A biological invasion modifies the dynamics of a host-parasite arms race

Helminth absence and invasion success of blackchin tilapia (Sarotherodon melanotheron) in Thailand

Introduction

This study investigates the helminth absence in the invasive blackchin tilapia (Sarotherodon melanotheron) in Thailand, testing the Enemy Release Hypothesis (ERH). The ERH suggests that invasive species thrive in new habitats due to the lack of natural parasites that control their populations in native environments. The recent introduction of S. melanotheron in Thailand has raised concerns about its ecological and economic impacts.

Methods

We surveyed 164 blackchin tilapia from six different locations in Chumphon Province, Thailand, including the sea, estuary, canal, and shrimp farms, examining them for helminths. Fishermen provided data on the first capture dates in the surveyed areas to determine how long the populations have been established.

Results

No helminths were detected in any of the examined fish. The absence of parasites was consistent across all surveyed environments, suggesting a lack of parasitic burden in the population.

Discussion

The absence of helminths may contribute to the successful expansion of S. melanotheron in Thailand, supporting the Enemy Release Hypothesis.

Big-Hearted Invaders: The Impacts of Range Expansion and Parasite Infection on Heart Mass in Cane Toads

The invasion of cane toads (Rhinella marina) across tropical Australia has resulted in the rapid evolution of traits that enable higher rates of dispersal, and that adapt toads to hot dry climates. In anurans, a larger heart facilitates both locomotor activity and desiccation tolerance. Heart size is also often affected, either directly or indirectly, by parasite infections. To test the effects of invasion history and parasite exposure on heart size, we studied common garden-reared toads whose parents were sourced from diverse locations, and experimentally exposed them to larvae of a nematode lungworm (Rhabdias pseudosphaerocephala). Offspring of invasion-front parents had larger hearts than did conspecifics from long-established populations. Exposure to infective lungworm larvae decreased heart mass in toads from all populations. Our study suggests that cardiovascular function, like other traits, has evolved rapidly during the toad invasion; and that lungworm parasites can modify the cardiovascular function and hence aerobic capacity of their host.

Does a biological invasion modify host immune responses to parasite infection?

Biological invasions can disrupt the close and longstanding coevolved relationships between host and parasites. At the same time, the shifting selective forces acting on demography during invasion can result in rapid evolution of traits in both host and parasite. Hosts at the invasion front may reduce investment into costly immune defences and redistribute those resources to other fitness-enhancing traits. Parasites at the invasion front may have reduced pathogenicity because traits that negatively impact host dispersal are left behind in the expanding range. The host's immune system is its primary arsenal in the coevolutionary 'arms race' with parasites. To assess the effects of invasion history on immune responses to parasite infection, we conducted a cross-infection experiment which paired common-garden reared cane toads and lungworm parasites originating from various sites in their invaded range across northern Australia. Infected toads had larger spleens and higher concentrations of eosinophils than did uninfected toads. Infected toads also exhibited lower bacteria-killing ability, perhaps reflecting a trade-off of resources towards defences that are more specifically anthelminthic. The impact of infection intensity on multiple immune measures differed among toads and parasites from different parts of the invasion trajectory, supporting the hypothesis that invasion has disrupted patterns of local adaptation.

A biological invasion modifies the dynamics of a host-parasite arms race

By imposing novel selection pressures on both participants, biological invasions can modify evolutionary 'arms races' between hosts and parasites. A spatially replicated cross-infection experiment reveals strong spatial divergence in the ability of lungworms (Rhabdias pseudosphaerocephala) to infect invasive cane toads (Rhinella marina) in Australia. In areas colonized for longer than 20 years, toads are more resistant to infection by local strains of parasites than by allopatric strains. The situation reverses at the invasion front, where super-infective parasites have evolved. Invasion-induced shifts in genetic diversity and selective pressures may explain why hosts gain advantage over parasites in long-colonized areas, whereas parasites gain advantage at the invasion front.