The Past, the Present and the Future of Eutardigrade Taxonomy
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J. Limnol., 2013; 72(s1): 1-7 DOI: 10.4081/jlimnol.2013.s1.e1 The past, the present and the future of eutardigrade taxonomy Giovanni PILATO* Dipartimento di Scienze Biologiche, Geologiche e Ambientali, Università di Catania, via A. Longo 19, 95125 Catania, Italy *Corresponding author: [email protected] ABSTRACT The Author first recalls the past of eutardigrade taxonomy and indicates the main factors that for a long time restrained its progress. One consequence of a superficial analysis is that very wide individual variability has been erroneously attributed to many species, and this has become the main problem for tardigrade taxonomists. The situation began to change after 1969 because of the first attempts to eliminate the above mentioned problems. Novelties gave impetus to the revision of tardigrade taxonomy, and genuine systematics, fi- nally based on phylogenetics, became popular. Today the morphological characters are considered more in depth and studies utilising DNA sequences are more in fashion; they are surely useful and allow the distinction of morphologically very similar species. However, in the author’s opinion, this progress will be possible only if the molecular studies are always associated with careful morphological studies, which, in the meantime, will be more and more detailed also thanks to the use of theonly Scanning Electron Microscopy (SEM). Key words: tardigrade, taxonomy, evolution. A HISTORICAL EXCURSION lected or indicated subjectively (small, large, longer, shorteruse); in other cases they were indicated in absolute Tardigrade taxonomy in the past value or related to the body length (ms index), or to the I am sufficiently old, unfortunately, to be able to say pharyngeal bulb length (cph index), thus being nearly un- something on the past history of tardigrade taxonomy, as usable as the body and the pharyngeal bulb may vary in well as its present and future. At the beginning of my length due to a different degree of compression under the study of these very interesting animals (1965), I realised microscope cover slip. The drawings of the past descrip- from the literature and from the examination of many tions often do not help because they were too generic, or species from various geographic areas, that, notwithstand- the structures were oriented in an inappropriate position. ing a first phylogenetic approach of Thulin (1928), tardi- Unfortunately, the type material of many old species got grade taxonomy was absolutely, and expressly, phenetic; lost for various causes or never existed because some au- therefore, I tried to understand what rendered it unsatis- thors did not mount permanent slides. Besides, many au- factory. Until the beginning of the twentiethcommercial century de- thors, while describing a new species, did not give a scriptions of tardigrade taxa were usually brief and thorough description, limiting themselves almost to men- defective, even those of the most careful authors, such as tioning only the characters by which that species differed Murray (1905, 1906, 1907, 1910, 1911) and Thulin (1911, from the already known ones. As a consequence, it might 1928). Few characters Nonwere taken into consideration, have occurred that an author found a new species, equal whereas others were totally neglected. This fact clearly to an already known one for the characters indicated in comes out in Marcus’s monographs (1928, 1936) and Ra- the description, but different for other characters not care- mazzotti’s monographs of 1962 and of 1972. It can be fully described or totally neglected; the result might have noted that there was no notice about the buccal armature; been that the new species was not recognised and the about the apophyses for the insertion of the stylet muscles specimens attributed to the already known species. Com- on the buccal tube of the Hypsibiidae, it was reported only bining different species under the same name had quite that they could be ridge- or hook-shaped, without any serious consequences: the differences that someone could other specification; the presence of a cuticular bar on the note, between what were believed to be populations of the legs was reported for only two species of Isohypsibius same species, were attributed to intraspecific variability. (which in that monograph was still considered a sub- The same mistake might occur many times, thus produc- genus); the presence of a spiral thickening in the pharyn- ing a vicious circle. All this led scholars to believe that geal tube in the species of Diphascon (considered as a tardigrade species were usually quite variable, thus feed- subgenus of Hypsibius too) was reported for just five of ing that vicious circle and creating great confusion. It is the 32 species known at the time for that taxon; about the also possible that a new species was described because it claws, only their shape, not the structure, was taken into appeared different from a known species for a character consideration; the metric characters were sometimes neg- overlooked in the description of the latter. 