Sand eels are slender, elongated marine fish that play a fundamental role in Northern Hemisphere coastal ecosystems. Despite their name, they belong to the family Ammodytidae, making them true fish rather than eels. They are highly abundant in the temperate and polar waters of the North Atlantic and North Pacific, often found near sandy seabeds. Their diet is foundational to the health of the ocean food web, dictating the flow of energy from microscopic organisms to large predators.
The Primary Diet
The diet of sand eels is overwhelmingly composed of zooplankton. Their preferred food source is often copepods, small crustaceans that are rich in lipids and highly nutritious. They also consume other small planktonic organisms, such as cladocerans and the eggs of various marine species.
The composition of their diet shifts with the seasons and the size of the fish. During spring plankton blooms, the smallest life stages of zooplankton, such as nauplii, form a large part of their intake. As the foraging season progresses, adult sand eels target larger, more energy-dense prey. Larger individuals consume larger copepod species, and occasionally, small euphausiids and amphipods become part of their diet.
Feeding Mechanics and Behavior
Sand eels are specialized filter feeders that emerge from the sandy seabed to forage in the water column during daylight hours. They employ a ram-feeding technique, swimming with their mouths open to strain plankton from the water. This method is efficient when prey density is high, allowing them to rapidly accumulate necessary energy reserves.
Foraging occurs in dense schools that move together through the water. Schooling provides defense against predators while maximizing the collective efficiency of finding and exploiting plankton patches. When encountering high concentrations of specific prey, such as fish larvae, their behavior can shift to a more active “pursuer” style of hunting.
When not actively feeding, sand eels retreat to the sandy substrate, burrowing tail-first into the sediment. This behavior is a survival mechanism used at night, during low light, or throughout the overwintering period to conserve energy and hide from predators. They rely on finding sand with a specific grain size (0.5 to 2.0 millimeters) to effectively bury themselves and remain safe.
The Ecological Importance of Their Diet
The plankton-based diet positions the sand eel as a foundational “forage fish” species. They act as a crucial link, converting the energy of zooplankton into a usable, highly nutritious form for higher trophic levels due to their high lipid content.
A wide range of marine predators relies heavily on sand eels for survival. This includes numerous seabirds, such as puffins, kittiwakes, and terns, which depend on them to feed their chicks during the breeding season. They are also a primary food source for commercially significant fish, including cod, whiting, and mackerel, as well as marine mammals like seals and some whale species.
The health of the sand eel population is directly tied to the availability and nutritional quality of their zooplankton prey. Sand eels function as an indicator species, reflecting the overall productivity and condition of the marine environment. Fluctuations in the plankton community, often driven by changes in ocean temperature, quickly impact the sand eels’ body condition and affect the breeding success of their many dependent predators.