Some of the most important animals sustaining a habitat are the most unassuming. The seas around Europe are teaming with life that rely on much smaller animals to sustain them – sand eels. While not actually eels, one alternate name for them is ‘sand lances’, these fish belong to a family of closely related fish mostly found in Europe, although some members are found elsewhere.
Taxonomy and Evolution
There are 28 species of fish which we normally call sand eels or sand lances split into three genre: Ammodytes, Ammodytoides, Bleekeria, Gymnammodytes, Hyperoplus, Lepidammodytes, and Protammodytes. These genre belong to the wider sand lance family, Ammodytidae, although some non-European fish called sand eels do not belong to this family. For this post we will largely look at two species, the lesser sand eel (Ammodytes tobianus) and the greater sand eel (Hyperoplus lamcealotus). Despite their name sand eels are not related to eels, in fact they belong to an entirely different order of fish that generally stick close to the sea bed. The oldest fossils belong to sand eels date back 55 million years ago to the Eocene with Ammodytes being the oldest and the others splitting off around 15 million years ago.
Biology and Behaviour
These fish earned their name of ‘sand eels’ or ‘sand lances’ due to their elongated bodies which give them a resemblance to eels or lances. They definitely are not one of the prettiest looking fish having a prominent bottom jaw which ends in a structure that resembles a tooth in several species. Sand eels do not have teeth on the lower haw and instead suck up food where the long lower jaw helps create a small tunnel to help it eat. They are not the largest of fish with the greater sand eel being the largest at 30 centimetres in length, and often when species live alongside each other the size is one of the only ways to tell them apart. Around Britain the lesser and grand sand eel are largely distinguishable by the smaller size of the lesser eel, 20 centimetres. As expected by their name, sand eels create burrows in the sand so have evolved to take advantage of this. Being so thin is helpful but they have evolved other traits – the have a long dorsal fin, have small pectoral fins (to aid in turning), and have no pelvic fins. Relying so heavily on burying themselves into the sand they have evolved to not have a swim bladder as this would make it harder otherwise to do this.
Breeding happens at different times for different species, so we will largely look at the breeding of the lesser sand eel. Depending on where they are lesser sand eels spawn in the spring and summer or the summer and autumn when the males release their sperm onto the eggs of the females. The fertilised eggs, between 2000 and 20,000, are then laid in the sand itself and will stick to the sand grains until they hatch after a few weeks. These newly hatched sand eels are incredibly small and will float in sea currents for the next three months until they reach 5 centimetres in length. At this time they will be large enough to burrow into the sand themselves, but it will take until their first year before they reach sexual maturity. Sand eels upon reaching maturity have a lifespan of between 4 and 7 years. They form giant shoals as a safety mechanism, safety in numbers after all, and when danger is detected they will dart into the sand and hide out. Like chameleons, sand eels can independently move each eye which gives them many opportunities to spot predators. As a further safety mechanism they are largely nocturnal although they are most common during the dawn and dusk as their large eyes give them better vision during low light. During the winter they hibernate, burying themselves up to a metre into the sand.
Distribution and Habitat
Sand eels are found worldwide, but most species are found in Europe. Lepidammodytes macrophthalmus, discovered as recent as 1994, is only found in the waters around Hawai’i and in fairly deep water for their family, as far down as 200 metres. The lesser and greater sand eels are meanwhile found in shallow waters, only 30 metres in depth. Most of the more recently discovered species, such as the Pitcairn sandlance discovered only in 2000, are found in the deep waters of the Pacific compared to their better known relatives in the Atlantic. As mentioned, the most common species are found in shallow European waters, mainly in the English Channel, Mediterranean, Irish Sea, and North Sea. Lesser and greater sand eels are most common in the North Sea, Irish Sea, and English Channel where they form the backbone of the food web. Sand eels need sandy sea beds without rock underneath for at least a metre due to their reliance on sand for shelter and safety.
Diet and Predators
Sand eels have a simple diet consisting of zooplankton, plankton, fish eggs, and small crustaceans. As mentioned above, several species entirely lack teeth so they instead suck up the tiny organisms. Meanwhile, they are on the menu for so many animals, so much so that if sand eel numbers decline this impacts the numbers of many other animals. Larger fish, sharks, seals, dolphins, whales, and sea birds all eat sand eels, with sea birds being the most impacted by any drop in the number of sand eels. There are so many iconic photos of puffins with their beaks full of sand eels, and this post was inspired by seeing one such image in a David Attenborough documentary. Humans have also relied on sand eels for food, either by directly eating them or using them to feed larger fish. Thanks to all this, sand eels are some of the most important members in their food web with so many other layers relying on the tiny fish.
Conservation and Threats
Sand eels have not been properly assessed by the IUCN which is quite frankly mind boggling due to how important they are for ecosystems. While some are understandable due to their deep water homes, it is less understandable for the lesser and greater sand eels. We do know that sand eel numbers are decreasing, and there are two reasons for this. The first is overexploitation. Whether directly being used for food or as bait humans have increased their exploitation of sand eels which have had a big impact on their numbers. The second is climate change. Warming temperatures causes shifts in sea currents which disrupts when they spawn and their access to food. Dropping sand eel numbers has reverberations further up the food web, with sea birds like puffins, cormorants, and kittiwakes being noted examples. The Scottish government has designated several areas of their habitat as a protected marine zone which has at least protected part of their habitat, but more needs to be done.
Bibliography:
- S.J. Rowley, ‘Lesser sand eel’, The Marine Life Information Network, (2008), [Accessed 20/05/2023]
- A. Ruiz, ‘Greater sand eel’, The Marine Life Information Network, (2008), [Accessed 20/05/2023]
- Joseph S. Nelson, Terry C. Grande, and Mark V. H. Wilson, Fishes of the World, Fifth Edition, (Hoboken: Wiley, 2016)
- ‘Our Precious Isles’, Wild Isles, directed by Hilary Jeffkins, BBC, (23/03/2023)
- ‘Sand Eels’, The Wildlife Trusts, [Accessed 19/05/2023]
- ‘Sandeels’, NatureScot, [Accessed 19/05/2023]
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