What is the orange stuff in crabs?

The orange stuff in crabs is eggs. Crab eggs are also known as roe or coral, and the presence of the eggs is an indication that the crab is a female. Crab coral is a delicacy often used in recipes.

A female crab's egg mass begins as bright orange but darkens as the embryos grow, turning black. Female crabs spawn several times during the year. During spawning, the eggs are expelled from the crab's ovaries and create a mass on the abdomen until they hatch. This mass consists of an average of 2 million eggs, though very few of the eggs survive to maturity.