Specimen Showcase｜Crab in Name Only?
- Hong Kong Biodiversity Museum
- Apr 8
- 4 min read
In the vast ecosystems of the oceans 🌊, evolution plays a fascinating game of creativity and repetition. Among the many marine creatures, crabs 🦀are particularly captivating. These crustaceans have evolved remarkable adaptations to survive in diverse environments, from rocky shores to coral reefs, from continental shelves to the twilight zone, or even inland and in freshwater for the most terrestrial ones. What makes them even more intriguing is how their physical traits and behaviours echo across different species😱! Can you believe that some of these "crabs" are unrelated species, from various families and subfamilies? Here, we present one of these "crab wannabes", 𝘗𝘦𝘵𝘳𝘰𝘭𝘪𝘴𝘵𝘩𝘦𝘴 𝘫𝘢𝘱𝘰𝘯𝘪𝘤𝘶𝘴 (De Haan, 1849). They are widely distributed in tropical and temperate Asian coasts, in intertidal cobble and boulder zones along shores; with Hong Kong as part of their Southeast distribution.
The genus 𝘗𝘦𝘵𝘳𝘰𝘭𝘪𝘴𝘵𝘩𝘦𝘴 belongs to a diverse family called Porcellanidae, known as Porcelain crabs. 𝘗𝘦𝘵𝘳𝘰𝘭𝘪𝘴𝘵𝘩𝘦𝘴 is the most diverse genus in the family, with over 100 species identified. They possess a well-calcified carapace that often appears smooth and shiny✨—hence their common name, "Porcelain." Their general body shape is typically flattened, allowing these crabs to fit into crevices and hide from predators. The carapace usually features an anterior extension near the eyes called the rostrum, which varies in size depending on the species. Porcelain crabs are mostly omnivorous feeders, consuming small invertebrates, algae and other detritus🐚, yet their feeding habits are quite diverse. Some use their claws to sift through sand and debris on the ocean floor, scavenging for food. 𝘗. 𝘫𝘢𝘱𝘰𝘯𝘪𝘤𝘶𝘴 is a bit different though, using their large claws to forage. The species also traps detritus and plankton with their long setae located near the mouth.
The breeding season of 𝘗. 𝘫𝘢𝘱𝘰𝘯𝘪𝘤𝘶𝘴 occurs from May to September, a period when we frequently observe egg-carrying female crabs🤰. If you observe these crabs carefully, eggs are attached to their abdomen, under swimming legs called ”pleopods”. Also, during this season, 𝘗. 𝘫𝘢𝘱𝘰𝘯𝘪𝘤𝘶𝘴 performs an interesting growing pattern of their“claws”.
The claws of porcelain crabs are called “cheliped”, and is one of their most notable features. 𝘗. 𝘫𝘢𝘱𝘰𝘯𝘪𝘤𝘶𝘴 shows astonishing sexual dimorphism in its chelipeds, as an outcome of their reproductive and competitive strategies. Males have significantly larger chelipeds compared to females, which is probably an outcome of sexual selection. During the breeding season, males allocate more energy to the growth of their major cheliped, which becomes wider and more robust with increasing body size💪🏻. These larger claws aid in male-male competition for mates and possibly function as a display to attract females.
In contrast, females exhibit more balanced growth in both chelipeds and maintain a more slender chela shape regardless of body size. Instead of cheliped development, females prioritize energy allocation to their abdomen, which is essential for carrying eggs during reproduction.
Interestingly, the larvae (zoea) of 𝘗. 𝘫𝘢𝘱𝘰𝘯𝘪𝘤𝘶𝘴 are even more identifiable❗️Porcelain crab larvae have a long, elongated rostral spine. Recent studies have discovered that these spines streamline the body, reducing drag and facilitating smooth lateral movement, which is crucial for agility and predator evasion. This streamlined shape allows for efficient swimming🏊, making lateral movements faster and less energy-intensive. Additionally, these spines improve directional control: the anterior spine being essential for maintaining a straight trajectory, while its removal leads to erratic swimming, indicating its role in stabilizing forward movement. The posterior spines aid in backward swimming and help the zoeae in making quick directional adjustments. Together, these spines enhance the larvae's adaptation and survival in the marine environment.
So, how do we distinguish porcelain crabs from true crabs? The easiest way is to observe their legs! True crabs "walk" with four pairs of legs, while porcelain crabs use only three pairs. These three pairs of legs, known as pereopods, are well-developed and primarily used for locomotion. They are slender and long, enabling the crabs to move quickly across substrates or climb over rocky surfaces🧗. The fourth pair of legs is generally reduced and often tucked under the carapace, shaping a streamlined body for better manoeuvrability in tight spaces.
Now. if porcelain crabs aren’t “true” crabs. What defines a “true” crab? Only species under infraorder Brachyura are considered true crabs. Many "crabs" we commonly refer to are not true crabs. An interesting phenomenon called "carcinization" occurs among decapods🦞. Porcellanidae, along with many animals under the infraorder Anomura, are morphing into crab-like forms! Sounds amazing, right?
In fact, processes similar to carcinization are more common than you might think. These processes are part of convergent evolution, where unrelated species develop identical traits or adaptations in response to comparable environmental challenges or ecological niches. As crustaceans🦐 adapt to similar lifestyles and ecologies, evolution has shaped their bodies into similar forms, characterized by a broad, flattened body, a reduced abdomen, and a more robust carapace. This is one of nature's best solutions to survival!
Does the porcelain crab raise your interest in ecology and biodiversity? If your answer is positive, come to the Hong Kong Biodiversity Museum where we have a wide variety of specimens, from corals to mammals, and you can learn more about their fascinating ecology. If you found this article📝 engaging, share it with your friends and stay tuned for our captivating biodiversity narratives released weekly!
Text: Max
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