Hydrobiologia 417: 25–35, 2000. V. Alekseev, G.A. Wyngaard & F. D. Ferrari (eds), Advances in Copepod Taxonomy. © 2000 Kluwer Academic Publishers. Printed in the Netherlands. 25 Components of morphological variation in Baikalian endemial cyclopid Acanthocyclops signifer complex from different localities Dmitry Lajus * & Victor Alekseev Zoological Institute of the Russian Academy of Sciences, Universitetskaya nab.1, 199034, St. Petersburg Russia E-mail: dlajus@popbio.zin.ras.spb.ru; E-mail: victor@zoology.spb.ru Key words: morphological variation, fluctuating asymmetry, taxonomy, Copepoda, Cyclopidae, Acanthocyclops signifer, Lake Baikal Abstract Use of traditional methods for morphological studies only permits the analysis of a small part of the information embodied in morphological structures. Besides comparing populations using the mean values of characters which allows one to estimate their morphological similarity, analysis of variation among individuals within a population can be informative. Variation among individuals consists of factorial and stochastic components. The factorial component is an upper estimate of genetic heterogeneity and thus permits one to evaluate the population’s adaptab- ility. The stochastic component (estimated by fluctuating asymmetry, i.e. random deviations from perfect bilateral symmetry), being a measure of developmental stability, is an indicator of a population’s fitness. Assessment of measurement error is necessary for assessment of the true value of the stochastic component and for selection of the most informative characters. Such analysis allows one to extract additional information from morphological data in comparison with methods traditionally used on copepods. This approach was applied to an analysis of morphological variation in the study of the Baikalian endemic cyclopoid Acanthocyclops signifer (Mazepova) from three different isolated localities. Characters typically used in studies of taxonomy of this group are considered here. Measurement error was rather high (more than 50% of the stochastic component), which can be explained by technical difficulties of measuring the characters. All populations differ in the mean values of the characters. This shows the taxonomic heterogeneity of this group and reveals the necessity of its taxonomic revision. Populations also differ in the level of stochastic and factorial components of the total variance. The data are interpreted from the point of view of taxonomy and the possible evolution of the group. Introduction Much attention traditionally has been paid to morpho- logical variation in studies of taxonomy and evolution of the Copepoda. Copepods have many morphological structures with complex shapes and these are rather convenient for different analyses. However, the mor- phological methods only permit analysis of a small part of the information embodied in morphological structures. A significant part of the information is not available for analysis. The goal of the present study is to apply some new approaches that permit extraction of additional information from morphological data tra- * Author for correspondence ditionally used for the study of a Baikalian endemic copepod, Acanthocyclops signifer Mazepova, 1978. A. signifer is a common species inhabiting both the near-shore and deep water zones of Lake Baikal. Prob- ably, it is a meiobenthic species that inhabits biotopes with various sediments (Mazepova, 1978). A. signifer exhibits high morphological diversity, which is typical also for many other Baikalian endemic invertebrates: other Cyclopoida (Mazepova, 1978), Amphipoda (Bazikalova, 1945), Ostracoda (Mazepova, 1989), Nematoda, Turbellaria (Tsalolikhin, pers. comm.). Special attention was paid to explaining the high level of morphological variability within these groups. Pos- sible reasons for the geographical and biotopical isol- ation that are typical of Baikal fauna were suggested by Mazepova (1978). But until now, no experimental