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Some Forms of Stephanodiscus Species
Ar ch. P r ot ist enk. 125 (198 2): 357 -371
U niversit y of Bi rst ol , Departm ent of Botany, Brist ol
Some Forms of Stephanodiscus Species By F . E.
RO UND
With 8 Pla te s
Introduction H aving described in some detail th e structure of S tephano discus ni aqorae, S . rotula and S . minutula using samples from the classical type material (ROUN D 1981) the probl em arose of th e identity of many mod em and some fossil Stephanodiscu« examples collected during t he last decade. Almost all examples differed somewhat from th e species descriptions mad e from the t yp e material and t he qu estion aro se how mu ch va riation in st r uct ure from that of t he t ype can be permitted whilst still allocating exam ples t o t he t ype. This is of course a very common pr oblem in t axonomic st udies but it seems to be par ti cularl y acute in t he genus St ephamodiscus. The rather awful prospect t hat each lake or river population is discrete and distinguishable mu st be rai sed . If t his turns out t o be- so, we t hen still have t o clust er th e form s, hopefully into the recognised (macro) species , which must be based on carefully describ ed types. The fact that the "type description" does not contain all the information necessary to encomp ass all form s has long been recogni sed. Th ese forms, however , ma y be ver y st able tax onomic entities livin g in parti cular envir onments over long periods of ti me - a beau tiful exa mple of t his is t he occurrence of distinct colonial morphologies of 'I'abellari a [locculosa in t he English Lake District (K NUDSEN] 955) and similar (morpholo gically identical forms ?) in Irish Loughs (R OUN D & BRO OK] 959). They all belong to th e same species but are not identi cal. Simil ar differences, but t his ti me in physiological char act eristics, are record ed in some marine forms, e.g. t he clones of Thalassiosira nana having different half-saturation constants for nitrate uptake (CARPENTER & GUII-LARD 1971). Th e latter also differ slightly in valve morphology. It. is necessary to distinguish these variants just as th e Drosophila geneticist dist inguishes the multitude of natural and artificially induced mutants, and t hus a trinomial sys tem will have t o be develop ed . Of course there is also th e existi ng system of varieties and forms but these ar e on the whole used for distinctive morph ological deviant s from the species, recognisable by light microscopy. The forms of St ephanodiscus t o be discussed below may be diffi cult to characterise in the light microscope and t he physiological variants may be morphologically id entical - we do not know yet .
358
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Observations The first to be discussed are those which can be allied with the three species described in Rouxn (1981) starting with S. niagarae. A form from Endamura, Japan (Figs. 1-4), has the typical characters of the species with the additional feature of a slight rib or ray between the spine and the exit of thc fultoportule. This feature is more prominent in samples from Alaska (Figs. 5, 6). Terrebonne (Figs. 7-12) and Klamath (Figs. 13, 14). In all these samples the valves are curved without a great change in angle passing from valve face to mantle. All have an internal ring of fultoportules (e.g. Fig. 9) and the internal organisation is of the three tubes around the one central (Figs. 4, 12). The Endamura material has the fultoportulae very close to the spines but whatever the depth of the valve mantle the fultoportulae always appeal' to be slightly less than half-way bet" een the spines and the end of the areolae on the edge ofthe valve mantle. The valve mantle may be "flanged" (Fig. 11) and with vertical slit-like markings which may be merely a remnant of the girdle band. "Fig. 11 also shows that the valve mantle areolae are simple tubes through the siliceous framework whereas in the valve face they are internally enlarged (see ltOUND 1981, Fig. 5). The Alaska sample may have a somewhat steeper valve mantle than the others (Figs. 5, 6). A sample from Sandusky Bay, I.. ake Erie, has the very characteristic internal structure to the fultoportules (Fig. 18) and internal ring of valve face fultoportules ; but is a rather variable form with prominent central fultoportule openings (Fig. ] 6) and no rays between the files of areolae (Figs. 15, 16). In this respect the two valve shapes differ somewhat - the one with the central depression (left-hand one on Fig. 15 and Fig. 16) having almost no separation of the files of areolae. The valves have rather pronounced central elevations/depressions (Fig. 15) and rather unusual areolae between the spines and fultoportules and bars across some of the valve face areolae (Fig. ] 7). I think it must remain as a form of 8. niaqarae. Samples from Henningfield (Figs. ]9,20), Combermere (Figs. 21-2:1), Queen Mary Reservoir (Fig. 24), Farmoor Reservoir (Figs. 25--28), all have