The Fascinating Lives of Colonial Animals
A physonect siphonophore colony observed during an exploration of the Central Pacific Basin. (Image Credit: NOAA Office of Ocean Exploration and Research)
Appreciating nature can become increasingly challenging when the environment and the creatures within are unfamiliar to us. Many of us are probably best able to identify with animals that are closely related to humans and appreciate them because of their similar appearance and behaviour to us. We are more likely to empathize with a chimpanzee playing with its friends, or a seal rolling around on the beach, yawning – maybe reminding us of ourselves loafing around on the couch on a Sunday afternoon.
Yet evolutionary processes led to very diverse outcomes, including animals whose appearance is so different from our own that it becomes almost impossible for us to see their value and understand their place in nature. Let’s take a look at one such form of life that is easily overlooked, but far more diverse and abundant than they may seem: colonial animals.
To our terrestrially trained eye, many aquatic organisms look very different, and may even remind us more of plants than animals. They might look like a tree or mushroom, having a bushy growth with repetitive elements, maybe having a stem with branches and a root. Even naturalists in the past confronted with some organisms asked themselves “is this a plant, an animal or a mixture of both?”. This also led to the original term for such species – Zoophytes (animal-plant). The Zoophytes encompassed all organisms whose place on the tree of life scientists at that time were uncertain about.
Fortunately, you can forget about that term here and now. With the help of advanced observation methods, our knowledge of the tree of life grew and led to the invention of systematics as we know it today. Nowadays we know that corals and sponges, among others, are undoubtedly animals.
Among these sit the colony-forming animals, which have probably evolved independently several times in evolutionary history and are spread throughout the animal kingdom: cnidarians (such as some hydrozoans or corals), bryozoans (commonly known as moss animals), platyhelminths (e.g. the class Catenulida), chordates (Salps and ascidians), hemichordates (Pterobranchs), entoprocts, phoronids (one species – Phoronis ovalis), rotifers (for example the genus Sinantherina) and poriferans (sponges). A detailed overview of the taxa can be found here.*
But what exactly are colonies? The definitions are almost as diverse as the taxa that the animals hail from. However, most of these organisms do have one thing in common: they are made up of individual units that form a larger association and can’t exist on their own. These individual units are often called zooids and are physiologically connected to others by tissue or skeleton. Put simply, these units are a bit like LEGO bricks, which come in different shapes and sizes and can only be connected in a certain way.
Individual Action, Collective Power
Zooids cannot survive on their own. Their success depends on their colony the same way a queen bee depends on its workers. The most widely known are perhaps corals, which can form massive reef complexes and can grow for hundreds of years. A zooid alone couldn’t manage such an achievement – but together in their colony, they are invincible (climate change aside). And while zooids are small individuals on their own, colonies can be quite complex. Zooids within a single colony can come in different functions and morphologies – a bit like the organs in our bodies. This is known as zooid polymorphism and is associated with the division of labour within the colony.
Read more: A Story About Mortality
The siphonophores (cousins of the corals), for example, form some of the most complex colonies on the planet. To be able to float around in the open ocean and elegantly catch smaller zooplankton, siphonophore zooids are divided into special task forces within the siphonophore colony. Some zooids are specifically designed to catch food (the gastrozooids), others are designed for defence and protection (called bracts, which sit around the delicate polyps) or as swimming and orientation devices (the pneumatophores). In fact, siphonophores combine both polyp and medusa morphologies that usually represent different and separate life stages in other cnidarians.
Another fascinating group of animals are bryozoans. They include species that form colonies with only one type of identical zooids and others that include several specialized types with a wide variety of species-specific forms and modifications. Some bryozoans even have zooids which act as tiny brooding chambers where the embryos are incubated until they are fully-fledged.
Clones in A Living Community
Normally, individual zooids within a colony are clones of each other and are therefore genetically identical. A colony starts with a single unit, to which more and more zooids are attached in the course of development. This copy-and-paste function is a particularly efficient form of asexual reproduction (often called budding) that leads to the growth of the colony. At some point, however, there will also be recombination with another colony. In addition to asexual budding, sexual reproduction is necessary – without genetic recombination, you can’t evolve, and without evolution, you lose the ability to adapt when necessary.
