Joseph Edward Ironside

Enhanced genetic diversity arising from interspecific hybridisation and from multiple introductions from different source populations has played a major role in the evolution of invasive traits in introduced populations. Across Europe, mitochondrial haplotypes associated with the Japanese mitten crab Eriocheir japonica have been discovered in populations of the Chinese mitten crab Eriocheir sinensis, one of Europe’s highest priority aquatic invasive non-native species, suggesting a possible cryptic invasion of a second Eriocheir species. Analysis of the nuclear genomes of mitten crabs from one of these populations based on single nucleotide polymorphisms (SNPs) obtained through genotyping by sequencing, combined with landmark analysis of carapace morphology indicates that the crabs resemble E. sinensis in their morphology and nuclear genomic background, regardless of whether they carry an E. sinensis or E. japonica mitochondrial haplotype. This suggests interspecific introgression of mitochondrial DNA originating in the Japanese mitten crab Eriocheir japonica at some point in the population’s history, either in its native range or following introduction to Europe. A discrete cluster of SNP genotypes is associated with the E. japonica mitochondrial haplotype in samples from 2015 but not from 2020, suggesting an intraspecific cryptic invasion of genetically distinct mitten crabs into a previously established population, followed by interbreeding. Genetically diverse and novel hybrid biological invaders require particularly careful monitoring since they are likely to display enhanced evolutionary potential, with their life cycles and ecology possibly diverging from that of ancestral populations in their native range. In addition, mitochondrial environmental DNA and DNA barcoding of mitochondrial markers are increasingly relied upon to detect and identify aquatic non-native species. The potential for mitochondrial introgression between species to confuse these methods must be considered.

Introductions of invasive, non-native species in the marine environment are increasing as human activity within coastal areas rises. Genetic datasets are useful tools to identify source populations, track routes of invasions, and illuminate the role of genetic variation in the establishment and subsequent spread of novel introductions. Here, a microsatellite dataset is used to estimate the genetic diversity and population structure of 7 introduced Didemnum vexillum populations in Britain and Ireland, 4 of which are associated with aquaculture and 3 with marinas. Genetic differentiation observed between these populations indicates human-mediated transport as the main mechanism underlying the population structure of D. vexillum in Britain and Ireland. In addition to elucidating patterns of population structure we found that aquaculture sites showed significantly higher genetic diversity (measured as allelic richness) in comparison to the marina sites. We discuss these findings in relation to the history of each invasion, the complex life history of D. vexillum, and available evidence of the relative invasiveness of these populations. Our results show numerous interesting patterns which highlight further research avenues to elucidate the complex factors underlying the global spread of this successful invader.

Understanding demographic processes over multiple spatial scales is vital for the optimization of conservation/management strategies, particularly for commercially harvested taxa such as the brown crab (Cancer pagurus L). Brown crab population genetic structure was investigated at (i) a local scale within the Irish Sea, which included comparisons with the Lundy No Take Zone (NTZ) and (ii) across the NE Atlantic. The results indicate that the brown crab does not exhibit strong spatial structure either within the Irish Sea or at the regional level, suggesting high gene flow within and among the Irish Sea, English Channel and North Sea. Comparisons between the Lundy NTZ and harvested areas revealed similarly high levels of genetic diversity. An intriguing result was that the Lundy NTZ sample exhibited a degree of genetic patchiness (ephemeral geographically unpatterned differentiation) which may indicate elevated recruitment skews within the NTZ. Overall, the results support the view that brown crabs within the sampled area belong to a single genetically panmictic stock and that if breeding stock sizes are maintained genetic drift will not be strong enough to reduce neutral genetic diversity. The highly connected nature of this species requires international cooperation for sustainable management, an important component of which will be the application of more powerful population genomic approaches to assess finer scale aspects of stock structure as well drivers of genetic patchiness reported for the species. This is a timely consideration in light of potential future misalignments between biological and geopolitical stock boundaries in the Irish Sea following Brexit.

Within populations of the amphipod crustaceans Orchestia gammarellus and Orchestia aestuarensis, a proportion of females produce thelygenic (female-only) broods, which often contain intersexual individuals. This phenomenon is associated with the presence of two putative feminizing parasites, the paramyxid Paramarteilia orchestiae and the microsporidian Dictyocoela cavimanum, which frequently co-infect the same host. In order to determine which of the parasites causes feminization, Orchestia were resampled from the type locality of P. orchestiae in France and from another population in the UK. Breeding experiments indicated that female O. gammarellus infected with P. orchestiae produced a significantly higher proportion of female and intersex offspring than uninfected females, even in the absence of D. cavimanum. There was no difference in mortality between infected and uninfected broods, indicating that the paramyxid alters the sex ratio through feminization rather than male-killing. Although D. cavimanum also displays a female-biased prevalence in Orchestia populations, this is due to co-infection with P. orchestiae, indicating that the paramyxid, rather than the microsporidian, is the cause of feminization in these Orchestia populations.