www.bryozoans.nl - Reproduction of freshwater bryozoans

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Bryozoans are special as they have three ways to reproduce:

  1. Sexual reproduction
  2. Survival capsules (a-sexual)
  3. Budding (a-sexual)

All three ways to reproduce are used, depending on the circumstances.
The sexual reproduction provides the species with an advantage in changing circumstances. The species can adapt through increased genetic variation.
The a-sexual reproduction has an advantage in stable circumstances. The species can maintain and extend its position with less effort. A-sexual reproduction produces genetically identical individuals (clones).

Sexual reproduction

Depending on the species, the individual bryozoan (zooids) or the colony is both male and female (hermaphrodite). On a given moment, when circumstances are sufficiently beneficial, the zooid forms spermatozoids and/or egg cells that fertilize each other within the individual or colony.
Depending on the species the embryo will be expelled earlier or later (some species breed their embryos), after which the embryo lives as plankton for several hours. The embryo has flagellate cells that enable limited motion. When a good spot is found the embryo will attach to a substrate and develop into a full zooid.
In most species the embryo is negative photo tactical when it is sufficiently developed to actively find a spot to attach. This means that the embryo actively searches for a place that is shaded. The more recent explanation is that shade usually also means protection against silt settling down on the colony. This is important for species that are not able to actively remove it from the colony and are in danger of being swamped (all but C. mucedo).
Older explanations were based on the idea that bryozoans need low light conditions to survive. This explanation is discarded.

Survival capsules

The two bryozoan groups (Phylactolaemata and Gymnolaemata) have different survival capsules: statoblasts and hybernacula. Survival capsules are intended to survive adverse condition (cold, lack of food, even dry conditions) and to aid the species in colonizing the current habitat that has proven to be beneficial to the specis.

Phylactolaemata

The Phylactolaemata species make statoblasts. A statoblast contains one or at most two zooids protected by a tough chitin enclosure. Research has shown that some species' statoblasts, that have hooks on them, will attach to waterfowl and are thereby transported to other habitats (lakes, ponds and streams). Another way to travel longer distances is through the waterfowl's intestins, when colonies and/or survival capsules are eaten as they are attached to aquatic plants.
There are two types of statoblasts:

  1. Floating, also called floatoblasts
  2. Non-floating, also called sessoblasts

Not all Phylactolaemata species make both types.

The floating statoblasts (floatoblasts) aid a species in colonize a proven beneficial habitat in a wider range as well as transport the species to other habitats. The floatoblasts have a gas-filled ring that makes them float to the surface so they are dispersed by water currents and wind.

Non-floating statoblasts (sessoblasts) can be attached to the zooid wall when formed. In that case they remain attached to the place where the zooid lived and ensure a colony is formed on a proven beneficial place.
If sessoblasts are formed free in the zooid body cavity they will fall when the zooid dies and colonize lower areas of the habitat.

Some species make one statoblast per zooid, others many.

There are significant differences in statoblast form that are good identification aids for a species.

Gymnolaemata

The Gymnolaemata bryozoans do not make statoblasts, but hybernacula. These are tough, brown and irregularly shaped capsules that house a zooid bud that rests waiting for better times.
Hybernacula are attached to the colony and will remain close to the place the colony lived when the colony dies.

Budding

If a new zooid has attached itself or an existing colony is in sufficiently good circumstances to extend itself, a new growth bud is formed that will develop in a full zooid. This way the colony will extend fairly rapidly in multiple directions.

Reproduction and environment

Statoblasts and hybernacula are formed in reaction (preparation?) to:

  1. Reduction of food availability below a critical level
  2. Reduction in water temperature towards (below) a critical level

Personal observations

In my favorite diving spot many colonies develop rapidly when the water temperature rises above about 14 C. About one and a half month later (beginning of June), when the spring algae blossoms has passed and food becomes scarce, many colonies die, to reappear around the end of July.
About mid October the temperature has fallen and little development is left. When the temperature drops below 9 - 10 C (end of October or beginning of November) all bryozoan colonies disappear almost overnight.

There are major differences in reproduction between species.
In Cristatella a few colonies develop in May and reproduce sexually after a few weeks, leading to large numbers of colonies (hundreds) close to the originating colonies. After around six weeks these colonies die, packed with statoblasts. A few weeks later new colonies form. During summer and fall there are two more generations to be observed. I am not sure about more than one sexual reproduction cycle.
Pectinatella has only one generation per year, at least in The Netherlands, possibly multiple generations form in warmer climates.
I have observed Plumatella and Fredericella less carefully, but at least two, possibly three generations can be recognized in one growing seazon. Paludicella also has at least two generations.

Sexual reproduction is impossible to observe in the field for a diver.