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Keywords:

  • diatom flora;
  • East Africa;
  • homonymy;
  • replaced name;
  • taxonomy

Summary

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and Methods
  5. Results and Discussion
  6. Acknowledgments
  7. References
  8. Supporting Information

Based on epilithic diatom samples collected from the rocky littoral zone of Lake Malawi (102 diatom taxa belonging to 34 genera were listed in the Supporting Information) we proposed the transfer of three taxa to new genera. Afrocymbella brunii (Fricke) comb. nov. was transferred from Gomphonema because of its dorsiventral valve and its transapically elongated dorsal stigma. Afrocymbella rossii (Kociolek & Stoermer) comb. nov. was also transferred from Gomphocymbella, which is actually a synonym of Gomphonema. Aulacoseira euareolata (O.Müller) comb. nov. et nom. nov. was transferred from Melosira because of the presence of linking spines and mantle areolae, and its specific epithet was replaced because of homonymy with Aulacoseira areolata Moisseeva.


Introduction

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and Methods
  5. Results and Discussion
  6. Acknowledgments
  7. References
  8. Supporting Information

In the African Great Lakes, diatoms are frequently abundant among the periphytic algae and are consumed selectively by some cichlid fishes (Yamaoka 1983; Genner et al. 1999). Floristic information on epilithic diatoms is thus important for studies on the food habits and microhabitats of cichlid fishes. In this study, we conducted underwater sampling of epilithic diatoms in the rocky littoral zone at seven sites in Lake Malawi, where cichlid fishes are abundant and diverse (see Appendix S1–12 in the Supporting Information for a full species list and microscopic photographs). To the best of our knowledge, the epilithic diatom flora of the rocky littoral zone has never been studied intensively, although differences in species composition between the pelagic and epilithic algae of Lake Marawi have been suggested (Vyverman & Cocquyt 1993).

No studies of the diatom flora of this region have been conducted since Müller (1903, 1904, 1905, 1910) and Hustedt (1949), with the exception of some piecemeal studies on the taxonomy of genera such as Cyclostephanos and Stephanodiscus (Klee & Casper 1992, 1995). During recent decades, the taxonomy of diatoms has been repeatedly reconsidered (Krammer & Lange-Bertalot 1986, 1988, 1991a,b; Round et al. 1990), but no corresponding revision of the Malawian diatom flora has been done. On the basis of a detailed examination of the diatom flora, taxonomic recombination some taxa that were previously assigned to the genera Gomphonema, Gomphocymbella and Melosira became necessary. Therefore, we proposed the transfer of three taxa to new genera.

Materials and Methods

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and Methods
  5. Results and Discussion
  6. Acknowledgments
  7. References
  8. Supporting Information

Lake Malawi is located at the southernmost end of the Great Rift Valley in Africa between Malawi and Mozambique with a surface area of approximately 30 000 km2 and a maximum depth of 700 m (Fig. 1). It is a meromictic lake in a subtropical region and the average surface water temperature is 24–27°C with an average pH of 7.7–8.6 throughout the year (Beauchamp 1953; Msiska 2001; Guildford et al. 2003).

figure

Figure 1. The locations of seven sampling sites in the rocky littoral zone of Lake Malawi.

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Diatom samples were collected from the rocky littoral zone at seven sites along the Malawian shore of the lake: Chilumba (10°40′S, 34°25′E), Likoma (12°07′S, 34°75′E), Chembe (14°02′S, 34°81′E), and Boadzulu (14°25′S, 35°14′E) during July−September, 2006, and Nkhata Bay (11°06′S, 34°30′E), Kande (11°64′S, 34°31′E), and Maleri (13°90′S, 34°62′E) during August, 2008 (Fig. 1). At each sampling site, stones that measured approximately 30 cm in diameter were collected at depths of 3–7 m by SCUBA diving. The algae present in 3 × 3 cm squares on the stone's surfaces were brushed off and fixed in 10% formalin. The algae were sampled from the upper, lateral, and lower sides of each stone to include various environmental conditions, such as differences in light availability. Two species of cichlid fish, Labeotropheus fuelleborni Ahl and L. trewavasae Fryer, were also caught using gill nets at the same locations during July to September, 2006. The gut contents of 15 individuals from each species were used as additional periphyton samples because these cichlids are known to be specialized epilithic algae feeders that seldom migrate along the shore (Konings 1991).

