Over the past few weeks, the CEI reef team has made great progress with their West Indian spider crab (Mithrax Spinosissimus) project. Dr. Iain McGaw, a visiting scientist, is leading the charge to investigate the potential use of M. spinosissimus to remove biofouling organisms from aquaculture cages. When organisms settle upon net cages, their growth can reduce water flow, and has the potential to cause the nets to sink. The process of using one aquatic organism to feed or control another is known as integrated multi-trophic aquaculture (IMTA). This often reduces the input of wastes or the need for chemical treatment which can release pollutants into the surrounding environment. Dr. McGaw hopes to show how spider crabs could be used as a sustainable, environmentally friendly alternative. Currently biofouling is controlled using chemical or power-washing methods which introduce a lot of dead material into the environment.

Spider crabs are a great candidate for IMTA because they are large, non-aggressive and are keen grazers on algae and encrusting invertebrates. Their consumption of algae has been found to rival that of other reef dwelling fish and they tend to have a quite broad preference with respect to the types of organisms they will consume. However, there has been little research into the ability of spider crabs to clean algae from aquaculture cages. The cage being used in this study was built by CEI in 2012 to house cobia (Rachycentron canadum). There were several problems with this aquaculture project, mainly the issue of large predatory fish finding a way into the cage and eating the study species! Several years on and the netting is now covered in green algae, fire coral, sponges and a wide variety of other encrusting organisms.  The top of the cage floats at 40 feet below the surface whilst the bottom rests on the sea floor at 80 feet, with algae-encrusted nets suspended in between.

Divers explore the old aquaculture cage. Photo credit: Kelly Martin

Divers explore the old aquaculture cage. Photo credit: Kelly Martin

At the start of March, the reef team took ten spider crabs from the CEI wet lab to the cage to begin the experiment.  The divers placed one crab on ten of the twelve sections that comprise the top half of the sphere-shaped structure.  The crabs are each enclosed by a small wire cage, which are secured to the algae-encrusted netting with zip ties. Several days later, the reef team went to check up on the crabs in their new homes.  They swam around the cage and checked on each of the ten crabs; all of them appeared healthy, but unfortunately one crab had died.  Dr. McGaw believes this crab was likely molting its exoskeleton, which would have made it more vulnerable to the change in environment from the wet lab to the open ocean. However, the other nine crabs appeared happy and healthy and were clearly eating the algae on the large cage.

A spider crab rests on the algae-encrusted netting. The crabs are enclosed in small cages to prevent them from escaping which allows the team to monitor their consumption of algae. Photo credit: Daisy Buzzoni

A spider crab rests on the algae-encrusted netting. The crabs are enclosed in small cages to prevent them from escaping which allows the team to monitor their consumption of algae. Photo credit: Daisy Buzzoni

Last weekend the reef team returned to the dive site to assess how the crabs were doing after their first week on the cage.  Once again, the crabs appeared quite content and some had completely cleared the netting of growth. The next phase of this experiment will be to collect more spider crabs from the patch reefs and place them inside the aquaculture cage in order to determine long term survival rates and assess feeding, growth and health of the individuals. This exciting project is just starting to take off, so stay tuned for more updates!