Crabs are decapod crustaceans, a group that also includes shrimp and lobsters, and they reproduce by laying eggs. They are classified as oviparous, meaning they do not give live birth to fully formed young. The confusion often arises because the female carries the developing eggs externally on her body for an extended period. This unique reproductive strategy involves distinct stages of mating, delayed fertilization, and extensive parental care.
The Mating Process and Fertilization
Reproduction in many aquatic crab species is closely linked to the female’s molting cycle, the process of shedding her hard exoskeleton. Before the female molts into her soft-shell state, she often releases chemical signals called pheromones to attract a male. The male, sensing her readiness, frequently guards her for several days, embracing her to ensure he is present when she sheds her shell and is most vulnerable.
Mating occurs while the female’s new shell is still soft. The male uses specialized appendages called gonopods to insert sperm into the female’s paired genital openings, or gonopores. Following copulation, the female stores the sperm in an internal structure, often called a spermatheca, sometimes for up to a year or more. This internal storage allows her to fertilize multiple batches of eggs later without needing to mate again. Fertilization is technically external and occurs much later when the female extrudes the eggs, as the stored sperm flows over them.
Carrying the Eggs The External Sponge Stage
Once the female is ready, she extrudes the fertilized eggs, attaching them to the small, feathery appendages on her abdomen called pleopods. These pleopods are located under the abdominal flap, which is broader and more rounded in females than in males. The mass of eggs is collectively referred to as a “sponge” or “berry” due to its dense, spongy appearance.
The number of eggs in this sponge can range from a few hundred thousand to several million, depending on the species and size of the female. During this external carrying phase, the female incubates and protects the developing embryos. She constantly aerates and cleans the eggs by waving her pleopods, ensuring proper oxygenation and preventing fungal or bacterial growth. The eggs often change color as they mature, typically starting as bright orange due to the yolk and gradually darkening to brown or black as the embryos develop eyes. The incubation period can last anywhere from a few weeks to several months, depending on the species and environmental temperature.
The Journey from Larva to Juvenile Crab
When the eggs are fully mature, the female releases the tiny, newly hatched young into the water column. These young do not look like miniature crabs; instead, they emerge as planktonic larvae called zoea. Zoeae are very small, floating freely, drifting with the ocean currents and feeding on microscopic organisms.
The zoea stage involves multiple molts, typically progressing through four to seven stages as the larva sheds its outer skeleton to grow. After this period, the larva transforms into the next major stage, known as the megalopa. The megalopa is a transitional form that begins to resemble a crab, possessing claws and walking legs, though it retains a prominent, lobster-like abdomen. After a final molt, the megalopa undergoes metamorphosis and settles onto the seabed, transforming into a juvenile crab. This miniature crab is a recognizable version of the adult, with the abdomen tucked underneath the body.