8 July 2009 Location: NE Atlantic, Whittard Canyon
Sea cucumbers- animal, vegetable or mineral?
The deep sea is a strange environment with vitually no light, extremely high pressures and low temperatures. This results in some pretty weird and wonderful creatures, perhaps none more weird or wonderful than the mighty holothurian, or sea cucumber. Often when people see a sea cucumber for the first time they are at a total loss as to what it might be animal, vegetable or mineral?! In fact sea cucumbers are animals; they are most closely related to sea stars, brittle stars and sea urchins and although they can be found in shallow waters they are amongst the most abundant large organisms in the deep sea, especially at abyssal depths.
They can take many beautiful or downright bizarre forms, such as the pretty pink Peniagone which are quite small (~10cm) and appear to be very abundant in the Whittard Canyon. The large green species of Benthodytes looks like something straight out of a science fiction novel, as does Deima with its body covered in papillae which wave in the currents like a multitude of tiny arms. The massive purple Psychropotes, which we have also found here in the Whittard Canyon, can reach half a metre in length and have large fleshy ‘sails’ which scientists believe may help them manoeuvre in the currents. Many sea cucumbers have jelly-like bodies, largely composed of water, which are neutrally buoyant, helping them to stay on the sea floor where they live. Others have tiny armoured plates within their skin called spicules, which come in many different shapes and sizes from round wheel like forms to tiny crosses. The shape and abundance of these spicules can help biologists to separate closely related sea cucumbers into different species.
For the inhabitants of the deep sea the only source of food comes in the form of sinking particles from the surface and upper layers of the ocean in the form of dead phytoplankton and zooplankton and even, rather unappetisingly, their faecal pellets. This ‘marine snow’ may not be to our tastes, but with no other options, the organisms inhabiting the deep sea floor have adapted to surviving on this food source alone.
So apart from looking like creatures from the outer reaches of the galaxy and possessing a dubious taste in food, why are scientists so interested in sea cucumbers? Firstly, as some of the most abundant large organisms in the deep sea it is important that we understand their functional role within the ecosystem. Also, by looking at the species distribution and abundance of sea cucumbers across the globe we can begin to deduce patterns of biogeography and understand what may be the most important biological and physical factors controlling the distribution of deep sea organisms. In addition, by looking at the genetics of deep sea cucumbers we can understand how connected populations in the deep sea are, and estimate how far these animals can disperse. These issues are of primary importance when considering the location and design of marine reserves in the deep sea. With increasing human pressures on these areas from oil companies, commercial fishing and even from climate change, we must strive to understand the mechanisms controlling species distributions and the role those organisms play in deep sea ecosystems. This will help to protect these fascinating and poorly understood environments for future generations.
ROV footage of the sea cucumber Peniagone swimming.