The systematics of the Sellaphora pupuZa complex ...

Download PDF
2 downloads 0 Views 8MB Size Report
Comment
Blackford Pond, a small eutrophic lake lying in an urban park in Edinburgh, were ... equivalent Blackford demes (Mann 1999; unpublished observations by V.A. ...
LANGE-BERTALOT-FESTSCHRIFT

STUDIES ON DIATOMS

The systematics of the Sellaphora pupuZa complex: typification of S. pupuZa

David G. Manni

Dedicated to Prof Dr. Dr. h.c. Horst Lange-Bertalot on the occasion a/his 65th birthday

Abstract Mann, D.G. (2001): The systematics of the Sellaphora pupuZa complex: typification of S. pupuZa. - In: Jahn, R., J.P. Kociolek, A. Witkowski & P. Compere (eds): Lange-Bertalot-Festschrift: 225-241. Gantner, Ruggell. - ISBN 3-904l44-26-X. Recent studies have shown that Sellaphora pupuZa (Ktitzing) Mereschkowsky is a complex of sympatric and allopatric genodemes, which differ subtly in their valve morphology and appear in several instances to be reproductively isolated from each other. It is highly likely that the complex will need to be subdivided into numerous (~ 102 or more) species. Before this can be done, the typification of S. pupuZa must be made secure and unambiguous. A previous attempt to do this, when variation within the complex was thought to be essentially continuous and without taxonomic significance, involved designating a slide as the lectotype. However, this slide contains at least two morphologically distinct entities belonging to the S. pupuZa complex. The lectotype therefore needs to be made unambiguous by a second-step lectotypification, which is done here via a single specimen on BM slide 17918. The morphological characteristics of S. pupuZa sensu stricto are described and the distribution of this species discussed on the basis of the few reliable records in the literature.

Introduction Since 1984, Sellaphora pupula (Ktitzing) Mereschkowsky (which is often still referred to as Navicula pupula, although studies by Mann 1989a showed clear differences from Navicula Bory sensu stricto) has been established as a model system for studying the nature of species in diatoms. In 1984, I reported that the ~s. pupula populations in Blackford Pond, a small eutrophic lake lying in an urban park in Edinburgh, were heterogeneous, containing five or more morphologically distinct entities, which differ in valve size and shape, striation density and striation pattern (Mann 1984). In this and subsequent papers, these entities h~ve been referred to as 'demes', a deme being "any assemblage of taxonomically closely\ related individuals" (Gilmour & Gregor 1939). Later work established that six de~_ were present (Mann 1988, 1989b) and 1 Royal Botanic Garden, Edinburgh EH3 5LR, Scotland, UK e-mail: [email protected]

226

D.G. Mann

demonstrated differences between them in reproductive biology and parasite sensitivity, as well as morphology (Mann 1999). A seventh deme has been detected recently; despite intensive studies at all times of the year for several years during the 1980s, this deme was never seen before the 1990s and it appears to be a recent invader (unpublished observations ). Early studies, using mixed seminatural populations, suggested a high degree of reproductive isolation between the Blackford demes (Mann 1984) and all subsequent work, with seminatural populations or clonal cultures, has confirmed this (Mann 1989b, 1999, Mann et al. 1999). Cells of different demes cannot recognize each other nor bond to each other during the early stages of sexual reproduction. All of the Blackford demes have morphologically similar counterparts elsewhere (Mann & Droop 1996) and in several cases it has been possible to show that these are capable of interbreeding with the equivalent Blackford demes (Mann 1999; unpublished observations by V.A. Chepumov and D.G. Mann). The Blackford demes tend to occur in highly eutrophic lakes. In more oligotrophic lakes and other habitats other demes are found, with different morphologies (Mann & Droop 1996, Taylor 1996). Indeed, so many demes exist and they are so difficult to separate morphologically that it is not surprising that the consensus among diatomists until the 1980s, even among those who had studied the complex in some detail (e.g. Ross 1963, Schoeman & Archibald 1976-1980, Krammer & Lange-Bertalot 1986), was that S. pupu/a represents a single, highly variable species. Ross's (1963) comment summarized this admirably: "When a large number of populations are examined ... the various forms are found so to intergrade that there seems little justification for the taxonomic recognition of them at any level." Consequently, before 1990, many varieties that had been described within S. pupula were abandoned and several taxa previously considered to be separate species, such as Navicula nyassensis O. Muller, N. pseudopupula Krasske and N. mutata Krasske, were either synonymized with S. pupula (Ross 1963) or included within it as varieties (Krammer & Lange-Bertalot 1986). The result was a coarse-grained but eminently practical classification, in which the principal species of Sellaphora Mereschkowsky could be identified with ease. First, there were two species with polar bars - thickened transapical ribs at the poles, linking the helictoglossa to the valve margin (for SEM, see Mann 1989a and Round et al. 1990). Sellaphora pupula comprised individuals with a narrow axial area, feebly developed or no longitudinal grooves alongside the raphe, and a strongly expanded central area that was usually rectangular or shaped like a bow-tie. S. bacillum (Ehrenberg) D.G. Mann, on the other hand, comprised cells with small, rounded central areas and well-developed longitudinal grooves covered by conopea (Sims & Paddock 1979, Mann 1989a). Then there was a third species, S. americana (Ehrenberg) D.G. Mann, which lacked polar bars but possessed a well-developed conopeum and a small central area; it also differed from S. bacillum in the width of the axial area (much greater in americana; Sims & Paddock 1979, Krammer & Lange-Bertalot 1986). Finally, there was S. laevissima (Kutzing) D.G. Mann, which lacked polar bars and conopea (though there can be a groove alongside the

