Review
Cryptic species as a window on diversity and conservation

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The taxonomic challenge posed by cryptic species (two or more distinct species classified as a single species) has been recognized for nearly 300 years, but the advent of relatively inexpensive and rapid DNA sequencing has given biologists a new tool for detecting and differentiating morphologically similar species. Here, we synthesize the literature on cryptic and sibling species and discuss trends in their discovery. However, a lack of systematic studies leaves many questions open, such as whether cryptic species are more common in particular habitats, latitudes or taxonomic groups. The discovery of cryptic species is likely to be non-random with regard to taxon and biome and, hence, could have profound implications for evolutionary theory, biogeography and conservation planning.

Section snippets

Cryptic species: the biodiversity wildcard

Increasing worldwide destruction and disturbance of natural ecosystems are precipitating catastrophic extinctions of species [1]. Given that most species remain undescribed, efforts to catalogue and explain biodiversity need to be prioritized. Research on cryptic species has increased exponentially over the past two decades (Figure 1), fuelled in large part by the increasing availability of DNA sequences. Identifying cryptic species (Box 1; see Glossary) challenged biologists and naturalists

What are cryptic species?

The literature is brimming with definitions of cryptic or hidden species. Most authors regard cryptic species to be synonymous with ‘sibling species’ [16], whereas others specify that ‘sibling’ connotes more recent common ancestry than does ‘cryptic’, implying a sister-species relationship [17]. Although not yet universally adopted, we feel that differentiating the term ‘sibling species’ in this way gives the phrase enhanced utility. Some advocate that sibling species graduate to the category

Description and discovery of undetected diversity

Large genetic distances within traditionally recognized species, often in combination with morphological, geographical and other subtle differences, have revealed cryptic species in most types of organism and habitat, from deep-sea clams [20] to freshwater fish [21], and from tropical butterflies [22] to arctic plants [23]. We surveyed literature referenced in ISI Web of Science (http://scientific.thomson.com/products/wos/) and Zoological Record Plus (//www.csa.com/factsheets/zooclust-set-c.php

Why are cryptic species cryptic?

Most sensory information processed by the human brain is visual, perhaps explaining why morphological characters feature more prominently than chemical and auditory characters in our classification of the natural world. Integrative taxonomists look for concordant changes in more than one characteristic of an organism, and corroboration from independent data (e.g. molecules, morphology or mating signals) is frequently deemed to be good evidence for separating species [4] (Box 1). However, there

Correcting cryptic assumptions

Several common but perhaps incorrect assumptions about cryptic species pervade the literature (Figure 1). One of the most common assumptions is that most cryptic species result from speciation that is so recent that morphological traits or other diagnosable features have not yet evolved. Although undoubtedly true for some taxa such as coccolithophores (see Ref [16]), the view that cryptic species are recent is challenged by studies of bonefish [40], amphipods [39] and copepods [41], which show

Paradigm shifts in ecology

Newly discovered cryptic species complexes have overturned traditional notions about ecological specialization in interspecific interactions. Investigations of many antagonistic interactions have found that some exploitative species tend to have more specialized diets than was previously thought. Studies of herbivorous beetles [47], Lepidoptera [22] and whole guilds of other herbivorous insects [48] have shown that presumed dietary generalists are complexes of dietary specialists. Similarly,

What we don’t know might hurt us

Several lines of evidence underscore the importance of recognizing cryptic species. Conservation planning, bioprospecting, biological control and the treatment of diseases and snakebites all hinge on the identification of species in cryptic complexes. An inability to identify biologically important species therefore hampers our efforts to conserve, study, contain or utilize them.

Summary

Molecular techniques (primarily DNA sequencing) have transformed the ability of scientists to describe and define biological diversity. Although they are not a panacea for species delimitation, molecular data are important and useful when combined with other types of data. Studies using non-morphological characters, such as chemical and auditory signals, to discriminate otherwise indistinguishable species are being published at an increasing rate. These newly discovered cryptic species present

Acknowledgements

We thank Glenn Adelson, Allen Herre, Dan Janzen and members of the Conservation Ecology Laboratory at the National University of Singapore for discussion and comments on earlier versions of this review, and the Singapore Ministry of Education for funding to pursue the study of cryptic species (R-154–000–270–112).

Glossary

Allopatric
populations or species with geographically separated ranges.
Alpha taxonomy
the description and naming of organisms.
Clade
a monophyletic group of organisms.
Cladogenesis
the evolutionary splitting of groups of organisms or, literally, the generation of a clade.
Cryptic species
two or more distinct species that are erroneously classified (and hidden) under one species name.
Karst
irregular (often limestone) formations often including structures such as caves, sink holes, underground water ways

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