For children growing up in the countryside go “jab” in agricultural dams is a rite of passage. the common yabby (Cherax destructor) is the most widespread Australian crayfish, inhabiting rivers and wetlands in south-eastern Australia.
And while the humble Yabby isn’t as cute and cuddly as some better-known Australian icons, we argue they might be more important from an ecosystem perspective.
Yabbies are a staple for platypus, many species of waterfowl, and fish such as Murray cod and goldfish. And the diet of yabbies consists largely of seaweed, rubble (dead organic material) and small animals. That means they connect energy from the bottom up food chain to peak predators at the top.
And yet little is known about how their diet affects their growth and alters their quality as a food source. Our current research is beginning to close this critical gap.
We found that yabbies in wetlands are a better food source for fish than those in rivers because yabbies in wetlands consume more food rich in high quality fatty acids. Although more research is needed, these results show how higher quality yabby diets can increase totals biomass from predators like Murray cod that can support river ecosystems.
Untangling the food web
food webs describe what eats what in ecological communities and provide a useful way to illustrate how energy moves through the environment.
But it’s more complex than big fish eating small fish. Within food webs, organisms can be divided into two groups:
- autotrophs: Organisms that obtain energy from the sun through photosynthesis, such as e.g. plants
- heterotrophs: Organisms that gain energy by eating other organisms such as bacteria, fungi, and animals.
If you grew up in the 1980s, you probably knew the term “polyunsaturated fat” from its association with margarine (although few probably understood its meaning back then). We hear the term around seafood more often these days.
We are encouraged to eat oily fish as it contains omega-3 and omega-6 fatty acids that the body needs for brain function and cell growth. We get these fatty acids from fish thanks to algae, which underpin many aquatic food webs. Fatty acids are essential for the growth of all animals, including yabbies.
The other primary source of energy in freshwater comes from detritus – organic waste and decomposing material. In wetlands and rivers, detritus collects from falling leaves and branches along the banks, which can be washed into the rivers at high water levels.
But while detritus is often plentiful, it is considered inferior quality because it is difficult to digest and has low levels of some important fatty acids. And in food webs, poor quality food makes for less bounce, so to speak.
The yabby is omnivorous—algae, detritus, and other animals are its food, but we know little about how these different energy sources affect the yabby’s growth and survival—or how it might affect animals that rely on yabbies for food .
You are what you eat
Our research examined how different quality fatty acid diets affected yabby growth and how this might affect other animals in the food chain.
We found yabbies in the laboratory that were fed poor quality food, consisting only of dead plant matter and barely growing. These yabbies also provided an inferior food source for predators.
In contrast, yabbies fed mixed diets high in high-quality polyunsaturated fat grew the most — more than doubling in mass in a 70-day trial. They also retained higher levels of these fatty acids in their body tissues, making them a good food source for other animals.
Yabbies are tough. Well adapted to Australia’s extremes, they can survive arid conditions by dormant in dried-up waterways. During wetter periods, they can travel long distances in search of a new home – usually wetlands or rivers.
So how might their environment affect their diet? We found that wild yabbies living in wetlands ate foods with higher concentrations of these fatty acids than yabbies living in rivers.
And as with our lab-fed yabbies, wild wetland yabbies that eat high-quality food also provided a better food option for fish than river yabbies. This is likely because wetlands contain higher proportions of diatoms (unicellular algae) and green algae, both of which synthesize long-chain polyunsaturated fatty acids.
What does this mean for freshwater ecosystems?
Australia’s flood flows are dynamic. High-flow wet periods connect rivers to wetlands that lie on the floodplain. During dry spells with low runoff, this connection is disrupted, leaving wetlands in floodplains isolated and sometimes even drying out completely.
Connectivity between rivers and their Aue is important for many reasons. It offers habitat and breeding opportunities for birds and fish, revitalizes plants and ensures an exchange of nutrients.
Water in the Murray-Darling Basin is shared between irrigation systems, municipal water supplies and the environment, and is largely regulated by infrastructure such as dams and weirs.
Our research is an example of the many benefits that come from ensuring we have enough water for the environment. Our work shows that an important aspect of the connection is to allow riverine predators access to high-value food resources – yabbies – in floodplain wetlands.
As yabbies thrive and essential fatty acids pass up the food chain, populations of popular recreational fish such as Murray cod and goldfish will benefit accordingly.
It is crucial that we improve our understanding of these complex relationships. This includes recognizing other drivers of river population success such as competition, habitat, life history traits and spawning cues to ensure Australia’s river animals can thrive.