Phycologia (1999) Volume 38 (6), 437-495 Published 10 December 1999 PHYCOLOGICAL REVIEWS 18 The species concept in diatoms DAVID G. MANN* Royal Botanic Garden, Edinburgh EH3 5LR, Scotland, UK D.G. MANN. 1999. Phycological reviews 18. The species concept in diatoms. Phycologia 38: 437-495. Diatoms are the most species-rich group of algae. They are ecologically widespread and have global significance in the carbon and silicon cycles, and are used increasingly in ecological monitoring, paleoecological reconstruction, and stratigraphic corre­ lation. Despite this, the species taxonomy of diatoms is messy and lacks a satisfactory practical or conceptual basis, hindering further advances in all aspects of diatom biology. Several model systems have provided valuable insights into the nature of diatom species. A consilience of evidence (the 'Waltonian species concept') from morphology, genetic data, mating systems, physiology, ecology, and crossing behavior suggests that species boundaries have traditionally been drawn too broadly; many species probably contain several reproductively isolated entities that are worth taxonomic recognition at species level. Pheno­ typic plasticity, although present, is not a serious problem for diatom taxonomy. However, although good data are now available for demes living in sympatry, we have barely begun to extend studies to take into account variation between allopatric demes, which is necessary if a global taxonomy is to be built. Endemism has been seriously underestimated among diatoms, but biogeographical and stratigraphic patterns are difficult to discern, because of a lack of trustwOlthy data and because the taxonomic concepts of many authors are undocumented. Morphological diversity may often be a largely accidental consequence of physiological differentiation, as a result of the peculiarities of diatom cell division and the life cycle. CONTENTS I. Introduction 438 II. Historical review 44 1 a. Pre-Darwinian concepts of species in diatoms 44 1 b. Post-Darwinian concepts of species in diatoms 44 1 c. Fantasies 443 d. Currents and countercurrents: the last 25 years 444 III. Evidence concerning the nature of diatom species 447 a. Detailed studies of morphology and morphometrics 447 b. Complications of polymorphism and phenotypic plasticity 45 1 c. Morphological changes brought about during the normal life cycle 457 d. Physiological and genetic evidence 459 e. Variation in reproductive behavior: Geitler's 'Sippen' 462 f. Ecological differentiation and discrimination by parasites 463 IV. To wards a synthesis concerning the nature of diatom species 465 a. Interpretation of discontinuous infraspecific variation in sympatry 465 b. Extension to allopatry 47 1 c. Speciation, evolution, and biogeography 472 V. Conclusions 482 VI. Acknowledgments 483 VII. References 483 * E-mail: [email protected] 437 438 Phycologia, Vol. 38 (6), 1999 [T]he determination of species in the Diatomaceae is at all times a task of difficulty, ... much enhanced when the observer does not enjoy the opportunity of examining these organisms in their living state, or has only prepared or fossil specimens, within reach of his observations (Smith 1855) Diatoms! you might as well collect and attempt to classify wall-papers (unattributed remark in Comber 1897) Scis quod dicunt: uno diatomate viso, omnia visa sunt (adapted from Beard 1990) I. INTRODUCTION atom taxonomy has become, by reference to a group of small­ celled Stephanodiscus Ehrenberg species. Here, misidentifi­ At least two dangers face anyone who tries to review the cation, nomenclatural errors, changes in the circumscription species concept with reference to a particular group of organ­ of taxa, and a failure to document research properly have cre­ isms, such as the diatoms. One is that the review focuses so ated an impressive body of information about a cosmopolitan strongly on the practicalities of species recognition in the diatom that does not exist! Without greater attention to ·the group that it becomes little more than a list of useful char­ species concept in diatoms, such problems will recur and get acters, of no general interest. The other is that the review worse (see, for instance, section II d). becomes an essay on species concepts in general, peppered The species concept is one of the most widely debated with examples and illustrations from the group in question, themes in biology and no single review can now do justice to chosen not because of their intrinsic worth, but because they the literature. The controversies that still surround the subject are the only ones available that fall within the remit of the could suggest that we are attempting to define the undefinable, review. I have tried to follow a middle course, with the extra or that we