Day 27: 17 July 2020

This purple dude is about 50cm in size. They exist in all shapes and sizes, colours and haptics
This purple dude is about 50cm in size. They exist in all shapes and sizes, colours and haptics

The cruise is slowly coming to an end and we get a sense of home as we cross 54° N latitude. However, sniffing familiar air again doesn’t mean that there is no more work – on the contrary: we have reached the deep abyssal plains of the North Atlantic and start our measurements from 3500m down to 4500m water depth to get an idea about species distribution along a depth transect. Abyssal plains are the remote result of a spreading sea floor that is constantly being pulled sideways and stretched, meanwhile being covered in a fine grained sediment layer of silt and clay. Those plains make up almost 50% of the Earth’s surface – and they play a major role in the ecosystem.

To the untrained eye, there is not much to see: basically ‘only’ mud, wormholes, a fish every now and then, and countless numbers of sea cucumbers in all forms and sizes. For an expert however, the vast sediment desert and its residents are crucial to the fundamental carbon circle and, therefore, to the entire nutrition cycle. Anything that is washed into the oceans by rain, rivers, and organic matter from dead biomass eventually settles on the seafloor. Abyssal plains therefore act as carbon sinks, with carbon being held in the sediment until it is reworked by crucial fauna. The time taken for material to reach the seafloor can be incredibly slow, with sedimentation rates in the order of mm/1000 years.

This is also what life is like down in the deep:  Unhurried and relaxed. Predators are opportunist feeders – waiting for prey to swim by rather than actively hunting it. Less so are the sea cucumbers or holothurians, who constantly devour the sediments and filter it for nutrients. Through their transparent bodies, this process is clearly visible and basically what goes in, comes out again, once particular organic matter has been digested. They leave (sea-cue-)cumbersome traces on the sediment as they gradually sweep over the seafloor– and sometimes, these trails end all of a sudden in the middle of nowhere, with no one in sight anywhere. This is when sea cucumbers decide to travel for a longer distance – they flood their body with water until they float and then go with the flow, wherever it may take them. Some specimens also have a sail that enables them to aim in a certain direction.