Enypniastes eximia: Can a sea cucumber be extreme?
We’re all probably familiar with sedentary (inactive) sea cucumbers that grace the shallow subtidal waters of the ocean. But Enypniastes eximia (E. exima) isn’t just any regular inactive sea cucumber that belongs to the phylum echinodermata and order Elasipodida. As E. exima is classed as being an active swimming sea cucumber. The swimming sea cucumber was first videotaped and collected at 717 metres depth on the 7th of April 1984 just off Great Abaco Island in the Bahamas. This was a rare first sighting as E. exima are less frequently seen at shallower depths above 1000 metres. This is down to the fact that it’s a prominent benthic boundary layer dweller, so it spends most of its time lurking around 50 metres above the sea bed. So, you’re probably all wondering what characteristics can a swimming cucumber possible have that can merit the title of being extreme? Well, it certainly has a strange look about it and even a couple of tricks up its sleeve.
At a first glance its highly adapted body may look extremely strange to us, but it’s functional for swimming. They have a distinctive bulbous barrel shaped body with a large webbed vampiric veil which is made up of 12 conically shaped podia (feet) at the front of its body (Figure 1). Along with 2 rectangular shaped veils that are composed of 10 to 15 podia, which are located at the back of the animal (Figure 1). The rhythmic pulsations of the vampiric-like webbed veil provides lift while the back two triangular veils provide steering and stability.
Its body ranges from 6 to 20 cm in length, but Individuals have been measured up to 25 cm in length. One of E. exima most notable features is that its gelatinous body is fully transparent, meaning that its internal organs such as its coiled up intestine can be easily viewed through its body wall. Although, the body is fully see-through the colouration can vary between individuals. It is thought to be linked to the organisms age, as juveniles are pale pink while large adults are dark brown-red to crimson in colour.
Unlike most other sea cucumbers that you’re probably more familiar with, the swimming sea cucumber only lands on the seabed to feed and spends most of its time swimming. A study by Suguru Ohta, estimated that at one moment 90% of his studied population were swimming while only 10% were on the sea bed feeding. This ability of E. exima to stay afloat for extended periods is rarely seen in other shallow water sea cucumbers. When the sea cucumber approaches the seabed to feed it erects the vampiric-like veil at the front of its body, which then through a wave like motion propels the animal downwards.
Once contact has been made with the sea bed the podia (feet) become buried into the sediment to act as an attachment point, effectively anchoring the organism to the seabed. Shortly after the animal has safely attached to the sea bed feeding begins. Sediment particles are carried towards the mouth via a series of non-specific grasping motions made by bifurcate (two pronged) tentacles (Figure 2). The feeding tentacle grasping motions can be described as being non-specific, because the organism doesn’t actively select particle sizes.
The animal can also move itself across the sediment while feeding by using the following two methods. Its erect webbed umbrella like veil acts as a sail to catch water currents that flow across the sea bed. Or the action of its grasping tentacles while feeding pull the sea cucumber across the sediment.
When the swimming sea cucumber has taken its fill of sediment it takes off in a spectacular fashion by thrusting its front facing webbed umbrella downwards in sync with its two back facing veils. To aid with take off the body can also be seen to curl downwards to increase the amount of thrust needed to generate lift. The feeding tentacles are then retracted back into the body, ready for the next feeding session.
This whole feeding process described above is demonstrated by the following video:
Figure 3: Note how the sea cucumber swims using its vampiric-like webbed veil and the 2 veils at the back of its body. Also note how it uses its grasping bifurcate (two pronged) tentacles to feed, and how its body thrusts downwards during take off. Source.
Unsurprisingly, like most other extreme deep sea dwelling creatures the swimming sea cucumber has bioluminescence abilities. Bioluminescence is when organisms emit light from their bodies via the chemical break down of luciferin. The sea cucumber emits light from its podia (feet), tentacle tips and from small points scattered around its body surface. The source of bioluminescence comes from specialised cells called granular cells (light producing cells) which lie just below the organisms skin.
The bioluminescence observed is thought to be used as an anti-predator defence strategy, as it can only be locally stimulated through external contact with objects in its environment (Figure 4). When a predator attacks the sea cucumber it glows, which reveals the presence of it attacker to its own visually cued predators, which effectively scares the predatory away.
If this isn’t enough to scare off the would be attacker the sea cucumber has another trick up its sleeve. Its skin is fragile and sticky, meaning when a predator attacks it gets temporarily marked by a cloud of glowing tissue, which makes the predator more conspicuous to its own predators. It takes around 1-5 days for the sea cucumber to regenerate both its skin and bioluminescence powers.
The peculiar defensive strategies of sea cucumbers aren’t just exclusive to this deep sea wonder, as others undergo evisceration (throwing up their internal organs) to scare away would be predators. However the swimming sea cucumber has a more extreme form of anti-predator defence, as it literally creates a dazzling cloudy light show of its own skin. Proving that the flying sea cucumber is as amazing as it is disgusting.
Figure 4: Note in the beginning how E. exima’s whole body is bioluminecent, but as the video progresses how other individuals can show more localised bioluminescence. Source.
So, can this sea cucumber be viewed as extreme?
The swimming sea cucumber is not only extreme in regards to it’s deep sea life style. But also in terms of the vampiric layout of its transparent (see through) body and mostly mid water life style, which is rarely seen in other inactive shallow water sea cucumbers. When E. exima should be resting on the sea bed feeding like other inactive sea cucumbers – it still chooses an extremely active life style by moving via its two pronged feeding tentacles and vampiric cape. It also has an extreme anti-predator defence mechanism, involving the loss of its own glowing flesh. This mechanism is extreme even in the context of other sea cucumbers that eviscerate – so well done E. exima for proving that the humble sea cucumber can have a extreme side.