Fiddler crabs, also known as "violinist crabs" in Spanish (cangrejo violinista), are small, semi-terrestrial crustaceans commonly found in salt marshes, brackish inter-tidal mud flats, lagoons, and swamps around the world. These fascinating creatures play a vital role in coastal wetland ecosystems, acting as both detritivores and a food source for various wetland animals. This article delves into the intricate details of the fiddler crab's diet and feeding habits, exploring their unique adaptations and ecological significance.
Physical Characteristics and Sexual Dimorphism
The fiddler crab's carapace (shell) typically ranges from 1 to 2 inches (2.5 to 5 cm) in length. Their coloration varies, ranging from violet to black, with males often displaying more vibrant hues. A unique characteristic of fiddler crabs is their ability to change color, appearing darker during the day and lighter at night due to a constant circadian rhythm.
One of the most distinguishing features of fiddler crabs is their sexually dimorphic claws. Males possess one significantly larger claw (cheliped) and one small claw, while females have two claws of the same size. The large claw of the male is used for attracting females and for fighting other males. The movement of the smaller claw from the ground to their mouth during feeding illustrates the crabs' common given name; it looks as if the animal is playing the larger claw like a fiddle!
Habitat and Behavior
In the wild, Fiddler crabs prefer to live in the open mudflats under the shade of mangrove trees (usually between 10 - 30 cm (4 - 12 inches) below the surface in these mangrove habitats). Fiddler crabs are a burrowing genus of crabs and readily construct dens in the substrate of their environment. These crabs also prefer to stay close to their burrows. When Fiddler crabs are not active on the surface, they remain inside their burrows. When active on the surface, crabs periodically return to their burrows and emerge a few minutes later. Although it is easy to scare them, Fiddler crabs are not especially shy crabs and can easily be observed scavenging for food and interacting with one another.
Dietary Preferences and Feeding Mechanisms
Fiddler crabs are omnivorous feeders that require water for the feeding process. In their natural habitat, they survive on bits of organic matter, insects, the detritus of decomposing plants and benthic algae they find in the sand along the shoreline. Ideally, you need to give them different types of food to get all the necessary microelements. In captivity, these small crabs enjoy eating flake, thawed freeze-dried, pellet, or thawed frozen foods - alternating proteins and algae foods.
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The fiddler crab's diet primarily consists of algae, bacteria, fungus scraped off of sand particles, and detritus (dead and decaying plant and animal matter). Their feeding mechanism is quite fascinating. Fiddlers use their claws to pick up sediment and scrape food particles into their mouths. The movement of the smaller claw from the ground to their mouth during feeding illustrates the crabs' common given name; it looks as if the animal is playing the larger claw like a fiddle! The crab's smaller claw picks up chunks of sediment from the ground, bringing it to its mouth, and sifting through the sand and soil for anything edible before rolling up what is left into a tiny ball. The presence of these types of sediment balls near the entrance of a sand burrow is a good sign of a crab living there.
Some Fiddler species have modified mouthparts that allow them to feed on the sand. They possess specialized setae that scrape off the organic material so that the sand particles are discarded before the organic material is passed to the buccal cavity.
Detritivores and Ecosystem Engineers
Their diet makes Fiddler crabs “detritivores”: meaning creatures that gain nutrition from eating decomposing plant and animal material. Fiddler crabs play an important role in the salt marsh community. They eat detritus (dead or decomposing plant and animal matter) and are themselves food for a number of wetland animals. Scientific research suggests that the feeding habits of fiddler crabs play a vital role in the preservation of wetland environments - by sifting through the sands, they aerate the substrate and keep the soil saturated with air. Burrow excavation efficiently oxidized anoxic subsurface sediment layers.
Burrowing Behavior and its Impact
In their natural habitat fiddler crabs burrow into mud and sand along the shoreline. Sand is the most popular substrate choice. The burrows provide privacy for mating, sleeping and "hibernating" during the winter months. Fiddlers also burrow into the sand to escape from predators and abandon their temporary burrow once the danger has passed. During high tide, fiddler crabs pack sand into the entrance to their burrows and wait until the tides goes down again. Sand or mud pellets "sprinkled" around the opening of a crab burrow means that a crab lives there.
Biologists studied burrow morphology and observed that generally, all Fiddler crab’s burrows were nearly vertical and straight, mostly unbranched with an enlargement at the terminal end. In general, their length was about 10 - 20 cm (4 - 8 inches). In the wild, Burrows did not necessarily contain standing water but were usually only damp at low tides.
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Molting and Regeneration
Like all crabs, fiddler crabs shed their shells as they grow. Fiddler crabs reabsorb their shells rather than shedding them as they grow. If they have lost legs or claws during their present growth cycle, a new one will be growing when they molt. If the large fiddle claw is lost, males will begin to develop one on the opposite side after their next molt. Newly molted crabs are very vulnerable because of their soft shells, so it is best to leave them to molt and recover peacefully. Don’t be alarmed to find a molting Fiddler crab lying on its back as this is normal! During a molt and shortly after, they are reclusive and hide until the new shell hardens. If you leave their shell in the enclosure afterward, though, they will eat the remaining discarded shell for it is a good source of calcium! The molting process starts in 8 - 12 days when the upper layer of the cells starts separating from the cuticle. At that time you will notice that Fiddler crab starts eating less and less the closer is the moment of molting. The process of molting does not take much time (5 - 15 minutes). Like all invertebrates, Fiddler crabs need adequate calcium supplements and trace minerals in the molting process.
Male fiddler crabs are capable of partial molting. Unlike most crabs, male fiddler crabs have the ability to quickly regrow their large claw without a complete molt, if it is damaged in a fight. These quickly regenerated claws are often not as strong, or as durable as their original claws.