The systematics of the Sellaphora pupu/a complex: typification of S. pupu/a

227

raphe externally), but had a narrow axial area and rounded central area, like those of S. bacillum (see, for example, Krammer & Lange-Bertalot, pIs 67, 68; Mann 1989a). However, all four species have proved to be heterogeneous and must now be regarded as species complexes, not single species (Mann 1989b, 1999 and unpublished observations, Mann & Droop 1996, Lange-Bertalot & Metzeltin 1996). The demonstration of reproductive barriers within S. pupula, between sympatric or allopatric demes, and also between demes of other common freshwater diatoms (Mann 1989b, 1999), has been accompanied by a change in the working species concepts of many diatornists, although the link may be coincidental (Mann 1999). Since 1990 a narrower species concept has begun to be adopted, with the result that many more species are now being recognized than hitherto. Species delimitation has grown correspondingly more difficult. Some of the expansion in numbers has been achieved by restoring taxa that had previously been rejected, such as Krasske's species N. pseudopupula and N. mutata, which were transferred to Sellaphora by Lange-Bertalot (in Lange-Bertalot et al. 1996), or by promoting infraspecific taxa to species rank, such as Sellaphora alastos (M.H. Hohn et Hellerman) Lange-Bertalot et Metzeltin and S. rectangularis (W. Gregory) Lange-Bertalot et Metzeltin, which were originally described as varieties (Lange-Bertalot & Metzeltin 1996). Others have been created de novo, including a fascinating series of Sellaphora species described from tropical South America by Metzeltin & Lange-Bertalot (1998). There needs to be much more rigorous and extensive testing of the new paradigm that underlies the proliferation of new species, but available data are generally consistent with the idea that the species concept was previously too broad (Mann 1999). For example, there appear to be large numbers of species still to be described, even from well studied areas like central and northern Europe. Of 823 'taxa' recorded by Lange-Bertalot & Metzeltin (1996) in three oligotrophic lakes in Germany, Austria and Finland, a quarter could not be identified. Elsewhere in the world - in many parts of S America, Africa and SE Asia, for example - diatom studies have scarcely begun and it is inevitable, therefore, that many new species will have to be described in the next few decades. Each of them will have to be compared with existing species and for these comparisons to be valid, it is important that reference is made to type material. Hence, if as seems likely, the Sellaphora pupula complex needs to be split into tens or hundreds of segregate species, it is essential that we know exactly what the type of S. pupula is; an extra reason is that S. pupula is the type of the name of the genus Sellaphora. Unfortunately, there is no holotype for S. pupula. A lectotype was designated by Ross (1963) and the material studled-by- Ross was illustrated photographically by Schoeman & Archibald (1976-1980), who clearly labelled particular micrographs (op. cit., figs 1, 5-8) as ' type'. However, these studies were undertaken in the context of a broad circumscription for S. pupula and as a result the typification was inexact, as I hav.~ noted elsewhere (Mann 1998, p. 81). It is common in diatoms to designate a slide or other preparation as the type, rather than a single specimen, and this is allowed under the

228

D.G. Mann

International Code of Botanical Nomenclature (ICBN), Article 8.2 (Greutecet al. 2000). This policy can lead to problems, however, if the circumscription of the taxon changes. The lectotype chosen for S. pupula by Ross (1963) was a slide (together with the corresponding unmounted material) and unfortunately, as Schoeman & Archibald's photographs show, this slide does not fix the use of the name unambiguously, since two or three different 'pupula' demes are present. I have therefore re-examined Ktitzing's material, to provide a firm foundation for future taxonomic treatments of the S. pupula complex. This can only be done if a single specimen is chosen as the type; an individual cannot be heterogeneous. Material and Methods The slides studied are listed in the text and were obtained from the Natural History Museum, London. All are derived from material in Kiitzing's herbarium (Ross 1963) and are the slides studied by Ross (1963), Patrick & Reimer (1966) and Schoeman & Archibald (1976-1980). England Finder (Graticules Ltd, Sovereign Way, Tonbridge, Kent, UK) references are given for individual specimens. In the Polyvar photomicroscope used, slides are inserted from the front of the specimen holder, which bears a fixed jaw on the left-hand side; the positions of slides are thus fixed by their top (distant) and left-hand edges, as viewed from the front. Consequently, as explained by Droop (1996), the England Finder has to be inserted the wrong way round, with its bottom and right reference edges against the fixed jaw of the microscope. The bottom and right reference edges must therefore be co-located with the top and left edges of the specimen slide for the Finder references to apply. Photographs were taken on Kodak Technical Pan film.

Observations and Discussion Kiitzing's original description All or most of the following criteria must be met durin-g the selection of lectotypes for diatom taxa (see also Greuter et al. 2000). The material selected must be shown to have been in the possession of the author before the taxon was described; the collection or source data must agree with the information given in the protologue; and the material must contain specimens that agree with the protologue and illustrations, once due allowance has been made for the methods, equipment and general accuracy of the author. Ideally, of course, the slide or herbarium packet will have been annotated by the author, clearly linking it to the taxon in question and its published description, and making lectotypification straightforward. Ktitzing (1844) gave the following description of Navicula pupula: "N. laevis minuta, a latere secundario oblongo-elliptica apice parum producto late rotundato. - Long. 1/85"'. Lebend in stissem Wasser bei Nordhausen!" Two specimens are illustrated in his plate 30, fig. 40 (reproduced here as Fig. 1), which are said to be at a magnification of x420. On the basis of the proto logue, Ross (1963) was able to rule out as unsuitable for typification two sets of material in Ktitzing's collection that Ktitzing said contained N. pupula, namely packets #925 and #1468. These had the wrong provenance (the River Main and Falaise, respectively: see below) and one of them (the Main sample) was only

