Macropodia rostrata
A decorator crab that uses algae for camouflage and food.
Macropodia rostrata, commonly called the common spider crab, long-legged spider crab, or long-legged crab, is a marine crab belonging to the family Inachidae. It mimics other small crabs in appearance but stands out due to its long legs. By attaching algae to these legs, it resembles a seaweed stem, which helps it avoid predators and ambush prey—fish seeking shelter in seaweed beds may mistake it for part of the plant. This camouflage is less common in larger crabs or those living at great depths, such as giant Japanese spider crabs.
The crab looks hairy or fuzzy because of the algae it applies for camouflage. Its carapace and legs range from greyish to yellowish or reddish-brown, sometimes with white markings. The carapace, pear-shaped due to a narrowing front, reaches a maximum length of 16 mm (28 mm in some cases), measured from front to back along the midline. Its surface is covered with hook-like setae. From above, the eye stalks are fully visible, and the crab uses its eyes and other senses to find food. Spider crabs are omnivorous scavengers, eating algae, mollusks, and small fish. The algae on their limbs also serves as an emergency food source.
This species is found along European and West African coasts, from 65° N in Norway to South Africa, including the Mediterranean and parts of the Black Sea. Unlike giant Japanese spider crabs, it does not live on the deep ocean floor. It typically occurs at depths of 0–50 meters, occasionally up to 150 meters, often on muddy sand or kelp. Slow-moving and easily preyed upon, it relies on its algal camouflage. When startled, it waves its pincers over its head as a main defense against predators like gulls and shorebirds. Large fish and sharks may prey on it but are often deterred by its hard shell.
Reproduction involves a “molt of puberty” required for both sexes—females do not ovulate before it, and males do not produce sperm. After this molt, copulation can occur at any time. Sperm are stored in a spermatheca, allowing females to use them for several ovulations. Eggs are about 0.6 mm in size, and the number produced depends on various factors. Macropodia rostrata breeds continuously, with no specific mating season. In males, sexual maturity is marked by swelling and whitening of the sperm ducts due to sperm enclosed in spermatophores. In females, maturity is indicated by changes in ovary shape and diameter.
Quick Facts
- Genus
- Macropodia
- Species
- rostrata
Facts from the source article.
Lore & Background
Macropodia rostrata has a hairy or fuzzy appearance due to algae it applies to itself for camouflage. Both the carapace and pereiopods are greyish to yellowish or reddish-brown, with some specimens showing white markings. The carapace grows to a maximum length of 16 (28) mm, with the anterior part narrowing to give it a pear shape. The surface contains numerous hook-setae, and from a dorsal view the eye-stalks are completely visible. It uses its eyes and other sensory organs to locate and catch prey. Spider crabs are omnivorous and mostly underwater scavengers, eating anything from algae to mollusks and small fish. Decorator crabs like Macropodia rostrata use the algae covering their limbs as camouflage and an emergency food source.
It is found along the coasts of Europe and western Africa, from 65° N in Norway to South Africa, including the Mediterranean Sea and parts of the Black Sea. It occurs chiefly at depths of 0–50 metres, though occasionally up to 150 m, commonly in muddy sand and on kelp. It is slow-moving and easily preyed upon. When startled, spider crabs wave their pincers over their heads as their main defense against predators such as gulls and shorebirds. Large fish and sharks will prey on them but are often dissuaded by their hard shells.
Reproduction involves a 'moult of puberty' required for both sexes to reproduce. Males show swelling and color change of sperm ducts due to sperm enclosed within spermatophores. Females show change in shape and diameter of ovaries. After copulation, sperm are stored in a spermathecae for several ovulations. Eggs are about 0.6 mm in size. Macropodia rostrata are continuous breeders with no mating seasons. The species was first described by Carl Linnaeus in 1761 under the name Cancer rostratus.
Reader's Guide
Macropodia rostrata is significant as a decorator crab that exemplifies adaptive camouflage in marine environments. By attaching algae to its long legs, it mimics seaweed stems, providing both defense against predators and a predatory advantage by luring unsuspecting fish seeking shelter. This behavior is noted to be absent among larger crabs and those at great depths, such as giant Japanese spider crabs. The species is also notable for its reproductive biology: scientists observed that the size of carapace length and genital papillae did not correlate in males, contrary to expectations. The 'moult of puberty' is required for reproduction, and females can store sperm for multiple ovulations. Its distribution spans European and West African coasts, from Norway to South Africa, including the Mediterranean and parts of the Black Sea, typically in shallow waters. The species was first described by Carl Linnaeus in 1761, adding historical significance. Its omnivorous scavenging role and use of algae as both camouflage and emergency food highlight its ecological adaptability. The species' slow movement and reliance on pincer-waving defense underscore its vulnerability, yet its hard shell offers some protection against larger predators.
Did You Know?
- Macropodia rostrata can attach algae to its thin legs to resemble seaweed stems, serving as both camouflage and a predatory lure.
- Male Macropodia rostrata can reach a maximum post-puberty carapace length of 23 mm, while females reach 20 mm.
- Macropodia rostrata are continuous breeders and do not have mating seasons.
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