Safety in a shoal or vital colony
A shoal is a group of animals in water, usually fish, swimming together. It is the equivalent of a flock, herd or swarm. A shoal is usually consisting of individuals of the same species, but sometimes different species swim together. Fish benefit from the proximity of each other; when they swim in numbers and close together, they make it harder for predators to catch them. It can also be easier to find food if there is a lot of help. Some predatory fish hunt together in shoals. Aquarium fish that are shoaling, but are forced to live alone or with only a few members of their own species in a confined tank, become ill from stress. This due to having lost the security they get from the shoal.
Animals living in colonies are a large number of individuals living more closely together than in a shoal. Some colony-dwelling species, such as ants, honeybees and the Portuguese man o’ war, are entirely dependent on the colony for their survival. Colony forming species often specialise in a particular type of food or shelter that they can only obtain by living so closely together. But there is a danger, and it is that predators that eat animals that live in colonies can consume a large number of individuals in one go when they attack the colony.
Sardines live in shoals, because there is safety in numbers. The main advantage of living in shoals is protection from predators. It is difficult for a predator to distinguish an individual in a large shoal of fish.
Ants live in colonies, and build anthills where all ants have important tasks. In large red wood ant colonies, there can be between 100,000 – 400,000 workers and 100 queens.
Photo: Wilder Kaiser CC-BY-SA
Termites are not closely related to ants, but live in a similar way, in a social structure where all individuals are important for survival. They build nests that sometimes resemble tall towers. They are complex buildings where the colony lives together.
The Portuguese man 'o war looks like a jellyfish, but they are not closely related. It is actually a colony of animals in the group of hydrozoas, which are so specialized in living together that they would not survive at all on their own. All individuals of the Portuguese man o' war are of the same species, but have completely different appearances and tasks.
Photo: Sylvere-Corre-CC-BY-NC-SA
Neighbors or room mates
Ants live in colonies. In the ant colony, different individuals have different tasks, and the whole colony must work together to survive and reproduce. Only the queen lays eggs, and most individuals in the hive are sterile. They only occupy themselves with building, and feeding the queen. A large ant colony is almost impossible for predators to stop, even if they were to eat a big portion of the ants in it. The land that the ants have seized is passed down through generations of ants.
The garden eel is a small, elongated fish, with many individuals living close together in a colony. Each one has its own small sand burrow. Garden eels enjoy excellent protection from predators, partly from the burrows and partly from living so close together. They stick part of their body out of the burrow, catching any food that floats by.
Garden eels live in colonies, but have their own burrows. It is common for several different species of garden eels to form colonies together.
Photo: NOAA-CCMA-Biogeography-Team-CC-BY
The sandcastle worm builds colonies that resemble coral reefs or termite nests, by making tubes of sand and other marine material.
Many corals consist of colonies of thousands of individuals, called polyps, each of which is a single animal with a mouth, stomach, and tentacles.
The zebra shelldweller is a freshwater fish that lives in colonies on the shallow bottoms of Lake Tanganyika. They form colonies where the availability of shells is high, lay their eggs, and raise their young in empty seashells.
Photo: berzentei-CC-BY-SA
Blue mussels grow together in colonies.
Photo: Michael-Palmgren-Marint-Kunskapscenter
Building colonies with seashells
Shell dwelling fish, usually cichlids, use abandoned shells to spawn and lay their eggs. The shells protect both the parents and the eggs from predators. Some species of shell dwellers can form colonies, using an area of the bottom with many abandoned shells close together. All the fish in the colony are male or female and can fertilise or lay eggs.
Mussels, such as the blue mussel, inhabit rocky and stony bottoms. They produce thin, sticky threads that they attach to algae, rocks, cliffs or other surfaces. Crowds of mussels attach themselves close together in giant colonies.
Sand martins build colonies in sandy hills during the breeding season. Each pair has its own burrow, but everyone in the colony are close neighbors.
Photo: Abc10-CC-BY-SA
A colony of breeding gannets.
Photo: Maria-Azzurra-Mugnai-CC-BY-SA
A cave in Mexico holds the world's largest bat colony - with around 20 million bats.
Colonies with wings
Bats sometimes also live in colonies. They live close together in caves and holes in trees, but also in attics and under the roofing tiles on houses. They protect and keep each other warm, and fly out together to feed. Bats benefit greatly from being so many in the same location and maintaining a close-knit group, because it increases reproductive success, and it is important for rearing pups. Many bird species also form colonies during rearing season.