smaller valves and the spine/areolaejfultcportule placements are closer than in S. niaqarae types. They also have less prominent rimoportule exit tubes and internally the fultoportules are smaller and of the "are" type and there is not a central ring though there may be isolated fultoportules halfway between the centre and valve mantle. The valve mantle may be spinulose (]'ig. 28), areolae are smaller and more distinctly separated from those of the valve face -- this is especially noticeable in the valves which have a depression internal to the ring of spines (e.g. Fig. 22). A sample from Lough Leven has very close spine insertions on the valve, often at the end of every ray and the fultoportule is extremely close to the spine (Fig. ~O). The valve mantle is shallow (3-4 areolae below the fultoportule) and if the fultoportules were located beneath each spine it would resemble the classical S. rotula. It is however interesting that in this sample the two valve types vary in the frequency of spine insertion and distance below the fultoportule insertion on the two valves (compare Figs. 29 and 30). In this, as in the previous three samples, the internal openings of the fultoportules are only slightly developed and of the "are" type. Rimoportules are scarce and there is a clear ring of valve face fulto-
Some Forms of Stephunodiscus Species
359
portules. A form from Lough Corrib also shows the dimorphism of the valve (compare Figs. 31 and 32). The only other form I have encountered which has closely spaced spines and fultoportules is one from L. Victoria (Figs. 33, 34) but in this form there is a considerable gap between the spines and exit tubes of the portules. This may have to become a new taxon though I am reluctant to take this step at the moment. Forms from Chew Valley Reservoir near Bristol fall within the same section but these have a Iultoportule beneath each spine (Figs. 35, 36). These forms from Henningfield to Chew all fall into what I believe most workers have termed 8. asiraea (cf. HAKANSSON 1976) but since this is an invalid combination (ROUND 1981) a new name must be found. This I do not intend to pursue since the remaining EHRENBBRG material is now under survey by HAKANSSON & LOCKBR (LOCKER, pel's. comm.) and should one of the EHRENBFJRG types have the general form of this section then the name of that will have to he used. What I am fairly certain of, is that the material of KUTZING on which 8. roiula KUTZ. is based seems to be a rather rare taxon and S. rotula should not be used indiscriminately for forms previously ascribed to the non-existent S. astraea, However another form occurring in Abbot's Pool, near Bristol, and tentatively referred to S. rotula by HAl'PEY (1970) is difficult to fit into the above series -- its cells are much more barrel-like (Figs. 37, 39), it has a much flatter valve face (Figs. 38, 40), more files of areolae near the outer valve face (Fig. 40) and a tendency for the development of fultoportules beneath alternate spines (Fig. 38). The cells have rather deep valve mantles with many areolae beneath the fultoportules (Fig. 38), though this seems to decrease somewhat in small cells (Fig. 40). Internally the fultoportules are of the "arc" type. There is also a rather sharp angle between valve face and valve mantle. In this form there seems to be hardly any difference between sibling valves, which is as expected in a cell with flat valves. The only other Stephomodiscus species with such flat valves and "vertical" valve mantle is S. binderanus (Figs. 41-44) but this has the "binderanus" characteristic of forked spines, whilst the flattish S. tenuis and S. hanczsdiii are much smaller. It is of course just possible that this is a post-auxospore stage but it is a taxon which requires further study when the complete EHRENBERG collection has been analysed. Figs. 41/42 and 43/44 also illustrate the 8. binderanus val'. binderamus and S. binderanu8 val'. oestrup1:i forms showing different degrees of valve spine forking (see also S'l'OERMER et a1. 1979). The smallest Stephamodiscus described in HOUND (1981) was S. minutula and modern examples of this species are figured from Slapton Ley (Figs. 45, 46) and Blelham Tarn (Figs. 47,48). They vary little from those in KUTZINO'S original material. In almost all samples of Stephanodiscus populations some cells are found with valves which have a somewhat different structure to the majority - they often have the areolae occluded, sometimes slit-like, as though a skin over the surface had split. Fig. 3 is a possible example in the S. niagarae series. HAKANSSON (1976) also illustrates these in her Figs. I.G.H. and STOERMER et al. (1979) have shown very neatly that in S. binderanue such valves act as "separation valves" to break up the filaments into
360
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ROU ND
short lengths (cf. t he similar sit uat ion in M elosira , (CRAWFOltD 1979). If t his is t he norm al sit uation t hen a problem which has been puzzling me for some years is solved.
