When it comes to crabs🦀, we usually think of them as "walking sideways↔️", but exceptions exist as in the case of today’s protagonist, 𝘔𝘪𝘤𝘵𝘺𝘳𝘪𝘴 𝘣𝘳𝘦𝘷𝘪𝘥𝘢𝘤𝘵𝘺𝘭𝘶𝘴 (Stimpson, 1858), a crab that walks forward↕️🙂↕️. Soldier crabs inhabit the lower parts of sandy coastlines from the Ryukyu Islands to Hong Kong and the Philippines, typically in sand areas and mud flats🏖️.
The species 𝘔𝘪𝘤𝘵𝘺𝘳𝘪𝘴 𝘣𝘳𝘦𝘷𝘪𝘥𝘢𝘤𝘵𝘺𝘭𝘶𝘴 (Brachyura: Mictyridae) is abundant on Hong Kong soft shores. It can easily be recognized with its spherical body🏐, and its carapace reaching about 1.5 cm in length and width. It possesses a smooth and shiny surface, resembling a shaven head of a monk👨🏻🦲. Adult crabs have bluish-purple🔵 carapaces and white limbs🦵🏻 with red joints, while crablet are yellow-brown in color. There are no noticeable differences, however, between males and females, and their sex can only be determined by examining the ventral part of their abdominal structure🔎. Due to their round-shaped carapace, they walk forward instead of sideways. When encountering predators or disturbances, they curl their bodies into a ball or rotate🔄 their bodies to burrow into the sand🕳️, hiding until it's safe to emerge and continue moving forward.
When eating time comes⌛️, soldier crabs adopt two feeding strategies: surface feeding by scooping up mud🥄 and subterranean feeding in tunnels🕳️. They are gregarious animals, and during favorable weather conditions at low tides, they venture out in groups to forage. They primarily feed on algae. This mass movement of crabs foraging in the same direction is quite remarkable and resembles a marching army, hence the nickname "marching crabs." Although they appear impressive as a "crab army💂♀️💂💂♂️," they are not actually cohesive. Each crab is solely focused on foraging🍴, and when one crab in the front slows down, the crabs behind will rush forward, often resulting in trampling each other.
During inclement weather⚠️, male crabs often refrain from venturing out and instead engage in "tunnels" beneath the surface. Immature individuals and females also tend to engage in tunnels. They excavate tunnels underneath the sand surface and deposit the filtered sediment (pseudo-feces💩) on the ground. This feeding behavior is similar to that of a mole digging tunnels, hence the term "feeding tunnels". Their powerful claws and legs are used for digging into the sand to create tunnel structures. These tunnels typically have one entrance and one or multiple passages, connecting them to the water to ensure the flow of water💧 and oxygen supply. Female crabs, especially during the breeding season, tend to dig deeper tunnels for egg-laying and protecting their offspring🤰🏻.
The tunneling behavior of soldier crabs also has implications for other organisms. Apart from being an important food source for birds and fish🍴, their tunneling activities modify the ecology of the mudflat, facilitating the cycling and exchange🔁 of organic matter and sediment. This behavior holds significant ecological value. When the crabs excavate tunnels, they bring up sediments and organic matter from the bottom to the surface. This behavior helps to recycle nutrients🔄, allowing nutrients from the bottom to re-enter the water, providing nourishment for other marine organisms. Excavated burrows of soldier crabs not only protect themselves from predators but also provide a safe habitat for other organisms🦺. Many small invertebrates and fish utilize these burrows to avoid predators and seek food and protection within the burrows⛑️.
So, come and visit the Hong Kong Biodiversity Museum🙆🏻♀️ as we explore the lives of soldier crabs🦀 and uncover the mysteries of their fascinating behaviors and those of thousands of other species🤫. If you enjoy this post, please share it with friends and follow us for more posts each week.
Text: Gemini
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