Introduced pathogens and native freshwater biodiversity: a case study of Sphaerothecum destruens

Abstract

A recent threat to European fish diversity was attributed to the association between an intracellular parasite, Sphaerothecum destruens, and a healthy freshwater fish carrier, the invasive Pseudorasbora parva originating from China. The pathogen was found to be responsible for the decline and local extinction of the European endangered cyprinid Leucaspius delineatus and high mortalities in stocks of Chinook and Atlantic salmon in the USA. Here, we show that the emerging S. destruens is also a threat to a wider range of freshwater fish than originally suspected such as bream, common carp, and roach. This is a true generalist as an analysis of susceptible hosts shows that S. destruens is not limited to a phylogenetically narrow host spectrum. This disease agent is a threat to fish biodiversity as it can amplify within multiple hosts and cause high mortalities.

Figure 1. Kaplan-Meier survival curves for Abramis brama, Rutilus rutilus and Cyprinus carpio following infection with Sphaerothecum destruens.

Cumulative proportion of (A) Bream Abramis brama, (B) Roach Rutilus rutilus and (C) Carp Cyprinus carpio surviving following exposure to S. destruens. Treatment fish (solid line) were exposed to an average concentration of 8.6×10⁴ S. destruens spores ml⁻¹ whilst control fish (dotted line) were sham exposed. Time: days post exposure.

Figure 2. Mortality pattern in Abramis brama as a result of infection with Sphaerothecum destruens.

The cumulative percentage mortality in the treatment groups (n = 60 individuals in total) and daily mortalities are presented for 26 days post exposure with S. destruens.

Figure 3. Host phylogeny and susceptibility to Sphaerothecum destruens.

Genetic distance between all known susceptible species to S. destruens was plotted against the susceptibility distance to Sphaerothecum destruens for all the species combinations. The two families, Cyprinidae (□) and Salmonidae (♦) show different relationship patterns between genetic and susceptibility distances. Genetic distances were based in the pairwise analysis of ten Cytochrome b sequences. Analyses were conducted using the Tajima-Nei method in MEGA4 . All positions containing gaps and missing data were eliminated from the dataset. There were a total of 249 positions in the final dataset.