A colonial association it is a system of organization in which two or more organisms live in an intimate relationship. The association can be physical, and the individuals that make up the colony can be connected.
We find colonial associations throughout the tree of life: from cellular organisms to multicellular ones. Similarly, the colony can be made up of clones (individuals with identical genetic material) such as a colony of bacteria, or it can be made up of more genetically heterogeneous individuals, such as a colony of insects..
Generally, the association translates into a mutual benefit for the individuals that compose it. For example, improve defense skills against attack by predators, or improve predation skills.
In some species, the formation or not of the colonial association is in the hands of environmental conditions - the colony is “facultative”. In contrast, the survival of other species is mandatorily dependent on colonial formation.
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Although it seems trivial to define what an "individual" organism is, it is a complex and imprecise concept - even for biologists..
From a physiological and genetic point of view, an organism can be defined as a genome within a body. We use the term "genome" to refer to the set of genes that exist in a particular organism.
The definition of "individual organism" has important consequences, particularly in evolutionary biology. We tend to affirm that natural selection (a mechanism of evolutionary change) acts at the level of the individual.
Some organisms are clearly a individual: a mouse, a fly, a dog. Nobody doubts in these cases the discrete character of the biological entity. However, there are certain systems that challenge this concept: colonial organisms.
It is known that organisms do not live in isolation - in fact, they establish multiple relationships with other individuals, forming complex networks of interaction. Some organisms carry these relationships very intimately and encourage the formation of colonies..
Below we will describe the most important aspects of these biological associations and the most prominent examples in the literature..
A colonial association or simply a "colony", is a grouping of individuals. The association is characterized by being intimate, from the physical point of view and in some cases the individuals that compose it are connected to each other.
Colonies are cooperative systems, where the presence of other individuals benefits their colonial companions..
In certain cases, individuals in the colony tend to divide up tasks - not just basic actions like foraging; in colonies there may be “reproductive” individuals and individuals that do not reproduce.
Thus, in the more complex colonial systems, we could think that each of the individuals in the colony behaves like the "cells" or systems of a discrete organism..
In this article, we will classify colonies according to the type of organism that composes them - that is, whether it is unicellular or multicellular..
A colony of bacteria is the association of unicellular organisms that originates from the division of a stem cell and gives rise to all the individuals that make up the colony. For this reason, the members of the colony are "clones" and are identical to each other (except for the sites where mutations have occurred).
When bacteria grow in culture medium, colonies are clearly visible to the human eye (no need for microscopes or magnifying glasses).
There are cases where the association of microbes is formed by different species. These bacterial ecosystems are called biofilms or biofilm.
Green algae are organisms that have chloroplasts and can be unicellular, colonial or multicellular..
The most iconic example of colonial organisms in literature is a freshwater genus called Volvox. The colony of these organisms is made up of hundreds, or even thousands, of flagellated cells.
The cells of the colony are linked by cytoplasm "strands" in a gelatinous, roe, and mobile sphere. This colony represents a very advanced degree of association.
The division of labor is clear in the colonies of Volvox. Certain cells are responsible for vegetative reproduction and others for sexual reproduction.
Protists are single-celled eukaryotic organisms. Although some species can live solitary, many of them live in colonies.
Protist colonies are made up of multiple cells. However, each of these exhibits an identity that allows it to carry out the basic tasks of a living being, such as reproduction and survival..
The imprecise term "slime mold" is used to describe more than six groups of eukaryotes whose life cycle form multinucleated or multicellular aggregates that have the ability to move through the soil in search of food. Although the name tends to be confusing, they do not belong to the group of fungi..
The model genus for molds is Dictyostelium. These amoebae have the ability to produce a substance that promotes bonding in multicellular bodies. The secretion of the substances generally occurs in times of drought and with little availability of food..
Multicellular organisms form colonies with different types of integration between the members. There are colonies of animals that live in a nearby area and we have examples of more intimate associations, such as eusocial insects.
Colony formation occurs quite frequently in marine animals, mainly invertebrates. Examples of this are corals, anemones, bryozoans and sea squirts. In these cases, there is a union (that is, a continuity) between the organisms.
As we increase complexity in the animal kingdom, we find other levels of colonial associations. Most notable are the eusocial insects, such as bees and some other members of the Order Hymenoptera.
The social interactions that occur inside these colonies are so close and so complex that some authors call the entire colony a superorganism..
As we saw in the example of Volvox, in bees there is also a very clear division of labor, both of daily activities (which include foraging, defense, among others) and reproduction. Only the queens reproduce and the rest of the colony works to contribute to this task..
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