Raitt's sand eel (Ammodytes marinus), also known as the lesser sand eel, is a small semi-pelagic ray-finned fish found in the North Atlantic Ocean. The Raitt's sand eel is member of the family Ammodytidae which includes all 31 species of sand eels, often referred to as sand lances. Contrary to their name sand eels, including Raitt's sand eel, are not true eels and instead belong to the order of “weever-like” fishes, the Trachiniformes.
The Raitt's sand eel is a small elongated fish that is usually about 20 cm long. The maximum length reported for this species of sand eel is 25 cm. They are silver all over with darker scales on their dorsal side.Raitt's sand eels have a short tail fin that is deeply forked. They have a long and low dorsal fin that is twice as long as the tail fin and runs along almost the entire body. The scales on their underside are arranged randomly instead of in a distinct pattern as in many other fish. The Raitt's sand eel has a pointed jaw where the lower jaw projects further than the upper jaw.
It is difficult to distinguish this species from the other members in its genus ‘’Ammodytes’’ as they are all very similar in appearance.
The Raitt's sand eel is found across the Northeast Atlantic Ocean and is particularly abundant in the North Sea. They live in both the shallow open ocean and coastal waters. Four other species of sand eel are also distributed in this part of the Atlantic. The fisheries here are dominated by Raitt's sand eel suggesting the Raitt's sand eel is the most abundant in the area.
The southern boundary of the Raitt's sand eel is predicted to move further North in the future due to climate change. The Raitt's sand eel survives better in cooler waters and may move North to avoid warming sea temperatures.
Sand eels live amongst sandy sediment of the sea bed where they spend most of the year burrowed avoiding predators. They are also found living in bedrock and kelp habitats. Sandbanks are the most important habitat for the sand eel as they are used as nursery, spawning and resting grounds.Raitt's sand eels live in turbulent areas of the ocean and do not live in depths below 100m.
Raitt's sand eel have very specific habitat requirements. They are specialised to live in waters high in oxygen but with low levels of silt and clay.These requirements mean the Raitt's sand eel is found in distinct patches across the North Atlantic and instead of one continuous population.
The Raitt's sand eel is a keystone species in the North Sea food web. They are the prey of a significant number of species including seabirds, marine mammals and larger fish. Seabirds in particular rely heavily on the Raitt's sand eel to feed their chicks. When the sand eels leave their burrows to feed seabirds dive into the water to catch them and then return to their nests. Atlantic puffin, black-legged kittiwake, common guillemot and razorbills are all known predators of this species. Common guillemots feed exclusively on Raitt's sand eel during the breeding season.
In the sea many larger fish species prey on the Raitt's sand eel including cod, haddock, whiting, saithe and mackerel. Many of these are very commercially valuable species and declines in sand eel have caused declines in their numbers due to lack of prey. Marine mammals also feed on Raitt's sand eels but less regularly. Mostly species such as grey seals, harbour seals and harbour porpoise consume many sand eels in the spring and early summer.
Raitt's sand eels feed mainly on zooplankton. By consuming zooplankton they act as a link through to the higher trophic level predators that eat sand eels.
Sand eel species switch between open water swimming and burying themselves in the sand to avoid nearby predators. When they are not burrowed they live in shoal formations. They spend most of the winter burrowed in the sediment but then enter the open water between March and June to feed as the abundance of prey increases. Even in the summer months when Raitt's sand eels emerge to feed they still burrow in the sand in the night.
Raitt's sand eels breed in December and January when they briefly leave their winter burrows in the sediment.They lay their eggs in the sand which incubate there until they hatch in February and March. The hatched sand eels live in the open water above the sediment until metamorphosis. After this, they return to burrow in the sediment.Most Raitt's sand eels live for only 3 or 4 years as they have a high level of natural death.
Due to their lifestyle, it has been very difficult to estimate the population size of the Raitt's sand eel. Most monitoring techniques are inappropriate and cannot detect the burrowed sand eels.
Biological indicators, or bioindicators, are now being used to assess the size of the Raitt's sand eel population. The main indicator used for sand eels are black-legged kittiwake seabirds. These coastal breeding birds are heavily reliant on sand eels and therefore their population size changes in response to the amount of Raitt's sand eel present. This can give a lot of information about the abundance of sand eels in the area.
The Raitt's sand eel is known from fishing records to have regular population fluctuations. Overfishing, climate change and food web effects have all been shown to have contribute to these fluctuations. As the Raitt's sand eel is found in the North Atlantic Ocean its climate is altered by the North Atlantic Oscillation. This weather fluctuation can alter the recruitment of sand eel. Warmer temperatures decrease the number of sand eel juveniles that survive to adulthood and so yearly changes to temperature can cause fluctuating population size. Changes to the abundance of species connected to the Raitt's sand eel in the food web has also caused variable population size. Calanus copepods are an important prey item for sand eels. The population of Raitt's sand eel has been related to changes in the availability of this prey. Young Raitt's sand eels have lower survival when copepods are less abundant.
The Raitt's sand eel is currently not assessed by the IUCN Red List of Threatened Species.
