Lennart Edsman

A highly virulent pathogen, Aeromonas hydrophila, from the freshwater crayfish Pacifastacus leniusculus

Aeromonas spp. are characteristic bacteria of freshwaters and many of them can be components of the bacterial flora of aquatic animals and may become pathogens on animals including humans. In this study Aeromonas hydrophila was isolated from the freshwater crayfish, Pacifastacus leniusculus, and was found to be a highly pathogenic bacterium among many isolated bacteria. Mortality reached 100% within 6h when 200 microl of 1.24 x 10(7)CFU/ml was applied by injection. Histopathological studies of moribund crayfish showed that extensive necrotic nuclei and clump-infiltrated hemocytes were found in observed tissues including gill, heart, hepatopancreas and the circulatory system. To verify how crayfish are susceptible to this bacterium, crude extracellular products (ECPs) obtained from culture supernatant of A. hydrophila was studied either in vivo or in vitro. ECPs (200 microl) were able to kill crayfish by injection. In an in vitro study, ECPs induced cytotoxicity of hemocytes as well as hematopoietic cells in a dose- and time-dependent manner after 30 min post inoculation. Two genes coding for endotoxins were also found in this isolate of A. hydrophila. This indicates that the bacterial endotoxins are the causative agents of crayfish mortality. Moreover, the effect of temperature on the infectivity of A. hydrophila to crayfish was also studied. At 4 degrees C, all crayfish survived, whereas at 20 degrees C the animals died rapidly after bacterial challenge. At this low temperature A. hydrophila did not replicate or replicated at a very low degree and hence crayfish could probably mount effective cellular reactions towards A. hydrophila.

Potent infection reservoir of crayfish plague now permanently established in Norway.

Noble crayfish Astacus astacus is threatened in Europe due to invasive crayfish carrying the crayfish plague agent Aphanomyces astaci. Norway is among the last countries in which the introduction of non-indigenous crayfish has been limited through strict legislation practices. However, North American signal crayfish Pacifastacus leniusculus were recently discovered in a water-course that has been repeatedly hit by the plague. We mapped the distribution and relative density (catch per unit effort) of signal crayfish within this lake, and performed agent-specific real-time PCR to estimate the prevalence of A. astaci in the population. The resulting length frequencies and relative density estimates clearly demonstrate a well-established signal crayfish population, in which 86.4% of the analysed individuals were confirmed carriers. The success of detection was significantly higher (84.1%) in the crayfish tailfan (i.e. uropods) than in the soft abdominal cuticle (38.4%), which is commonly used in prevalence studies. We therefore propose tailfan (uropods and telson) as the preferred tissue for studying A. astaci prevalence in signal crayfish populations. The likelihood of detecting an A. astaci-positive signal crayfish increased significantly with increasing crayfish length. Further, large female crayfish expressed significantly higher PCR-forming units values than large males. In surveys primarily exploring the presence of A. astaci-positive individuals in a population, large females should be selected for molecular analyses. Our study demonstrates that a potent crayfish plague infection reservoir, evidently originating from the illegal human introduction of signal crayfish, has permanently been established in Norway.

Microsatellite markers reveal clear geographic structuring among threatened noble crayfish (Astacus astacus) populations in Northern and Central Europe

Noble crayfish (Astacus astacus L.), the most highly valued freshwater crayfish in Europe, is threatened due to a long-term population decline caused mainly by the spread of crayfish plague. Reintroduction of the noble crayfish into restored waters is a common practice but the geographic and genetic origin of stocking material has rarely been considered, partially because previous genetic studies have been hampered by lack of nuclear gene markers with known inheritance. This study represents the first large scale population genetic survey of the noble crayfish (633 adults from 18 locations) based on 10 newly developed microsatellite markers. We focused primarily on the Baltic Sea area (Estonia, Finland and Sweden) where the largest proportion of the remaining populations exists. To allow comparisons, samples from the Black Sea catchment (the Danube drainage) were also included. Two highly differentiated population groups were identified corresponding to the Baltic Sea and the Black Sea catchments, respectively. The Baltic Sea catchment populations had significantly lower genetic variation and private allele numbers than the Black Sea catchment populations. Within the Baltic Sea area, a clear genetic structure was revealed with population samples corresponding well to their geographic origin, suggesting little impact of long-distance translocations. The clear genetic structure strongly suggests that the choice of stocking material for re-introductions and supplemental releases needs to be based on empirical genetic knowledge.

Incomes, Attitudes, and Occurrences of Invasive Species: An Application to Signal Crayfish in Sweden

This article analyzes and carries out an econometric test of the explanatory power of economic and attitude variables for occurrences of the nonnative signal crayfish in Swedish waters. Signal crayfish are a carrier of plague which threatens the native noble crayfish with extinction. Crayfish are associated with recreational and cultural traditions in Sweden, which may run against environmental preferences for preserving native species. Econometric analysis is carried out using panel data at the municipality level with economic factors and attitudes as explanatory variables, which are derived from a simple dynamic harvesting model. A log-normal model is used for the regression analysis, and the results indicate significant impacts on occurrences of waters with signal crayfish of changes in both economic and attitude variables. Variables reflecting environmental and recreational preferences have unexpected signs, where the former variable has a positive and the latter a negative impact on occurrences of waters with signal crayfish. These effects are, however, counteracted by their respective interaction effect with income.