Crabs Aren’t Just a Dinnertime Favorite

Humans are mainly appreciative of the delicious taste of crabs, and not particularly of their importance as ecological kingpins. But the numerous crabs present in saline habitats are often extremely important in their food webs. Fiddler crabs are often used as bait and can be numerous on muddy tidal flats. They also serve as food for birds such as yellow crowned night herons and have a large role in transfer of nutrients from detritus and algae to higher levels of the food web. Scientists have long known that they have a distinct internal tidal “clock” that allows them to know when to come on the surface to forage. 

Blue crabs are not just delicious, their role as predators/scavengers in shallow water habitats is significant and removal of too many may have serious ecological impacts on the soft bottom community. Certainly they are one of the best defended invertebrates and use their claws very effectively. The photo illustrates one of the safest ways to hold a blue crab without being pinched, by the rear swimming flipper. Males have blue claws and females red-tipped claws; in essence the females have “lipstick” which attracts the attention of males.
Anytime you are in mangrove areas you will notice a small crab scurrying around in the trees. This crab, Aratus, feeds on leaves, detritus and some animal protein and is thought to have an important influence not only on nutrient cycling in mangrove forests, but to affect the types of mangroves present. A much rarer and larger crab in our area of SW FL is the mangrove root crab, Goniopsis cruentata. I found one of these for the first time recently in Oyster Creek. The scientific literature indicates that the feeding of this crab can increase the number of red mangroves present in the tidal forest relative to white and black mangroves. This occurs because it does not like to eat red mangrove seedlings with high amounts of tannins. You will note that the mangrove root shown in the photo is quite red inside with tannins. These chemicals not only can be used to tan animal hides, but will greatly reduce the ability of an animal to digest its food by binding with proteins in its gut. Humans have figured out that milk added to coffee will bind to tannins and decrease the bellyache from their ingestion. The mangrove root crab is likely only an occasional tropical visitor to our shores. You will be very lucky to see one. I ran into this individual while walking a trail that I had walked many times before. This proves the point I often make which is that repeated visits to the same natural areas will yield interesting bits of natural history gradually over time.