Discovering cryptic species in the pelagic: the results of molecular research

Biogeography and biodiversity in the pelagic realm are not yet fully understood. Pelagic
species are widely distributed in a horizontal plane and have three-dimensional
distributions. At the present when we speak about taxonomic diversity, we usually mean
the inventory of described species, or known species richness.
However, we do not know very much about variation, either morphological or genetical,
within so-called species in different parts of their range. Recent research of the genetic
population structure of wide-spread zooplankton and micronekton species poses the
question whether we are really looking at one highly variable species or to a species
complex consisting of several more or less cryptic species or infraspecific categories.
In those species complexes the morphology of the components only show very slight
sometimes hardly discernible differences in different geographic areas. The marked
limitation in morphological variation may be the result of the constraints imposed by the
environment, which allows little variation on the best possible morphological option, but
the underlying genetic and physiological variation might be much greater than we
perceive. This question is especially relevant for widely distributed species or for species
with interrupted distributional ranges in the pelagic environment. Examples are given
from the literature of copepods and fishes, and from research on chaetognaths of the same
phenomena e.g. that we find variation in consistent geographic patterns. The recognition
of the true dimensions of the species diversity and variation in the ocean may give us a
new view about the so-called species-poor pelagic system.