Deep sea piggy back ride? The curious case of the the creepy carrier crab.
The crab species Paromola cuvieri is known to the Danish as a witch (heksekrabbe) and to the Algerians as diable, the devil. It is usually observed at depths of over 150 m, up to 1212 m. Here, it creeps along muddy sea floors and cold water coral and sponge communities. Much of the biology and behaviour of this species remains a mystery, despite it being relatively abundant in the East Atlantic and the Mediterranean. This crab is often observed carrying materials collected from its environment in it’s shortest, specialised pair of legs. Its relatives from shallower waters exhibit similar behaviours. They carry materials which are highly abundant in their environment, in order to camouflage their bodies from predators. But in Paromola cuvieri‘s deep sea habitat, where it is extremely dark, what is the point in camouflage?
Paromola cuvieri is usually observed carrying live cold water corals and sponges and even plastic litter, despite these not being all that common in their environment. This indicates that these materials are not carried in an attempt of camouflage. So this creepy crustacean may be smarter than it appears.
What is a cold water coral?
Each individual coral animal consists of a fleshy polyp. Some species form vast, branching colonies and others remain as solitary polyps. Some species produce hard, stone-like cups in which each polyp lives and others do not.
Unlike tropical corals, which rely on the photosynthetic organisms within their tissues to provide energy for them, cold water corals do not require sunlight to survive. Therefore, they can make their homes in the murky depths of our oceans. Cold water coral polyps feed by filtering particles from the water with their tentacles, so they are relatively large in order to maximise this process.
What is a sponge?
Sponges are relatively simple, multi cellular animals and are almost sessile. They inhabit sea floors worldwide, in shallow and deep waters. They feed by drawing water into their bodies through small pores all over their surface. They have an internal network of chambers connected by canals through which the water flows. Here, food particles are filtered out of the water, then the water flows back out of the body through more pores.
A creature of the deep
The lack of light in the deep sea means sources of food are very different to those in most other environments. This is because photosynthetic organisms (those which are able to create energy using sunlight, like plants), which usually provide energy to the animals which eat them (and these animals provide energy to those which eat them and so on) are not able to survive. Like many deep sea species, Paromola cuvieri scavenges upon dead animals such as whales, fish and sharks, which have fallen to the sea floor from above. However, these sources of food are relatively difficult to find. Another challenge for those living in the deep are very cold temperatures, which are no higher than 4⁰C in most areas. There are also crushing pressures of up to one thousand times that which we experience in our atmosphere at sea level. These extreme conditions demand for P. cuvieri to be highly specialised, which it certainly is. It moves very slowly, which allows it to save as much energy as possible while it searches for food in this extreme environment. It also has a narrow pair of claws, specially adapted for rummaging in the sediment for buried food.
However, there are possible costs of these adaptations. The scavenging diet of this crab could increase the likelihood that it will come into contact with predators or competitors. This is because when large animal carcasses fall to the sea floor, they attract a wide diversity of animals, including sharks and other crabs. The slow moving nature of P. cuvieri could make it vulnerable to attack from faster moving predators, as it cannot escape. Also, the ability of this species to defend itself from predators and competitors is limited, due the narrow shape of its claws. So perhaps this species carries hard or robust materials such as plastic, corals and sponges in an attempt to protect itself?
The materials that P. cuvieri chooses could have a role in protection from predators. Corals have the ability to release stinging cells into the water to defend themselves and sponges have sharp spines which deter predators. It is thought that our crab utilises the defense strategies of these animals by carrying live individuals above its body, hoping to increase it’s chance of survival.
P. cuvieri‘s carrying behaviour could possibly be beneficial for the animals it carries too. Many cold water coral and sponge species have the ability to reproduce by fragmentation (where a whole new organism grows from a piece of another). Therefore, as these crabs carry pieces of coral and sponge around, putting them down in new locations, they could be helping these species to spread around the sea floor. This is particularly important for cold water corals, as they grow incredibly slowly (less than 25mm a year). Therefore they are incredibly vulnerable to disturbances such as damaging fishing practices and oil and mineral extraction. The wider the distribution of these species, the more likely they are to find a good habitat where they will thrive, increasing their populations. So, perhaps this crab deserves to be called something nicer than a witch or a devil?