Hemimysis anomala

H. anomala is a small mysid shrimp native to the Ponto-Caspian region. In the 1950s and 1960s it was used to stock reservoirs in Eastern Europe to promote fish production (Gasiunas, 1968; Grigorovich et al., 2002). Following successful population establishment in these reservoirs, H. anomala was passively transported to the Baltic Sea, where the first accidentally introduced invasive populations were reported in the Curonian Lagoon off the Lithuanian coast in 1962 (Gasiunas, 1964). Further range expansion has been facilitated by a wide salinity tolerance (Ellis and MacIsaac, 2009), which has enabled the mysid to be transported over great distances in ballast water. It has now been observed in numerous countries across mainland Europe, in the United Kingdom and in North America and Canada. The mysid’s euryhalinity has allowed invasive populations to become established in a wide variety of habitat types, including coastal waters, lagoons, estuaries, rivers, canals and reservoirs. It can reach high population densities in newly invaded habitats (Holdich et al., 2005; Pothoven et al., 2007; Wittmann, 2007) and acts as a top-down regulator of the plankton community; it can therefore have a significant impact on the ecology of a receiving environment (Ketelaars et al., 1999). No methods have been developed to control invasive mysid populations.

H. anomala is of Ponto-Caspian origin, and is regarded as native only to freshened parts of the Black Sea (Pothoven et al., 2007) and the estuaries and lower reaches < 60 km upstream) of rivers draining into the Black Sea, Sea of Azov and the eastern Caspian Sea (Audzijonyte et al., 2008, and references therein). The species’ anthropogenic range expansion began in the 1950s and 1960s, when it was successfully inoculated into reservoirs in Eastern Europe to promote fish production (Gasiunas, 1968). These intentional introductions gave H. anomala downstream access to the Baltic Sea. From the Baltic and also from its native range, the species has spread west across Europe, this range expansion being facilitated by ballast water exchange by international vessels. It has recently reached both the United Kingdom and North America as a result of leisure craft activities and international shipping.

Deliberate introductions of H. anomala largely ceased in the 1980s, as awareness of the detrimental ecological impacts of such activity increased (Rieman and Falter, 1981; Wittmann, 2007). Furthermore, many fish species are unable to exploit H. anomala as prey (Ketelaars et al., 1999), and H. anomala may outcompete planktivorous fish for zooplankton food resources (Spencer et al., 1991); introductions are therefore potentially counterproductive. Despite the cessation of intentional releases, accidental introductions of H. anomala and other aquatic invasive species (AIS) continue to be reported. Current legislation enacted to prevent these accidental introductions does not appear to be effective, specifically that related to ballast water exchange (Ellis and MacIsaac, 2009). Therefore, further introductions of H. anomala and other AIS into international ports through ballast water exchange is anticipated (Pothoven et al., 2007). The species can only reproduce sexually (Mauchline, 1980), and therefore a female carrying fertilized eggs or a combination of male and female individuals must be introduced for population growth to occur. In addition, certain environmental criteria must be fulfilled for introduced individuals to thrive in a newly colonized habitat. However H. anomala can tolerate a wide range of environmental conditions, and in practice there appear to be few barriers preventing population establishment (Pienimäki and Leppäkoski, 2004). H. anomala has only a very limited ability to swim against a current (Stubbington, 2006; Wittmann and Ariani, 2009), and this may limit its upstream range extension following an initial introduction. However, its distribution in countries including England indicates that H. anomala has some capacity to migrate upstream through slow flowing habitat (Stubbington et al., 2008). Based on records of its spread to date, H. anomala is considered to have the capacity for more rapid range expansion than any other Ponto-Caspian AIS (Wittmann, 2006; Wittmann and Ariani, 2009). Pienimäki and Leppäkoski (2004), for example, predict that H. anomala will arrive in the Finnish Lake District in the near future.