The systematics of the Sellaphora pupu/a complex: typification of S. pupu/a

229

collected in 1847 (Ross 1963). Ross studied various samples from Nordhausen and found one, and apparently only one (packet #58), that contained S. pupula sensu lato. However, as noted above, other aspects of the protologue need to be examined besides collection details. Kiitzing's description of frustule morphology is short. The salient points are that the valve is linear-elliptical and has slightly produced, broadly rounded apices. These features are also evident in the illustrations (Fig. 1). Kiitzing's drawings are clearly of cells that still contained protoplasts (they probably represent dried material) and help confirm that his 'Navicula pupula' was indeed a Sellaphora species: the two lateral lobes of the chloroplast are visible, together with the median connection between them. In addition, there is evidence of the prominent volutin granules that are present in the vacuoles of most Sellaphora species (Mann 1989a). Finally, we must examine the information Kiitzing gave on the dimensions of N. pupula, but here we encounter a problem. Kiitzing (1844) states that the valves measure 1/85"'. The triple prime symbol denotes the ligne or line, used for example by watch-makers, which is equivalent to 2.256 mm (e.g. Walker 1988). Schoeman & Archibald (19761980: 'Explanatory notes [2]') note that Kiitzing explicitly stated elsewhere that he used the Paris ligne; slightly later authors, such as Rabenhorst (1853), used metric units. Kiitzing's (1844) measurement is therefore 27 )lm. However, his illustrations differ from this by a factor of two: the two cells measure 5.7 and 6.2 mm, which, at x420, correspond to 13.6 and 14.8 )lm. None of these measurements are unreasonable for S. pupula sensu lato (e.g. Mann 1989b). The discrepancy between text and illustrations is present elsewhere too. For example, for the Neidium Pfitzer species that Kiitzing called Navicula firma, the text gives dimensions of 150-160 x 38 )lm (1/15"'-1114'" x 1/60"'), while the single specimen illustrated would be 81 x 19 )lm. Which is correct? One way to judge between the text and the figures is to examine how well they agree for large-celled species whose identity is fairly certain and about which there has rarely been any disagreement since Kiitzing's time. Examples are Cymatopleura elliptica (Brebisson) W. Smith, Craticula cuspidata (Kiitzing) D.G. Mann, Cymbella cornuta (Ehrenberg) R. Ross (= C. lanceolata) or Cymbella ehrenbergii Kiitzing. On the whole, the illustrations appear to be more accurate than the text. Thus, for example, for Cymbella ehrenbergii, Kiitzing states in his description that there are 12 punctate striae in 11100"', i.e. 5.3 in 10 )lm. Measurements from his illustrations (pI. 6: fig. 11), however, give 8- 9 striae in 10 )lm across the central part of the valve. Hustedt (1930) gives 7- 9 in 10 )lm for C. ehrenbergii and Krammer & Lange-Bertalot (1986) say there are 6-9 striae near the centre. The widths of the valves shown in Kiitzing's fig. 11 are apparently 21-22 flm, which is reasonable for valves produced late in the size reduction cycle (and the specimens shown are indeed fairly small for the species, judging by the aspect ratio). The cells of Craticula cuspidata shown (as Navicula cuspidata, pI. 3: fig. 37) also represent late stages in size reduction, again judging by the shape and ratio of length to widtb. Measurements from the illustrations (at a magnification of x420) indicate sizes of 56-59 flm long by circa 19 flm wide. Again, these sizes are plausible for C. cuspidata

230

D.G. Mann

cells towards the end of the life cycle (Hustedt 1930, Krammer & Lange-Bertalot 1986). In the text, however, Kiltzing gives a length range of 1123 to 1126"', equivalent to 87- 98 /-lm. While C. cuspidata does attain these lengths, they are impossible for a cell with the appearance shown in the illustrations. Simi lar calculations can be made for other diatoms and generally indicate that the illustrations may be more accurate than the text. I cannot account for the discrepancy between the text and the illustrations and have found no reference to it elsewhere. It is interesting to compare Ktitzing's (1844) data with the information given in one of the next major works on diatoms, by Rabenhorst (1853). Rabenhorst quoted other diatomists' (Ktitzing, Ehrenberg, Nageli) measurements in lines when the species were unknown to him, but otherwise substituted his own measurements, in millimetres (op. cit.: iv). For Navicula pupula, Rabenhorst gave lengths of 10-15 /-lm. Among the illustrations supplied is one (pI. 6: fig. 82c) very similar indeed to Kiltzing's drawings (Fig. 1), indicating continuity in the circumscription of the species. Rabenhorst's measurements agree with the lower estimates made here, from Ktitzing's drawings, and support the view that Kiltzing's illustrations, not the text, should generally be given priority during choice of a lectotype.

1

Fig. I . Reproductions of Kiitzing's original (1844, pI. 30: fig. 40) illustrations of Navicula pupula, which were clearly made from uncleaned (live or dried) cells. The printed originals measure circa 5.7 and 6.2 mm. The H-morphology of the single chloroplast, with two large lobes beneath the girdle, linked by a central isthmus, is clearly visible in both cells. In addition, structures are indicated closer to the poles (e.g. at arrows) that probably reflect the presence of volutin granules in the vacuoles, as in many Sellaphora species.

Figs 2- 16. Sellaphora pupula. Frustules from the lectotype slide BM 17918, Nordhausen material. Figs 2- 6. Lectotype (second-step emendation: see text), at England Finder reference M45/2. Figs 2, 3. Bright field optics showing the upper (Fig. 2) and lower valves. Figs 4, 5. As Figs 2, 3, but Die optics. Fig. 6. Focus to show the polar bars (arrows), orientated at right angles to the raphe system. Figs 7- 10. Isolectotype, at Finder reference L39/ 1- 3.

The systematics of the SelLaphora pupu/a complex: typification of S. pupu/a

231

Figs 7- 9. Upper valve, at slightly different foci and with bright field and DIe optics. Fig. 10. Lower, broken valve. Figs 11 - 12. Isolectotype at Finder reference M37/2, ole optics: upper (Fig. II) and lower valves. Figs 13, 14. Isolectotype at Finder reference S35/2- R35/4, DIe optics: upper (Fig. 13) and lower valves. Figs 15, 16. Isolectotype at Finder reference 132/2-4, ole optics: upper (Fig. 15) and lower valves. Scale bar = 10 11m (Figs 2-16).