The samples were heated with sulfuric acid, and potassium nitrate was added to remove any organic materials from the diatom frustules. The cleaned frustules were mounted on slides in Pleurax mounting medium (refraction index >1.50). At least 400 valves were observed in each sample. An Olympus BX61 light microscope equipped with a Nikon DS-Fil digital camera was used for diatom identification, measurement, and imaging. A field emission scanning electron microscope (JSM-6301F, JEOL, Tokyo, Japan) was also used for species identification, as necessary.

Results and Discussion

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and Methods
  5. Results and Discussion
  6. Acknowledgments
  7. References
  8. Supporting Information

We found 102 diatom taxa, which belonged to 34 genera (see Appendix S1–12Supporting information in the Supporting Information for the full species list and microphotographs). The three taxa described below represent new nomenclatural combinations as a result of their generic reassignment proposed herein.

Afrocymbella brunii (Fricke) comb. nov. (Figs 2, 3)

figure

Figure 2. Afrocymbella brunii. Light microscopy (LM). Bar: 10 μm.

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figure

Figure 3. Afrocymbella brunii. Scanning electron microscopy (SEM). a, c, e, External views. b, d, f, Internal views. (a) Head pole without apical pore field. (b) Head pole showing helictogrossa. (c) Valve center showing the round dorsal stigma and central pores. (d) Valve center showing the transapically elongated dorsal stigma and antler-shaped raphe fissure. (e) Foot pole showing the apical pore field and terminal raphe fissure. (f) Foot pole showing helictogrossa. Bars: 1 μm.

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Basionym

Gomphonema brunii Fricke (1902) in Schmidt et al. (1874–1959) fig. 238: 12–14.

Synonym

Gomphocymbella brunii (Fricke) O.Müller (1905).

Length 37–114 μm, width 8–20 μm; striae 10–15 in 10 μm, areolae 13–26 in 10 μm.

We found a wide variation in size among the strongly dorsiventral Afrocymbella specimens collected from Lake Malawi. According to the maximum likelihood method, their bivariate length-width distribution was better approximated by a two-component mixture of bivariate lognormal models (AIC = −156.9) rather than by a single bivariate lognormal model (AIC = −151.4) (Fig. 4). This indicated that the specimens comprise two groups. The group consisting of larger specimens was identified as Afrocymbella pergracilis Krammer (Appendix S3 a–b). The specimens in the other group were highly variable in their length, width and areola density, but the distributions of these attributes showed no discontinuity (Figs 4, 5); therefore, we regarded them as morphological variations within a single species. The extremely wide variation of the striae density is not normal in raphid diatoms, but it has been reported in Cymbella ehrenbergii Kützing (Kawashima & Mayama 2001).

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Figure 4. Relationship between valve length and width of the strongly dorsiventral Afrocymbella specimens of two species collected from Lake Malawi. Confidence ellipses were estimated by the maximum-likelihood method, assuming a two-component mixture in bivariate lognormal distributions. ○, Afrocymbella brunii, with a solid 95% confidence ellipse; ×, Afrocymbella pergracilis, with a dashed 95% confidence ellipse.

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Figure 5. Scatter plots of striae density (a) and areola density (b) versus the valve length for Afrocymbella brunii with respective 95% confidence ellipses.

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Gomphocymbella, with its type species Gomphonema vulgaris (Kützing) Rabenhorst, is synonymized with Gomphonema, because the type species is very similar to Gomphonema subclavatum Grunow. Therefore, the genus Gomphocymbella is actually a synonym of Gomphonema and hence any endemic African taxa assigned to Gomphocymbella require revisions (Krammer 2003). The dorsiventral valves and the transapically elongated dorsal stigma show clearly that G. brunii belongs to the genus Afrocymbella (Krammer 2003).