In its native range, H. anomala is a deep water species that avoids exposure to daylight (Lindström, 2000). In the Black Sea, it is commonly found at depths of 6 - 10 m, although it has been observed at depths of 20 m in the Black Sea, 30 m in the Caspian Sea (Bacescu, 1954), and as deep as 50 m in the Dneiper River (Zhuravel, 1960). Many invasive populations are also found in deep water, in both sublittoral coastal areas and inland habitats. Salemaa and Hietalahti (1993), for example, observed swarms at depths of 2 - 12 m along the Baltic shoreline, and Verslycke et al. (2000) collected individuals from a depth of 13 m in a brackish water pond. In deep water, the species undergoes diurnal vertical migrations, entailing the extension of vertical distribution into shallower water after dark (Mauchline, 1980; Salemaa and Hietalahti, 1993; Ketelaars et al., 1999).

Genetics

To date, only one study has considered the genetics of H. anomala (i.e. Audzijonyte et al., 2008), and this study only sequenced a fragment of a single mitochrondrial gene. Audzijonyte et al. (2008) analysed 130 individuals from across H. anomala’s entire range (including native and invasive populations) and differentiated between nine haplotypes in three distinct clusters. Mean divergence among the three clusters was 0.9-1.5%, where 0.5% divergence is considered to be relative genetic uniformity and 5% divergence indicates deep genetic subdivisions (Audzijonyte et al., 2008).

Reproductive Biology

Ioffe (1973) studied the lifecycle of a H. anomala population introduced into the Ukrainian Zaporozhie reservoir and noted the start of the breeding season in April, with the first ovigerous females being observed as water temperature reached 8-9°C. At 11-12°C, neonates were observed in the marsupium, the first young were released in late May, and reproduction continued until October. Other studies have noted a similar breeding season, for example Salemaa and Hietalahti (1993), studying an invasive population in the coastal waters of the Baltic, observed females with fully developed ovocytes as early as April, with breeding females being present until the end of October. Pothoven et al. (2007) noted that females carrying young were still present in Lake Michigan in November, when water temperature was 8°C.

Ioffe (1973) observed reproduction in females of the first generation of the year 45 days after release, with the second and third generations developing even more rapidly and reaching maturity within one month. In total, four generations were produced in one year, the highest recorded in any study (Ioffe, 1973). Borcherding et al. (2006) observed only two generations (in April/May and September) in an invasive population in a gravel pit in Germany, but considered a third generation, released during the summer months, to be likely. Similarly, Mordukhai-Boltovskoi (1970) noted that most female H. anomala produce at least two broods per year. Ioffe (1973) recorded a clutch size of between 14 and 66 eggs per female in the Zaporozhie reservoir, depending on female size and season. Other studies have considered the number of young rather than eggs: Ketelaars et al. (1999) recorded a mean brood size of 13 young per female in a Dutch reservoir; Borcherding et al. (2006) noted a mean brood size of 29 young in April and 20 young in September in the German gravel-pit population; and Pothoven et al. (2007) found that clutch size varied between 2-17 young in the Lake Michigan basin.

Factors affecting fecundity include water temperature, trophic status (in lakes) and female length (Ketelaars et al., 1999), the latter being influenced by salinity, with larger individuals found in saltwater compared with freshwater (Bacescu, 1969). Invasive populations in relatively warm, brackish waters therefore have the potential for the most rapid population expansion.

Fish species vary in their ability to prey upon H. anomala, due to the latter remaining hidden during daylight hours. Certain species have been shown to consume considerable numbers of H. anomala, in particular adult perch (Perca fluviatilis), which is native throughout the mysid’s current European range including the UK. Ketelaars et al. (1999) analyzed the stomach contents of five perch and found three to be ‘completely stuffed’ with them, and Borcherding et al. (2006) also noted that perch preyed heavily on the mysid, leading to above average growth in young-of-the-year individuals. This predation pressure, however, appears insufficient to have a significant impact on H. anomala abundance, with both Ketelaars et al. (1999) and Borcherding et al. (2006) reporting high density mysid populations.