232

D.G. Mann

However, Kiitzing's working practices are insufficiently well known for us to be sure where errors are most likely to have occurred. By contrast, his contemporary e.G. Ehrenberg recorded the identities of individual specimens (valves or frustules) on mica slips that are still preserved in his collections today, and observations of these specimens show that the magnifications of Ehrenberg's illustrations are generally accurate (Regine Jahn, personal communication and 1995a, b). This has effect of undermining still further any confidence we might have in Kiitzing's data, since Kiitzing himself noted (1844, p. 34) that the magnifications of Ehrenberg's illustrations appeared to be at variance with his own. Kiitzing said that, although Ehrenberg's . figures were said to be prepared at x300, "Ich finde indessen, dass seine [Ehrenberg's] Figuren da, wo er dieselben Formen als ich darstellt, nicht kleiner sind, als die meinigen, obgleich ich immer eine 420malige Vergrosserung anwendete; es kommt sogar vor, dass seine Figuren die meinigen noch an Grosse iibertreffen" (I am most grateful to Regine Jahn for drawing my attention to this passage). Mayama & Kobayasi (1988) have also noted difficulties with Kiitzing's 1844 data, finding that the dimensions he gives there (p. 108) for Amphora alomus Kiitzing (= Navicula alomus (Kiitzing) Grunow) disagree with the dimensions of specimens on his slides. To summarize: we cannot trust either the text or the illustrations of Kiitzing (1844) to give reliable information about the dimensions of specimens he studied.

Kiitzing's Nordhausen material As noted by Ross (1963), packet #58 in Kiitzing's collection fulfils some of the criteria for a lectotype: it comes from the right locality (Nordhausen, Germany), and was apparently collected at the right time. Furthermore, Ross (1963) and Schoeman & Archibald (1976-1980) have demonstrated that this material contains specimens of the diatom that has generally been known as Sellaphora (Navicula) pupula since 1844. However, given that S. pupula has now been revealed to be a complex, heterogeneous taxon, for the lectotypification to be valid it must be shown that the specimens in #58 do conform in detail with Kiitzing's original description and illustrations, interpreted in the light of the discrepancies noted above. Figs 17- 31. Demes in the Sellaphora pupula complex present in Ktitzing samples but which apparently are not conspecific with the emended lectotype. DIe optics (except Fig. 31, bright field). Figs 17- 26. BM 18725, Falaise material. Figs 17, 18. Deme 4, with almost linear valves, corresponding to Blackford 'rectangular' deme. Fig. 19. Deme 5, cf. Blackford ' capitate' deme. Figs 20- 26. Deme 3, the predominant deme on BM 18725 , at various stages of size reduction. Fig. 27. BM 17918, Nordhausen material , England Finder reference 038/2- 039/1. Deme 2 frustule (upper valve shown), with narrow poles, ell iptical central area and grooves alongside the raphe. Figs 28, 29 . BM 17918, Nordhausen material, Finder references F33/0- 3 and N30/4-2. Broken frustu les, which cou ld represent a third deme on the lectotype slide, but may belong to the same deme as the emended lectotype (deme I). Figs 30, 31. BM 18726, River Main material. Deme 3 valves (cf. Figs 20- 26, from Falaise) . Scale bar = IO 11m (Figs 17- 31).

The systematics of the Sellaphora pupu/a complex: typification of S. pupu/a

233

234

D.G. Mann

BM slide 17918 was made from Ktitzing's #58 and bears a note from Ross stating that it is the type. Specimens of S. pupuJa are rare on it and most or all of them are illustrated in Figs 2-16, 27-29. Some of these have already been illustrated by Schoeman & Archibald (1976-1980): their fig. 5 corresponds to Fig. 11; figs 6, 7 to Figs 2, 4; and fig. 8 to Fig. 27. The specimen illustrated in Fig. 27 certainly belongs to a different deme from those shown in Figs 2-16, which are very similar to each other in valve shape and size, striation pattern, and striation density; these specimens will be referred to as deme 1. Deme 2, represented by Fig. 27, has finer striation, a longitudinal groove (possibly associated with a conopeum) immediately adjacent to the raphe on either side, an elliptical rather than a bow- tie-shaped central area, and much narrower, protracted poles. This specimen corresponds to none of the demes in Blackford Pond, nor to any illustrated by Mann & Droop (1996). The specimens shown in Figs 28, 29 may represent a third deme, since their central areas appear to be unlike those of Figs 2-16; however, they are so broken that it is impossible to be sure. The deme 1 valves on BM 17918 measure 18- 22 x 7.1-7.9 /lm, while the single deme 2 valve ' is 27 x 7.5 /lm. Hence deme 1 cells are too small if Ktitzing's numerical measurement is accepted as correct, and too large if priority is given to the illustrations. The deme 2 cell is the right size according to the numerical measurement, but much too big if the illustrations are trustworthy; however, this cell can in any case be ruled out as a type, because it has the wrong shape (compare Figs 1 and 27). It is possible that the packet (#58) from which BM 17918 was made may contain further demes; or it may contain smaller or larger cells of demes 1 and 2, bridging the gap between specimens and protologue. Indeed, the broken valves shown in Figs 28-29 are slightly smaller, and also apparently more slender than in deme 1, approaching the aspect ratio shown in Ktitzing's figures (Fig. 1). However, in view of the uncertainties associated with the protologue, no wholly convincing argument can be constructed for choosing any deme on BM 17918 as 'typical'. The same difficulty would apply to any demes not present on BM 17918 but occurring in the parent material of packet #58. Given the rpixture of pupuJa demes and valves on BM 17918, none of which correspond well to Ktitzing's description, Ross's lectotypification can be seen to be flawed: it fails one of the principal criteria listed above. Annotated material in Kiitzing's collection We have direct evidence of Ktitzing's own concept of S. pupula from two annotated packets in his collection. Packet #925 was collected by A. Braun from the River Main, Germany, in 1847, and the material in it used later to prepare BM 18726. Sellaphora pupula is rare in this material, which is dominated by small Navicula species. However, at least one deme is present, which has been illustrated by Schoeman & Archibald (19761980, figs 9-10) and is shown here in Fig. 31 . This deme, which will be referred to as deme 3, has approximately the same valve outline as in Ktitzing's illustrations (Fig. 1) but differs from demes 1 and 2 from Nordhausen: it lacks the longitudinal groove of