Reproduction and Life Cycle
At one to two years, the fiddler crab reaches sexual maturity. Spawning season is from June through August. Eggs incubate for 14 days before hatching. Fiddler crabs live up to 1.5 years. Fiddlers have a remarkable mating ritual: male fiddlers line up beside the tidy burrows they have dug and move their chelipeds (large claws) back and forth in a fiddling motion to attract females. Females, fresh from foraging, will walk past and if a male catches a female's eye, he will tap the ground with his claw. If she is still receptive to his advances, she will enter his burrow. They mate inside the burrow and she emerges two weeks later to release her sponge (fertilized eggs) into the water.
Fiddler crab mating is an interesting ordeal. First of all, these species show two types of mating behavior in sex-mixed colonies. A male courts a nearby female, usually the nearest one. The male leaves his burrow and approaches the female. A burrow holding male induces a wandering female into his burrow. If a female is ready to mate she will choose one of the waving males. In case the mating is not successful, the female will emerge from the burrow in a few moments and continue wandering. If it is successful she will not emerge, but the male will come out after about 5 minutes and begins to gather up sand to block the entrance to his den. Each male consistently courted females within 1.5 m of his burrow. After underground mating, the male will leave the den and make, or fight for another one. The female stays underground for about two weeks (11-14 days), while she incubates her eggs. Like with shrimp, the clutch size of Fiddler crabs significantly increases with female size. After the incubation period, the female will enter the water and release her eggs, the larvae will hatch immediately. The larvae are called zoea and the closely resemble water fleas. After three or so molting periods, into different stages of zoea, the baby crabs will molt into their final larva state. This stage is called megalopa and is similar to a crayfish in appearance. They are still free swimming at this stage but tend to settle. Rearing of Fiddler crab larvae is only possible in marine water. However, in the wild, breeding periods are not coincident with the wettest times of the year, but with the months following these times. Furthermore, this coincided with the beginning of the growth of new cohorts.
Keeping Fiddler Crabs in Captivity
The usual mini or fiddler crabs you can find at your local pet store are semi-terrestrial brackish crabs, which will mean they need some aquarium salt in their water as well as access to occasional air and dry land. Fiddler and other mini crabs require at least a 5-10 gallon tank, with preferably as many as or fewer than 4 crabs per 10 gallons. Be careful choosing a filter and aquarium top, too, as they are amazing escape artists and will often climb away to hide in your filter or be found outside your aquarium! This is also why providing a place for them to hide in the enclosure and substrate in which to burrow is so important as well. Fiddler crabs do well at a range of temperatures between about 72-85 degrees Fahrenheit and an additional heat source may be needed to maintain this temperature. They live up to 2-3 years, are nocturnal, and typically grow only approximately 2-3 inches wide.
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Fiddler crabs are not fully aquatic, they require an enclosure that offers both water and dry land. The best setup for their needs is a paludarium tank. When done properly a Fiddler crab may also be housed in a fully aquatic tank that has a floating land area, with a dry substrate where the crab can exit the water. A partially filled aquarium with large rocks above the water level can also be used. These kinds of setups need to be done carefully to ensure the crab has easy access to the dock, but can not escape the enclosure.
Most Fiddler crabs in the pet trade come from brackish swamps with low salinity levels, because of this some pet stores mislabel them as freshwater crabs. Fiddler crabs can live in water with a low salinity level, but they can not survive in freshwater for a long time. The preferred saline ratio for fiddler crabs is teaspoon of marine salt, for every gallon of water (~4.5 liters) of water. It is advisable to use Instant ocean marine salt for brackish setup. Chlorine is dangerous to Fiddler crabs, so it is important to dechlorinate any water before it is added to the tank. Salinity level between 1.01 and 1.08. Constant temps between 25 - 31C (~75 - 87F), with no major fluctuations, pH level of 7.2 - 8.2. Adequate aeration from an air pump, or filter outlet. Although Fiddler crabs can osmoregulate powerfully over a wide salinity range. They do not like the high level of salinity.
Tankmates and Compatibility
Keeping Fiddler crabs with other creatures can be difficult due to their semi-terrestrial needs, but it can be done. Whenever you keep a community tank, it is important to ensure the needs of all included species are fully met. In the case of Fiddler crabs, any tankmates must be saltwater animals that can survive in the lower salinity levels of brackish water. Fiddler crabs are small, so keep this in mind when choosing tank mates, larger predators may decide to snack on it. Do not keep them with other crab species. It is recommended that you keep this group as a harem, one male will multiple females, as males will fight. They don’t require a ton of space and up to 4 crabs can easily be housed in a 10-gallon aquarium.
Territoriality and Social Behavior
Fiddler crabs are a very active and social crab species. Males are highly territorial and will fight each other, for burrows and space. They are a burrowing genus of crabs and readily construct dens in the substrate of their environment. These crabs also prefer to stay close to their burrows. When Fiddler crabs are not active on the surface, they remain inside their burrows. When active on the surface, crabs periodically return to their burrows and emerge a few minutes later. Although it is easy to scare them, Fiddler crabs are not especially shy crabs. They can easily be observed scavenging for food and interacting with one another.
According to the observations, Fiddler crabs always maintain and defend territories around the burrow area. They delimit territories by waving the male major claw chela and by standing in a statuesque pose known as “posing”. Although, males use their major claws as the main weapon in combat and defense of their territory. The length of a territorial dispute depends upon the value of a burrow as a resource, the cost to maintain it, and the local population density. Food levels can affect time spent fighting as well.