Research report: Expansions of cosmopolitan Copepoda Acartia tonsa Dana 1849 in the Black, Caspian and Baltic seas: morphological and genetic analyses

Acartia tonsa is often described as cosmopolitan, but that label conceals a complicated history. The species thrives in productive estuaries and coastal zones from tropical and subtropical regions to high latitudes, tolerating salinities from 0.3 to 36.9 parts per thousand and temperatures from minus 1 to 32 degrees Celsius. In many bays, fjords, and inland canal networks it is the dominant copepod, forming a crucial trophic link between primary producers and the fish that feed on zooplankton.

In warmer regions it produces two reproductive peaks each year, one in spring and another from late summer into autumn, while in colder waters only a single late-summer peak is possible. In the Baltic Sea it appears in the plankton only in late summer and autumn, surviving the rest of the year as so-called resting eggs, a dormant stage between diapause and ordinary eggs that allows the species to endure unfavorable conditions and to disperse widely in ships' ballast water. One of the central problems the researchers faced is that species of the genus Acartia look frustratingly alike.

A. tonsa coexists with A. clausi, A. longiremis, and A. bifilosa across much of its range, and misidentifications have plagued the literature for decades. Until now, no genetic methods had been applied to confidently distinguish these species, particularly in non-native populations. The new study set out to close that gap with three goals: a detailed morphological characterization of A. tonsa, a synthesis of the geographic distribution of native and non-native populations, and genetic analyses using the mitochondrial markers CO1, 16S, and Cytb together with the nuclear marker 18S.

The specimens came from research cruises conducted by the Atlantic Branch of the Shirshov Institute of Oceanology in the Baltic Sea in 2019, the Institute of Oceanology of the Bulgarian Academy of Sciences in the Black Sea in 2019, and the Iranian Caspian Sea Research Institute in Ecology in the Caspian Sea in 2023. Samples were fixed in 96 percent ethanol immediately after collection, and three females from each site were selected for molecular work. DNA was extracted using a direct lysis approach with Chelex and Proteinase K, and sequencing was performed on an Applied Biosystems 3500 Genetic Analyzer.

The resulting sequences were aligned with Clustal W, and phylogenetic relationships were reconstructed with Neighbor-Joining and Maximum Composite Likelihood methods in MEGA 11. The morphological work confirmed two distinct Acartia species in the studied regions, A. tonsa and A. clausi, and quantified their differences. A. clausi is generally larger, with females averaging 1.20 millimeters in total length compared with 1.02 millimeters for A. tonsa females; males measured 1.09 and 0.98 millimeters respectively.

The decisive diagnostic features lie in the fifth swimming legs. In A. tonsa females the exopod is long and slender with distinct lateral serrations, matching the invasive morphotype described in earlier studies of the Caspian and Baltic basins, while A. clausi females bear shorter, thicker terminal spines. A. tonsa males show a highly asymmetrical right leg with a large, smooth, curved inner projection and a hook-shaped terminal segment.

Across all three seas these traits proved remarkably stable, suggesting that despite very different salinity and temperature regimes the invasive morphotype remains consistent, and the measurements closely matched published data from the Black Sea. Analysis of the nuclear 18S marker, based on a 656-position dataset, found identical sequences in every sample, confirming that all examined specimens belong to a single species. The mitochondrial markers told a richer story.

Drawing on 459 CO1 samples, 55 16S samples, and 8 Cytb samples from GenBank and the authors' own material, the phylogenetic trees showed that copepods from the Black, Baltic, Caspian, and North seas share common evolutionary origins with North Atlantic populations. A haplotype network built from 458 samples grouped them into 159 distinct haplotypes. In contrast, Black Sea samples form their own cluster, indicating they arrived long ago and have since accumulated mutations, and the Caspian population shows the same pattern of isolation.

These patterns translate into a coherent invasion history. The native habitat of A. tonsa lies along the Atlantic coast of North America, from which the species expanded into the North and Baltic seas. It first appeared in Dutch coastal waters in 1916, reached Denmark's Ringkøbing Fjord in 1921, and entered the Baltic in 1924 in the Gulf of Riga, spreading to the Gulf of Gdansk in 1925, the Gulf of Finland in 1934, and waters near Helsinki by 1939.

The Black Sea record is more recent and more tangled: earlier researchers had confused A. tonsa with the similar A. clausi, but a re-examination of archived samples showed the species was present in Sevastopol Bay as early as 1976. From the Black Sea the species moved into the Caspian, most likely transported in ballast water after the opening of the Volga-Don Canal. It was first officially recorded in the central Caspian in 1982 and the northern Caspian in 1983, though it was probably present earlier and misidentified as A. clausi.

Once established, it became the most significant invasive copepod in the Caspian, displacing the native Eurytemora grimmi in the Middle and Southern basins and replacing Calanipeda aquaedulcis in coastal zones. The numbers are stark: in the Southern Caspian, A. tonsa reaches densities of up to 3,100 individuals per cubic meter, while C. aquaedulcis collapsed from 563 to 0.3 individuals per cubic meter in the west and from 968 to 68 in the east. A. tonsa also became the primary food source for commercially important kilka, a role that became especially critical after the invasion of the comb jelly Mnemiopsis leidyi devastated native zooplankton.

What makes A. tonsa such a formidable colonizer is a suite of adaptive traits documented across decades of ecological research. Its nauplii and adults feed on diatoms, flagellates, planktonic ciliates, rotifers, and even the nauplii of other copepods, switching opportunistically between suspension feeding when phytoplankton is abundant and ambush predation on motile prey such as ciliates. Reproduction is equally impressive: each female produces 20 to 30 eggs daily for three to four weeks, egg development takes as little as nine days at 22 degrees Celsius, and the first nauplii emerge roughly 48 hours after spawning at 25 degrees.

In colder northern environments the species produces benthic diapause eggs that overwinter and hatch in spring, while in southern regions it can reproduce year-round, and in the Gulf of Maine it completes up to three generations between May and December. The genetic evidence also carries a conservation-relevant message: after the initial establishment of A. tonsa in the Black and Caspian seas, no further introductions have occurred, leaving those populations genetically isolated with no interchange. The authors argue that climate-mediated warming has opened a thermal window favoring A. tonsa over stenothermal native species, and that its displacement of native copepods makes it not merely an addition to local faunas but a transformative driver of zooplankton community structure.

Subject of Research: Genetic and morphological analysis of the invasive copepod Acartia tonsa in the Black, Caspian, and Baltic seas

Article Title: Expansions of cosmopolitan Copepoda Acartia tonsa Dana 1849 in the Black, Caspian and Baltic seas: morphological and genetic analyses

Article References: Shiganova, T. A., Stupnikova, A. N., Stefanova, K. B., & Tabari, M. R. (2026). Expansions of cosmopolitan Copepoda Acartia tonsa Dana 1849 in the Black, Caspian and Baltic seas: morphological and genetic analyses. Nature Conservation, 64, 41-57. https://doi.org/10.3897/natureconservation.64.194441

Image Credits: AI Generated

DOI: 10.3897/natureconservation.64.194441

Keywords: Acartia tonsa, copepods, invasive species, Black Sea, Caspian Sea, Baltic Sea, CO1 barcoding, phylogenetics, ballast water, zooplankton, marine ecology, resting eggs