The systematics of the Sellaphora pupula complex: typification of S. pupula

235

deme 2 and it has polar bars that are reflexed towards the centre (Fig. 31), rather than being at right angles to the raphe, as they are in deme 1 (Figs 5, 6, 9, 11, 13). This is correlated with a difference in the stria pattern, the polar striae being convergent in deme 3 but parallel in deme 1; geniculate striae are often present a little distance from the poles (Fig. 31). These stria features were noted also by Schoeman & Archibald (19761980). In addition, the central area of deme 3 is a flamboyant bow-tie compared to the more rectangular area in deme 1 (compare Fig. 31 with Figs 3, 4, 8, 10-16), and the stria density is slightly but consistently lower (circa 20 in 10 flm in deme 3, as against 23-24 in deme 1). Some individual valves in BM 18726, such as the broken valve in Fig. 30, appear at fIrst sight to be more similar to deme 1, but closer examination reveals that they have the polar bar orientation, stria density and polar stria pattern of deme 3. BM 18725, prepared from packet #1468 (Falaise), contains many specimens of S. pupula sensu lato; as in the Nordhausen material, more than one deme is present. One deme is especially common and many stages in size reduction can be found (Figs 20-26). This deme appears to be the same as deme 3 from the River Main, with the same valve outline, valve dimensions, stria pattern and density, central area shape, and polar bar orientation. Individual valves (e.g. Figs 20-24) are not dissimilar in shape to Ktitzing's illustrations (Fig. 1), although again there is the problem of matching measurements between Ktitzing's specimens, text and illustrations. Fig. 20 shows a valve 29 flm long, which is not far from Ktitzing' s measurement of 1/85'" and could easily have come from a cell with the shape illustrated by Ktitzing (Fig. 1), given the slight bulging of the girdle that occurs in vivo in many diatoms. However, if Ktitzing's illustrations are correct, this valve is much too large. Altogether, BM 18725 would be much more suitable than BM 17918 as a type, were it not for the insuperable obstacle that it is from the wrong locality. The sample belonged to A. de Brebisson, who collected it at Falaise, in Normandy, France, and it was de Brebisson who fIrst identifIed Navicula pupula as being present in the material (Ross 1963), implying that it was collected after Ktitzing's monograph was published in 1844. Ross makes a convincing case, however, that Ktitzing himself confIrmed the identifIcation. As well as deme 3, BM 18725 contains at least two other demes, not illustrated by Schoeman & Archibald (1976-1980), both with linear to linear-Ianceolate valves and broad, slightly capitate apices (Figs 17-19). Deme 4 (Figs 17-18) appears to be identical to the 'rectangular' deme from Blackford Pond, deme 5 (Fig. 19) to the 'capitate' deme (Mann 1989, 1999, Mann & Droop 1996, Mann et al. 1999). In Blackford Pond, these demes are known to be reproductively isolated. Both are already known from several sites in the UK (Mann & Droop 1996, Taylor 1996) and the ' capitate ' deme has been recorded from the Ukraine (Mann 1999, V.A. Chepumov, unpublished observations); a valve illustrated from Ehrenberg material collected at Santa Fiora, Italy (Schoeman ~ Archibald 1976-1980, fIgs 13, 14), probably also belongs to the ' rectangular' deme. BM 18725 does not appear to contain either deme 1 or 2 from N ordhausen.

236

D.G. Mann

Choice of a type specimen Given the prevalence of deme 3 in the Falaise material, very probably confirmed to contain Navicula pupula by Kiitzing himself, and its presence also in the River Main sample, which was definitely stated to contain N. pupula by Kiitzing, we can be virtually certain that Kiitzing would have referred to deme 3 as N. pupula. By contrast, we cannot be sure that the Nordhausen sample selected by Ross (1963) is the one Kiitzing refers to in 1844 and so we cannot be sure that demes 1 or 2 would have been included in N. pupula when he described it. On the other hand, Ross's arguments for typifying N. pupula via BM 17918 are plausible and cannot be set aside, especially given the uncertainties introduced by the mismatch between description and illustrations: BM 17918 is from the right geographical area and contains specimens of Sellaphora pupula sensu lato. The principle of priority applies to lectotypification, unless it can be shown that the lectotype "is in serious conflict with the protologue and another element is available that is not in conflict with the protologue" (lCBN, Article 9.17: Greuter et al. 2000, my emphasis). On balance, therefore, I believe we must accept Ross's decision that BM 17918 (and packet #58) contains type material, despite the difficulties it entails. What must be done, however, is to modify Ross' s typification so that it refers to a single deme, which can be done by designating a single specimen as type. According to the latest version of the ICBN (Article 9.14), lectotypification can be emended (' second-step lectotypification') to narrow its application to a single specimen ('specimen' is defined in Article 8.2) (Greuter et al. 2000) and this is what I propose to do. The second-step lectotype selected is very close to the edge of the cover slip on BM slide 17918, at England Finder reference M45/2; it interprets and modifies Ross's (1963) designation of the whole of BM 17918 as lectotype. This specimen belongs to deme 1 and is illustrated in Figs 2-6; photographs of it have also been provided by Schoeman & Archibald (1976-1980, figs 6-7). The other specimens of de me 1 on BM 17918 (Figs 716) can be regarded as 'isolectotypes', but are subsidiary to the (second-step) lectotype for determining the use of the epithet pupula. Deme 1 was chosen, rather than deme 2, because it _agrees better with Kiitzing's illustration. The new lectotype is a whole frustule: Figs 1, 3 illustrate the nearer valve, and Figs 2, 4 the further valve. The frustule measures 19.7 x 7.1 11m and has 22- 24 striae in 10 11m, measured adjacent to the raphe. It is narrowly lanceolate with very slightly produced, broadly rounded apices (as required by the original description: see above and Kiitzing 1844). The polar bars are at right angles to the raphe (i.e. strictly transapical: Figs 5-6) and the central area is transapically elongate, reaching approximately half-way to the valve margin. Over most of the valve, the striae are slightly radial, becoming parallel near the poles; at the centre, they are alternately long and short. The striae cannot be resolved into lines of areolae using normal bright field or DIC optics (NA 1.32). These features are common to all the isolectotypes, which differ only in dimensions. The largest valves observed on BM 17918 (in the frustule shown in Figs 15-16) are 21.7 11m long; the smallest (in the frustule in Figs 11-12) are 18 11m.

