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Ecology

Two Crabs and an Alga- a story of protection and evolution.

 The Paper 

Bishop, M. J. and Byers, J. E. (2014), Predation risk predicts use of a novel habitat. Oikos. doi: 10.1111/oik.01967

Background

The predator-prey dynamic shaped organisms into what they are today. Strategies to better catch and eat food have always been met with strategies to stay safe. The blue crab (Callinectes sapidus), a notorious bully of the estuarine world, can efficiently gobble up the smaller black-fingered mud crab (Panopeus herbstii). The mud crab doesn’t stand much of a chance fighting back so they choose the noble “flee and find shelter” option.

 

Hungry, big blue crabs.

Big, hungry, predatory blue crabs.

On the east coast of the United States, P. herbstii evolved in a world along with oysters, which happen to be the best refuge against predatory blue crabs. But the world is changing – the red alga Gracilaria vermiculophylla is native to the northwest Pacific but since the 1980s has been spotted on the Atlantic coast as well. The alga forms an intermediately good refuge for mud crabs.

The authors set out to investigate the nature of the new third player in the predation game, the red alga. If evolution had not taught the mud crab to use algae as protection how would the crabs be able to assess the refuge compared to the native oyster refuges options?

 Methods

Predation Experiments:

First our investigators had to determine the relative refuge value of different habitats. They compared oyster reef, red alga, and mudflat habitat (which does not offer much protection).

Mud crabs were placed in one of three habitats and then a blue crab was let loose. After 12 hours the blue crab was removed and the surviving mud crabs were counted.

Habitat Selection Experiments:

The experimenters then watched the habitat choices of mud crabs when given the three options with and without the presence of a blue crab predator.

Field Experiments:

Mud crabs were given habitat options at varying distances to determine if convenience changed their decision-making in respect to suitable protective habitat. The investigators performed field experiments with and without the presence of blue crabs to view potential predator induced changes.

Results

Fig 1. Mean (+ SE) number (out of ten) of Panopeus herbstii mud crabs surviving across 10 replicates in each of three habitats (oyster = Crassostrea virginica, alga = Gracilaria vermiculophylla, mud = bare) over 12 h of deployment with a blue crab.

Fig 1. Mean (+ SE) number (out of ten) of Panopeus herbstii mud crabs surviving across 10 replicates in each of three habitats (oyster = Crassostrea virginica, alga = Gracilaria vermiculophylla, mud = bare) over 12 h of deployment with a blue crab.

Fig 2. Mean (+ SE) number of Panopeus herbstii mud crabs colonizing replicate patches (0.16 m2 crab traps) of Gracilaria vermiculophylla with (+C) and without (−C) a predatory blue crab and placed at distances 3 m (black bars), 6 m (light grey bars) or 9 m (dark grey bars) from an oyster reef (n = 4).

Fig 2. Mean (+ SE) number of Panopeus herbstii mud crabs colonizing replicate patches (0.16 m2 crab traps) of Gracilaria vermiculophylla with (+C) and without (−C) a predatory blue crab and placed at distances 3 m (black bars), 6 m (light grey bars) or 9 m (dark grey bars) from an oyster reef (n = 4).

With available oyster habitat, mud crabs successfully avoided predation significantly better than with algae or mud habitat. The red alga was in turn better habitat than the mud, just as predicted.

When given a choice, mud crabs more often chose oyster habitat over algal or mud and this tendency was even stronger with big bad blue crabs around.

In the field experiments more mud crabs used red algae for habitat when oyster reef habitat was further away. Also, in the presence of blue crabs, when safety is key, mud crabs are less likely to find shelter in mediocre algal habitat.

Significance 

This paper raises a lot of interesting issues such as predator-prey coevolution as well as how non-native species can change the behaviors of other organisms in unexpected ways. Non-native or invasive species can have non-obvious effects to an ecosystem and food web.

Sarah Giltz
I am a doctoral candidate in Ecology and Evolutionary Biology at Tulane University. My research focuses on the larval dispersal and development of the blue crab in the Gulf of Mexico.

When not concerning myself with the plight of tiny crustaceans I can be found enjoying life in New Orleans with all the costumes, food, and music that entails.

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