Introduction to freshwater bryozoans

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There are many families and species of animals that live underwater. One of the more unobtrusive groups are the bryozoans, of which most species lead a hidden existence.
This web-site provides information on bryozoans that live in the Dutch fresh and brackish water. There are many species that live in the Dutch seawater, but these are not covered in this web-site.
This web-site is created and maintained by an interested scuba diver. Most people that study bryozoans do that by collecting material from the waterside and study it in a laboratory or at home under a microscope. As scuba diver there is a unique opportunity to study and photograph the species in their habitat.

What are bryozoans?

Bryozoans are a group of relatively primitive animals that (almost) always live in colonies. Most species live in the sea, Some in fresh water and even less in brackish water.
In The Netherlands there are about 12 fresh water species and 1 brackish water species.

Bryozoans are fairly complex animals with a retractable polyp, a complete gut with the anus outside the tentacles, a nerve ganglion for coordination, some muscles to retract and expand the tentacles and reproductive organs.

The freshwater species each have a survival strategy and related to that a body shape.

Bryozoans have existed for millions of years and many species are found as fossil, especially the seawater species, that often make a calcified enclosure that fossilizes well. It is thought that the freshwater species are more recent/


Bryozoans live in standing and flowing water. Examples are ponds, lakes, streams, canals and rivers, but also water mains and filter units of water supplies.
Bryozoans need a firm substrate, such as rocks, sticks, submerged trees, tin cans, old boots, waterplants, sunken boats and so forth. Only P. magnifica is known to produce floating colonies. No bryozoans live on silt bottoms.

Stones and specially wood seam more popular than glass and metal (personal observation). Also soft materials, such as carpet dumped in a lake is not a popular substrate.

Some species search for a place that stands free in the water, such as sticks may do. Many species cannot handle silt and aim for a more hidden place under a rock or a piece of timber, provided enough water flows around it to provide food. I have also found many colonies on a submerged piece of a fence. In my experience aquatic plants are less popular. Branches hanging in the water, ok. I have searched many water lilies and other plants and have only found C. mucedo growing on top of them. In reed roots sometimes a colony can be found. Exception is V. pavida that seemed to favor reed canes.

The settling silt problem

All bryozoans must find a solution of the settling silt problem, or they will be covered and die by suffocation or lack of food. To illustrate the problem i too two pictures of the same spot shortly after eachother. In between i made a small movement with one hand.

Before After

It will be clear that silt is an important issue and that the risk of becoming covered can be significant.
Different species solve this in different ways. Taking shelter underneath something is a common strategy. Working together (C. mucedo) also works. Find a spot with a mild current to take the silt away (P. fungosa) is also a solution.

Food availability

It seems that bryozoans need fairly clean but sufficiently food rich water. I tried to cultivate P. magnifica in an aquarium but failed due to lack of food. It is unclear to me if bryozoans live in food rich (eutrophous) waters. Some literature refers to a decline of C. mucedo due to eutrophication.
There is a clear relation between number and health of bryozoan colonies and the availability of food, measured by sight-depth as experienced by scuba divers. In my favorite lake bryozoan colonies decline as soon as the sight depth increases above about 3 meters for a week or so. New colonies form once de sight depth decreases below about 2 meters for a week or so. These changes happen naturally with the seasons and with the amount of rainfall that brings new food into the lake. Near streams that enter the lake often more and bigger colonies can be found.
If after a period of abundance, for example the spring algae bloom in April - may, the amount of food in the water decreases below the minimum amount needed for survival, most bryozoans will react by producing survival capsules (statublasts or hybernaculae, depending on the species) and will die.
Given the natural high and low food periods there are two good times in the year to go bryzoa hunting:

  1. In the second half of the spring algae bloom in may and the weeks after
  2. In august to October, depending on rainfall. In October there is a secondary algae bloom and therefore a bryozoa increase

I rarely find bryozoa colonies in the upper 1 to 2 meter of the lake, especially not in these parts of the lake where wave action is induced by the wind. In sheltered areas bryozoa colonies can be found for instance on branches that hang into the water.
In streams bryozoa can be found in shallow water. Deep water often is silty and serious wave action is prevented by the size and shape of the stream. Most species can handle limited water flow. After a serious rainfall i have had to cling to a firm point in order not to be carried with the flow when i wanted to make pictures. P. magnifica and P. fungosa were not harmed at all. In fast flowing streams i can imagine bryozoa to be prevented to settle and therefore not to be able to survive. C. mucedo is less well-attached

It seems to me that bryozoa need fairly oxigen rich water. While diving in local lakes i have rarely found bryozoans below 5 meters. That is about the depth a thermoclyne exists in summer and below that low-oxygen conditions arise as summer progresses. My only find of bryozoa below 5 meters was C. mucedo living in a clear lake on top of aquatic plants. Locally enough oxygen must have been produced for C. mucedo to survive.

The bryozoa that live in the Netherlands do not like cold conditions. When the water cools below 8 to 9 degrees Celsius, colonies die off and the species survives in statoblasts or hybernaculae (see: survival capsules).


Bryozoans are not static things. If observed well they have several types of behavior, sometimes over longer periods of time:

  1. As soon as a more abrupt than normal water flow suggests a threat the tentacles retract rapidly into the body cavity. Not long after they tentatively come out again. As photographer it is mandatory to move slowly and carefully and regularly wait for a few minutes to catch the tentacles on a photo.
    Time to extend the tentacle after retraction differs per species. P. articulata can wait several minutes, while C. mucedo extends after a few seconds.
  2. The polyp and the individual tentacles are in constant motion in an attempt to attract more food with the water stream created by the ciliate cells on the tentacles. If attracted the food will be actively sucked in by the mouth and swallowed, as can be seen in this short video (1,5 MB).
  3. If bryozoans reproduce sexually and produce larvae, the larvae exhibit interesting behavior (as described in the literature):
    1. In the first period of their free swimming existence of several hours the larvae swim actively towards the light - they are positively photo tactic. This way they have a better chance to find a good new place to live as they depart from their parent's hiding place and enter the faster flowing areas.
    2. Shortly before they are ready to select a good spot to build a colony the larvae actively swim away from the light - they are negatively photo tactic. This way they have maximum chance to find a location to live that protects them from sediment.
      An interesting question is whether C. mucedo larvae also develop negative photo tactic behavior. I expect they do not as colonies usually grow on top of the substrate
  4. As the average lifespan of a zooid is maximum 5 - 6 weeks and often limited to 3 weeks, bryozoans are continuously focussed on reproduction, both sexually and a-sexually
  5. C. mucedo colonies, especially young ones, are described to be able to crawl several centimeters a day in order to move away from another colony, they might have split off from.


Bryozoans eat by actively use their cilia (whiphairs) that cover the tentacles to create a current towards the mouth. From this current they select small plancton, such as:

  1. Green algae
  2. Bacteria
  3. Flagellates
  4. Other small one-celled animals

Part of the plancton escapes through the tentacles, so the real food abundance must be higher than the food need.
Not all plankton can be used as food. This is dependent on the size of the mouth, that differs per species, and the size of the food object. For example Cyclos species are too large to be eaten by bryozoans.