The systematics ofthe Sellaphora pupula complex: typification of S. pupula

237

Explicitly excluded from the isolectotypes are the valves of deme 2 and an unspecified deme on BM 17918 (Figs 27-29), and all valves of demes 3- 5 on BM 18725 and 18726 (Figs 17-26, 30-31), despite Ktitzing's own identification of deme 3 as Navicula pupula. Literature records and the known distribution of Sellaphora pupula As shown already, Sellaphora pupula was interpreted in a broad sense even by its original author, Ktitzing, if we assume that the Nordhausen sample #58 (BM 17918) is indeed the material Ktitzing referred to in 1844. The first illustrations by a different author, Rabenhorst (1853 , pI. 6: fig. 82a-c, ) reinforced the broad concept and even extended it to include ± linear and elliptical specimens, as well as cells like those shown in Fig. 1. The concept was broadened still further by Grunow (in Cleve & Grunow 1880), Hustedt (1927-1966, 1930) and others. Hence, records of S. pupula are almost all imprecise: except in the few cases where photographs are supplied, information about Sellaphora or Navicula pupula could refer to anyone of many tens or hundreds of demes. Infraspecific taxa have often been recognized within S. pupula. In general, 'typical' specimens, identified as S. pupula var. pupula or 'var. genuina' to distinguish them from other named varieties, resemble Ktitzing' s original drawings (Fig. 1) and the emended lectotype in overall shape (for example, Cleve-Euler 1953, fig. 890a-c; Ko-Bayashi et al. 1986), as one would hope they would. However, this is not always so. Hustedt (19271966, fig. 1254a-g) included a wide variety of demes within var. pupula, as did Krammer & Lange-Bertalot (1986) and Lange-Bertalot & Metzeltin 1996 (pI. 25 : figs 6-8; fig. 8 is similar to the lectotype except that its stria density is slightly but probably significantly lower). However, none of the valves of var. pupula illustrated by Cleve-Euler (1953), Ko-Bayashi et al. (1986), Hustedt (1927-1966), Germain (1981) or Krammer & LangeBertalot (1986) can be assigned to the same deme as the emended lectotype of S. pupula and so it is unlikely that any of them should in fact be classified as S. pupula. All of them have the wrong shape, size, stria pattern, or stria density. There are a few published records that can be assigned with some confidence to S. pupula sensu stricto. Among them are some of my own micrographs (Mann 1989, fig. 1e, f; Mann & Droop 1996, fig. 5), which indicate that S. pupula sensu stricto is the same as the Blackford ' neat' deme. This deme is also known from other lakes in Edinburgh, including Figgate Loch (Mann & Droop 1996, fig. 11), Inverleith Pond, Dunsapie Loch and St Margaret' s Loch (Taylor 1996); Rae Loch, near Blairgowrie, E Scotland (Taylor 1996); and Fenemere, Shropshire, C England (Mann & Droop 1996). Measurements of valves from these localities extend the known range of size to 18.5- 35 x 6.5-7.3 ~m, although, until the full life cycle has been studied in culture, these ranges must be regarded as provisional. The striation density is always circa 23- 24 in 10 ~m. Unfortunately, the 'neat' deme has proved to be one of the most recalcitrant of all the demes we have attempted to culture in the last 5 years, growing slowly if at all in OUF media and growth conditions (see e.g. Mann et al. 1999). It is also the rarest deme of the

