An animal colony is a biological structure formed by multiple individual organisms living in close association, often physically connected, that function as a unified whole. These complex groupings represent a unique point in evolution where the line between a single organism and a collective group becomes blurred. Most colonial animals are marine invertebrates, and their collective existence allows them to achieve sizes, functions, and efficiencies that a solitary individual could not manage alone. The study of these organisms provides insights into how division of labor and specialization first developed in the animal kingdom.
Defining Colonial Life
A colonial animal is defined by the permanent physical connection and shared genetic identity of its constituent members, known as zooids or modules. These zooids are clones, meaning they originate from the same single, sexually produced founder and are genetically identical. The individuals are physiologically integrated, often sharing a common circulatory system or continuous tissue, which makes them highly interdependent. The colony as a whole is frequently considered a “superorganism” because the survival of the group overrides the independence of the individual zooid.
This arrangement fundamentally differs from that of merely social animals, such as ant colonies or wolf packs. In social groups, the individuals are separate, genetically distinct organisms that cooperate but can often survive independently. Colonial animals, by contrast, have physically attached members that are clones of one another, and many individual zooids cannot survive if separated from the colony.
How Colonial Animals Form and Grow
The initial founding of a colonial animal begins with sexual reproduction, typically involving a single fertilized egg that develops into a larva. This larva settles onto a substrate and metamorphoses into the first zooid, sometimes called the primary zooid. This founding individual then begins the process of colony expansion through asexual reproduction.
The growth of the colony occurs through the continuous addition of new, genetically identical zooids via processes like budding or fission. In budding, a new zooid grows as an outgrowth from the body wall of an existing zooid or shared tissue, similar to how a plant grows a new shoot. Fission, another method, involves the original zooid splitting into two or more distinct but still-connected individuals.
The Spectrum of Colonial Organization
Colonial animals display a wide spectrum of integration and specialization among their zooids, a concept known as polymorphism. At one end of the spectrum are colonies where all zooids are structurally similar generalists, each performing all necessary life functions, such as feeding and reproduction. At the other end are highly complex colonies where the zooids have evolved into distinct, specialized forms.
In these highly integrated colonies, zooids are specialized into castes, each performing a dedicated function for the entire group. For example, a colony might contain gastrozooids, which are specialized for digestion and feeding the whole colony through the shared tissue. Other forms include dactylozooids, which are highly modified for defense and prey capture, often equipped with stinging cells.
Examples in the Animal Kingdom
Colonial animals are found in several phyla, often classified by their mobility. Many well-known examples are sessile, meaning they are permanently attached to a surface for most of their lives. Corals are perhaps the most famous sessile colonial animals, where thousands of tiny polyps are connected by a thin layer of living tissue and secrete a shared, large calcium carbonate skeleton.
Bryozoans, or “moss animals,” are another group of sessile colonial organisms that build intricate, often lace-like exoskeletons, with specialized zooids for feeding, cleaning, and defense. In contrast to these attached forms, some colonial animals are pelagic, or free-floating, and represent a high degree of specialization. The siphonophores, which include the Portuguese Man o’ War, are complex floating colonies made up of multiple types of interdependent zooids. These specialized units include the pneumatophore (a float), nectophores (for propulsion), and the feeding and reproductive units.