Krammer (2003) included two specimens of Gomphocymbella (Gomphonema) brunii with narrow valves (Schmidt et al. 1874–1959; fig. 238: 13, 14) in the synonymy of Afrocymbella beccarii (Grunow) Krammer. However, our specimens at a similar length showed a wide variation of valve width (Fig. 4), which corresponded to the type drawings of Gomphonema brunii (see Schmidt et al. 1874–1959; fig. 238: 12, 13). Moreover, many of our specimens were larger than the initial cells of Afrocymbella beccarii (maximum, 75 μm long and 14.1 μm wide; Krammer 2003) and they corresponded better with the description of Gomphocymbella brunii (33–99 μm long and 12.5–25 μm wide) given by Müller (1905), although he may have confused them partly with other taxa that possess broader valves such as Afrocymbella pergracilis Krammer. We regarded the present taxon as Afrocymbella brunii (Fricke) comb. nov., which is distinct from Afrocymbella beccarii.

This taxon was observed at Chilumba, Nkhata Bay, Kande, Likoma, Maleri, Chembe, and Boadzulu.

Afrocymbella rossii (Kociolek & Stoermer) comb. nov. (Fig. 6)

figure

Figure 6. Afrocymbella rossii. Light microscopy (LM). Bar: 10 μm.

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Basionym

Gomphocymbella rossii Kociolek and Stoermer (1993) p. 81, fig. 31–33, 41, 42.

Length 29–35 μm, width 5–7 μm; striae 11–15 in 10 μm.

The observed valves tended to be wider and the striae were somewhat finer than in the original description, although the characters were mostly consistent with the latter. The transapically elongated dorsal stigma (see Kociolek & Stoermer 1993) shows clearly that this taxon belongs to Afrocymbella (Krammer 2003), whereas it does not belong to the genus Gomphocymbella, which is a synonym of Gomphonema (see above).

This taxon was observed at Nkhata Bay, Kande, Maleri, and Boadzulu.

Aulacoseira euareolata (O.Müller) comb. nov. et nom. nov. (Fig. 7)

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Figure 7. Aulacoseira euareolata. a–b, Light microscopy (LM). c–d, Scanning electron microscopy (SEM). (a–c) Overall view. (d) linking spines. Bars: a–c, 10 μm; d, 1 μm.

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Basionym

Melosira areolata O.Müller (1904) p. 288, fig. 19.

Mantle height 14–17 μm, diameter 10–21 μm; mantle striae 5–7 in 10 μm.

The ratio of the mantle height to the diameter was 0.6–1.2. The mantle areolae were in quincunx and formed straight or slightly sigmoid striae. The presence of linking spines shows clearly that this taxon belongs to the genus Aulacoseira (Crawford 1981).

Moisseeva (1971) named a fossil diatom Melosira areolata, but this was illegitimate because it was a later homonym of M. areolata O.Müller (1904). Moisseeva (1986) transferred the former to Aulacoseira and the transferred name Aulacoseira areolata Moisseeva was legitimate according to IAPT Art. 58-1. Thus, the new combination of M. areolata O.Müller, resulting from the reassessment of this species to Aulacoseira, required a new name.

This taxon appears very similar to Aulacoseira canadensis (Hustedt) Simonsen (= Melosira canadensis Hustedt; Simonsen 1987, p. 381, fig. 577: 1–4) under light microscopy (LM), but it has spatulate-rhombic linking spines and areolae that comprise volae without cribra, whereas A. canadensis has triangular linking spines and areolae with cribra (cf. Haworth & Sabater 1993, figs 19–27). Simonsen (1979) noted that this taxon is possibly a later synonym of Aulacoseira nyassensis (O.Müller) Simonsen, because they share similar ultrastructures. However, we maintain them as separate because the former has externally hollow and almost round areolae, whereas the latter has almost flat and elongated ones.

This taxon was found only in cichlid gut content samples from Chembe and Boadzulu.

