A group of eels is usually called a swarm. Lists of collective nouns also give a bed of eels and a fry of eels. These are traditional English terms rather than scientific ones, and in real life most eels live alone. The genuine exceptions are garden eels, which live in colonies of hundreds to thousands, and young freshwater eels, which arrive at coasts and push upriver in enormous numbers.

What is a group of eels called?

“A swarm of eels” is the answer given by most grammar guides and lists of animal collective nouns, and it is the safest choice if you are writing. Diving organisations and popular lists, including a PADI blog on names for groups of marine life, also mention “a bed of eels” and “a fry of eels”. Fry is an old word for young fish in general, so “a fry of eels” fits small eels best. Bed suits animals gathered on or in the bottom, which describes a garden eel colony better than any other term.

Scientists rarely use any of these. A marine biologist describing eels together will usually say a group, an aggregation or, for garden eels, a colony. Fishers and river managers speak of a run of glass eels or elvers when young eels move upstream in numbers. None of these words is wrong; they come from different traditions and suit different situations.

Where do animal collective nouns come from?

From medieval hunting. Words for groups of particular animals are known as terms of venery, and the tradition grew out of the hunting culture of late medieval England, which borrowed a fashion for elaborate hunting vocabulary from France. The earliest source in any European vernacular is the Treatise written by Walter of Bibbesworth in the mid-1200s, which also records animal noises. By the beginning of the fourteenth century, specialised terms were already part of French and English hunting language, and over that century it became a courtly fashion to extend them. Hunting manuals such as the Venerie of Twiti and The Master of the Game even had separate words for the droppings of different animals.

Lists of group names took off in the fifteenth century. A manuscript known as Egerton MS 1995, dated to about 1452, has 70 “termis of venery”, and the Book of Saint Albans, printed in 1486, runs to 164. Many entries were jokes, aimed not at animals but at people: “a Sentence of Juges”, “a Melody of harpers”, “a Fightyng of beggers”. The book was hugely popular in the sixteenth century, and its lists were reprinted and expanded, with the result that many terms first meant as wordplay settled into standard English.

The game never really stopped. In 1968 the American writer James Lipton published An Exaltation of Larks, a book collecting old terms and inventing new ones, and modern lists mix authentic medieval terms with recent coinages. That is why the same animal can have three or four “official” group names, and why eels, which medieval hunters had little reason to name, ended up with swarm, bed and fry. No authority decides which is correct, so usage settles it.

Do eels actually live in groups?

Mostly not. The word eel covers the order Anguilliformes, which includes freshwater eels, morays, congers, garden eels, snake eels and many deep-sea families, along with a handful of unrelated fish that share the long body. Most of these are solitary, ambush or nocturnal predators that spend the day in a hole, crevice or burrow. A moray guarding a reef crevice or a conger in a wreck is behaving the way most eels do.

That does not mean eels never share space. Several morays may occupy the same ledge on a good reef because the holes are good, not because they form a social group. Freshwater eels can be found at high densities in productive rivers and lakes, and they bunch up below barriers during migrations. In both cases the crowd is a by-product of habitat or movement rather than true group living. For the varieties of morays, see types of moray eels.

Which eels live in real colonies?

Garden eels. These small members of the conger family, in the subfamily Heterocongrinae, live in burrows in sand and poke their heads and front bodies up into the current to pick plankton out of the water. Because they live in groups, a colony looks like a field of plants growing from the seabed, which is how they got their name. There are 36 known species, most of them in the Indo-Pacific. The largest reaches about 120 cm, but most are under 60 cm.

Colonies can be very large. FishBase describes colonies of the spotted garden eel, Heteroconger hassi, of up to several hundred animals, and colonies of garden eels in general can cover as much as an acre and number up to several thousand individuals. Each eel keeps its own burrow, lined with mucus, and the whole colony withdraws into the sand when a diver or predator approaches, then slowly re-emerges. If any group of eels deserves to be called a bed, this is it. See garden eels and garden eels out of sand.

When do eels gather in huge numbers?

During their migrations. Freshwater eels of the genus Anguilla have one of the strangest life cycles of any fish. European and American eels spawn far out in the Atlantic, in the Sargasso Sea. Their larvae, flat transparent leptocephali, drift toward the coasts for months; European eel larvae take about 300 days to reach Europe. Near land they transform into transparent glass eels, which enter estuaries in vast numbers each year. In rivers they darken into elvers and begin moving upstream, climbing weirs, dam walls and even waterfalls. These runs of young eels are the closest thing to a real swarm of eels, and they have long been fished with basket traps in the estuaries of western Europe.

Adults spread out to live in rivers, lakes and coastal waters as yellow eels for about 5 to 20 years. When they mature, their eyes grow larger, their sides turn silver and they head back to sea as silver eels. A 2016 study in Science Advances led by David Righton tagged 707 European eels from four regions of Europe and received data from 206. Many tracks ended early because the eels were eaten, but the team reconstructed the routes of more than 80, which headed from western Europe past the Azores, more than 5,000 km toward the Sargasso Sea, at speeds of 3 to 47 km a day. All the eels dived deeper by day and rose to shallower water at night. The study suggested that eels do not all arrive together; some migrate fast and spawn the first winter after leaving, while others arrive in time for the following season.

