Killer shrimps wreak havoc through terror

The fear of invasive “killer shrimps” can stop native organisms fulfilling their vital role in river ecosystems, new research shows.

Even if the predatory shrimps do not attack, their mere presence is enough to debilitate other species. And this fear is already having an impact in Europe’s rivers.

Mark Briffa, from the University of Plymouth, UK, together with independent consultant Calum MacNeil, studied the impact of the invasive shrimp Dikerogammarus villosus, which has been displacing members of the native shrimp genus Gammarus in rivers across Europe for 30 years.

The voracious predator consumes a vast range of other species, and has been linked to ecosystem changes and local extinctions since spreading into nearly all Europe’s major rivers from its home in the Black Sea and Caspian Sea.

The decline in Gammarus populations is especially bad news because these tiny freshwater shrimps carry out the vital task of shredding fallen leaf litter, making it accessible as food to other species.

Gammarus is therefore seen as a “keystone species” – one that plays a unique and crucial role in the way an ecosystem functions. {%recommended 6663%}

Writing in the journal Acta Oecologica, the scientists say that while the predatory impacts of the invader have been well studied, theirs is the first research into “non-consumptive effects” – that is, effects other than being eaten, including the fear response.

The researchers put Gammarus shrimps in tanks, and then dropped a closed cage containing Dikerogammarus villosus into half of them. 

The behaviour of the Gammarus was assessed over several days, with researchers measuring whether they shredded leaves as they would in their natural environment.

The results showed that after four days the Gammarus shrimps sharing a tank with the caged killer shrimps had become “significantly” less efficient at leaf shredding, compared to the tanks where it was absent. 

The presence of the invasive shrimp made the Gammarus shrimp expend more energy in simply avoiding the predator in a bid for self-preservation, rather than focusing on core ecosystem tasks, the researchers write.

Far from getting used to their enclosed – and therefore harmless – enemies, the Gammarus shrimp became even less efficient at leaf shredding as time went on.

In the river environment, such behaviour would reduce the amount of energy available to the entire ecological community.

While it might seem common sense that sharing a tank with a predator would make anyone edgy, the researchers ensured that their Gammarus shrimp had not previously been exposed to their invasive rival and would, therefore, not have been familiar with its behaviour and “alarm cues”.

“This study demonstrates the potential for fear of invasive predator presence alone to impact on ecosystem function,” the researchers write. 

“It shows that the mere presence of an invader can influence resident prey behaviour, in this case the feeding efficiency of naive residents.

“A better understanding of the role of non-consumptive effects during biological invasions could enhance our ability to predict their progress and, in some cases, the wider ecosystem level ramifications.”