238

D.G. Mann

S. pupula complex in Blackford Pond and we have discovered relatively little about it from studies of natural and semi-natural populations; an isolated observation suggests that it may be autogamous or parthenogenetic (Mann 1989). Another published micrograph resembling the S. pupula lectotype sufficiently to allow confident identification is pI. 7: fig. 13 of Metzeltin & Witkowski (1996, but not figs 10-12, in which there are longitudinal grooves accompanying the raphe, probably indicating the presence of a short conopeum). This specimen was collected on Bj0ffi0ya (Bear Island, Norway) in the Barents Sea. Reichardt (1984, pI. 13: fig. 1) also illustrates a valve that appears to represent S. pupula sensu stricto, from the river Altmlihl in Bavaria, Germany. Other possible records are those of Foged (1982, pI. 20: fig. 3), from the island of Bornholm, Denmark, and Reavie & Smol (1998, pI. 18: figs 14-15) from the St Lawrence River, Canada. The two valves of S. pupula ' var. pupula' from New Caledonia illustrated by Moser et al. (1995, pI. 58: figs 1-2) resemble the lectotype in shape and size, but the striae are more strongly radial and appear coarser and I am unconvinced that they are conspecific with S. pupula sensu stricto. This illustrates how difficult it is to determine the distributions of taxa using literature records alone, even when excellent micrographs are provided in support. However, I can say with confidence that the vast majority of published micrographs of Sellaphora or Navicula pupula, in Bibliotheca Phycologica, Bibliotheca Diatomologica or Iconographia Diatomologica, or in earlier papers by Foged and others, do not correspond to the lectotype of S. pupula and are extremely unlikely to represent the same species. At present, all of the records of S. pupula sensu stricto that I regard as proven or probable (see above) are from cool temperate or subarctic parts of the N Hemisphere. However, the full geographical range of the species and its ecological preferences are unknown. Final comments This paper would not have been necessary if Article 8.2 of the ICBN (Greuter et aI. 2000) did not allow slides to be designated as types for small plants and if diatomists did not take ~dvantage of this permission. The whole purpose of the type method is to determine the use of names, especially when the circumscription or status of taxa are altered. Typifying taxa via an undefined population of diatom frustules on a slide will always be dangerous, because one can rarely be sure that later research will not show that the population is heterogeneous, consisting of several distinct taxa; if it is heterogeneous, the type will be ambiguous and will need to be redefined or supported by an epitype. The holotype, lectotype or neotype of a diatom taxon should therefore be a single specimen in all cases, whether or not the ICBN requires this (Mann 1998). The example of S. pupula also shows, only too clearly, the inadequacy of the documentation in many ecological and floristic papers on diatoms. I have commented on this before (Mann 1999), but the matter is important enough to merit further discussion. There can be few studies of meso- or eutrophic lakes and rivers anywhere in the world in which S. pupula has not been recorded (almost always as Navicula pupula), yet with a

The systematics of the Sellaphora pupuZa complex: typification of S. pupuZa

239

change to a narrower species concept, it has become almost impossible to say anything meaningful about the ecology and biogeography of this species. The different component species (demes) of the S. pupula complex do not all share the same ecological and physiological characteristics, nor do they have the same distributions (e.g. Mann & Droop 1996, Taylor 1996, Mann 1999). We cannot use the literature to discover these characteristics, however, because we do not know which demes were being recorded under the name pupula: authors have not recorded what they saw, only their identifications of what they saw, and in many cases they have not discussed or preserved the evidence. One might have hoped that it would be possible to say something about the ecology of distinctive morphological variants within S. pupula sensu lato that have been recognized as named infraspecific taxa. However, several of the varieties and forms that are often recognized, such as vars rectangularis (W. Gregory) Grunow, elliptica Hustedt and capitata Hustedt (see Hustedt 1930), are themselves heterogeneous (unpublished observations), so that literature records of these too convey little or no useful information (var. capitata sensu Hustedt 1930 is in any case predated by var. capitata Sk:vortzov et Meyer). In groups whose taxonomy is sound and stable, like most vertebrates (and also vascular plants in well-explored areas like Europe), such problems rarely arise and so it is unnecessary to document in great detail every species that is recorded in a surveyor studied in the laboratory: it may even be superfluous to refer to a flora or fauna to indicate how taxa were circumscribed and identified. For diatoms, however, if research is to have long-term value, it must be backed by evidence (photographic images, gene sequences, genetic profiles) of the taxa studied and by voucher specimens deposited in museums that curate their collections responsibly and make specimens available for study by the scientific community. Sadly, most of the floristic and ecological works on diatoms published in the 19 th and 20 th centuries were not supported and validated in this way and so they are now almost worthless. Acknowledgements I thank Pat Kociolek and Regine Jahn for organizing this volume to pay tribute to Professor Dr Horst LangeBertalot; Horst himself for his tireless work, without which this paper and so many others would have been much the poorer; Debbie White for reproducing Kiitzing' s illustrations; David Williams for taking the trouble at short notice to check for me that the facsimile edition of Kiitzing's book is a faithful copy of the original; and Pat Kociolek, Elisabeth Fourtanier and Regine Jahn for helpful comments and advice. References CLEVE, P.T. & A. GRUNOW (1880): Beitrage zur Kenntniss der arctischen Diatomeen. - Kong!. Svenska Vetenskapsakad. Hand!. 17 (2) : 1-120. CLEVE-EULER, A. (1953): Die Diatomeen von Schweden und Finnland. Teil III. Monoraphideae, Biraphideae 1. - Kong!. Svenska Vetenskapsakad. Hand!. , Ser. 4, 4 (5): 1-255. DROOP, S.J.M. (1996): The identity of Diploneis splendida (Bacillariophyta) and some related species. Phycologia 35: 404-420. FOGED, N. (1982): Diatoms in Bomholm, Denmark. - Biblioth. Phyco!. 59: 1-175.