For almost a century the Sargasso spawning ground was inferred only from larval surveys, ever since the Danish scientist Johannes Schmidt proposed it in the early 1900s. No eggs or spawning adults had ever been caught there. In 2022 Ros Wright and colleagues, writing in Scientific Reports, reported satellite tags on 26 eels from rivers in the Azores, tracked for 40 to 366 days at 3 to 12 km a day, and provided the first direct evidence of adult European eels reaching the Sargasso Sea. The full migration is 5,000 to 10,000 km, the longest of any anguillid eel. Whether the eels form spawning aggregations once they arrive, and what such a gathering should be called, remains unknown.

Do eels ever cooperate?

Occasionally, and with a surprising partner. In the Red Sea, roving coral groupers, Plectropomus pessuliferus, recruit giant moray eels, Gymnothorax javanicus, to hunt with them. A 2006 study in PLoS Biology by Redouan Bshary and colleagues showed that the partnerships were not random, that groupers signalled to morays with head-shaking to start a joint hunt and to show where prey was hiding, that groupers signalled more when hungry, and that both partners benefited. The moray can follow prey into crevices the grouper cannot enter, while the grouper catches fish that flee into open water.

That is a two-species team rather than an eel group, but it shows that eels are capable of coordinated behaviour when it pays. Within their own species, morays and most other eels remain loners, and even garden eels, for all their numbers, each defend a personal burrow.

Why are swarms of young eels disappearing?

Because the European eel is in serious trouble. The species is listed as Critically Endangered by the IUCN and is on Appendix II of CITES, which regulates international trade. Wright and colleagues noted that recruitment of young European eels has fallen by about 95% since the 1980s, a decline attributed partly to problems during the eels’ marine life stages. Barriers such as dams and weirs, habitat loss, pollution, disease and fishing all add pressure, and glass eels are valuable enough to drive large-scale illegal trade.

Breeding eels in captivity, which could relieve pressure on wild stocks, has proved extremely difficult. Researchers at the Technical University of Denmark were the first to achieve some success. By 2014 larvae at their facilities typically survived 20 to 22 days, and by 2022 they were surviving up to about 140 days, well into the leptocephalus stage, but the full life cycle has still not been completed in captivity. Expeditions to the Sargasso Sea suggest that wild eel larvae feed on a variety of plankton, especially microscopic jellyfish, which helps explain why feeding captive larvae has been so hard. Until that problem is solved, every eel on a plate or in a farm still starts life as a wild glass eel. For the wider eel picture, see the eels section.

Why are most eels loners?

Their way of life gives little reason to gather. Most eels are ambush or nocturnal predators that rely on a secure hiding place, a crevice, burrow or hole in a wreck, and on striking or seizing prey that comes close. A long, flexible body is ideal for slipping into tight spaces and turning around inside them, but a hole usually has room for one owner. Sharing would mean competing for the same shelter and the same prey, with little to gain in return.

Group living tends to pay off for animals that face predators in the open, that hunt prey too large or fast to catch alone, or that feed on patchy food that is easier to find together. Open-water fish form schools for those reasons. Most eels avoid open water during the day, hunt alone in the dark and rely on a burrow for protection, so they get few of those benefits. Garden eels are the exception that proves the rule: they feed on drifting plankton that passes over the whole colony, and many watchful heads may help spot danger, so living close together carries little cost and some benefit.

What are some other marine collective nouns?

Eels are not the only sea creatures with colourful group names. Fish in general come in schools and shoals. Biologists use shoal for any loose social group of fish and school for a group swimming in a coordinated, synchronised way, so every school is a shoal but not every shoal is a school. Dolphins and whales form pods, and seals on land form colonies or rookeries. Jellyfish appear in lists as a smack or a bloom, and scientists speak of blooms when jellyfish numbers surge. Sharks are sometimes said to come in a shiver, a modern addition to the tradition rather than a medieval one.

Like eels, many of these animals rarely do what their group names suggest. Most sharks are solitary hunters, and a shiver is more a writer’s flourish than a description of shark behaviour. The terms are part of the enjoyment of the English language, and the old hunting lists were never meant as biology. When you want to describe what animals actually do, the plain words, group, aggregation, colony, school, are usually clearer.

What is a baby eel called?

It depends on the stage. Eels hatch as leptocephali, flat, leaf-shaped transparent larvae that drift in surface waters feeding on marine snow and other small particles. Freshwater eels then become glass eels, still transparent, as they reach the coast, and elvers once they darken and move into fresh water. Growing adults are yellow eels, and mature adults heading back to sea to spawn are silver eels. Marine eels such as morays and congers also start life as leptocephali but do not go through the freshwater stages.