The ocean is a vast and mysterious realm, filled with creatures that seem to dance through the waves🌊 with flair and finesse. One such oceanic luminary is the blue swimming crab🦀️ 𝘗𝘰𝘳𝘵𝘶𝘯𝘶𝘴 𝘱𝘦𝘭𝘢𝘨𝘪𝘤𝘶𝘴 (Linnaeus, 1758), a true performer of the underwater ballet, a visually striking crustacean found in warm coastal waters, such as Indian Ocean and the western Pacific Ocean. As we peel back the curtain on the marine world, we uncover some truly enchanting facts about this stylish crustacean.
As their name implies, blue swimming crabs are adept swimmers, a trait uncommon among crab species. They have a pair of specially adapted, flattened back legs that function as paddles, allowing them to efficiently swim in the water. Most other crabs are primarily walkers and are not as capable in open water. The crab uses a rowing motion with its swimmerets to swim, which is different from the walking or scuttling movement typical of other crabs. The Blue Swimming Crab can move with agility and speed in the water, using its swimmerets to swim backwards, which is a common escape response.
Blue swimming crab features a distinctive, rounded carapace (shell) that exhibits vibrant shades of blue, green, and brown. They have blue legs and claws and bodies that can be olive to grayish-blue. Adult crabs are about 23 cm wide, 10 cm long, and weigh 0.5 to 1 kg. Males 🚹are a bit bigger and have bright blue claws, while females 🚺have claws with red tips. If you flip the crab over and look at its belly, you can guess how old it is and if it’s a boy or a girl. Males have a belly that looks like a “T”. Adult females have a rounded dome-shaped belly, while immature females have a triangle-shaped belly. It’s like a special code to know more about these cool crabs!
The blue swimming crab, also known as the blue crab or flower crab, gets its name from the distinctive blue coloration on its carapace (shell). The blue color🔵 originates from the pigments present in the crab’s shell, including a combination of red (astaxanthin) and green (biliverdin) pigments (🔴+🟢=🔵). Astaxanthin pigments are responsible for the reddish color observed in many crustaceans, including in other crabs, shrimps 🦐 and lobsters🦞. In the case of the blue swimming crab, the combination of astaxanthin with the greenish pigment biliverdin results in its characteristic blue color. The blue coloration of the crab’s shell, however, can vary based on factors such as diet🍽, habitat, and molting cycle🔁.
While many crab species are known for producing many eggs, the Blue Swimming Crab is particularly prolific. A single female can produce from several hundred thousand to a few million eggs in its lifetime, which is higher than many other crab species. In the past decades, this high fecundity has been crucial for the species’ survival, given the heavy fishing pressures it faces. As blue swimming crabs are commercially significant and are harvested for their meat, this high fecundity makes their reproductive strategy unique in comparison to many other crab species and have significant implications for their ecology, conservation, and fisheries management.
If you want to unveil more about the elegance of the Blue Swimming Crab, come visit the Hong Kong Biodiversity Museum!🤩
Text: Mei
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