Blue Crab Life Cycle

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Range

The range of the blue crab is from Nova Scotia, down the east coast of North America (including the Chesapeake Bay), off Bermuda, throughout the Gulf of Mexico and the Caribbean Sea (including the Antilles Islands), and down the east coast of South America to northern Argentina. Although the blue crab is rarely found north of Cape Cod, it has been seen in Maine and Nova Scotia following consecutive warm years.

The blue crab has been introduced, probably via ship ballasts into Europe, North Africa, and southwest Asia. Introductions into the Mediterranean Sea and adjacent waters have produced breeding populations whereas others were probably temporary occurrences. The blue crab also has been introduced into Japan.

Food

Blue crabs are classified as general scavengers, bottom carnivores (eats other animals), detritivores (eats decaying organic matter), and omnivores (eats either other animals or plants). At various stages in the life cycle, blue crabs serve as both prey and as consumers of plankton, benthic macroinvertebrates, fish, plants, mollusks, crustaceans (including other blue crabs), and organic debris. Food is located by a combination of chemoreception (chemical sense) and taction (touch). Blue crabs may play a significant role in the control of benthic populations.

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. When air temperatures drop below 50°F (10°C), adult crabs leave shallow, inshore waters and seek deeper areas where they bury themselves and remain in a state of torpor throughout the winter.

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. She releases pheromones in her urine when she is getting ready for this final molt to attract a mate.

When a mature male encounters a female that is about to molt to sexual maturity (terminal molt), 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 her upside down so that their abdomens are touching, clasping her with his walking legs. This is known as a “copulatory embrace”.

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 below). The male inserts his gonopods (see male anatomy below) 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.

The sperm packets are stored inside the female in special receptacles, or sacs, known as spermathecae, which lie just inside the gonopore. These receptacles store the male's sperm so that it can be used for egg fertilization at a later time. Viable sperm can live in the female's seminal receptacles for well over a year and will be used for two or more spawnings.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.

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 produce 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.

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 prepares for her migration to higher salinity waters so that she may spawn. Mass migration of schools of "sook" can be observed. The male stays put and will mate with several additional females. During the female's migration, the egg cells in her ovaries will begin to develop and will be ready for fertilization within one to two months after mating.

Spawning

After the female matures and mates, the newly molted "sook" must regain strength and build muscle mass in order to prepare for her migration to high-salinity spawning grounds where she will produce a "sponge" (fertilized eggs mass). To prepare for this journey she will remain and forage in the same general area where she mated for a period of weeks to months (Turner, Wolcott, Wolcott, and Hines 2003). It isn't until late September or October that she will begin her migration to the lower Chesapeake Bay to spawn. The "pregnant" females that survive the gauntlet of crabpots, scrapes, trotlines and dredges will usually spawn for the first time the following season since they reach the spawning grounds late in the season. In other words, sooks won't release any eggs until approximately 2-9 months after mating, usually May-August of the next season.

When the female is ready to spawn, egg cells (oocytes) are forced from the ovaries through the seminal receptacles where they are fertilized from sperm stored in the male’s spermatophores packet. The fertilized eggs, which are about 0.25 mm in diameter, are then extruded into a large, cohesive mass or "sponge" that remains attached to fine hairs beneath her abdomen until they hatch. The average sponge contains about two million eggs, but may contain anywhere from 750,000 to 8 million eggs, depending on the size of the crab. The sponge is formed in about two hours and is roughly one third the size of the female crab's body. The eggs take about two weeks (14 days) to fully develop and hatch. Initially the egg mass is orange-yellow in color and gradually darkens to black. The color change is caused by absorption of the yellow yolk and development of dark pigment in the eyes and on the body of the embryos. On the average, only one out of every million eggs survives to become a mature adult. Mortality of eggs has been attributed to fungal infection, predation, suffocation in stagnant water, and exposure to extreme temperatures.

Growth Stages

Growth and development of the blue crab, as in other crustaceans, consist of a series of larval, juvenile, and adult stages during which a variety of morphological, behavioral, and physiological changes occur. These changes are most dramatic when the animal molts (sheds its rigid exoskeleton) permitting growth and changes in body shape. Before molting, a new shell is formed underneath the old exoskeleton, which then loosens and is cast off. The new shell is initially soft, but it expands and hardens in a few hours.

Larvae

Stage 1 - Zoeae

First stage larvae, called zoeae, measure approximately 0.25 mm at hatching. They bear little morphological resemblance to adults (Hopkins 1943), are filter feeders, and live a planktonic existence in the high-salinity surface waters near the spawning grounds (Pyle and Cronin 1950; Darnell 1959). Tagatz (1968) found more zoeae near the water's surface than at the bottom. Evidence suggests that blue crab zoeae hatch in the Chesapeake Bay, Chincoteague Bay, Delaware Bay, and other estuaries and drift out to sea, where they feed and grow. These larvae may migrate vertically in the water column to reach flood and ebb tides, which transport them back into the bay area.

