Hunters Become the Hunted – The Only Place where Crabs Eat Frogs

Broughton Island is a small oasis of life off the coast of Myall Lakes National Park, three hours north of Sydney. This island lays claim to many interesting animals including the endangered Green and Golden Bell Frog (Litoria aurea). The bell frogs are one of two frog species found on the island, the other being the Striped Marsh Frog (Limnodynastes peronii). Both of these frogs thrive on the island due to an almost absence of predators and human disturbance.

 

Green and Golden Bell Frog (Litoria aurea) | Copyright Chad Beranek 2014

Green and Golden Bell Frog (Litoria aurea) | Copyright Chad Beranek 2014

Striped Marsh Frog (Limnodynastes peronii) | Copyright Chad beranek 2014

Striped Marsh Frog (Limnodynastes peronii) | Copyright Chad beranek 2014

 

There is a catch for these frogs though… Most of the breeding ponds found on the island are in very close proximity to the ocean which makes them in the center of the rocky shore biome. This is known to be the ideal habitat for the Purple Rock Crab (Leptograpsus variegatus). This set of unusual circumstances has paved the way for a very interesting rivalry between frog and crab…

I had heard of these predation attempts of crabs on frogs but I thought they were a chance occurrence. I thought that the crabs must be utilising opportunistic situations to their advantage to get ahead. However I slowly began to realise that the crabs on this island are definitely targeting frogs and tadpoles, and possibly utilising them as a large part of their diet. It was common place to see Purple Rock  Crabs ‘fishing’ and picking off small tadpoles in some of the coastal ponds. This was always seen after large frog spawning congregations where tadpoles were swarming in thousands.

The shock and realisation set in when I witness a large Purple Rock Crab eating a juvenile Green and Golden Bell Frog. It was literally picking it to pieces. After that observation, I was equally disturbed and curious. Why is this the only known location in the world where crabs eat frogs? I’m sure there’s other places that have frog and crab habitat overlapping. However, the overlap is very thorough on Broughton Island, making it very likely that frog and crab come into contact. From the crabs perspective, it’s a great opportunity to gain resources if a frog jumps on your lap. It’s a simple matter of supply and demand in the end.

The last observation I made was by far the most disturbing but the most compelling. After a long night out collecting data we stumbled upon a large Purple Rock Crab gripping an adult male Striped Marsh Frog near the largest frog breeding pond on the island. The crab had a vice grip on the helpless frog, with one pincer clamping down on the frogs hip, while the other pincer was severing the frogs ankle.

 

IMG_1368

 

Our first instinct was to ‘save’ the frog, however common sense got the better of us. This is the natural course of things and what nature intends. We watched as the crab man handled the resistant frog into the water. The frog held on with all its remaining might till eventually the crab got the better of it and dragged it into the dark algae stained depths of the pond water. Every now and then the frogs head poked up. We could see it giving large bursts of energy to try to stay surfaced. Eventually there was no movement and no frog in site… The crab had won.

 

Leptograpsus variegatus vs Limnodynastes peronii IMG_1378

 

We moved on to continue the night of data collecting. A few moments later we rechecked and were surprised to see the frog had hastily escaped the death claws of the crab. However the victory of the frog was short lived. It had suffered severe injuries to the claws of the crab. The once alpha male marsh frog was barely hopping. The crab had fled, perhaps giving up after the long struggle, so we analysed the frog to see just how powerful and damaging the crab had been in its predation attempts. As we observed, the frog was barely breathing. There was significant internal bleeding around the hip area and also near the ankle which appeared to be broken. Moments later the frog drew its last breath.

 

Dying marsh frog - crab attack

 

This observation showcases the brutality of nature but also highlights, from a scientific standpoint, how varied and unpredictable ecological systems can be. Random and opportunistic encounters can develop between two very different animals which always result in unpredictable outcomes. To find out the science behind this peculiar phenomenon of frog verse crab  on Broughton island, check out this scientific article:

Pyke G. H., Ahyong S. T., Fueseel A. and Callaghan S. (2013) Marine crabs eating freshwater frogs; Why are such observations so rare? Herpetology Notes 6, 196-199.

Comment below if you have witnessed any bizarre wildlife battles!