Discussion Wi th t he exce ption of t he Abbot's P ool mat erial ("Figs. 37- 40) t he t hree species considered fall into a section of Stepluimodi scus whi ch has distin ctl y conce ntrically undulate va lves - the "gewolbte Schale" of InKAN ss o~ (1976) who uses this fea ture opposed t o "flaehe Schale" as t he first bifurcati on of her key . T his I believe provides a va luable distinction which will help t he ecologist separate out t he small " hantzscliii; tenuis, invisitatus " group in pa rticular, from S . minutula, In R OU ND (1981) a det ailed generic descrip tion was provided and in vestigati on of these form s provides only a few ad ditional points. Th e Lake Victo ria sa mple has t he files of areo lae reduced almost. to a single row, som e va lves can have almost a complete break in the sequence of areolae just internal to the ring of spines, others have some of t he areolae elongated aro und spines or exit tubes of t he portulcs and some h ave gra nula rjs pinulose valves. Th ese features in no \yay alte r the basic concept of the gen us but anyt hing more drastic could hardly be permitted , hen ce the removal of S . tlubius t o a new genus Cyclostephano8 (R OUN D 1982). One new feature emerging from t he pr esent study is that the arra ngement of areolae, spines and fult oportules in anyon e species can vary slightly on sibling valves and t his is parti cularl y noticea ble when t he valve s ar e prominent ly , concentrically waved. In t hese taxa both va lve types should be figured and described iIJ t he specific ac-count. In addition, as point ed out fi rst by ST01'1H1I1E R et al. (1979) t here are separat ion valves, wit h distinct morph ology even in this genm; whic h is not prone to form long filam ent s. It is features like t his, that ha ve confused me and pr ot ract ed t his st udy for which I apo logise to all who have so kindl y sent me samples an d patiently waited for some ans wers . Regre ttably I still feel there are many problems I ha ve not solved and all form s cannot be allocate d t o species. J n particular the lar ge com plex of t he old in valid S . astraea st ill has no name. Th e st udies of H J KAN ~S O N & LU CKl';h on the Stephano discus form s in the EHRE N lJ E1W collection will be invaluable and probably yield a classical name. It, would be a great help if t he tw o valve types of these concent rically waved val ves could hav e some form of design ation. The simplest feature of the two types is the central elevat ion on one and depr ession on the other and so I suggest they be t ermed the elevated valve and the depressed valve. Sugge sted Latin designations are "scut ate " and " lacunat e" . Th e elevated va lve h as the raised edge just in t ernal to the spines and it is t his whi ch, in some species, has t he break in t he files of areolae. Th e depressed va lve h as a more eve n curve in this region and t ends to hav e a more regul ar areola arrangement. A further feature encountered in earlier work (e.g. ROUND 19i2) is the vertically groov ed valve mantl e edge (e.g. in Fig. 2). Thi s feature occur s also in some oth er cellt ric genera , e.g. Biddulphia and is not as I on ce thought a rem nan t of a fimbriat e mantle.
Some Forms of Stephanodiecus Species
361
Finall y, to return to the early problem of the distinction between Stephanod?:scus and Oyclotella -- the latter has no ring of spines, though there may be many scattered small spines or nodules, there is always a clearly distinguished central area to the valve face even in species which have some scattered areolae in the centre, though in these cases there are usually also scattered fultoportulae. Internally the structure of the two genera is equally distinctive - in all Cyclotella species there is a chambering of the valve mantle region such that there is the appearance of ribs with apertures between. Often the internal valve face is almost plane with a few fultoportulae and domed cribra. There is, as far as I know, only a single rimoportule in Oyclotella and the exit tubes of the fultoportules tend to be close to the valve mantle edge and very short or even reduced to a circular aperture.
Acknowledgements I wish to thank the many diatomists who have sent me samples over the years, in particular Mr. R. 1. FIRTH, Miss P. A. SIMS, Dr. D. YOUNG and Prof.•J. BRUNEI<.
Literature CARPENTER, E .•r., & GUILLARD, R. R. L.: Intraspecific differences in nitrate half-saturation constants for three species of marine phytoplankton. Ecology 52 (1971): 183 -185. CRAWFORD, R. M.: Filament formation in the diatom genera Melosira C. A. AGARDH & Paralia HEIBERG. Beih. Nova Hedw. 64 (1979): 121-133. HAKANSSON, H.: Die Structur und Taxonomie einiger Stephanodiscus-Arten aus eutrophen Seen Schwedens. Bot. Not. 129 (1976): 25-34. HAPPEY, C. M.: The effects of stratification on phytoplanktonic diatoms in a small body of water. .J. Ecol. 58 (1970): 635-651. KNUDSEN, B. M.: The distribution of 'I'abellaria in the English Lake District. Proc. Int. Ass. Theo· ret. & Appl. Limnol. 12 (1955): 216-218. ROUND, F. E.: The diatom genus Stephanodiscus, An electronmicroscopic view of the classical species. Arch. Protistenk. 124 (1981): 447-462. Cyclostephanos - a new genus within the Sceletonemaceae. Arch. Protistenk. 125 (1982): 323-329. & BROOK, A.•J.: The phytoplankton of some Irish loughs and an assessment of their trophic status. Proc. R. Irish Acad. 60, Sect. B. (1959): 167 -191. STOERMER, E. F., KINGSTON, J. C., & SroKO-GOAD, L.: The morphology and taxonomic relationships of Stephanodiscus binderanue val'. oestrupii (A. CL.) A. CL. Beih. Nova Hedw. 64 (1979): 65-78. Author's address: Prof. Dr. F. E. ROUND, Department of Botany, University of Bristol, Bristol, BS8 lUG, U.K.
