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Ichthyoplankton is a term used in science to describe the early life history stages (eggs and larvae) of marine fish. It is found in the upper part of the water column, in depths less than 200 metres. Ichthyoplankton studies provide a valuable source of information on spawning and nursery areas of commercial fish, as well as their recruitment. Studying ichthyoplankton is also important for fisheries management, as it allows to better understand the dynamics of marine fish populations and provide information for forecasting the future abundance of adult fish stocks.

Identification of fish larvae to species level requires high level of taxonomic expertise and it may sometimes prove challenging even for larval fish experts. Traditional methods of identifying ichthyoplankton to species level are based on morphological features. But the changes that occur in the course of the development of the individual organism make larval fish identification difficult, particularly in ecosystems with high species diversity, where no supporting/reference materials (e.g. ichthyoplankton guides) exist. Inconsistent or inaccurate identification of ichthyoplankton can also have consequences in a much wider context. For example, errors in taxonomic classification of eggs or larvae may lead to considerable misinterpretations of larval fish community or create ambiguity about fish species spawning and nursery grounds. For commercially important species, this may have an impact on future stock assessments and might lead to inadequate management measures. Thus, traditional methods of studying ichthyoplankton are not always sufficient. Molecular approaches (i.e. DNA barcoding, a method of species identification using a short section of DNA from a specific gene or genes) have proved to be a promising tool for assessing marine biodiversity. DNA barcoding studies can help recognize species of complex morphological features, identify rare or fragile organisms or else, discover new species. The molecular method offers at the same time an opportunity for rapid and accurate species identification at all life stages. The efficacy of this technique for identification of early-life stage of fish has been tested on a variety of fish species and families inhabiting different ecosystems, including coral reefs and many tropical ecosystems  Improving knowledge on the ecology of the early life stages of fish is one of the main goals under Theme 1 of the Science Plan (“early life history, recruitment and mortality of exploited fish stocks”) of the EAF-Nansen Programme. The combination of morphological methods and DNA barcoding is an ongoing initiative in recent EAF-Nansen research surveys.

“We are using integrated morphological and molecular approaches to facilitate accurate identification of larvae to species level. This allows us to investigate the plankton diversity of poorly studied marine ecosystems both around Africa and the Indian Ocean,” explains Stamatina Isari, a scientist responsible for Theme 1 in the EAF-Nansen Programme at the Institute of Marine Research (IMR) in Norway.

A recent example of using combined morphological and molecular approaches comes from an ecosystem survey conducted in Sri Lankan waters in 2018 onboard the R/V Dr Fridtjof Nansen. Ichthyoplankton samples were collected during the survey and analyzed by a team of scientists from the National Aquatic Resources Research and Development Agency of Sri Lanka and IMR (Rathnasuriya et al., 2021). The study revealed very high larval fish diversity in the tropical central Indian Ocean, exceeding significantly previously reported data from the same region. Based on morphological criteria/features, identification of species to family-level was achieved for 70% of the samples, but integration of DNA barcoding enabled identification to species level in all cases, providing new records of species in the Indian Ocean and Sri Lankan waters. A total of 80 different species belonging to 69 families were documented during the research, including eight new species for the surveyed area. The study additionally identified spatial differences in the larval fish diversity between eastern and western regions of Sri Lanka. The EAF-Nansen study is the first detailed analysis of larval fish diversity and composition in the region documented with the use of both morphological and molecular methods.

“Molecular method is undeniably a useful tool for identifying ichthyoplankton in poorly-studied areas of the world, and it should always be coupled with archiving voucher specimens and photographs” says Stamatina Isari. “In case of the EAF-Nansen Programme, these efforts should be combined with good taxonomic descriptions to produce open access online regional larval fish guides,” she further explains.  

References
Rathnasuriya M.I.G., Mateos-Rivera A., Skern-Mauritzen R., Wimalasiri, H.B.U.G.M., Jayasinghe R.P.P.K, Krakstad J.O. and Dalpadado P. (2021) Composition and diversity of larval fish in the Indian Ocean using morphological and molecular methods. Marine Biodiversity 51:39 (1-15)