Crabs are an underappreciated species. Whilst living in a harsh and arid environment they dedicate their lives to keeping the beach pristine.
Swift-Footed Rock Crab
The swift-footed rock crab can easily go unnoticed due to its elusive behaviour. It inhabits rocky shores at mid to high tide level and so can be found around beach rocks, boat ramps, rock walls and jetties. These crabs are fast moving and are generally only seen at night, unless disturbed. Then you may observe them jumping from rock to rock trying to find a new refuge.
Their colouration can be mesmerising with a multitude of blue, green, purple, orange, white and black. The crabs encountered on Gili are more blue, green and purple with white stripes. The shell (carapace) can be up to eight centimeters wide and is flattened, compared to other crabs which have shells that are more rounded.
The crab feeds on algae, detritus, small vertebrates, barnacles, limpets and snails. They use their claws to break into the shells of other animals and to tear off pieces of their prey to be transferred into their mouth. This species is predated upon by a variety of animals including birds, octopus and fish, so it isn’t safe for them in the ocean or out of it!
Fiddler crabs are a small and short lived species of crab (up to two years) and are closely related to ghost crabs. They are found in mangroves, brackish water, mud flats, lagoons and swamps. The colouration of the crabs change in correlation with circadian rhythm – during the day they are dark and at night they are light.
Fiddler crabs are well known for their sexual dimorphism – the male’s major claw is much larger than the females. If the large claw has been lost the male will develop a new large claw on the opposite side, which will appear after molting. The female’s claws are the same size. The crabs use their claws in communication, courtship and combat. The male claw is used in waving displays which signals to the female that they are ready to mate. A more vigorous waving display indicates a healthier male and a larger claw indicates a wider burrow which will provide better temperatures for egg incubation. Females chose their partners based on claw size and the quality of waving. Once a female has been attracted she will reside in the male’s burrow whilst the eggs are being laid. The female will carry her eggs on the underside of her body for a two week gestation period. After this period the female will venture out of the burrow and release the larvae into a receding tide.
During feeding the crabs move their smaller claw from the ground to their mouth. This movement looks like the crab is playing the smaller claw like a fiddle – hence the name, fiddler crab. The smaller claw is used to pick up the sediment which is then sifted through in the crab’s mouth. Algae, microbes and fungus are the preferred diet of the crab. After the nutrients are extracted the sediment is placed back onto the ground in a ball. The feeding habits may play a vital role in preserving the ecosystem as they aerate the soil. The fiddler crab can be seen when visiting local islands, especially in the mangrove area at low tide
PADI’s guest blogger Emma Bell introduces herself:
I am a marine biologist and scuba diver from England. I have had the privilege of working in Greece, Seychelles and Maldives. I have worked in an aquaculture research centre where I focused on hormonal manipulation of a pelagic fish species. In addition, I have experience with coral restoration projects including frames and ropes; habitat restoration – crown of thorns, drupella and invasive plant species removal; educational activities and social media updates including blogs. I have also monitored population dynamics of bird, turtle, shark and cetacean species to aid in their conservation. I started my career working in the Maldives and I have done a round trip via Greece, England and Seychelles, I hope to increase my skills set and knowledge further whilst I am at Gili Lankanfushi, Maldives.