24 Arch. Protistenk. Bd. 125
362
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ROUND
Explanation of Plates Plate 1. Stephunotliscus niagarae.
1- 4 Endamura material 1. Low magnification outside. Mag. X 1,700. 2. Valve mantle showing striations on mantle edge, pads where spines have fallen off and the external opening of one rimoportule. Mag. X 4,500. 3. Valve mantle showing irregular areolae around the spine bases and fultoportule openings. Mag. X 4,100. 4. Internal view of two fultoportules and a Ringle rimoportule. Mag. X 8,600. 5 - li Alaskan material 5. Outside view of valve. Mag. X 1,200. li. Valve mantle showing small openings of both fultoportnles and a rimoportule. Mag. X4,900.
Plate 2. Siephrinodiecus niagarae from Terrebonne
7. Valve showing distinct "stripes" beneath spines. Mag. X675. Enlargement of valve with raised centre showing irregular areolae in region of spines and internal ring of fultoportnle opening. :'Ilag. X 860. 9. External view of valve with depressed centre showing a very distinctive ring of fnltoportule openings. Mag. X 1,950. 10. Mantle edge showing pads from which spines have fallen and elongate areolae around these. Mag. Xli,OOO. 11. Break in valve mantle showing simple linear passages in the mantle areolae and the expanded edge. Mag. X 5,300. 12. Inside valve mantle showing two fultoportules and one rimoportule. Mag. X 10,900.
s.
Plate 3. Stephanodiscus niaqarae
13 -14 From Klamath 13. External view showing a rut her smooth valve surface and a small areolae. Mag. X 2,liOO. 14. Enlargement of cent.ro showing bars across some areolae. Mag. X 5,300. 15-18 Material from Sandusky Bay, Lake Erie 15. Low power of the two valve morphologies. Mag. X 1,200. HI. Valve showing an internal ring of fultoportule openings (arrow). Mag. X2,100. 17. Valve mantle of the elevated valve form showing bars across some areolae. Mag. X4,900. 18. Internal view showing ring of valve mantle fultoportules and ring of valve face fultoportules. Mag. X 1,100.
Plate 4. Stephnnodiscue sp. previously placed in S. astraea,
19 - 20 From Henningsfield 19. Part of valve showing, pads at base of spines and fultoportule openings near spine. Mag. X6,300. 20. Valve form with scattered areolae around spine region. Mag. X 7,100. 21- 23 From Combermere 21. The two valve types. Mag. X 1,100. 22. Valve form with clear area internal to spines. Mag. X5,100. 23. Edge of other valve type. Mag. X5,300. 24. From Queen Mary Reservoir. Whole cell. Mag. X 1,800.
Some Forms of Stephanodiscus Species
363
Plate 5. Stephanodiscus sp. as previous plate. 25 - 28 From Farmoor Reservoir 25-26. Two valves of the elevated type. Mags. X 1,050 and X2,050. 27. Higher magnification showing external openings of isolated internal fultoportules (arrow). Mag. X 5,300. 28. Form with a spinulose valve mantle. Mag. X 6,400. 29 - 30. From Loch Leven 29. Showing the break in the aerolae internal to the spines. Mag. X 2,100. 30. A form with bars breaking up the areolae. Mag. X 6,400. Plate 6. Stephanodiscus sp. as previous plate 31-32. 33-34-. 35-36. X 5,500
Two valve forms from material from Lough Corrib. Mags. X4,900 and X7,200. Stephanodiscus - unknown species from Lake Victoria. Mags. Xl,500 and X5,100. External and internal view of another S. aetraea type from Chew Valley Lake. Mags. and X 2,300.
Plate 7. Stephanodiscus sp, from Abbot's Pool 37. Two uncleaned cells showing the flat valves. Mag. X2,050. 38. Two valves with steep valve mantles and exit tubes of fultoportules close to spines. Mag. X12,800. 39. Cell immediately after division showing the "barrel" form. Mag. X 2,150. 40. A small celled valve showing the increase in areolae in the files near the valve face margin. Mag. X 5,600. 41- 42. Stephanodiscus binderanus. Two filaments with spines forking to different degree. 41. var. binderanus, Mag. X 4,900. 42. var. oestrupii, Mag. X 3,500. Plate 8. Stephanodiscus binderanus 43. Typical var, binderarcus, Mag. X 5,600. 44. Typical val'. oestrupii. Mag. X 11,300. Stephanodiscus rninutula 45-46. Material from Slapton Ley. Two valve forms. Mags. X5,300 and X5,600. 47 -48. Material from Blelham Tarn. Outside and inside views. Mags. X5,900 and X5,900.
