A River of Saw Grass dominates the Water Conservation Districts of southern Florida. See other interesting phenomena from Florida and the Bahamas.
Figure 2: The T shaped apron on the male blue crab abdomen
Figure 3. A mature female U shaped apron on its abdomen.
Blue crabs have a diverse habitat due to heir broad tolerance ranges of te2mperature and salinity. In the United States their range is from Massachusetts to Florida, Louisiana and Texas (2).
The Blue crab mates and spawns in the Gulf or lower bays where the salinity is approximately 20ppt. A female blue crab only mates once in its life. Like most crabs the female becomes sexually mature only after its pubertal molt. The females shell is soft after the molt and it is at this time that mate will occur. The female blue crab can spawn eggs within a period of two to nine months after mating. The fertilized eggs are extruded as a cohesive mass, or "sponge," which remains attached to her abdomen until the larvae emerge. The average sponge contains about two million eggs and is formed in about two hours.
Life Cycle & Feeding Habits
Like other crustaceans the blue crab also goes through a series of growth stages such as larval, juvenile and adult. The first larval stage or zoeae feed on small plankton later develop into second and final larval stage (megalops), which are omnivorous and in turn becomes a juvenile. The larvae hatch in the bay area and are carried to sea by the tides. They tend to migrate within the water column when at sea. After about a year to a year and a half the blue crabs reaches adult hood and is sexually mature. The female stops molting and growth after this point but the male continues to grow. 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. Adult Blue crabs are known to be scavengers, bottom dwellers and detritivores and omnivores. They locate their food using a combination of chemical senses and touch.
The ecological significance that Blue crabs have is that they maintain benthic populations and are important prey to animals higher up on the food chain. Like the fiddler crab the blue crab has the ability for autotomy to prevent capture. These limbs are later regenerated.
Fiddler Crab ( genus Uca)
Figure 4. The Atlantic marsh fiddler crab(Uca pugnax)
Fiddler Crab or Atlantic marsh fiddler crab is the common name for Uca pugnax. It is is of the Ocypodidae family within the Decapoda order. They are also one of the most thoroughly studied shore crab studied in North America, because of their ecological role in salt marshes. Crabs belonging to the Uca genus are moderate to large in size and are characterized by the males are distinguished by their one large cheliped that makes them asymmetrical. There are currently 97 different species and subspecies of fiddler crabs. There are three common species namely, the Atlantic marsh/mud crab (U. pugnax), Red Jointed crab(U. minax) and the sand fiddler crab(U. pilator) (figure 5).The geographic range of the fiddler crab extends from the inter tidal marshes from Providence town, MA to Daytona Beach, FL.
Figure 5. The different sub species of Atlantic marsh fiddler crabs comparing the size of their large claw.
The fiddler crab is mostly brown but has a blue- blue green coloration on its anterior part of its carapace and the eyestalks. The Atlantic marsh fiddler crab does not have red or purple spots on its carapace like the Red Jointed and Sand Fiddler crabs. The large claw of red jointed fiddler crab is the largest, while the sand fiddler crab has the smallest claw, which is about 20mm. The fiddler crab is the most conspicuous and abundant invertebrate in most salt marshes (4). The abundance of this crab on a particular day is proportional to the tidal height.
The fiddler crabs can be found emerging in large numbers during low tide to feed. They feed by scrubbing organic matter of muddy particles in the substratum. The use their small claw for feed, which accounts for the male spending more time feeding than the female crab.
The mating rituals of these crabs include a series of visual and acoustical displays. The male usually waves his large claw in a circular fashion in the air while stamping its walking legs on the substrate (3). The female follows the male into the burrow where the copulation takes place. The fiddler crab unlike other crabs mate while the female’s exoskeleton is hard. It is found that the female lays 1500 to 94000 eggs. The size of the sponge or number of eggs per crab depends directly on the size of the size of the female. These eggs can hatch over a two hour period (5). Studies have shown that the larval releases coincide with the lunar cycles. There are high releases of larva seen during the spring and neap tides. This phenomenon is viewed as a survival technique. During the spring and neap tides the zoeae (crab larva) have a chance of being swept from the marsh to the coastal waters and the chances of predation for the female in low.
The crabs go through 5 zoeae stages and one megalop stage each lasting from 7days to a month. After the megalop stage they go through 2 crab stages during which they are weak and cling on to the substrate. The fiddler crab takes about a year to mature and molts at least 1 to 2 times a year. The molting of these crab’s exoskeleton is temperature dependant. Molting is inhibited at 20 o C or less.
