Cryptic lineages—same but different?
Ecotoxicology assumes that traits of species, like sensitivity toward chemical stressors, only vary around one mean value. However, the concept of cryptic lineage complexes, which has been considered a taxonomic challenge for nearly 300 years (Bickford et al. 2007), is largely ignored in ecotoxicology. These complexes are defined as 2 or more distinct lineages with at least a superficially morphological equality that are misleadingly merged as one nominal species but still, however, exhibiting meaningful genetic differentiations. Moreover, as cryptic lineages can occur sympatrically, indirect evidence exists for a reproductive isolation within cryptic complexes (Witt et al. 2006).
The technical terms, “cryptic lineages” and “sibling species,” should not be confused, as the latter describes closely related species (e.g., Gammarus fossarum and Gammarus pulex) that are evidently dissimilar to each other. The number of uncovered complexes of cryptic lineages—in various habitats—has substantially increased over the past decades as a result of relatively cheap and fast DNA sequencing methods (Bickford et al. 2007). The benthic environment in freshwater ecosystems, which provides conditions leading to strong selective constraints on morphological evolution, is a habitat encouraging the development of cryptic lineage complexes (Sturmbauer et al. 1999). Resulting genetic differentiations among cryptic lineages can reach levels similar to those between nominal species (Feckler et al. 2012). Deviations regarding behavioral and physiological characteristics within cryptic complexes are conceivable (Bickford et al. 2007).
POTENTIAL IMPLICATIONS OF CRYPTIC LINEAGES IN ENVIRONMENTAL MONITORING AND RISK ASSESSMENT
To the best of our knowledge, only 2 publications have investigated the impact of anthropogenic stressors on cryptic lineage complexes. Feckler et al. (2012) uncovered a statistically significant difference in sensitivity among the cryptic complex of G. fossarum (Crustacea; Amphipoda) in terms of a 6-fold higher sensitivity of the Eastern European lineage A compared to the Central European lineage B when exposed to 2 pesticides with a distinct mode of action (i.e., thiacloprid and tebuconazole). Sturmbauer et al. (1999) illustrated that different mitochondrial lineages of Tubifex tubifex (Oligochaeta, Tubificidae) varied in their sensitivity toward Cd by a factor of up to 8. These studies hence support hypothesized deviations in behavioral and physiological characteristics among lineages of a cryptic complex.
Special attention should be dedicated to representatives of cryptic lineages used as test organisms for environmental monitoring and risk assessment. Tubifex tubifex, for instance, is one of the freshwater invertebrates recommended to assess sediment-associated contaminants (ASTM 1997). As cryptic T. tubifex lineages may differ in their response toward chemical stress (Sturmbauer et al. 1999), the results of such ecotoxicological investigations may be inaccurate depending on the lineage used.
Another species recommended for investigations on bioaccumulation of sediment-associated contaminants (ASTM 1995), Lumbriculus variegatus (Oligochaeta, Lumbriculidae), represents at least 2 cryptic lineages occurring both in North America and Europe (Gustafsson et al. 2009). Specimens used for ecotoxicological research are usually obtained from stores selling L. variegatus that are apparently allocated to cryptic lineage I. However, L. variegatus of cryptic lineage II can frequently be found in wild populations (Gustafsson et al. 2009). Hence, results received from such laboratory tests may incorrectly estimate bioaccumulation of sediment-associated contaminants by L. variegatus in the field.
Furthermore, genetic analyses of Hyalella azteca (Crustacea; Amphipoda), a widely distributed species throughout Northern America and used for ecological risk assessments by the US Environmental Protection Agency (USEPA 1994), revealed a cryptic complex of at least 7 lineages in Eastern Canada (Witt et al. 2006). Moreover, another 33 provisional lineages were uncovered in the Southern Great Basin of California and Nevada, with the assumption of many additional undescribed lineages elsewhere (Witt et al. 2006). Hence, comparative field and laboratory studies may be confounded by the genetic differences and consequent deviations in behavioral and physiological characteristics among H. azteca lineages.
Against this background, it seems to be inadequate to assess the sensitivity of species that are known to be part of a cryptic lineage complex by assuming equal sensitivity among lineages (Feckler et al. 2012). Hence, a correct identification of test organisms, by also considering molecular biological methods, may be required before ecotoxicological testing, especially if field sampled species are used (Sturmbauer et al. 1999). However, the focus on genetic heterogeneity as the sole driving factor for differences in behavioral characteristics among cryptic lineage complexes may be insufficient. There is a need to account additionally for potential confounding factors that influence organisms' sensitivity toward stress (e.g., lipid content of test organisms and land usage around sampling sites) (Feckler et al. 2012).
The current species identification system, which is based on morphological deviations (i.e., the classification system of Linné), seems to be inaccurate and misleading regarding the assessment of invertebrates sensitivity. As cryptic lineages may differ in other traits than morphology (i.e., their behavioral and physiological characteristics), their consideration in comparative field and laboratory studies is urgently needed. Ecological risk assessments and biodiversity assessments should proactively consider the discussed genetic heterogeneity in terms of cryptic lineage complexes by including a correct genetic identification.