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U niversit y of Bi rst ol , Departm ent of Botany, Brist ol
Some Forms of Stephanodiscus Species By F . E.
RO UND
With 8 Pla te s
Introduction H aving described in some detail th e structure of S tephano discus ni aqorae, S . rotula and S . minutula using samples from the classical type material (ROUN D 1981) the probl em arose of th e identity of many mod em and some fossil Stephanodiscu« examples collected during t he last decade. Almost all examples differed somewhat from th e species descriptions mad e from the t yp e material and t he qu estion aro se how mu ch va riation in st r uct ure from that of t he t ype can be permitted whilst still allocating exam ples t o t he t ype. This is of course a very common pr oblem in t axonomic st udies but it seems to be par ti cularl y acute in t he genus St ephamodiscus. The rather awful prospect t hat each lake or river population is discrete and distinguishable mu st be rai sed . If t his turns out t o be- so, we t hen still have t o clust er th e form s, hopefully into the recognised (macro) species , which must be based on carefully describ ed types. The fact that the "type description" does not contain all the information necessary to encomp ass all form s has long been recogni sed. Th ese forms, however , ma y be ver y st able tax onomic entities livin g in parti cular envir onments over long periods of ti me - a beau tiful exa mple of t his is t he occurrence of distinct colonial morphologies of 'I'abellari a [locculosa in t he English Lake District (K NUDSEN] 955) and similar (morpholo gically identical forms ?) in Irish Loughs (R OUN D & BRO OK] 959). They all belong to th e same species but are not identi cal. Simil ar differences, but t his ti me in physiological char act eristics, are record ed in some marine forms, e.g. t he clones of Thalassiosira nana having different half-saturation constants for nitrate uptake (CARPENTER & GUII-LARD 1971). Th e latter also differ slightly in valve morphology. It. is necessary to distinguish these variants just as th e Drosophila geneticist dist inguishes the multitude of natural and artificially induced mutants, and t hus a trinomial sys tem will have t o be develop ed . Of course there is also th e existi ng system of varieties and forms but these ar e on the whole used for distinctive morph ological deviant s from the species, recognisable by light microscopy. The forms of St ephanodiscus t o be discussed below may be diffi cult to characterise in the light microscope and t he physiological variants may be morphologically id entical - we do not know yet .
358
F. E.
ROUND
Observations The first to be discussed are those which can be allied with the three species described in Rouxn (1981) starting with S. niagarae. A form from Endamura, Japan (Figs. 1-4), has the typical characters of the species with the additional feature of a slight rib or ray between the spine and the exit of thc fultoportule. This feature is more prominent in samples from Alaska (Figs. 5, 6). Terrebonne (Figs. 7-12) and Klamath (Figs. 13, 14). In all these samples the valves are curved without a great change in angle passing from valve face to mantle. All have an internal ring of fultoportules (e.g. Fig. 9) and the internal organisation is of the three tubes around the one central (Figs. 4, 12). The Endamura material has the fultoportulae very close to the spines but whatever the depth of the valve mantle the fultoportulae always appeal' to be slightly less than half-way bet" een the spines and the end of the areolae on the edge ofthe valve mantle. The valve mantle may be "flanged" (Fig. 11) and with vertical slit-like markings which may be merely a remnant of the girdle band. "Fig. 11 also shows that the valve mantle areolae are simple tubes through the siliceous framework whereas in the valve face they are internally enlarged (see ltOUND 1981, Fig. 5). The Alaska sample may have a somewhat steeper valve mantle than the others (Figs. 5, 6). A sample from Sandusky Bay, I.. ake Erie, has the very characteristic internal structure to the fultoportules (Fig. 18) and internal ring of valve face fultoportules ; but is a rather variable form with prominent central fultoportule openings (Fig. ] 6) and no rays between the files of areolae (Figs. 15, 16). In this respect the two valve shapes differ somewhat - the one with the central depression (left-hand one on Fig. 15 and Fig. 16) having almost no separation of the files of areolae. The valves have rather pronounced central elevations/depressions (Fig. 15) and rather unusual areolae between the spines and fultoportules and bars across some of the valve face areolae (Fig. ] 7). I think it must remain as a form of 8. niaqarae. Samples from Henningfield (Figs. ]9,20), Combermere (Figs. 21-2:1), Queen Mary Reservoir (Fig. 24), Farmoor Reservoir (Figs. 25--28), all have smaller valves and the spine/areolaejfultcportule placements