2 G. Pilato That confusion led to another consequence: some an- it is not equally certain that Macrobiotus always has buccal imals collected in very distant geographic areas (even dif- tube with ventral lamina and that it is always lacking in ferent zoogeographic regions) have been attributed to the Hypsibius […] as a matter of fact, not all the species have same species, with very wide geographic distribution at- been examined from this point of view, and, besides, we tributed to it. This sort of error affected plenty of species. know that species of Hypsibius in which the ventral lamina The possibility of passive dispersal let that belief appear is present exist: they are those species which Pilato gathers justified and this made confusion increase for it induced in the genus Doryphoribius. It is therefore evident that Ra- attribution to intraspecific variability of all the slight dif- mazzotti did not conceive that a possible tardigrade with ferences that could be noted between populations living all characters as Macrobiotus but without ventral lamina in even very distant geographic areas. should simply be ascribed to a different genus, nor did he For many decades, the milestones for the study of tardi- conceive that a tardigrade with all characters as Hypsibius grades were the monographs of Marcus (1928, 1936) and but with ventral lamina could not be ascribed to Hypsibius of Ramazzotti (1962, 1972), and then also that of Ramaz- but to a different genus. The fact that a species with ventral zotti and Maucci (1983). In their monographs, those authors lamina had been ascribed to the genus Hypsibius was for reported practically all data from the previous literature. In him a definitive truth, while in my opinion that species had the paper that opened the 3rd International Symposium of simply to be transferred from Hypsibius to another genus. 1980 in Johnson City, Tennessee (USA), Ramazzotti and It must also be stressed that Ramazzotti overlooked the fact, Maucci (1982) wrote about Marcus’s monographs: The di- which I realised, that the presence or absence of the ventral agnosis and discussion of each species was extremely ac- lamina always implies otheronly differences; as a matter of fact, curate, particularly for the oldest and therefore more if the ventral lamina is lacking, the buccal tube has ventral controversial species; for which type material is usually and dorsal apophyses for the insertion of the stylet muscles non-existent or unusable. Marcus’s descriptions are ade- so that the symmetry of the buccal-pharyngeal apparatus quate today, particularly those descriptions of many com- with respect touse the frontal plane changes completely. Be- mon and widespread species. For them, modern taxonomy sides, the idea that before instituting a genus it is necessary was based on Marcus’s revision more than on the original to know first all the species cannot be supported; in that descriptions, which are sometimes not very accurate, and case, no genus could ever be instituted because it is never often scattered in virtually unobtainable publications. […] possible to be sure of knowing all the existing species (in- At present for all these species, and others as well, a con- cluding also the extinct ones). In my opinion, and not only solidated taxonomy is based on Marcus’s interpretations. mine, it is instead necessary to manage to establish, on the It is therefore evident that in Ramazzotti’s (1962, 1972) and basis of our experience, which characters individualise dis- Ramazzotti and Maucci’s (1983) monographs, the notes on tinct phyletic lines. Obviously, one must be able to evaluate many species came from Marcus’s interpretations and I characters, and must be also ready to change their opinion wonder what Marcus’s revision exactly means, since this if novelties prove the past choices wrong. author often did not have the type materialscommercial or even the During the first years of silent study, I became con- original descriptions of the species in hand. It cannot be ex- vinced that it was no longer possible to unconditionally cluded that, at least in some cases, his interpretations might accept all the data in Marcus’s and Ramazzotti’s mono- have given the idea of some characters which did not cor- graphs, and I found essential two things: to always take respond to the concrete exactly. NonNotwithstanding the nov- into consideration primarily the original description of a elties which had been increasing (Pilato, 1969) about the species, though insufficient and defective, and to build as systematic arrangement of Eutardigrada, in the 1972 edition rich as possible personal collection. I therefore started of his monograph, Ramazzotti followed the classic works pestering colleagues asking them for specimens of the of Marcus. I think that in a historical essay it is opportune species they had found, not only those described by them.