Copepods are small crustaceans, on average 1-5 mm, found in all aquatic environments from freshwater lakes to deep sea benthos. They have developed a variety of modes of life and may be free-living or parasitic. In zooplankton ecosystems, copepods are often the dominant group, representing over 55% of the total biomass. They are an extremely important link between primary producers and higher trophic levels, providing a food source for larger copepods, fish (including herring and cod) and even whales and sea birds. It is therefore important that native populations are monitored to ensure the stability of local food chains. However, invasive copepods have the potential to disrupt whole food webs and ecosystems. For example, in the Chehalis River estuary of western America invasive copepods have led to declines in native copepods and altered feeding behaviours of fish populations. Monitoring for invasive and non-native copepods is therefore important to aid in maintaining ecosystem integrity.
Passive transport of planktonic copepods over large distances to regions such as the North Sea by ocean currents is considered unlikely since the time required would exceed the lifespan of the individual. Therefore, most planktonic non-native copepods are introduced via human pathways. Although they may be transported via fisheries and aquaculture, the most common vector is through ship ballast water.
The Ballast Water Management Convention is an international treaty developed by the International Maritime Organization which ensures all ships which cross international waters undergo a ballast water management protocol to reduce the transfer of non-natives, including copepods. Ships either exchange ballast water in the open ocean, away from coastal waters where species are more likely to survive and establish, or treatment systems are used to destroy any potential non-natives transported within the ballast tanks. Early detection of non-native spread is crucial and, analysing ballast water discharge can indicate if management techniques have been effective and whether non-natives survived.
Certain planktonic copepod species are more easily spread via methods such as ballast water. For example, the native habitat of the Asian copepod Pseudodiaptomus marinus in Japan is brackish, temperate waters, meaning it is also well suited for life in UK estuaries, and particularly ports with a similar environment.
Approximately half of all described copepod species are parasitic. These are specialized to infect a range of hosts including bony fish, marine mammals and invertebrates including polychaete worms and bivalves. Potentially more damaging to ecosystems than non-parasitic species, non-native parasitic copepods can be spread via the transfer of their non-native hosts and have the potential to infect local species. For example, the Pacific oyster (Magallana gigas) has been linked with the introduction of two species of non-native parasitic copepods into northern Europe, Mytilicola orientalis and Mytilicola intestinalis. Once introduced these were discovered to not only infect their original host but also able to parasitize native bivalves including the commercially important blue mussel (Mytilus edulis) and common cockles (Cerastoderma edule).
Other parasitic copepods such as sea lice, which target finfish, as hosts pose a huge risk to the aquaculture industry. If a new non-native parasitic copepod was transferred to a fish farm area it could cause huge economic issues by increasing fish mortality and decreasing growth rates.
Defence against non-native copepods requires regular, local monitoring via zooplankton sampling so any new species are detected as early as possible. Once a non-native species has been detected a rapid response approach is the best way to limit population growth and impacts to the ecosystem. This may include additional sampling so that the spatial distribution of the species can be established, identifying potential dispersal factors to provide an opportunity to limit spread, and investigating options and feasibility for eradication.
Skilled and experienced analysts, such as those employed by APEM, are required to identify non-native copepods, which are often morphologically similar to local species. Dissection of limbs and mouth parts, with observation under high powered microscopes, is often necessary to be certain of identification.
Multiple long-term studies carried out by APEM scientists have found the presence of Pseudodiaptomus marinus in estuaries in south east England. The species appeared to be established, as all life stages were detected, including brooding females. The long-term nature of these studies also showed seasonal variation within these populations, highlighting the importance of regular zooplankton sampling in the detection of non-native planktonic species. At times Pseudodiaptomus marinus was the most abundant taxon recorded; however, further research is required to understand if the introduction of this species will have a detrimental impact on the ecosystems involved.
Other non-native copepod species detected in samples processed by APEM’s specialists include Oithona davisae, Acartia tonsa and Eurytemora herdmani, whilst Tortanus (Boreotortanus) discaudatus, Eurytemora americana and Eurytemora pacifica have also been found in British waters by other researchers. Little research has been conducted on their potential impacts.