are closer than in S. niaqarae types. They also have less prominent rimoportule exit tubes and internally the fultoportules are smaller and of the "are" type and there is not a central ring though there may be isolated fultoportules halfway between the centre and valve mantle. The valve mantle may be spinulose (]'ig. 28), areolae are smaller and more distinctly separated from those of the valve face -- this is especially noticeable in the valves which have a depression internal to the ring of spines (e.g. Fig. 22). A sample from Lough Leven has very close spine insertions on the valve, often at the end of every ray and the fultoportule is extremely close to the spine (Fig. ~O). The valve mantle is shallow (3-4 areolae below the fultoportule) and if the fultoportules were located beneath each spine it would resemble the classical S. rotula. It is however interesting that in this sample the two valve types vary in the frequency of spine insertion and distance below the fultoportule insertion on the two valves (compare Figs. 29 and 30). In this, as in the previous three samples, the internal openings of the fultoportules are only slightly developed and of the "are" type. Rimoportules are scarce and there is a clear ring of valve face fulto-
Some Forms of Stephunodiscus Species
359
portules. A form from Lough Corrib also shows the dimorphism of the valve (compare Figs. 31 and 32). The only other form I have encountered which has closely spaced spines and fultoportules is one from L. Victoria (Figs. 33, 34) but in this form there is a considerable gap between the spines and exit tubes of the portules. This may have to become a new taxon though I am reluctant to take this step at the moment. Forms from Chew Valley Reservoir near Bristol fall within the same section but these have a Iultoportule beneath each spine (Figs. 35, 36). These forms from Henningfield to Chew all fall into what I believe most workers have termed 8. asiraea (cf. HAKANSSON 1976) but since this is an invalid combination (ROUND 1981) a new name must be found. This I do not intend to pursue since the remaining EHRENBBRG material is now under survey by HAKANSSON & LOCKBR (LOCKER, pel's. comm.) and should one of the EHRENBFJRG types have the general form of this section then the name of that will have to he used. What I am fairly certain of, is that the material of KUTZING on which 8. roiula KUTZ. is based seems to be a rather rare taxon and S. rotula should not be used indiscriminately for forms previously ascribed to the non-existent S. astraea, However another form occurring in Abbot's Pool, near Bristol, and tentatively referred to S. rotula by HAl'PEY (1970) is difficult to fit into the above series -- its cells are much more barrel-like (Figs. 37, 39), it has a much flatter valve face (Figs. 38, 40), more files of areolae near the outer valve face (Fig. 40) and a tendency for the development of fultoportules beneath alternate spines (Fig. 38). The cells have rather deep valve mantles with many areolae beneath the fultoportules (Fig. 38), though this seems to decrease somewhat in small cells (Fig. 40). Internally the fultoportules are of the "arc" type. There is also a rather sharp angle between valve face and valve mantle. In this form there seems to be hardly any difference between sibling valves, which is as expected in a cell with flat valves. The only other Stephomodiscus species with such flat valves and "vertical" valve mantle is S. binderanus (Figs. 41-44) but this has the "binderanus" characteristic of forked spines, whilst the flattish S. tenuis and S. hanczsdiii are much smaller. It is of course just possible that this is a post-auxospore stage but it is a taxon which requires further study when the complete EHRENBERG collection has been analysed. Figs. 41/42 and 43/44 also illustrate the 8. binderanus val'. binderamus and S. binderanu8 val'. oestrup1:i forms showing different degrees of valve spine forking (see also S'l'OERMER et a1. 1979). The smallest Stephamodiscus described in HOUND (1981) was S. minutula and modern examples of this species are figured from Slapton Ley (Figs. 45, 46) and Blelham Tarn (Figs. 47,48). They vary little from those in KUTZINO'S original material. In almost all samples of Stephanodiscus populations some cells are found with valves which have a somewhat different structure to the majority - they often have the areolae occluded, sometimes slit-like, as though a skin over the surface had split. Fig. 3 is a possible example in the S. niagarae series. HAKANSSON (1976) also illustrates these in her Figs. I.G.H. and STOERMER et al. (1979) have shown very neatly that in S. binderanue such valves act as "separation valves" to break up the filaments into
360
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short lengths (cf. t he similar sit uat ion in M elosira , (CRAWFOltD 1979). If t his is t he norm al sit uation t hen a problem which has been puzzling me for some years is solved.
