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Background: Cryptic species are two or more distinct but morphologically similar species that were classified as a single species. During the past two decades we observed an exponential growth of publications on cryptic species. Recently published reviews have demonstrated cryptic species have profound consequences on many biological disciplines. It has been proposed that their distribution is non-random across taxa and biomes. Results: We analysed a literature database for the taxonomic and biogeographical distribution of cryptic animal species reports. Results from regression analysis indicate that cryptic species are almost evenly distributed among major metazoan taxa and biogeographical regions when corrected for species richness and study intensity. Conclusion: This indicates that morphological stasis represents an evolutionary constant and that cryptic metazoan diversity does predictably affect estimates of earth's animal diversity. Our findings have direct theoretical and practical consequences for a number of prevailing biological questions with regard to global biodiversity estimates, conservation efforts and global taxonomic initiatives. Background Cryptic species are two or more distinct species that were classified as a single species due to their morphological similarity. Facilitated through technical advances such as PCR and direct DNA sequencing, many phylogenetic, phylogeographic and population genetic studies in the last two decades discovered – frequently without a priori intention – genetically divergent but morphologically cryptic lineages. These discoveries continue to increase exponentially and raise a number of general questions, such as: How frequent are cryptic species? Are cryptic species evolutionary young? Is morphological stasis upon speciation more often found in environmental extremes, such as the tropics, the artic or the deep sea? A recent review proposed that the distribution of cryptic species is non-random across taxonomic groups and biomes, which might have substantial consequences for biodiversity assessments, macroecology, biogeography, conservation management and evolutionary theory [1]. Biodiversity estimates of certain taxonomic groups might be largely underestimated, ecological interactions remain hidden, conservation efforts may be inappropriate and cryptic pathogens, parasites and invasive species might represent unrecognised threads to human health. To tackle these issues, we need more information on the proportion of cryptic species in different phyla and different biomes. Here, we analysed the Zoological RecordTM database (1978–2006) for the taxonomic and biogeographical distribution of cryptic metazoan species in relation to the number of described species. Published: 19 July 2007 BMC Evolutionary Biology 2007, 7:121 doi:10.1186/1471-2148-7-121 Received: 13 April 2007 Accepted: 19 July 2007 This article is available from: http://www.biomedcentral.com/1471-2148/7/121 © 2007 Pfenninger and Schwenk; licensee BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Page 1 of 6 (page number not for citation purposes) BMC Evolutionary Biology 2007, 7:121 http://www.biomedcentral.com/1471-2148/7/121 Results and discussion We found 2,207 cryptic species reports (CSR) under 771,931 studies that were suitable to detect cryptic species. Log10 number of CSR were correlated with the log10 of estimated number of described species in different metazoan taxonomic groups (Fig. 1, R2 = 0.53, F1,18 = 20.69, P = 0.0002). Deviations from the regression line, termed CSR taxon variation, are presumably composed of differences in study intensity and taxonomic practice in the respective research community, true differences among taxonomic groups and random error. In order to assess the impact of differential research intensity, we regressed the log10 number of studies on the log10 number of described species in the respective taxonomic group; the residuals were used as a measure of taxonomic study bias (R2 = 0.64, F1,18 = 8.63, P = 0.0088). This parameter explained part of the CSR taxon variation (Fig. 2, R2 = 0.47, F1,18 = 15.67, P = 0.0009). Assuming that the numbers of CSR are proportional to the true number of cryptic species, their distribution is nearly homogeneous across the taxa analysed. Only six groups fell outside the 95% confidence intervals: Mammalia, Amphibia and combined smaller Arthropoda classes exhibited an excess of CSR, while Bivalvia, Arachnida and combined smaller Mollusca classes exhibited a deficit of CSR. At least for vertebrates, we suspect that these results might result from taxonomic inflation [2]. Overall, differential taxonomic practice in the various research communities seemingly exerted no major impact on the results. Applying the same procedure to the classic biogeographical regions revealed a marginally significant correlation between the number of CSR and the proportion of described species in the respective region (Fig. 3, R2 = 0.59, F1,5 = 7.34, P = 0.0423). Study intensity of biogeographical regions was independent of estimated metazoan species richness (R2 = 0.41, F1,5 = 3.41, P = 0.1242). However, regressing the residuals against each other, the same The log10 of cryptic species reports (CSR) as a function of the log10 number of described species in the respective taxon Figure 1 The log10 of cryptic species reports (CSR) as a function of the log10 number of described species in the respective taxon. Deviations from the regression line represent CSR taxon variation. Dashed lines represent 95% confidence intervals. y = -1.31 + 0.71 * x 2 = 0.53, = 20.69, < 0.0002 R F P 1. 18 Echinodermata Porifera Other Metazoa Insecta