This guest post by Laura Lopez, a PhD candidate from the University of Wollongong who is looking at the behavioural interactions between the introduced Eastern Mosquito fish and native fauna. Below is a summary of her awesome work on glass shrimp. Laura is also assessing the impact of Eastern Mosquito fish on Australian Bass so hopefully we can get an update on this work later in the year!
Eat or be eaten: invasive fish forces shrimp to make tough life decisions
by Laura Lopez
Predation is often presented as a rather simple, albeit dramatic sequence. We might think of a cheetah chasing down a lone gazelle, or the aerial aerobics of a great white shark ambushing a rather unfortunate sea lion. While it’s true that the immediate impact of predation is one less prey animal and one satisfied predator, there is a whole other side to this interaction that we can’t easily see.
In reality, predation is actually a complex interaction, the outcome of which depends on predator behaviour and motivation, prey behaviour and the environment. Fear of a predator itself has a powerful effect on prey animals and can be both a blessing and a curse. For example, while hanging out on the beach might help a sea lion avoid a shark attack, it also prevents it from feeding. Therefore, while avoiding exposure to predators might help prevent being eaten (known as a consumptive effect) it also leads to a loss of fitness in the long term (known as a non-consumptive effect).
For prey to make the most of this trade-off, they need to be able to tell when it’s a good time to go out, or a good time to stay home. A further complication is the fact that predator density and therefore behaviour can vary, as can the time of day. I’ve been researching this conundrum by observing how different densities of the invasive freshwater Eastern Mosquito fish, Gambusia holbrooki, predate upon and alter the behaviour of a native glass shrimp, Paratya australis, during the day and at night. The Eastern Mosquito fish is a tiny but devastating introduced species which has been very successful in Australia. However, we don’t know much about how it’s interacts with native species as a predator.
Eastern Mosquito fish can vary hugely in their densities depending on the time of year. We found that at higher fish densities shrimp were approached and nipped more by fish, as we might expect. Interestingly however, a higher density of fish did not result in a higher predation rate. Sometimes aggressive interactions between predators can actually distract predators and lead to a lower predation rate, which is a possibility in our experiment.
Shrimp reacted strongly to Mosquito fish, regardless of whether they were in the company of a low or high number of the predators. Many chose to stay home in their shelter or give up swimming. Interestingly, when around a lot of fish, shrimp had to forage more, but only at night. This might suggest that mosquito fish were also competing with shrimp for food.
Our results indicate that mosquito fish have multiple negative effects on the native shrimp. They eat them, scare them and cause them to change their lifestyle. Ultimately, this means that nutrient cycling by these algae eating shrimp is being impacted, which is bad for the whole ecosystem. However, the good news is that shrimp responded to real risk of predation they were exposed to, and not the density of predator or time of day, which as it turn out didn’t alter the predation rate. It’s possible that having evolved with native fish predators, our shrimp came pre-prepared with the right response for the Eastern Mosquito fish.