Discussion Wi th t he exce ption of t he Abbot's P ool mat erial ("Figs. 37- 40) t he t hree species considered fall into a section of Stepluimodi scus whi ch has distin ctl y conce ntrically undulate va lves - the "gewolbte Schale" of InKAN ss o~ (1976) who uses this fea ture opposed t o "flaehe Schale" as t he first bifurcati on of her key . T his I believe provides a va luable distinction which will help t he ecologist separate out t he small " hantzscliii; tenuis, invisitatus " group in pa rticular, from S . minutula, In R OU ND (1981) a det ailed generic descrip tion was provided and in vestigati on of these form s provides only a few ad ditional points. Th e Lake Victo ria sa mple has t he files of areo lae reduced almost. to a single row, som e va lves can have almost a complete break in the sequence of areolae just internal to the ring of spines, others have some of t he areolae elongated aro und spines or exit tubes of t he portulcs and some h ave gra nula rjs pinulose valves. Th ese features in no \yay alte r the basic concept of the gen us but anyt hing more drastic could hardly be permitted , hen ce the removal of S . tlubius t o a new genus Cyclostephano8 (R OUN D 1982). One new feature emerging from t he pr esent study is that the arra ngement of areolae, spines and fult oportules in anyon e species can vary slightly on sibling valves and t his is parti cularl y noticea ble when t he valve s ar e prominent ly , concentrically waved. In t hese taxa both va lve types should be figured and described iIJ t he specific ac-count. In addition, as point ed out fi rst by ST01'1H1I1E R et al. (1979) t here are separat ion valves, wit h distinct morph ology even in this genm; whic h is not prone to form long filam ent s. It is features like t his, that ha ve confused me and pr ot ract ed t his st udy for which I apo logise to all who have so kindl y sent me samples an d patiently waited for some ans wers . Regre ttably I still feel there are many problems I ha ve not solved and all form s cannot be allocate d t o species. J n particular the lar ge com plex of t he old in valid S . astraea st ill has no name. Th e st udies of H J KAN ~S O N & LU CKl';h on the Stephano discus form s in the EHRE N lJ E1W collection will be invaluable and probably yield a classical name. It, would be a great help if t he tw o valve types of these concent rically waved val ves could hav e some form of design ation. The simplest feature of the two types is the central elevat ion on one and depr ession on the other and so I suggest they be t ermed the elevated valve and the depressed valve. Sugge sted Latin designations are "scut ate " and " lacunat e" . Th e elevated va lve h as the raised edge just in t ernal to the spines and it is t his whi ch, in some species, has t he break in t he files of areolae. Th e depressed va lve h as a more eve n curve in this region and t ends to hav e a more regul ar areola arrangement. A further feature encountered in earlier work (e.g. ROUND 19i2) is the vertically groov ed valve mantl e edge (e.g. in Fig. 2). Thi s feature occur s also in some oth er cellt ric genera , e.g. Biddulphia and is not as I on ce thought a rem nan t of a fimbriat e mantle.
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361
Finall y, to return to the early problem of the distinction between Stephanod?:scus and Oyclotella -- the latter has no ring of spines, though there may be many scattered small spines or nodules, there is always a clearly distinguished central area to the valve face even in species which have some scattered areolae in the centre, though in these cases there are usually also scattered fultoportulae. Internally the structure of the two genera is equally distinctive - in all Cyclotella species there is a chambering of the valve mantle region such that there is the appearance of ribs with apertures between. Often the internal valve face is almost plane with a few fultoportulae and domed cribra. There is, as far as I know, only a single rimoportule in Oyclotella and the exit tubes of the fultoportules tend to be close to the valve mantle edge and very short or even reduced to a circular aperture.
Acknowledgements I wish to thank the many diatomists who have sent me samples over the years, in particular Mr. R. 1. FIRTH, Miss P. A. SIMS, Dr. D. YOUNG and Prof.•J. BRUNEI<.
Literature CARPENTER, E .•r., & GUILLARD, R. R. L.: Intraspecific differences in nitrate half-saturation constants for three species of marine phytoplankton. Ecology 52 (1971): 183 -185. CRAWFORD, R. M.: Filament formation in the diatom genera Melosira C. A. AGARDH & Paralia HEIBERG. Beih. Nova Hedw. 64 (1979): 121-133. HAKANSSON, H.: Die Structur und Taxonomie einiger Stephanodiscus-Arten aus eutrophen Seen Schwedens. Bot. Not. 129 (1976): 25-34. HAPPEY, C. M.: The effects of stratification on phytoplanktonic diatoms in a small body of water. .J. Ecol. 58 (1970): 635-651. KNUDSEN, B. M.: The distribution of 'I'abellaria in the English Lake District. Proc. Int. Ass. Theo· ret. & Appl. Limnol. 12 (1955): 216-218. ROUND, F. E.: The diatom genus Stephanodiscus, An electronmicroscopic view of the classical species. Arch. Protistenk. 124 (1981): 447-462. Cyclostephanos - a new genus within the Sceletonemaceae. Arch. Protistenk. 125 (1982): 323-329. & BROOK, A.•J.: The phytoplankton of some Irish loughs and an assessment of their trophic status. Proc. R. Irish Acad. 60, Sect. B. (1959): 167 -191. STOERMER, E. F., KINGSTON, J. C., & SroKO-GOAD, L.: The morphology and taxonomic relationships of Stephanodiscus binderanue val'. oestrupii (A. CL.) A. CL. Beih. Nova Hedw. 64 (1979): 65-78. Author's address: Prof. Dr. F. E. ROUND, Department of Botany, University of Bristol, Bristol, BS8 lUG, U.K.
