Smith’s Swimming Crab (Charybdis smithii) is a very interesting swimming crab that exhibits swarming behavior. They were first found in False Bay near Capetown in South Africa in 1838 when a large swarm was discovered. They were only seen swarming again in Capetown in 1978 some 140 years later. They then reappeared in 1983 and 1993.
Swarming in other areas off the East Coast of Africa and Somalia has subsequently been documented as well as the East and West Coasts of India. The crabs in the images were photographed off the Tanzanian coast near Dar es Salaam in November 2017. They were in the upper 8 meters of the water in very large numbers, roughly one per two square meters, mainly singular but occasionally in groups of two or three. Interestingly they were not seen during night dives in the same area at the time.
Smith’s Swimming Crab Crab have a brown colored body with the legs being more of a red brown colour. The claws are orange colored. The body is roughly trapezoidal in shape and the rear legs are modified for swimming. The eyes are a brown color with black pupils . They grow to a maximum width of 12 cm. The crabs in the image in the post were all in the 10 cm range.
SMITH’S SWIMMING CRAB IN THE WILD
We have only seen Smith’s Swimming Crab during mid November 2017 for a period of ten days whereupon they disappeared. Very large numbers appeared in the water in the upper eight meters , approximately one crab per two square meters. It was difficult to judge the overall size of the swarm but it extended a long way. Surface water temperatures were 31 C and the temperature at 5 meters was 30 C.
They were very curious and often swam up to the divers particularly when we were at the 5 meter safety stop. They are extremely maneuverable and capable of rapid movement in any plane. Certainty it would be difficult for any predator to capture them. If one went too close to them then their claws came out in a threatening fashion.
We never viewed them feeding and neither did we view any predation on them. Only on rare occasions did we see more than one together and then these were groups of two or three. Interestingly when we did a night dive in the same area we saw none, yet they were back during the day on the next dive in the area. The depth of the area that they were in alongside Big T Reef was between 45 to 60 meters.
Talking to the local fishermen they say that they observe this phenomenon every year between September and early December off Latham Island off the Tanzanian coast.
Smith’s Swimming Crab have been found in the offshore waters of South Africa, East Africa, the Gulf of Aden, the Eastern Arabian Sea, the Bay of Bengal and Martaban Bay. Research indicates that they are more numerous on the bottom at depths of 140 to 200 meters but they have been found to depths of at least 600 meters. Aside from their swarming behavior the adults are found on the substrate. Small juveniles are pelagic. They are only found in South Africa when the water temperatures are higher than normal.
Smith’s Swimming Crab feed on a fairly wide variety of prey ranging from small fishes to ameboid’s , other crustaceans and mollusks. Cannibalistic behavior has been noted among the juveniles.
As with most crustaceans, the eggs are fertilised by the male and are carried under the body of the female. Once they hatch the larvae go through a planktonic stage before settling down and growing into their adult form. In research conducted in India females with eggs were only found on the substrate and the small juveniles were pelagic. The purpose of the swarming behavior is unknown and one can only speculate that it may be related to mating, but there is no evidence to suggest this.
COMMERCIAL EXPLOITATION AND PREDATION
Smith’s Swimming Crab is presently not exploited commercially although several studies indicate that it may be possible. They are an important food source for Tuna and other pelagic fish.
SMITH’S SWIMMING CRAB CLASSIFICATION
- Natural diet of the deep water crab
Charybdis smithii McLeay (Brachyura :
Portunidae) of the seas around India
C. p. BALASUBRAMANIAN’ AND C. SUSEELAN
Central Marine Fisheries Research Institute, Cochin – 682 014, India