April 20, 2018
During the Gulf of Mexico 2018 expedition, we have encountered many deep-sea holothuroids (sea cucumbers) that not only swim, but rise, sink, and hover. These behaviors suggest that some species are specially adapted to manage their buoyancy (the force that allows something to float, sink, or remain neutral in a liquid) remarkably well. How do they do it?
Some species show a common escape behavior of thrusting themselves up off the bottom with a strong flexing of the body that continues rhythmically in a swimming fashion. We have seen this with the large blue-purple animal of the genus Benthothuria. When disturbed, Benthothuria gracefully thrusts its colorful body upward, defecates to shed some ballast, floats, then slowly descends back to the bottom.
Another deep-sea holothuroid, of the genus Enypniastes (so bizarre and common in the deep Gulf of Mexico that industry remotely operated vehicle pilots nicknamed it the “headless chicken monster”), is a graceful purple swimmer that seems to spend most of the time a few meters above bottom. However, it does not feed there. Rather, it drops from a neutrally buoyant "hover" to the bottom and rapidly shovels sediment into its mouth with a ring of feeding tentacles. When finished or disturbed, it makes a strong push with a large fin-like structure and swims away, eventually becoming neutral again and drifting until the next feeding trip to the bottom. Animals living close to and interacting with the bottom are called bentho-pelagic.
Buoyancy control is a critical for these holothuroids. Experiments using freshly caught specimens have shown that some holothuroids float to the surface of aquaria if the gut is devoid of all ingested mud—this is easy to see, as both the animal and the gut are transparent. If there is mud remaining in the gut, they are either neutral or slowly sink to the bottom. So, the body density is due to a combination of heavy mud plus the low-density gel of the body wall.
But these animals do not float uncontrollably to the surface of the ocean after evacuating mud from their guts while drifting in the water column. Somehow, they get heavy again without mud, drop to bottom, and fill up with mud again. They seem to have active density control of their tissues. Questions about how the animal controls its buoyancy still remain.
Holothuroids are in the phylum Echinodermata, which is found at all ocean depths and can be especially abundant in the deep ocean–even in the deepest trenches. In shallow water, they are most abundant and diverse in the tropics. Although the overall body shape shows few hints of the echinoderm’s definitive five-part symmetry, there are five main muscles, nerve fibers, and vascular systems. In a swimmer such as Enypniastes, the tube feet are modified to form fins.
This video features some of the sea cucumbers seen swimming during Dive 07 on a previously unexplored mud volcano.
Text contributed by Robert S. Carney, Emeritus Professor of Oceanography, Louisiana State University