The salt marsh swarm, crabs carpeting the mud – Uca spp.
The salt marsh can be a challenging environment with the high fluctuations in water and air exposure, salinity and temperature. If that wasn’t enough storms and other natural effects have a much larger impact on these areas typically in North America. This habitat can be found all over the would however below 25N- 25S latitude the salt marsh gets replaced with mangroves. Salt marshes cover around 70% of the wetlands on the mid-atlantic coast in the North of America in comparison to the UK where the salt marsh only covers around 25% of the coast. But why should you care? Salt marshes in the UK are highly bio-diverse with around 15-20 species of flora and fauna on average, US salt marshes are far less bio-diverse however they provide protection for the coastline from storms and floods, useful fisheries as some of the crab species are considered to be a delicacy, carbon sequestration and for recreational use.
The salt marsh in which the swarm inhabits is located on the East coast of Virginia, USA. In this marsh some species of organisms appear to be a lot more at home and can even have a massive population in such a small area. These would be the fiddler crabs, Uca spp.
There are three main species of fiddler crabs which inhabit the salt marsh of Virginia, these are Uca minax commonly known as the red-jointed fiddler crab, Uca pugnax commonly known as the Atlantic marsh fiddler crab and Uca pugilator commonly known as the Atlantic sand fiddler crab (least common of the three). The large populations of these species cover larger areas of the marsh, living in burrows small round holes can be found all across the marsh however other crabs do inhabit the marsh such as Sesarma reticulatum the Purple marsh crab which also build burrows. These burrows can be identified due to them being irregular and oval shaped, the other species is Calinectes sapidus the Blue Crab these are highly aggressive to every species that comes too close to them, these can sometimes be found under the mud of the marsh however spend most of their time out in the water.
These crabs can be show sexual dimorphism which is when organisms from the same species have different characteristics or features depending on the gender, this is represented in fiddler crabs by female crabs having two claws of the same size where as the male has one claw larger than the other. Fiddler crabs primarily feed on algae, bacteria and decaying plant matter which is contained within the mud. When feeding on this the crabs produces a pseudo-faecal pellet which is a small round ball of mud located near the area where they feed by digging up the mud with the smaller claw. Due to the needing to be able to dig up the mud but also need to make burrows from it the ideal marsh mud would be firm lower down but slightly loose on top with lots of nutrients in the mud. The distribution of the fiddler crabs is high throughout the whole of the marsh however at the top of the marsh closest to land is the least populated as the mud is the driest and the nutrients severely lacking in this area, the mid marsh area is highly covered in fiddler crabs with the lowest point in the marsh having slightly less in numbers. This may be due to predation as the blue crabs advance up the marsh as the tide comes in, or it may be due to the mud being too loose to make burrows.
Other animals prey upon the fiddler crab such as many of the marsh birds, fish like the menhaden which makes life more dangerous for the crabs that would want to live closer to the waters edge. Another possible threat for the fiddler crab would be people, the small crabs are not much in terms of a snack however in the pet trade industry they have been know to be circulated and captured as a “fresh-water” crab as they don’t require high amounts of salinity to live. However when kept in the captive environment they can be known to live for around 3 years when in the wild the tend to only live till 2 years old, during the life cycle they tend to moult around 2 times a year. During this time the soft shelled crab spends its time within the burrow away from any predators that may harm the now defenceless crab.
When fiddler crabs are trying to attract a mate, the male bobs up and down whilst waving his major claw, this is to show the females that he has the largest claw compared to the other males which is the desirable trait when breeding within this species. If the male was to somehow lose this lose the other claw would grow to the size of claw which was lost.