Acknowledgments

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and Methods
  5. Results and Discussion
  6. Acknowledgments
  7. References
  8. Supporting Information

We sincerely thank Mr R. Zatha, Mr Y. Kazembe, and Mr A. Mangawa for their support in collecting samples, and Dr M. Yuma for his logistical support during the study. Ms M. Umemoto, Mr Y. Nakamura, and Ms E. Ishizumi supported our laboratory studies. We are deeply grateful to Dr M. J. Grygier and the anonymous reviewers and editors for their constructive comments on the manuscript. Financial support was provided by the Joint Research Center for Science and Technology of Ryukoku University.

References

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and Methods
  5. Results and Discussion
  6. Acknowledgments
  7. References
  8. Supporting Information
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  • Crawford, R. M. 1981. The diatom genus Aulacoseira Thwaites: its structure and taxonomy. Phycologia 20: 174192.
  • Genner, M. J., Turner, G. F. and Hawkins, S. J. 1999. Foraging of rocky habitat cichlid fishes in Lake Malawi: coexistence through niche partitioning? Oecologia 121: 283292.
  • Guildford, S. J., Hecky, R. E., Taylor, W. D., Mugidde, R. and Bootsma, H. A. 2003. Nutrient enrichment experiments in tropical great lakes Malawi/Nyasa and Victoria. J. Great Lakes Res. 29: 89106.
  • Haworth, E. Y. and Sabater, S. 1993. A new Miocene Aulacoseira species in diatomite from the ancient lake in La Cerdanya (NE Spain). Nova Hedw. Beih. 106: 227242.
  • Hustedt, F. 1949. Süsswasser-Diatomeen. In Damas, M. H. (Ed.) Exploration du Parc National Albert. Institut des parcs nationaux du Congo belge, Bruxelles, pp. 1199.
  • Kawashima, A. and Mayama, S. 2001. Diatom from Akan-ko (Lake Akan) in Hokkaido, Japan. 8. Raphid diatoms: Cymbella, Encyonema, Gomphoneis, Gomphonema, Gomphosphenia, Reimeria. Natl. Environ. Sci. Res. 14: 89109.
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Supporting Information

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and Methods
  5. Results and Discussion
  6. Acknowledgments
  7. References
  8. Supporting Information
FilenameFormatSizeDescription
pre12036-sup-0001-appendixs1.doc241K

Appendix S1. List of epilithic diatoms in the littoral zone of Lake Malawi.

pre12036-sup-0002-appendixs2.tif3671K

Appendix S2. Microscopic views of Achnanthes inflataAfrocymbella brunii.

pre12036-sup-0003-appendixs3.tif3485K

Appendix S3. Microscopic views of Afrocymbella pergracilis–Cavinula scutelloides.

pre12036-sup-0004-appendixs4.tif2781K

Appendix S4. Microscopic views of Cocconeis euglypta–Cyclostephanos malawiensis.

pre12036-sup-0005-appendixs5.tif3615K

Appendix S5. Microscopic views of Cymatopleura librileDiploneis smithii.

pre12036-sup-0006-appendixs6.tif3949K

Appendix S6. Microscopic views of Encyonema geissleraeGomphonema parvuloides.

pre12036-sup-0007-appendixs7.tif3920K

Appendix S7. Microscopic views of Gomphonema sphaerophorumNavicula sp. 4.

pre12036-sup-0008-appendixs8.tif1882K

Appendix S8. Microscopic views of Nitzschia aequalisN. obsoleta.

pre12036-sup-0009-appendixs9.tif1608K

Appendix S9. Microscopic views of Nitzschia paleaceaN. sp. 6.

pre12036-sup-0010-appendixs10.tif2666K

Appendix S10. Microscopic views of Placoneis gastrumRhopalodia gracilis.

pre12036-sup-00011-appendixs11.tif2981K

Appendix S11. Microscopic views of Sellaphora malombensisStephanodiscus nyassae.

pre12036-sup-0012-appendixs12.tif2969K

Appendix S12. Microscopic views of Surirella gradiferaUlnaria ulna.

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