Blue Crab Mating

Mating

The blue crab's activity begins in early spring, when the waters of the Chesapeake Bay warm and the crabs stir from their dormant state. All winter, the females have remained on the bottom of the Bay, most of them in the deepest water. The mature males have been buried in the sediments of the estuaries, and the juveniles have sheltered in shallow-water habitats. During this time, the crabs have not eaten or ventured far from their resting place. With the arrival of spring and warmer temperatures, the male and female crabs begin to move away from their wintering grounds to look for food or seek out a mate.

Mating occurs primarily in relatively low-salinity waters in the upper areas of estuaries and lower portions of rivers. Mating takes place in areas where female crabs normally go to molt—shallow areas with marsh lined banks or beds of submergent vegetation. Blue crabs mate in the Chesapeake Bay from May through October. The primary mating seasons for blue crabs in Louisiana are April through June and September through October. Extended periods of low temperatures will usually significantly shorten the mating season.

The male may mate during its third or fourth intermolt phase after it matures. Females mate only once in their lives immediately following the pubertal, or so-called terminal molt.

Precopulation

When a mature male encounters a female that is about to molt to sexual maturity, the male will perform a rather elaborate courtship ritual, or dance, to get the female's attention. Upon initial contact, the male will stand up high on the tips of his walking legs. He will then wave his claws, stretching them out wide, extending them fully outwards, and will begin to fan pheromones (a chemical scent which attracts females) contained in his urine towards the female with his swimming paddles. Finally, he will snap his body backwards and kick up sand with both his swimming and walking legs. Should the female fail to respond, he will repeat the process again.

The female shows her interest by rocking and waving her claws in and out. She may or may not approach the male, turn around backwards, and attempt to wedge herself under him. Still waving her claws, the male responds by tapping and rubbing her claws with his. Soon she quiets down, tucking her claws into a submissive posture, and allows the male to clasp and carry her with his walking legs. In this position, the mated pair is called a "doubler" or a "buck and rider." She is right side up and facing forward. This is also known as a precopulatory embrace.

The male will "cradle carry" the female for 2 to 7 days until ecdysis (molting) is imminent. This serves two purposes. First, the male is able to protect the female from predators. Second, it assures that the male will be present during the brief time that the female's shell is soft and she's able to mate.

Once the female begins to molt, the male releases her and stands guard over her by making a cage with his walking legs. The molting process may take several hours to complete. This terminal molt marks the female's transition into sexual maturity, and is usually the last time she will shed.

During this transition, her abdomen changes from a "V" shape to a more rounded "U" shape. She is now called a "sook" and will only mate this one time in her life while her shell is still soft. The female must rest briefly to fill the voids of her new shell with water. Next, the male turns the her upside down so that their abdomens are touching, clasping her with his walking legs. This is known as a copulatory embrace.

Copulation

The female extends her hinged abdomen, exposing two genital pores known as gonopores. The female gonopores are large triangular openings in the sterna of the sixth thoracic segment, in line with the third pair of legs (see female anatomy for more information). The male inserts his gonopods (see next paragraph) into the genital pores and transfers seminal fluid which contains microscopic packets of sperm, called spermatophores, to the female. Each spermatophore packet contains several thousand sperm cells. Copulation will last from 5 to 12 hours.

Males have two pairs of pleopods, located towards the front of the abdomen. Both function in the transfer of spermatophores to the female during copulation. The long, curved, tubular first pleopod is the gonopod. The gonopod, not the penis, is the intromittent organ used to deliver spermatophores to the female gonopore (the penis fits into a groove on the gonopod to which it delivers sperm). The second pleopod is much shorter and functions as a piston to push spermatophores through the hollow core of the gonopod (see male anatomy for more information).

The sperm packets are stored inside the female in special receptacles, or sacs, known as spermathecae, which lie just inside the gonopore. These sperm are believed to be viable for as long as the female is alive. Although a female will mate only once, she may produce many fertilized egg masses during her lifetime from this single mating. Fertilization occurs each time a new egg mass is produced by the ovaries until the sperm reserves are depleted. Studies in Florida found that some female crabs produce as many as seven broods (sponges) in one year from a single mating, and up to 18 broods over 2–2½ years. Chesapeake Bay female crabs are capable of producing multiple egg masses over several years, though most will not produce more than one or two masses due to their short average life span, typically 1–2 years.

The amount of sperm that a male crab transfers to a female during mating depends on both the size of the male crab and its mating history. Large males can product larger amounts of sperm than their smaller counterparts. Regardless of their size, males that mate frequently will transfer less sperm to each individual female than males that mate less often. A male can fully recharge his sperm stores in about 10 - 20 days. For the females, larger size at maturity can result in larger egg masses that yield more larvae. Conserving healthy numbers of females and large males in the Chesapeake Bay is important to protecting the overall reproductive potential of the entire blue crab population.

Postcopulation

Following copulation, the female is once again clasped and cradle carried, right side up, facing forward, until her shell has had a chance to fully harden, about 48 hours. This is known as the postcopulatory embrace.

Once released, the female instinctively begins a migration to higher salinity waters so that she may spawn. Mass migration of schools of "sook" can be observed and plotted down the bays in June and again in August. The male stays put and will mate with several additional females.

During the female's migration, the egg cells in the her ovaries will begin to develop and will be ready for fertilization within one to two months after mating. Most females spawn for the first time two to nine months after mating, usually from May through August the following season (they overwinter before spawning by burrowing in the mud.)

See Spawning for the next stage in the blue crab's life cycle.