereas the latter appears to be restricted to warmer seas, according to my observations. But the case of “P. aequatoriale” should be seen in a wider context.

Without detailed taxonomic footwork, the basic tenet that science should deal with defined entities will not be guaranteed. It has long been evident (e.g., Archibald 1984) that irreproducible ecological or biological studies on diatoms have been published because the organisms they purported to deal with were actually of some other, undefined, entity. Also, traditional views on distribution patterns and species diversity—both worldwide and regionally—may need revision. To illustrate the situation for the genera Gyrosigma and Pleurosigma, just for The Netherlands (not really a vast territory!):

- the regional flora of Van der Werff & Huls (1957-1974) described 20 taxa in these genera; for about a dozen of these the data are reliable.
- so far, I have observed some 60 different species of these genera in this area, many of these now typified.

Dutch politicians might thus claim that their professed efforts to protect the environment are exemplary, because in contrast to the worldwide trend of decreasing species diversity, in this country it has tripled in the past 25 years for this group of organisms alone!


I am greatly indebted to Dr. Paul C. Silva (University of California, Berkeley, USA) for expert advice and spirited discussions on the complex nomenclatural aspects of this case.

Marianne Hamnede, Naturhistoriska Riksmuseet, Stockholm, staunchly assisted me by supplying Cleve & Moeller slides and documentation.


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Observations and remarks on morphology and taxonomy of the diatom genera Gyrosigma Hassall and Pleurosigma W. Smith. II. Gyrosigma waitangiana sp. nov. and Pleurosigma sterrenburgii sp. nov. Nova Hedwigia 56: 139–153.
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Observations and remarks on morphology and taxonomy of the diatom genera Gyrosigma Hassall and Pleurosigma W. Smith. Gyrosigma fogedi sp. n. and some diatoms similar to G. fasciola (Ehrenb.) Griffith & Henfrey. Diatom Research 9: 213–224
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