Animal feed, fodder, forage

• Fishmeal

A variety of techniques have been used, intentionally and unintentionally, to observe and collect specimens from invasive H. anomala populations, including diving (Salemaa and Hietalahti, 1993; Dumont, 2006), vertical plankton hauls (Ketelaars et al., 1999), pond/dip nets (Verslycke et al., 2000; Pothoven et al., 2007), baited bottle traps (Borcherding et al., 2006; Wittmann, 2007), and searching by torchlight (Holdich et al., 2005; Stubbington et al., 2008). All techniques appear to be effective for initial surveys of the habitats in which they have been employed, with the mysid’s coloration, size and reflective eyes aiding preliminary identification in the field. Not all techniques are suitable in all habitats; however, for example searching by torchlight is only effective in shallow water. In all cases, collection and subsequent identification in the laboratory is necessary to confirm the specimens as H. anomala. Mysids can be identified to genus using Mauchline (1980); further information regarding identification to species level is provided in the ‘Similarities to Other Species/Conditions’ section.

H. anomala may be the sole representative of the Mysidacea in an invaded region (e.g. Holdich et al., 2005), in which case confusion with other species is unlikely. If other mysids are also present, the red coloration of H. anomala can be a useful identification aid in field situations. A UK identification guide to invasive freshwater shrimps and isopods includes species present in the UK and others that are invasive across Europe (Dobson, 2012). 

Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.

SPS Measures

In 2004, the International Maritime Organisation adopted the International Convention for the Control and Management of Ships' Ballast Water and Sediments, which will enter into force 12 months after it has been ratified by 30 States, representing 35% of world merchant shipping tonnage. However, to date (April 2009), only 18 States had ratified the convention, representing 15.36% of world merchant shipping tonnage (International Maritime Organization, 2009). Until the convention is ratified, some countries are following voluntary guidelines intended to prevent the spread of invasive species (Johnson, 2008). Particularly strict regulations controlling ballast water exchange were implemented in 1993 to protect the North American Great Lakes from further invasions (United States Coast Guard, 1993), however, even these measures do not appear to have succeeded in slowing the invasion rate (Ellis and MacIsaac, 2009).

Public Awareness

Efforts to raise public awareness of H. anomala are regionally variable and are most notable in the North American Great Lakes, being coordinated by the National Centre for Research on Aquatic Invasive Species (NCRAIS) and affiliated organizations. NCRAIS provide information enabling the public to recognize H. anomala and provide a mechanism for reporting sightings through the ‘Hemimysis anomala Survey & Monitoring Network’ (NCRAIS, 2009). Elsewhere, little has been done to raise public awareness of H. anomala.

Eradication

No records of any attempts to eradicate H. anomala have been published to date.

Containment/zoning

No attempts to contain invasive populations of H. anomala have been reported.

At present, no method has been developed to remove an invasive mysid populations that has become established.

Biological Control

In some habitats, high densities of planktivorous fish can potentially limit the population expansion of H. anomala through predation (Pothoven et al., 2007). However, to date, this predation has not been exploited to develop biological control strategies for H. anomala or any other mysids.

No control measures currently exist for established populations of invasive zooplankters such as H. anomala; extensive research is therefore required in this area and should be prioritized. Further research is also required regarding the impact of an invasive H. anomala population on the receiving ecosystem, for example to substantiate claims that the mysid may reduce the abundance of toxic blue-green algae, and may encourage the growth of submerged macrophytes (Stubbington et al., 2008). Determining those populations likely to have particularly pronounced detrimental impacts on the receiving environment will allow prioritization of the implementation of any newly developed control measures.

Europe: DAISIE - Delivering Alien Invasive Species Inventories for Europe, Web-based service, http://www.europe-aliens.org

Northern Ireland: QUERCUS - Biodiversity and Conservation Biology Centre, School of Biological Sciences, Queen's University,, Belfast, http://www.qub.ac.uk/sites/Quercus/

USA: NCRAIS - National Centre for Research on Aquatic Invasive Species, NOAA/GLERL 2205 Commonwealth Boulevard, Ann Arbor, Michigan, 48105, http://www.glerl.noaa.gov/

29/04/09 Original text by:

Rachel Stubbington, Loughborough University, Department of Geography, UK