Majidae

Majidae is a family of marine crabs in the superfamily Majoidea, order Decapoda, class Malacostraca. Commonly referred to as true spider crabs, members of this family are notable for their long, slender legs and a carapace that is often broad, spiny, or sculptured. Their appearance can vary considerably among species, and many individuals ornament their bodies with attached algae, sponges, or other organisms as camouflage.

Size and form within Majidae range from small to large. Some species have relatively compact bodies, while others possess very long legs in proportion to their carapace. The giant Japanese spider crab, Macrocheira kurombei, is one of the best known examples and can achieve an impressive leg span, reported up to several meters in extreme individuals, with a comparatively smaller carapace.

Habitat and ecology extend across temperate to tropical marine environments worldwide. Majidae species are primarily associated with rocky bottoms, coral reefs, kelp forests, and sandy substrates, often in shallow coastal waters but sometimes at greater depths. They are omnivorous scavengers and detritivores, feeding on algae, mollusks, small invertebrates, and organic matter. Their long legs aid in navigating uneven substrates, and camouflage is a common behavioral strategy.

Reproduction involves sexual maturation followed by the female bearing eggs under the abdomen, where they are brooded until release. Larval development typically passes through free-swimming zoea and megalopa stages before metamorphosing into juvenile crabs.

Taxonomically, Majidae sits within Majoidea, but the classification of genera within the group has undergone revisions, with some genera moved to related families in modern treatments. The family includes several well-known genera, such as Maja and Macrocheira. The fossil record for this lineage extends back to ancient seas, indicating a long evolutionary history.