Samantha Andrews

Marine biologist/ecologist and a science and environmental writer. She can be found talking or writing about our Earth in all its splendour—including the people and other animals who live here —and achieving a more sustainable future.

Predator parfum makes for unexpectedly bold crayfish

January 20, 2020 | 3 minute read

Rusty crayfish may not have a nose as such, but they do have an exceptional sense of smell. With a “sniff”, these crustaceans can glean all sorts of information about the world around them.

If rusty crayfish can “smell” a predator, such as largemouth bass, it stands to reason the crayfish will seek to reduce their chances of becoming dinner.

A new study in the Canadian Journal of Zoology found crayfish not only detect the presence of a predator as a threat, but before changing their behaviour they also consider how likely it is that a predator will eat them.

Based at Bowling Green State University, PhD candidate Tyler Wood and Professor Paul Moore fed various sized bass either commercial feed pellets or pellets made from one of three crayfish species—including rusty crayfish.

Over a series of trials in artificial ponds and streams, the researchers monitored the crayfish’s behaviour and the amount of plants they ate in the absence (no predator odours) and presence of a bass; the crayfish and bass were physically separated by a mesh so that the only contact the crayfish would have with the bass was the shared water—and the predator odours carried with it.

Sure enough, the crayfish exposed to bass odours changed their behaviours, but exactly how the crayfish changed depended on how big the bass were and what the bass had eaten.

Crayfish that were larger relative to the size of the bass spent more time eating and less time sheltered than their counterparts who were relatively smaller (or those who weren’t exposed to any predator odours).

The unexpected boldness of relatively larger crayfish was, however, somewhat tempered when the crayfish were exposed to bass fed crayfish-flavoured pellets.

Set up of experimental pond.

 

Like many predatory fish, a largemouth bass’s dinner choices aren’t so much limited by how many different species of prey it can eat as much as what it can eat in a single gulp.

Gape size—the size the mouth can open to—has a strong relationship with body size. The bigger the fish, the bigger the gape, the bigger the prey they can eat.

From the crayfish’s perspective, the smaller they are, the more likely they are to encounter a predator who can eat them.

“It’s a risk-assessment,” Wood explained, “you don’t necessarily want to change your behaviour if you don’t have to.”

Precisely how crayfish estimate the size of the bass remains a mystery. Wood and Moore are keen to note that it is unclear whether the changes in behaviour were cognitive decision-making.

“The change in behaviour may be driven by a desire to grow quickly to escape gape size,” Wood said.

“Or it may simply be that animals which eat more in the presence of predators have an evolutionary advantage that improves their chances of survival and reproduction,” Moore added.

“What we are seeing is a landscape of chemical fear is defining interactions that we are only just starting to understand.”

Read the paper: Big and bad: how relative predator size and dietary information influence rusty crayfish (Faxonius rusticus) behavior and resource-use decisions in the Canadian Journal of Zoology.

Samantha Andrews

Marine biologist/ecologist and a science and environmental writer. She can be found talking or writing about our Earth in all its splendour—including the people and other animals who live here —and achieving a more sustainable future.