A new review published in ICES Journal of Marine Science draws attention to a hidden challenge in plankton monitoring: taxonomic uncertainty. Cryptic diversity, lookalike species, and the limits of morphology-based identification can blur our understanding of biodiversity patterns, species dynamics, and ecosystem processes.
The paper, developed through close collaboration between two ICES expert groups (the Working Group on Integrated Morphological and Molecular Taxonomy (WGIMT) and the Working Group on Zooplankton Ecology (WGZE)), highlights the need to improve species-level resolution in zooplankton datasets across the North Atlantic and Mediterranean.
Common names, complex realities
Taxonomic uncertainty doesn't just affect rare or obscure species; it also concerns some of the most frequently recorded and ecologically important zooplankton in the ICES region. What looks identical under the microscope can, in fact, be very different. A key challenge: the same species names are often used across regions and time-series, even when they refer to genetically distinct organisms. If these differences go unnoticed, there's a real risk of misinterpreting species distributions, ecological preferences, or environmental responses.
Even well-known species are affected. The moon jellyfish for example, commonly referred to as Aurelia aurita, is not a single species but part of a complex of genetically distinct forms that look almost identical. Many remain undescribed and are only known by numbers. Similarly, Paracalanus parvus - long considered one of the most widespread copepods in the Atlantic - comprises several cryptic species that differ by region. The review highlights more than 20 frequently recorded but taxonomically challenging zooplankton taxa, ranging from copepods and jellyfish to pelagic tunicates and diverse larval stages, and shows where species-level uncertainty is most likely to distort ecological interpretation.
Seeing more clearly
Improving species-level identification is not a purely academic exercise. It enables better interpretation of long-term data, more accurate ecosystem modelling, and stronger detection of ecological change. For instance, shifts in the relative abundance of cryptic copepod species with different temperature affinities could indicate subtle but important community responses to warming.
“This review is intended as both an entry point and a way to raise awareness for non-taxonomists that even among our key players, different species may be hiding behind the same name," says lead author Janna Peters, German Centre for Marine Biodiversity Research, “This can seriously affect how we interpret ecological data. It also shows what's possible when taxonomic expertise comes together through the ICES working group structure."
Linking taxonomy and monitoring
WGIMT and WGZE work closely to integrate new taxonomic methods into routine zooplankton monitoring. Techniques such as DNA barcoding, metabarcoding, and high-resolution imaging make it possible to detect cryptic species that would otherwise go unnoticed. By linking this knowledge to existing time-series, both groups help ensure that taxonomic advances can inform ecological assessments and environmental analyses.
The review underlines the value of continued support for integrative taxonomy, well-curated reference data, and collaboration between research and monitoring communities, especially as biodiversity and ecosystem functions are increasingly under pressure.
Read the full paper Taxonomic uncertainty in North Atlantic and Mediterranean zooplankton limits species-level monitoring accuracy in ICES Journal of Marine Science.