240

D.G. Mann

GERMAIN, H. (1981): Flore des diatomees. Diatomophycees eaux douces et saumatres du Massif Armoricain et des contrees voisines d'Europe occidentale. - Boubee, Paris. GILMOUR, lS .L. & lW. GREGOR (1939): Demes a suggested new terminology. - Nature (London) 144: 333-334. GREUTER, W., 1 McNEILL, F.R. BARRIE, H.M. BURDET, V . DEMOULIN, T.S . FILGUEIRAS, D.H. NICOLSON, P.C. SILVA, J.E. SKOG, P . TREHANE, N.J. TURLAND & D.L. HAWKS WORTH (2000): International code of botanical nomenclature (St. Louis Code). - Regnum Veg. 138: 1-474. HUSTEDT, F . (1927-1966): Die Kieselalgen Deutschlands, Osterreichs und der Schweiz. - In: Dr. L. Rabenhorsts Kryptogamenfiora von Deutschland, Osterreich und der Schweiz 7. - Akademische Verlagsgesellschaft, Leipzig. HUSTEDT, F. (1930): Bacillariophyta (Diatomeae). - In: PASCHER, A. (ed.) : Die Susswasser-Flora Mitteleuropas 10 (2 nd ed.). - G. Fischer, Jena. JAHN, R. (1995a): Ehrenberg's marked species on five mica. - Iconogr. Diatomol. 1: 101-107. JAHN, R. (1995b): C.G. Ehrenberg's concept of the diatoms. - Arch. Protistenk. 146: 109-116. KO-BAYASffi, T., S. KATSUYAMA & H . FUKUSffiMA (1986) : Taxonomical studies on pennate diatom Navicula pupula Kutz. - Jap. 1 Water Treatm. BioI. 22 : 17-28 [In Japanese]. KRAMMER, K. & H. LANGE-BERTALOT (1986): Bacillariophyceae 1. Teil: Naviculaceae. - In: ETTL, H. , J. GERLOFF, H. HEYNIG & D. MOLLENHAUER (eds) Su/3wasserfiora von Mitteleuropa 2 (1). G. Fischer, Stuttgart & New York. KUTZING, F.T. (1844) : Die kieselschaligen Bacillarien oder Diatomeen. - W. Kohne, Nordhausen. LANGE-BERTALOT, H. & D. METZELTIN (1996): Oligotrophie-Indikatoren. 800 Taxa reprasentativ fUr drei diverse Seen-Typen: kalkreich - oligodystrophe - schwach gepuffertes Weichwasser. - Iconogr. Diatomol. 2 : 3-390. LANGE-BERTALOT, H., K. KULBS, T. LAUSER, M . NORPEL-SCHEMPP & M . WILLMANN (1996): Dokumentation und Revision der von Georg Krasske beschriebenen Diatomeen-Taxa. - Iconogr. Diatomol. 3: 2-358. MANN, D.G. (1984): Observations on copulation in Navicula pupula and Amphora ovalis in relation to the nature of diatom species. - Ann. Bot. (London) 54: 429-438 . MANN, D.G. (1988) : The nature of diatom species: analysis of sympatric populations. - In: ROUND, F.E. (ed.): Proceedings of the 9th International Diatom Symposium: 317-327. Biopress, Bristol and Koeltz, Konigstein. MANN, D.G. (1989a). The diatom genus Sellaphora: separation from Navicula. - Brit. Phycol. 1 24: 1-20. MANN, D.G. (1989b): The species concept in diatoms: evidence for morphologically distinct, sympatric gamodemes in four epipelic species. - PI. Syst. Evol. 163: 215-237 . MANN, D.G. (1998): Ehrenbergiana: problems of elusive types and old collections, with especial reference to diatoms. - Linnean, Special Issue 1: 63-88 . MANN, D.G. (1999): The species concept in diatoms. - Phycologia 38: 437-495. MANN, D.G. & S.J.M . DROOP (1996). Biodiversity, biogeography and conservation of diatoms. Hydrobiologia 336: 19-32. MANN, D.G. , V.A. CHEPURNOV & S.J.M . DROOP (1999): Sexuality, incompatibility, size variation and preferential polyandry in natural populations and clones of Sellaphora pupula. - 1 Phycol. 35: 152-170. MAYAMA, S. & H. KOBAYASI (1988): Morphological variations in Navicula atomus (Kutz.) Grun. - In: ROUND, F .E. (ed.) Proceedings of the 9th International Diatom Symposium: 427-435. - Biopress, Bristol and Koeltz, Konigstein. METZELTIN, D. & H. LANGE-BERTALOT (1998): Tropische Diatomeen in Sudamerika 1. 700 uberwiegend wenig bekannte oder neue Taxa reprasentativ als E lemente der neotropischen Flora. Iconogr. Diatomol. 5: 1-695. METZELTIN, D. & A. WITKOWSKI (1996): Diatomeen der Baren-Insel. Susswasser- und marine Arten. Iconogr. Diatomol. 3 : 3-232.

The systematics of the Sellaphora pupula complex: typification of S. pupula

241

MOSER, G., A. STEINDORF & H. LANGE-BERTALOT (1995): Neukaledonien: Diatomeenflora einer TropeninseL Revision der Collection Maillard und Untersuchung neuen Materials. - Biblioth. DiatomoL 32: 1-340. PATRICK, R. & C.W. REIMER (1966): The diatoms of the United States, exclusive of Alaska and Hawaii. L Fragilariaceae, Eunotiaceae, Achnanthaceae, Naviculaceae. - Monogr. Acad. Nat. Sci. Philadelphia 13: 1-688. RABENHORST, L. (1853): Die Siisswasser-Diatomaceen (Bacillarien). - E. Kummer, Leipzig. REAVIE, B.D. & J.P. SMOL (1998): Freshwater diatoms from the St. Lawrence River. - Biblioth. DiatomoL 41: 1-137. REICHARDT, E. (1984): Die Diatomeen der Altmiihl (Beitrage zur Diatomeenflora der Altmiihl 2). Biblioth. DiatomoL 6: 1-169. ROSS, R. (1963) : The diatom genus Capartogramma and the identity of Schizostauron . - BulL Brit. Mus. (Nat. Hist.), Bot. 3: 47-92. ROUND, FE , R.M. CRAWFORD & D.G. MANN (1990): The diatoms. Biology and morphology of the genera. - Cambridge University Press, Cambridge. SCHOEMAN, F.R. & REM. ARCHIBALD (1976-1980) : The diatom flora of southern Africa. - CSIR Special Report WAT 50, Pretoria. SIMS, P.A. & T.B .B. PADDOCK (1979): Observations and comments on some prominent morphological features of naviculoid genera. - Nova Hedwigia Beih. 64: 169-191. TAYLOR, N.G. (1996): Variation within Sellaphora pupula (Bacillariophyta) in relation to the species concept. - M.Sc. Thesis, University of Edinburgh, Edinburgh, UK. WALKER, P.M.B. (ed.) (1988): Chambers Science and Technology Dictionary. - Chambers, Edinburgh & Cambridge University Press, Cambridge. Received April 2000, accepted in revised form October 2000.