have not grasped some essential point about the danger that I will please no one. I believe the attempt is jus­ evolution of diversity; thus, having asked the wrong questions, tified for three reasons. First, diatoms are remarkably abun­ we are inevitably dissatisfied with the answers. Or, the fuss dant, playing a very important part in global cycling of many may be justified and the nature of species and speciation may elements, but particularly C and Si. Recent estimates of global be the central issue in evolutionary biology, so that Mayr net primary production suggest an overall total of 105 Pg (105 (1963) could write "species are the real units of evolution, as X 1015 g) of carbon fixed per year, of which 46% is oceanic the temporary incarnation of harmonious, well-integrated gene and 54% is terrestrial (Field et at. 1998). Of the oceanic com­ complexes ... [without] speciation, the production of new ponent, approximately one-quarter takes place in oligotrophic gene complexes capable of ecological shifts, ... there would regions where diatoms account for no more than 25-30% of be no diversification of the organic world, no adaptive radi­ primary production (Nelson et at. 1995); but in highly pro­ ation, and very little evolutionary progress. The species, then, ductive areas, diatoms predominate. Overall, therefore, it is the keystone of evolution". Or, perhaps species have no might not be unreasonable to estimate that diatoms could ac­ reality, so that defining them is arbitrary, although they may count for between 40 and 45% of oceanic production, pro­ nevertheless be useful for communication. One advocate of ducing perhaps 20 Pg of carbon per year, making them more this nominalist standpoint suggested that: "species have no productive than all the world's tropical rainforests (Field et al. actual existence in nature. They are mental concepts and noth­ 1998). Diatoms play a particularly important part in sustaining ing more ... conceived in order to save ourselves the labor fisheries, as the maj or producers of 'new' phytoplankton bio­ of thinking in terms of individuals, and they must be so mass, sustained by fluxes of nutrients from deeper waters as framed that they do save us labor" (Bessey 1908). The con­ opposed to nutrients recycled via the 'microbial loop' (Fal­ tinuing uncertainty about species certainly contradicts the as­ kowski et ai. 1998). Diatoms are also the major sources of surance of a recent dictionary, which reports modem usage of biogenic silica, transforming dissolved orthosilicic acid into the hydrated amorphous silica of their frustules; their contri­ 'species' for "a taxonomic grouping ranking next below ge­ bution to the global biogeochemical cycle of silicon greatly nus and subgenus, which contains organisms that are uniquely outweighs that of silicoflagellates and radiolaria (data of Lis­ distinguished from others by certain shared characteristics and itzin: Treguer et at. 1995). Thus, understanding diatoms and usually by an inability to interbreed with members of other their species diversity can scarcely be irrelevant ecologically, such groupings" (Brown 1993; actually, I think this is rather especially in the context of enhanced atmospheric CO2 levels good!). Similar definitions are found in many basic biology (Smetacek 1999). Second, diatoms are found in almost all texts (e.g. Roberts 1976; Villee et at. 1989; Avila 1995), al­ aquatic and many subaerial habitats, and exist in a huge va­ though in others the approach is more circumspect (e.g. riety of shapes and sizes; even on conservative estimates, they Campbell 1987). contain tens of thousands of species (e.g. Gordon & Drum The principal focus of discussion during the last 60 years 1994), yet the group has apparently existed only since the has been the 'biological species concept' (BSC) and it is the early Mesozoic (Medlin et ai. 1997). Their explosive diver­ assimilation of this concept into common parlance that is re­ sification demands an explanation, which must include how flected by the dictionary definition given above, which differs diatoms speciate. Third, diatoms are increasingly used in from earlier definitions in similar dictionaries (e.g. Craigie paleoecological reconstruction and ecological monitoring 1919: a species is "a group or class of animals or plants ... (Stoermer & Smol 1999), in which accurate identification of having certain common and permanent characteristics which species and knowledge of geographical distributions are es­ clearly distinguish it from other groups"). Various definitions sential. Here, the current state of diatom taxonomy at the spe­ of 'biological species' have been offered in the scientific lit­ cies level is a serious impediment to further progress.A recent erature. One of the most