The activities and by products of the fiddler crab can influence transfer of energy and nutrients within the marsh ecosystem. Theses crabs are very sensitive to pollutants and play and important role in balancing the salt marsh ecosystem. The three main ways in which fiddler crabs influence the nutrients cycles are:
q Burrow excavations, this differentially affects the biomass of spartina alterniflora, a sea grass, at different tidal heights. The burrows significantly increase the biomass of this grass that grows in the intermediate zone because of its effect on the sediment. Crab burrows increase soil drainage and soil oxidation -reduction potential thus increasing root function. Burrows also increase litter decomposition in the soil therefore increasing the availability of nutrients in the system.
q Feeding perturbation, has the most extensive effect on this ecosystem. It increases agal growth and thus decreases crowding. The first 5mm of the marsh surface is turned over at least once a year (3, 5).
q Fecal pellets is the third way in which crabs contribute to the nutrient input. Depending on the size of the marsh fecal pellets can over the entire area. An average amount of fecal pellets per crab per day is between 100to 200 and contributes about 9mg/m2 of organic nitrogen per day (5).
The Stone crab (Menippe mercenaria)
Figure 6. A fisherman holding a stone crab (on the right) their claws on the right.
The stone crab is from the Xanthidae family of the Decapod order. It is the largest crab in its family. The adult stone crabs found in Florida are light gray or tan dorsally on the carapace and cheliped, with dark brown spots and a creamish underside. They have distinctive yellow to white bands at the junctions of their segments on their brown legs. The juveniles are deep maroon to black with white tips on their chelae and four small dots on their dorsal carapace. They are formidable predators with massive claws well suited for breaking shells of mollusks open. The claws of these crabs are harvested and have the ability to regenerate within its first molt (6).
The geographic extent of this crab is from Cape Lookout, NC to Gulf of Mexico, to the Bahamas, Cuba, and Jamaica.
Another commonly found species of Stone crab in the strip reefs, near the west coast of Bimini also goes by the name of King crab. An armored shell that has many spikes and spines for protection covers its body (figure 7). It is well camouflaged with the reef, giving it additional protection.
Figure 7. The stone crab in Bimini blending in with it’s surrounding.
The female stone crab usually produces eggs from around spring and fall. The spawns go through 5 larval stages before the form a megalopal, which last for 4 to 6 days. The development of the larvae takes place in the estuaries. The juveniles are omnivores and feed on mollusks, crustaceans, flatworms and vegetative matter. Peak abundance of adult stone crab is seen around late summer. There are large inter annual fluctuations in the abundance of these crabs this limiting large scale harvesting of these crabs.
The stone crab found in Bimini is a plant eater that hunts at night. They venture out at night and crawl all over the surface of the coral heads to hunt for leaves of algae growing on the corals (8)
Like all crabs the importance of this crab in the food chain is significant, but it is also has an added commercial consumer market that makes its existence important. The commercial harvests of these crabs like most other crabs in this region have been declining. The statistics of the past 30years of fishing in this area shows a decline from around 7million lbs of stone crab in the 1960s and 70s to approximately 2 to 3 million lbs (8).
The species of stone crab in Bimini helps keep the coral clean of algae. The algae that grow on the coral compete and eventually kill the coral if the grow in excess. Other organisms depend on the coral reefs so the Stone crab plays an important role in this ecosystem (9).
1. www.blue-crab .org
2. US Department of the Interior. Biological Services Program. Matagorda Bay, Texas: Its hydrology, evcology and Fishery resources. Jan 1980.
3. US Army Engineer Waterways Experiment Station. Costal Ecology Group. Species Profile: Life Histories and Environmental Requirements of Costal Fisheries and Invertebratges( Mid Atlantic.Biological Report Washington D.C. 1989.
4. Montague, C.L. 1980. A natural history of temperate western Atlantic fiddler crabs (Genus Uca) with reference to their impact on the salt marsh. Contr. Mar. Sci. 23:25-55.
5. Krauter, J.N. 1976. Biodeposition by salt marsh invertebrates. Mar. Biol. 35:221-223.
6. Decourtsey, P.J.1979. Egg hatching rhythms in three speicies of fiddler crabs. Pages 399-406. E. Anylor and R.G. Hartnoll,eds. Cyclic phenomina in Marine plants and animals.Pergammon Press, NY.
7. http://pelotes.jea.com/fidcrab.htm \
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