The Leach's spider crab or Inachus phalangium, belongs to the order Decapoda, family Diogenidae. It is a species typical of the Atlantic Ocean and Mediterranean Sea, living on hard or soft substrates in the Atlantic region, and in the Mediterranean it is very common to find it in association with the anemone Anemonia viridis or Anemonia sulcata, in fact it is estimated that two thirds of the specimens of these anemones, have the commensal relationship with specimens of Inachus phalangium. It is common to find this species at depths of 10 meters, although there are records of specimens observed at depths of up to 50 meters.
The size of Inachus phalangium specimens can reach 2 cm in carapace width, possessing 10 limbs that are very striking for being extremely long compared to the size of the body. Two of these limbs, the most frontal ones, are strong-looking clamps in males, and somewhat less developed in females. The body of this species is triangular in shape, with the apex towards the front. The carapace is variable in color from brown to reddish-yellow, and has not very prominent spines on which it attaches algae that Inachus phalangium uses to go unnoticed. The rostrum is short and has two small spines curved towards the center.
The feeding of the Leach's spider crab is carnivorous, basing its diet on small invertebrates, and sometimes on particles that may have moved the anemones or the mucus of their own surfaces.
Females are usually sedentary and we will always find them around the same anemone. Males, on the other hand, take advantage of the night hours to look for other females and reproduce, which can occur throughout the year. It has been observed that when there are significant numbers of Inachus phalangium specimens, the males, once they have reproduced, leave in search of other females to reproduce with. However, when the density of the species is lower, males tend to monogamy, attending to females for much longer periods.Photos: