Possible mechanisms of invasion by the spiny waterflea
December 21, 2018
Species transported outside of their natural range to sometimes far-flung locations across the globe have earned them the label of “exotic species”. A minority of exotic species will spread and disrupt ecosystems in their new home thereby becoming “invasive species”. Transitioning from exotic to invasive depends on a species’ ability to survive and thrive in a new environment and the underlying mechanisms may be related to evolutionary changes in ecologically important traits.
Fiorino and McAdam (2018) were interested in local adaptation versus phenotypic plasticity as possible mechanisms of invasion by the spiny waterflea (Bythotrephes longimanus). Local adaptation involves natural selection processes where individuals of a species split into separate habitats can eventually become genetically distinct as a response to the environmental conditions specific to each habitat. Phenotypic plasticity is where a species has the capacity within its genetic makeup to respond to different environmental conditions, therefore, when separated into different habitats, individuals do not diverge genetically.
The spiny waterflea, is a tiny zooplankton predator native to Eurasia. It was first reported in North America in Lake Ontario in 1982 and can now be found in all Great Lakes and over 160 inland lakes in Ontario. It is considered an invasive species due to its heavy consumption of other zooplankton and corresponding effects on food web dynamics within aquatic systems. True to its name, the spiny waterflea is characterized by a long posterior spine that can protect it from fish predators. In their study, Fiorino and McAdam (2018) asked the question of whether spine length was controlled by local adaptation or phenotypic plasticity.
To address this question, they collected spiny waterflea from 6 Ontario lakes where the species had invaded at some point over the last 30 years. Three of the lakes were dominated by a species of fish considered a “gape-limited predator” while three other lakes were dominated by a “non-gape-limited predator”. The length of the spine can protect spiny waterflea from the former but not the latter. Fiorino and McAdam (2018) used a sub-set of the waterfleas collected from the lakes and reared them over two-generations in a lab setting in order to assess genetic and maternal influences on spine length.
The researchers found that spine length and natural selection on length differed between the lakes but was not influenced by gape-limitation of the fish predator present. Variations in spine length of the lab-reared waterfleas showed that differences were not genetically based. Overall, evidence pointed to spine length being driven by phenotypic plasticity rather than local adaptation. The results of this study provide important insights into a mechanism that may underlie the spiny waterflea’s transition from an exotic into an invasive species of aquatic ecosystems.
Read the full article (journal subscription required):
Fiorino GE, McAdam AG. 2018.Local differentiation in the defensive morphology of an invasive zooplankton species is not genetically based. Biological Invasions. 20(1): 235-250.