Species introductions pose a major threat to biodiversity (Lee 2002). Jellyfish, like other coastal marine species, can be accidentally introduced as organisms growing on ship hulls or in ballast water, or released unintentionally (or intentionally) as a result of the aquarium trade and aquaculture (Holland 2000; Grosholz 2002). The scale of species introductions is bewildering. For example, the USA receives between 20 million and 80 million tonnes of ballast water per year that is laced with an average of one billion bacteria per litre from foreign ports (Ruiz et al. 2000; ANS). The numbers are smaller for larger organisms, and only a subset become established, but the numbers are still impressive. At least 287 introduced, or 'exotic', marine species have become established in Hawai'i (Eldredge & Carlton 2002), between 212-335 in San Francisco Bay (USGS), and 120 in the Chesapeake Bay (ANS). Moreover, their effect is disproportionately large. Over 15% of exotic species cause serious harm, exotic species negatively impact at least 42% of endangered species, and the cost associated with major exotic species in the USA alone is on the order of US$100 billion per year (Lee 2002; ANS).

 

Causative links between introduced species of jellyfish (scyphozoans, hydrozoans, ctenophores), increases in the occurrence or severity of jellyfish blooms, and decreases in the abundance of endemic gelatinous zooplankton can still be somewhat obscure (Mills 2001). This may be due in part to the existence of 'cryptogenic' species, i.e. species whose endemic or exotic status is unclear (Carlton 1996; Mills 2001) and in part to difficulties in ascribing causal relationships when other influences such as eutrophication are also important (Mills 2001). However, several candidates are likely to be nuisance exotic scyphozoans. For example, Rhopilema nomadica was first recorded in the eastern Mediterranean in 1976, and has since become abundant off Israel each summer; it is hazardous to bathers and fishermen due to its sting and may also clog fishing nets. Presumably exotic Aurelia sp. have clogged seawater intakes and shut-down power plants in Australia, the Baltic region, India, Korea, Philippines, and Saudi Arabia (Mills 2001). Such blooms may be facilitated by small ecological advantages conferred on the exotics by 'genotype x environment' interactions (Dawson & Martin 2001). Other exotic scyphomedusae that have had negative impacts in recent years include Phyllorhiza punctata, in the Mediterranean and Gulf of Mexico, and Cassiopea spp., in the Mediterranean and Hawai'i (Devaney & Eldredge 1977; Mills 2001; Graham et al. 2003; Holland et al. unpubl.).