Bryozoans-Welcome to the colony

by Matthew Norton

Most of the time, natural selection is focused on how the individual can survive and reproduce, but not everyone can do it on their own. Some creatures are so dependent on other members of their species that they stick together, making it hard to work out where one individual ends and the next one begins.

Bryozoans are one such group of invertebrate animals that live together in colonies (with one exception). Each animal is joined together with its next-door neighbours with a series of tubes that carry resources, such as food, throughout the colony. These connections run so deep that each ‘individual’ animal is called a zooid because they are not independent enough to be recognised as individual animals. Bryozoans would probably find this insulting if they ever found out how we talk about them, but their lack of individuality does allow them to share the workload with each zooid playing its own role to keep the colony alive.

bryozoan blog image 1 — Some examples of bryozoan species with the seaweed shaped hornwrack (left) and sea mats growing on real seaweed (right). If you look really close, you can see each zooid in the bryozoan colony as a little ‘box’.

The most immediate concern for colony would be food with many zooids equipped with a feeding organ called a lophophore, a crown of hairy tentacles that catch small pieces of food from the water and flick them towards the mouth. However, with so many lophophores in such a small space these feeding zooids can easily get in each other’s way. The colony also has other needs for which a lophophore would be useless, so some of the zooids have replaced it with another tool.

bryozoan blog image 2 — You can see from this photo how close each bryozoan is to each other with their lophophores sticking out.

For defending the colony from predators or rival colonies, some of the bryozoan zooids have spines or a set of snapping jaws (some say they look like mouse traps). Some species go even further with zooid types that are built for holding up the colony, protecting fertilised eggs and sweeping away pests (algae, tiny animals etc) with bristles. Incidentally, these bristles can also be used for ‘walking’ and digging the colony out of sand, a useful trick in busy areas where the sand is thrown around by ocean currents and other sea creatures.

There is just as much diversity between species with bryozoans being one of the most varied animal groups I have seen, and the image below shows this better than I could do with a thousand words. The only thing I will add is that colonies can vary in shape and size from sheets growing on plants and animals to ‘free living’ forms that look like seaweeds, corals and other creatures.

bryozoan blog image 3 — These bryozoans were drawn by the naturalist Ernst Haeckel and it’s not the first time I’ve used his illustrations on my blog. They are beautifully detailed and coloured (it also helps that he has been dead long enough for the copyright to expire).

In reality, no one on this planet is truly alone. Every single plant, animal and human being on the planet is made up of millions of cells working in harmony to keep the body alive, most of the time. Even the single celled microorganisms such as bacteria and viruses are often sharing the air and water with millions, if not trillions of other microorganisms. Bryozoans are just a useful reminder of this universal fact.

From a human perspective

From time to time, we probably wish that we could live in a separate existence to the rest of life on earth (I suspect the feeling is mutual). No one likes catching a disease, being swarmed by flies, or being bitten by a wild beast. Bryozoans can also cause us a few problems.

Some of us, particularly fishermen and anyone handling their nets, have encountered the sea chervil (Alcyonidium diaphanum). This species of bryozoan is notorious for causing a skin condition called “Dogger Bank Itch” with symptoms including rashes and dermatitis which can be very uncomfortable in severe cases. Records of the condition in the North Sea date back to at least the 1930s, but it seems to have spread in recent years with recorded cases in Cornwall and the English Channel since 2000.

Other species can be a nuisance without coming anywhere near us. Similar to many other marine invertebrates, bryozoans can stick to our pipes, filters and any other hard surface we build in the water. This can lead to a messy situation when we need these pipes and filters to deliver clean drinking water and treat sewage, it’s bad enough when it’s our own crap causing these blockages (literally in some cases). Their tendency to stick to other sea creatures is also annoying if we are trying to farm them for food. Would you eat mussels in a restaurant if they were covered in weird squishy things?

bryozoan blog image 4 — These zebra mussels really do not look appetising with all those bryozoans growing on their shells.

Bryozoans can cause another problem for fish farms by helping diseases to spread and kill off their fish. There is a particularly serious, and costly disease affecting salmon farms called “Proliferative kidney disease” with symptoms including swelling of the kidneys, abdomen and damage to other body organs. The culprit is a myxozoan (jellyfish-like) parasite, but they can’t spread directly from fish to fish and so need a middleman to piggyback off, with bryozoans being the prime suspect

bryozoan blog image 5 — Proliferative kidney disease can affect wild salmon, but salmon in fish farms, such as the one above, are more vulnerable because it is easier for diseases to spread in confined spaces.

Despite all this negativity there is one species of bryozoan who could bring us some very good fortune. Bugula neritina, also known as brown bryozoans and common bugulas, produce chemicals called “bryostatins” which could be used to treat cancer and Alzheimer’s disease. I am saying “could” a lot because despite some promising results, this area of research is relatively young, and some clinical trials have raised doubts about their effects against certain types of cancer. An even bigger problem is how can we possibly meet the demand for bryostatins from this one species. A promising solution to this supply problem would be to develop synthetic copies that work just like bryostatins and can be easily mass produced, but this research is still ongoing.

Bringing humans and nature together can be a complicated business with interactions sometimes being unpredictable and dependent on the circumstances. Sometimes the benefits and drawbacks of such interactions are only realised after years of research, which is why it is important for humanity to do these scientific experiments, even some of the really wacky ones (within reason). In the case of bryozoans, we have been studying them for so long that I would have been surprised if we hadn’t found something useful from them by now. Imagine what else we might find in the near future.