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DescriptionInachus phalangium is a small spider crab with a triangular carapace up to 2 cm long and 1.7 cm across its base. It has a narrow V-shaped rostrum with an extremely narrow slit separating the rostral horns. The carapace bears three tubercles in a triangle formation on its upper surface, with the largest at the rear. It has a brownish-red carapace with slightly lighter limbs. The upper surface of the carapace and limbs are coverd in hook-like hairs. This species has very long, slender legs and sturdy claws, usually held folded under the body. These claws are particularly large in male individuals. The propodus is short and inflated, with the second limbs being thicker and longer than the others.Inachus phalangium is often found in association with Anemonia viridis. This species is egg bearing throughout the year, producing planktivorous larvae. The carapace and limbs of this species may be concealed by encrusting epifauna, primarily sponge and algae often, renduring it difficult to see.
Inachus phalangium may be confused with either Inachus dorsettensis or Inachus leptochirus. In Inachus dorsettensis, the tubercules on the carapace are more prominent, arranged with four tubercules at the front with a fifth behind them. However these features are often concealed by epiphyte growth. Therefore the U-shapped rostrum without the slit is a more reliable distinctive feature from Inachus phalangium. Inachus leptochirus has an identicle tubercule arrangement on the carapace but differs from Inachus phalangium in its widely separated rostral horns.