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Biological solution

Are spider crabs a cure for a common Bahamian aquaculture problem?

special feature: Innovation

Part of a special feature showcasing Memorial’s innovation ecosystem, a pan-university effort focused on supporting the development and success of innovators across Newfoundland and Labrador.  

By Kelly Foss

A five-month sabbatical in the Bahamas has led to a new research area and a new graduate student for Dr. Iain McGaw.

During his time on the island, Dr. McGaw, Department of Ocean Sciences, Faculty of Science, used the research facilities at the Cape Eleuthera Institute (CEI) to investigate the biology of the West Indian spider crab (Mithrax spinosissimus), a very large crab that, unlike others, grazes on seaweed.

He was interested in determining if the crustaceans could be utilized to solve a common problem with aquaculture pens.

Biological control

Seaweed and encrusting animals grow on the pens, which cuts down on water flow through the mesh. The fish become starved of oxygen, or the pens become so heavy they sink to the bottom, says Dr. McGaw.

“The pens have to be raised and power washed, which is time-consuming and expensive, and the seaweed and animals fall to the seabed and rot,” he said.

“The bacterial breakdown of this material in turn leads to “dead zones” on the ocean floor. Alternatively, you could impregnate the net material of the pens with chemicals that retard growth of newly settling organisms, but that could poison everything in the area and introduce toxins into the food chain. I wanted to look for a biological control, instead.”

His idea was to place adult spider crabs in the pens to see if they could resolve the seaweed issue — the idea being that the crabs would graze on the encrusting organisms and remove them. In turn, the crabs would grow, potentially leading to another revenue stream if the crabs could be sold as food.

1/ View from the sea floor

A spider crab inside the aquaculture cage.

Photo: Daisy-May Buzzoni

2/ A meal of algae

Spider crab grazing on algae in the net.

Photo: Logan Zeinert

3/ Research space

The Cape Eleuthera Institute wet lab where Mr. Zeinert will carry out some of his experiments.

Photo: Iain McGaw

4/ Experiment in progress

Crabs were placed in wire cages on top of the aquaculture pen, with the idea that they would graze upon, and remove, encrusting organisms.

Photo: Daisy-May Buzzoni

5/ Diving deep

Crabs were taken down from the boat and placed in cages on the aquaculture pen.

Photo: Daisy-May Buzzoni

6/ Crabs at work

Logan Zeinert (in the orange fins) checks on one of the penned spider crabs.

Photo: Daisy-May Buzzoni

7/ Biological control

Dr. Iain McGaw snorkels overhead and watches the dive team inspecting the aquaculture pen.

Photo: Iain McGaw


The multi-national dive team collects spider crabs at first light. Spider crabs are primarily nocturnal.

Photo: Iain McGaw

Preliminary research

While at CEI, Dr. McGaw met and worked with New Zealander Logan Zeinert.

As an undergraduate student, Mr. Zeinert spent four months at the institute, then returned to the institute to work as a research technician upon his graduation.

Mr. Zeinert eventually partnered with Dr. McGaw on the spider crab project, and is now completing a master’s degree at Memorial under Dr. McGaw’s supervision.

“It mainly started because Iain couldn’t dive to collect the crabs, so he needed someone to do that,” said Mr. Zeinert.

“The work we did together ended up being our preliminary research. I’ll be going back there in January for another five months to look at it in more detail.”

Population dynamics

While in the Bahamas, Mr. Zeinert plans to carry out two separate research projects.

The first project will look at the population dynamics of spider crabs to determine if the population is healthy and large enough to be harvested.

Researchers will collect 100 individuals each from of a number of different sites around Eleuthera and compile their measurements, including carapace and abdomen width, claw length, height, depth and diagonal length, as well as each crab’s weight and sex to determine when the population reaches 50 per cent maturity.

“Right now, nobody is really eating the spider crab.” — Dr. Iain McGaw

Mr. Zeinert will also look at the meat yields of the crabs and compare it to the individual’s size. Finally, he will test to see if the crabs have site fidelity — meaning, if they are transferred to another location, will they stay there.

“The whole idea is, if people are harvesting spider crabs, what size should they harvest them at so that they aren’t taking immature ones,” said Dr. McGaw.

“What’s the population size and how much meat can you get from them? Are they even worth harvesting? Right now, nobody is really eating the spider crab because you can only catch them scuba diving; since they only eat algae, you can’t catch them in a normal crab trap.”


The second phase will look at the spider crabs from an aquaculture perspective.

During his five-month stay, Mr. Zeinert will place the crabs in an aquaculture cage to see how well they grow. Individuals will go in small cages that will be placed on top of the large aquaculture pen to see if they will eat the algae that grows on the mesh and, if so, how fast and effectively they remove it.

“We’ll also do a feeding trial in the lab to see how much a crab eats in a 24-hour period,” said Mr. Zeinert.

“I am planning a foraging study to look at their periods of activity, when they choose to eat food and how much they forage during that period. We’ll have them in a raceway tank with a GoPro camera recording how far they move each hour and how many minutes they forage in that hour.”

Student research opportunity

The researchers are quick to recommend CEI to undergraduate and graduate students looking for research experience in another country.

In addition to providing facilities for researchers interested in local terrestrial and marine environments, CEI also focuses heavily on sustainable living. It makes its own biodiesel and all electricity is generated by wind or solar power. Much of the food is grown on site and waste is composted or fed to farm animals.

High school student groups from around the world come to Eleuthera for a semester to attend The Island School and learn about living more sustainable lives.

“CEI is always looking for interns and research technicians,” said Dr. McGaw. “They take people from all over the world and I think it would be a great opportunity for Memorial students to learn research techniques and work with scientists on their own projects.”

Research technicians receive free room and board at CEI, while interns pay for their accommodations. However, they are given the opportunity to teach research classes at The Island School to offset those costs.

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