Sea cucumbers are the humble vacuum cleaners of the sea, quietly tidying the environment as they creep across the sea floor on hundreds of tube feet. I consult with projects growing sea cucumbers for the lucrative Chinese market; so I have studied them for many years and find them fascinating – and decidedly weird.
Sea cucumbers are echinoderms, related to starfish, sea urchins and sand dollars. They inhabit all the seas of the world, from shallow water to the deepest ocean trenches. Only about seventy of the 1700 species have commercial value.
Holothuria scabra, the species we work with, eats sand and digests the organic matter it contains. This means the sand exits ‘cleaner’ than it entered!
These simple animals have no brain, relying on a nerve network instead – like the human reflex system. They have no eyes, yet they can detect change in light levels through their skin. And they breathe through their rear end, absorbing oxygen from the water through their vascular system.
Speaking of the anus, some species of sea cucumbers have a comensal relationship with a pearlfish (of the carapidae family) who live inside the cloaca via the anal opening.
One day in a country where I was working, I was inspecting a local species of sea cucumber with some fishermen from the village watching. As I was handling the animal, a pearlfish came out of the sea cucumber’s rear end. One of my audience members gave an exclamation of fear and they all backed away.
Surprised, I asked them why they reacted this way. The captain of one of the local boats explained that the pearlfish, in their traditions, had the ability to grow incredibly large and had been known to crush their fishing craft. I asked if they had witnessed this. None of them had, but they said their grandfathers had passed on the story as a warning.
Sea cucumbers cannot outrun predators; instead, they have various defences to protect them. Most, when startled, will expel half of their internal, which presumably scares predators (maybe – so many things we don’t know).
Some have sticky Cuvierian tubules they release to entangle predators. These tubules also contain a toxin called holothurin. Fishermen cleaning sea cucumbers often find fish kills down-current because of this toxin.
If you cut a sea cucumber in half, both sides will regenerate into a new animal. Some sea cucumbers can swim, like the Enypniastes in this video (also known as the ‘headless chicken fish’).
Sea cucumbers are just one of the infinite marvels of creation.