Swimming sea cucumber steals carbon before it sinks

A sea cucumber that never touches the seafloor has been caught intercepting carbon from the deep ocean. Around the Galapagos Islands, the swimming sea cucumber Pelagothuria natatrix hangs in the midwater and gathers sinking particles of marine snow long before they can reach the bottom, according to research reported by MBARI senior scientist Bruce Robison. The result, published in the journal Ecology, shows a sea cucumber actively short-circuiting the downward flow of organic carbon that feeds the deep sea. It adds a previously invisible middleman to one of the largest carbon fluxes on the planet.

Of the more than 1,700 described species of sea cucumbers, Pelagothuria natatrix is the only one that lives its entire life in the water column. It is a gelatinous, nearly transparent creature about 17 centimeters long. While most sea cucumbers crawl along the seabed and feed on sediment, this species holds a webbed veil of tube feet open like a funnel toward the surface, collecting dead algae, fecal pellets and other organic debris as it sinks. The compact body acts as ballast while the veil sweeps, and a ring of feeding tentacles around the mouth gathers the trapped particles.

A swimmer in a world of crawlers

Robison documented the behavior during 23 dives from the research submersible Johnson-Sea Link II along the steep island slopes of the Galapagos. The species was abundant at depths of 250 to 650 meters, at densities of roughly one individual per 300 cubic meters. The depth at which animals occurred, and their height above the bottom, varied independently of the seafloor depth itself, a sign that they were genuine midwater residents rather than visitors drifting up from below. The submersible held steady at neutral buoyancy to follow swimming individuals and to watch stationary ones feed for long stretches.

Most of the animals encountered were already eating. Of 104 individuals observed, 73 were in a characteristic passive feeding posture, with the body oriented vertically, the mouth pointed upward and the webbed podia spread outward like a funnel. Eight collected specimens were dissected: gut contents included phytoplankton debris, fecal pellets and detrital matter in every animal, plus pteropod shells, salp stomachs, fish remains, sand grains and ctenophore fragments in some. The species was first described in 1894 from specimens taken by the fisheries steamer Albatross in the eastern tropical Pacific, yet its natural history has stayed almost unknown until now.

Flux feeding and why it matters

Ecologists call this strategy flux feeding, a feeding mode described by Jackson in 1993 for grazers that harvest the downward rain of settling particles. The advantage for a midwater grazer is straightforward. Organic debris in the midwater is nutritionally richer than the same material after a long descent during which microbes have picked it apart, so intercepting it close to the source is a better deal than waiting for leftovers on the seabed. What the animal eats is either respired or packaged into lozenge-shaped fecal pellets, while its predators gain direct access to the intercepted energy. Robison found that the Galapagos midwater community looks different from the better-studied waters off Monterey, California, where salps and larvaceans dominate grazing. The Galapagos is swept by strong currents, and the strong-swimming sea cucumber appears to have claimed a niche there that weaker swimmers fill elsewhere.

The animal holds one more surprise. Gentle mechanical contact triggers a bright, sustained pulse of green bioluminescence, the brightest such light Robison observed at depth. Chromatophores in the skin can also flush an individual from transparent to bright pink. Unlike its close relative Enypniastes eximia, which sheds glowing, sticky skin to deter predators, Pelagothuria has tough skin that stays put. There is a practical reason to pay attention to all of this. Deep-sea mining of polymetallic nodules releases sediment plumes into the water column, and the same feeding strategy that makes this species an efficient carbon interceptor also makes it vulnerable to fouling by mining discharge. It occurs in the Clarion-Clipperton region of the Pacific, one of the areas targeted for nodule mining. For an animal that lives entirely off the seabed, the water it swims in is everything.

The broader point concerns how carbon moves through the ocean. Plankton at the surface convert carbon dioxide into organic material, and the usual picture sends that carbon sinking through the midwater to the deep seafloor, where scavengers and microbes lock it away. Pelagothuria interrupts that journey. What it intercepts enters the pelagic food chain instead of the slow detrital one, and in tropical waters, where this population is abundant, that rerouting may add up to a measurable term in the ocean carbon budget that models of the biological pump have not accounted for.

Read the reporting on the study at Phys.org and the original paper in Ecology.