24 Arch. Protistenk. Bd. 125
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Explanation of Plates Plate 1. Stephunotliscus niagarae.
1- 4 Endamura material 1. Low magnification outside. Mag. X 1,700. 2. Valve mantle showing striations on mantle edge, pads where spines have fallen off and the external opening of one rimoportule. Mag. X 4,500. 3. Valve mantle showing irregular areolae around the spine bases and fultoportule openings. Mag. X 4,100. 4. Internal view of two fultoportules and a Ringle rimoportule. Mag. X 8,600. 5 - li Alaskan material 5. Outside view of valve. Mag. X 1,200. li. Valve mantle showing small openings of both fultoportnles and a rimoportule. Mag. X4,900.
Plate 2. Siephrinodiecus niagarae from Terrebonne
7. Valve showing distinct "stripes" beneath spines. Mag. X675. Enlargement of valve with raised centre showing irregular areolae in region of spines and internal ring of fultoportnle opening. :'Ilag. X 860. 9. External view of valve with depressed centre showing a very distinctive ring of fnltoportule openings. Mag. X 1,950. 10. Mantle edge showing pads from which spines have fallen and elongate areolae around these. Mag. Xli,OOO. 11. Break in valve mantle showing simple linear passages in the mantle areolae and the expanded edge. Mag. X 5,300. 12. Inside valve mantle showing two fultoportules and one rimoportule. Mag. X 10,900.
s.
Plate 3. Stephanodiscus niaqarae
13 -14 From Klamath 13. External view showing a rut her smooth valve surface and a small areolae. Mag. X 2,liOO. 14. Enlargement of cent.ro showing bars across some areolae. Mag. X 5,300. 15-18 Material from Sandusky Bay, Lake Erie 15. Low power of the two valve morphologies. Mag. X 1,200. HI. Valve showing an internal ring of fultoportule openings (arrow). Mag. X2,100. 17. Valve mantle of the elevated valve form showing bars across some areolae. Mag. X4,900. 18. Internal view showing ring of valve mantle fultoportules and ring of valve face fultoportules. Mag. X 1,100.
Plate 4. Stephnnodiscue sp. previously placed in S. astraea,
19 - 20 From Henningsfield 19. Part of valve showing, pads at base of spines and fultoportule openings near spine. Mag. X6,300. 20. Valve form with scattered areolae around spine region. Mag. X 7,100. 21- 23 From Combermere 21. The two valve types. Mag. X 1,100. 22. Valve form with clear area internal to spines. Mag. X5,100. 23. Edge of other valve type. Mag. X5,300. 24. From Queen Mary Reservoir. Whole cell. Mag. X 1,800.
Some Forms of Stephanodiscus Species
363
Plate 5. Stephanodiscus sp. as previous plate. 25 - 28 From Farmoor Reservoir 25-26. Two valves of the elevated type. Mags. X 1,050 and X2,050. 27. Higher magnification showing external openings of isolated internal fultoportules (arrow). Mag. X 5,300. 28. Form with a spinulose valve mantle. Mag. X 6,400. 29 - 30. From Loch Leven 29. Showing the break in the aerolae internal to the spines. Mag. X 2,100. 30. A form with bars breaking up the areolae. Mag. X 6,400. Plate 6. Stephanodiscus sp. as previous plate 31-32. 33-34-. 35-36. X 5,500
Two valve forms from material from Lough Corrib. Mags. X4,900 and X7,200. Stephanodiscus - unknown species from Lake Victoria. Mags. Xl,500 and X5,100. External and internal view of another S. aetraea type from Chew Valley Lake. Mags. and X 2,300.
Plate 7. Stephanodiscus sp, from Abbot's Pool 37. Two uncleaned cells showing the flat valves. Mag. X2,050. 38. Two valves with steep valve mantles and exit tubes of fultoportules close to spines. Mag. X12,800. 39. Cell immediately after division showing the "barrel" form. Mag. X 2,150. 40. A small celled valve showing the increase in areolae in the files near the valve face margin. Mag. X 5,600. 41- 42. Stephanodiscus binderanus. Two filaments with spines forking to different degree. 41. var. binderanus, Mag. X 4,900. 42. var. oestrupii, Mag. X 3,500. Plate 8. Stephanodiscus binderanus 43. Typical var, binderarcus, Mag. X 5,600. 44. Typical val'. oestrupii. Mag. X 11,300. Stephanodiscus rninutula 45-46. Material from Slapton Ley. Two valve forms. Mags. X5,300 and X5,600. 47 -48. Material from Blelham Tarn. Outside and inside views. Mags. X5,900 and X5,900.
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