Sea cucumbers eat decaying organic matter, microscopic organisms, and algae that they sift from sand and sediment on the ocean floor. They are deposit feeders, meaning they vacuum up mouthfuls of sand, extract the nutritious bits, and expel the rest. This simple-sounding diet makes them one of the ocean’s most important recyclers.
The Main Components of Their Diet
A sea cucumber’s meal is mostly sediment. Mixed into that sediment is the food they’re actually after: organic detritus from decomposing algae and animals, bacteria, single-celled organisms, and tiny fragments of seaweed. In areas near seagrass beds or kelp forests, their diet leans heavier on decaying plant material. On coral reefs, they ingest coral sand and rubble to digest the organic matter clinging to each grain.
In aquaculture settings, sea cucumbers readily eat common seaweeds like kelp, sea lettuce, and sargassum when these are mixed with sea mud. Wild species don’t graze on living seaweed the way a fish might. Instead, they wait for plant and animal material to break down and settle onto the seafloor, then sweep it up along with everything else in the sediment.
How They Collect Food
Sea cucumbers have a ring of sticky tentacles around their mouth, typically 10 to 30 depending on the species. These tentacles come in different shapes. Deposit-feeding species (the majority) have flat, shield-shaped tentacles called peltate tentacles that press against the sediment like a mop. Suspension-feeding species, which are less common, have branching tentacles that catch drifting particles from the water column.
Food sticks to the tentacles through a combination of adhesive secretions and mechanical trapping. The animal pushes each tentacle into the sediment, collects a load of particles, then stuffs the tentacle into its mouth and wipes it clean against its throat. It cycles through its tentacles one or two at a time, creating a slow but continuous feeding rhythm. This process looks unhurried, but over the course of a day, a single sea cucumber can process a surprising volume of sand.
They’re Picky About Particle Size
Despite their reputation as indiscriminate bottom-feeders, most sea cucumbers are selective eaters. Around 85% of shallow-water species studied show a clear preference for organic-rich particles, and they actively choose specific grain sizes. One Mediterranean species, for example, gets about 75% of its diet from fine particles smaller than 250 micrometers, roughly the texture of fine beach sand. It extracts the most organic matter from these small grains, likely because finer sediment has more surface area for bacteria and detritus to cling to.
Preferred grain size varies by species. Some tropical species go for coarser particles up to 3,500 micrometers, while others stick to the 125 to 250 micrometer range. This selectivity matters ecologically because it means different sea cucumber species can coexist in the same habitat without directly competing for food.
Deep-Sea Species Eat Differently
Sea cucumbers living on the abyssal plain, thousands of meters below the surface, face a very different food landscape. Organic material is scarce, arriving mainly as “marine snow,” the slow rain of dead plankton and waste drifting down from above. Deep-sea species prefer much finer particles than their coastal relatives, selecting grains as small as 7 to 54 micrometers.
One notable difference: deep-sea species eat fecal pellets from other animals, while coastal species do not. When food is limited, nothing goes to waste. Deep-sea species also host fewer types of gut bacteria (about seven identified groups compared to 15 in coastal species), which likely reflects the simpler, less varied diet available at depth. Despite these constraints, 70% of deep-sea species still show selectivity for organic-rich sediment, actively seeking out the most nutritious patches on an otherwise barren seafloor.
How Much Sediment They Process
A single Australasian sea cucumber in laboratory trials consumed about 6.7 grams of wet sediment per day. That may sound modest, but scale it across a healthy population and the numbers become enormous. On coral reefs and seagrass beds, dense populations of sea cucumbers collectively rework tons of sediment per hectare each year, turning over the top layer of the ocean floor the way earthworms turn over garden soil.
This constant processing has measurable effects on ocean chemistry. As sediment passes through a sea cucumber’s gut, which is slightly acidic at a pH of about 6.7, calcium carbonate particles partially dissolve. On one studied reef at One Tree Reef in Australia, sea cucumbers were responsible for nearly 50% of nighttime calcium carbonate dissolution. That dissolution releases alkalinity into the surrounding water, which can partially buffer against ocean acidification and support the chemical conditions corals need to build their skeletons.
Their Role as Ocean Recyclers
Sea cucumbers don’t just clean the seafloor. They transform what they eat into forms other organisms can use. Their digestion increases the release of ammonium from enriched sediments, a form of nitrogen that stimulates algae growth and feeds the base of the food web. On coral reefs, this nutrient recycling produced two to three times more alkalinity than the dissolution caused by organisms boring into hard coral rock.
In coastal areas affected by excess organic waste, whether from aquaculture runoff or natural sources, sea cucumbers help break down and redistribute that material. Their constant feeding prevents organic matter from building up in sediments, where it would otherwise deplete oxygen and harm other bottom-dwelling creatures. This is why some aquaculture operations now raise sea cucumbers alongside fish or mussels, using them as biological cleaners that convert waste into a marketable product.
Feeding Timing
Many sea cucumber species are more active at night, and aquaculture researchers recommend providing food before the nighttime feeding peak. However, studies on the commonly farmed Japanese sea cucumber found no significant difference in foraging and defecation behavior between day and night, with digestive efficiency staying consistent across the 24-hour cycle. Feeding patterns likely vary by species and habitat. In areas with heavy predation pressure, nocturnal feeding helps sea cucumbers avoid visual predators, while species in low-predation environments may feed more continuously regardless of light.