… Or Fancy A Multigenerational Home?
As usual, there are always exceptions to the rule: some colonies do not only contain genetically identical zooids. Hydrozoans, ascidians, corals or bryozoans, for example, can form multigenerational colonies. Offspring (in many cases the larvae) settle on the parent colony and fuse with it. So they form a so-called chimaera colony, which consists of closely related, but not genetically identical zooids. This fusion of kin cells only works because the zooids can recognize other cells. Cells that are identified as closely related (for example from the child) are not responded to with a defensive reaction. Only cells with sufficient genetic similarity are accepted – others are not welcome and will be rejected. This form of recognition is called allorecognition.
Not a very integrative society, one might think, but this kinship recognition has some advantages: The genetic diversity of the colonies increases, which is not possible through asexual reproduction alone. It is also believed that chimaeras can grow faster, are more resilient against environmental threats, and that competition with less closely related neighbours is reduced.
What Do You Want To Be When You Grow Up?
Genetically identical clones, but which can look completely different depending on their function in the colony, can be a bit confusing. How is it decided which function each zooid gets allocated? How are signals conveyed within the colony and how do zooids communicate with each other? The determination of the appearance and function of zooids, as well as the general evolutionary history of colonial organisms, is still poorly understood. It is believed that a zooid’s fate is determined by complex interactions between the environment, developmental programming, and the underlying genotype (see for example here).
Evolutionarily Successful, But Almost Ignored In Our Theories
Coloniality in animals appears to be a successful organization and is the result of many different evolutionary paths. This fascinating diversity across the entire tree of life is, however, surprisingly overlooked in our biology classes, university courses and also our biological theories. Much of our thinking is based on individuals who merely reproduce sexually and our theories are often based on human viewpoints. Hence, the incredible diversity of colonial organisms and other asexually reproducing organisms are often ignored, probably biasing our perception of evolution and nature.
The sheer awe-inspiring complexity within colonies challenges some of our biological theories, and much can be learned in the future from further research with these organisms. It can help to answer relevant questions, even about our own existence, such as: How did multicellularity evolve? What is the story of individuality and its origin? How can cells recognize genetically similar cells and not reject them, but others? Bryozoans, siphonophores and all the other colonial animals (too many for me to describe them all here, but which are no less fascinating) deserve a lot more attention. But understanding their nature and biology is a first step towards appreciating them more.
Lara Beckmann just finished her master’s degree in ‘Biodiversity & Systematics’ and currently works for the invertebrate collections at he University Museum of Bergen, especially focusing on the diversity of hydrozoans. You can read more of Lara’s work at Ecology for the Masses at her profile here, or follow her on Twitter here.
* In this article I mainly focus on aquatic animals. But colonial animals are also present on land: Termites, ants or bees are so-called eusocial insects and form colonies of physiologically separated individuals that can belong to different groups in terms of morphology and behaviour. Unicellular organisms such as protists also form colonies that even enables individual cells to act collectively, for example, observed in some choanoflagellates.
Literature & Suggested Reading
Hiebert, LS, Simpson, C, Tiozzo, S. (2021). Coloniality, clonality, and modularity in animals: The elephant in the room. J Exp Zool (Mol Dev Evol). Special Issue: Evolution of Animal Coloniality and Modularity. 336, 3 (198– 211). DOI: 10.1002/jez.b.22944
Hiebert, LS et al. (2021). From the individual to the colony: Marine invertebrates as models to understand levels of biological organization. J Exp Zool (Mol Dev Evol). 336, 3 (191-197). DOI: 10.1002/jez.b.23044
Simpson, C et al. (2020). How colonial animals evolve. Science Advances 6, 2. DOI: 10.1126/sciadv.aaw9530
Lidgard, S et al. (2012). Division of labor and recurrent evolution of polymorphisms in a group of colonial animals. Evol Ecol 26 (233–257). DOI: 10.1007/s10682-011-9513-7