A Morphometric Analysis of the Euplotes charon Morphotype (Ciliophora: Euplotida)

Twenty-one different strains of Euplotes of the cirrotype 10, double dargyrome morphotype were collected in marine and brackish habitats from widely varying geographic locations. The strains were cultured under similar conditions prior to fixation and staining. The morphological features considered included the cell lengths and widths, the number and position of all the ventral cim, adoral membranelles and dorsal cilia, and the position of the peristomal shelf. Univariate and multivariate analyses revealed continua in these measurements among the strains, as well as significant correlations between cell length and the number of adoral membranelles, the number of dorsal cilia, and the length, width and area of the oral cavity. Based on data from this study and from previous studies, we conclude that the traditional criteria used for species descriptions within this morphotype are insufficient to provide robust distinctions. Therefore, we submerge all of the descriptions within this morphotype under the nomenclaturally superior Euplotes charon Miiller, 1786. Supplementary key words. Euplotidae, hypotrichs, principal components analysis, species problem, systematics. N E of the goals of taxonomy is t o describe the limits of 0 observable variation in nature by superimposing hierarchical levels of classification upon living organisms, in the hope that the resulting order will consistently reflect evolutionary relationships among them. As molecular phylogenies for protists are not common, the taxonomic divisions a t all hierarchical levels of this group are mostly based on morphological descriptions [9 ,4 I]. More robust methods of ascertaining evolutionary relationships within genera which exhibit asexuality involve molecular techniques in constructing phylogenies (e.g. rRNA sequencing and D N A hybridization). Despite the directness of these techniques, such studies need to examine numerous strains, preferably from widespread locations, t o be confident that any significant differences observed are not simply local extremes of a continuum of genetic variability. The taxonomy of the ciliate genus Euplotes is controversial, with n o consistent guidelines for defining the boundaries o f species [6, 11, 16, 18, 19, 29-35, 47, 51, 521. Assigning species within Euplotes, and within genera containing asexual strains, is problematic i n that reproductive isolation cannot always be used as a diagnostic criterion. General morphological descriptions have been the basis of the vast majority of species de' To whom correspondence should be addressed. scriptions within this and other ciliate genera. There exist, in the literature for the genus Euplotes, several remarkably similar descriptions of supposedly different species; but there have been very few attempts to quantify the morphological observations in these descriptions beyond such gross measures as cell length and/or width [6, 7, 11, 32, 35, 471. As long as morphology is to be used as a mainstay of taxonomy, the analysis of tentative differentiating traits should be performed i n such a way that the conclusions drawn are robust. This is particularly true i n ciliate taxonomy, where differences among strains are often subtle, and traits frequently form continua among strains. Therefore, morphological descriptions need to be quantitative and statistical in order t o avoid subjectivity and bias, and they should focus on the observed limits o f variation of the considered traits, rather than just the central tendencies [ 161. Furthermore, bias can arise, even in quantitative analysis, i n choosing only a few characters to examine. Such trait-dependent biases are lessened through multivariate analysis, in which several attributes are examined simultaneously. Examples of the power of multivariate ciliate morphometrics are studies o f morphological stability over time [22, 231 and of growth cycle changes [45] i n the Tetrahymena pyriformis complex, of syngenic ascertainment in the Paramecium aurelia complex of sibling species [24, 27, 381, and of ventral cirral patterns [15, 17, 18, 511, dorsal ciliary patterns [20], and population differentiation [2 11 in Euplotes populations.