How a swimming sea cucumber intercepts sinking carbon in the Galápagos

Reading path: Underwater images and evidence →

A sea cucumber in the water column

Most sea cucumbers live on or near the seafloor, feeding on organic material in sediments. Pelagothuria natatrix is an unusual exception: it spends its life in open water. A new Ecology paper published on 23 September 2026 examines how this swimmer feeds in the midwater around the Galápagos Islands. The research matters because it shows one route by which sinking organic particles can be intercepted before reaching the seabed. It does not measure a global carbon removal rate. [1] [3]

A web that gathers the marine snow

The animal’s feeding structure is made from long, modified tube feet joined by a thin web. In a typical feeding posture, it spreads this veil upward like a funnel. Sinking particles meet the web and are gathered by the tentacles around the mouth. The same structure also provides propulsion, so feeding and swimming are linked rather than separate tasks. That design helps explain how an echinoderm can occupy a drifting, midwater niche while many of its relatives forage on the bottom. [1]

What the submersible observations show

The author made observations, video recordings and collections during 23 dives in a research submersible across the Galápagos archipelago. Survey dives moved gradually through the water column and then followed the island slope. The study describes P. natatrix as a prominent grazer between roughly 250 and 650 metres, with an estimated density of one animal per 300 cubic metres within that depth range. That is a local estimate from the surveyed waters and method; it is not a census of the region’s entire midwater population. [1] [2]

From feeding behavior to food-web consequences

The argument about carbon is a food-web mechanism. Organic matter produced near the sunlit surface sinks as marine snow. Many particles are consumed or altered on the way down; particles that reach the bottom become food for benthic scavengers and microbes. By capturing some of that rain in midwater, Pelagothuria can redirect material into its own biomass and, in turn, make it available to predators in the pelagic food web. The study calls this flux feeding. It gives a concrete example of how an animal’s feeding position can change where material enters a food web. [1]

What the evidence cannot yet quantify

The distinction between a pathway and a rate is important. The paper documents behavior and a plausible transfer route, but the observations do not tell us how many tonnes of carbon this species moves each year, what fraction of sinking carbon it consumes, or how that contribution compares with other midwater grazers across ocean basins. The author examined gut contents from eight specimens, a small sample that helps identify what was eaten but cannot by itself establish the species’ full diet or seasonal variation. [1]

A useful comparison, with different sampling limits

A NOAA-hosted 2019 study reviewed ROV footage from 12 Central Pacific dive sites and recorded 97 Pelagothuria across depths from 196 to 4,440 metres. It found some sightings at or near the seafloor, a reminder that observation context and definitions matter when describing a species as wholly pelagic. Those records are not a direct replication of the Galápagos dives: the location, expedition, equipment and sampling design differ. Together, the studies show why new midwater observations can sharpen ecological questions while still leaving abundance and carbon flux uncertain. [4] [1]

What a stronger next measurement would add

To turn the mechanism into a regional carbon budget, researchers would need repeated measurements of animal density and feeding across seasons and locations, along with particle flux above and below occupied depths and estimates of how much captured material is assimilated, respired or returned as waste. The public Dryad record makes the underlying dive notes available, including sighting pages and field summaries, so readers can inspect the observational foundation. For now, the defensible conclusion is narrower and still useful: in the surveyed Galápagos midwaters, this swimming sea cucumber was common enough and fed in a way that can reroute sinking organic matter before it reaches the seabed. [2] [1]

Sources & further reading

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