The zoeae and all subsequent life stages can increase body size only by molting (Hay 1905; Pyle and Cronin 1950). Zoeal development may require 31 to 49 days, depending on salinity and temperature, but development time has been shown to be variable even in a single salinity-temperature regime (Williams 1965). Zoeae molt four to seven times before entering the next stage of development.

Stage 2 - Megalops

The final molt of the zoeae is characterized by a conspicuous change to the second larval stage, called a megalops (also termed megalopa [singular] or megalopae [plural]. Development to this stage requires 31 to 49 days. The megalops larva is more crablike in appearance than the zoeae, its carapace is broader in relation to its length, and has biting claws and pointed joints at the ends of the legs. It measures about 1.0 mm in width. The megalops swims freely, but generally stays near the bottom in nearshore or lower-estuarine, high-salinity areas (Tagatz, 1968). Megalops is a planktonic eater eating both phytoplankton and zooplankton. The megalops stage lasts 6 to 20 days, after which the megalops molts into the "first crab" stage, with proportions and appearance more like those of an adult.

There are usually seven zoeal stages and one postlarval, or megalopal, stage. On occasion, an eighth zoeal stage is observed.

Juveniles

The juvenile "first crab" is typically 2.5 mm wide (from tip to tip of the lateral spines of the carapace). These juveniles gradually migrate into shallower, less-saline waters in upper estuaries and rivers where they grow and mature (Fischler and Walburg 1962). Growth and maturation occur during a series of molts, each of which is termed a "crab" stage according to the number of molts that have occurred since the megalops stage. Molting and grown stop during winter (Churchill, 1921; Darnell 1959); growth resumes as waters warm, and juveniles generally reach maturity during the spring or summer of the year following their hatching.

Adults

In the Chesapeake Bay, sexual maturity is reached after 18 to 20 postlarval molts, at the age of 1 to 1½ years. Males continue to molt and grow after they reach sexual maturity. It is generally accepted that females cease to molt and grow (terminal molt) when they mature and mate. However, new research suggests that mature females (sooks) will continue to molt given the right set of circumstances.

Why stop molting for females and males continue to molt?

  • Molting takes energy; energy that is better used for reproductive output. Since it takes more energy to produce eggs, the theory is that mature female crabs don't grow as large or molt as frequently because of their reproductive energetics. The converse is that sperm production is cheap, so males don't put energy into reproduction, rather they put it into somatic growth.
  • Molting is energy dependent. Larger animals must store far more nutrients for molting than do smaller juveniles. Thus, a really big lobster only molts every 2 to 5 to 10 years. Similarly for blue crabs, the larger the crab, the more difficult to store energy for molting.
  • Molting is risky business. Larger animals may be at more risk, hence, they are not frequent in the population.

The maximum age for most blue crabs in the Mid-Atlantic Region is three years but 5-8 years have been documented; adults thus live an average of less than one year after reaching maturity

Interesting Fact: Autotomy & Regeneration

Blue crabs have the ability to sacrifice limbs (called autotomy) in order avoid capture. Missing limbs are regrown by a process called regeneration.

Blue Crab Life History Questions:

Directions: Answer the following questions on YOUR OWN PAPER and in COMPLETE SENTENCES.

  1. What is the U.S. range of the blue crab?
  2. What is the classification of the blue crab starting with Kingdom and ending with genus /species?
  3. What do blue crabs eat?
  4. What do blue crabs do during winter months?
  5. Why do crabs move from their wintering grounds in spring?
  6. Where does mating take place (location)?
  7. When is the mating season for blue crabs along the Gulf of Mexico?
  8. What does a “terminal molt” signify in female crabs?
  9. Describe how a male crab attracts a female (bullet points)?
  10. How does a female show her interest to the displaying male?
  11. Describe a “doubler” or “buck and rider” (the precopulatory embrace).
  12. Why does the male “cradle carry” the female? How long does he carry her like this?
  13. Describe the physical changes in the female crabs body as she goes through the terminal molt?
  14. How is the “copulatory embrace” different from the “precopulatory embrace”?
  15. Describe copulation in blue crabs (make bullet points). How long does it last?
  16. Is fertilization internal or external?
  17. What are gonopores? What are gonopods?
  18. Describe the spermatophores in crabs.
  19. The sperm are stored inside the female in special receptacles known as ______.
  20. If the female mates only once in her lifetime, how does she keep producing fertilized egg masses?
  21. A male crab can restock his sperm in ______days.
  22. Describe the “postcopulatory embrace”. Why does the male crab still need to embrace her?
  23. Egg cells in her ovaries develop and are ready for fertilization ______months after mating.
  24. What is a “sook”?
  25. What is a “sponge”?
  26. What happens to the egg cell when it is released from the ovary? (describe the path it takes to the outside of the female’s crab body)
  27. How many eggs does the egg mass contain? Why does the egg mass change color over time?
  28. How many eggs actually make it to mature adult crabs?
  29. Why is it better for females to only molt once in her lifetime?
  30. Name and describe the first larval stage in crabs.
  31. Draw this first stage of larval development (you need your book pg. 142)
  32. Name and describe the second larval stage in crabs.
  33. Draw this second stage of larval development (book pg. 142)
  34. What is autotomy?
  35. What is regeneration?