Statoblast introduction

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Statoblasts are formed by most freshwater bryozoa species (except Paludicella and Victorella), partially to survive severe circumstances and partially as asexual reproduction.
Cristatella for example makes large numbers of statoblasts, while other species, like Lophopus, make one or a few. Once the zooids die the statoblasts will be entombed in the colony sac or zooid tube until it desintegrates.

Statoblasten in Cristatella

Statoblasts consist of two halves or valves, that are both concave and house a proto-zooide with some food between them. The drawing below from [Allman] shows a Cristatella mucedo statoblast in side view.

Statoblast from Allman The brownish parts in the middle are the statoblast's actual valves. Surrounding it is a grayish border - the annulus. The annulus exists in some species in a more or less prominent fashion (see table below). The valves are made from a chitin like material.
In some species the annulus is filled with a bit of gas, making the statoblast float upon release. In other species the annulus is not gas filled or only minimally developed. The photo below is made in such a way that the annulus can be clearly seen. The dark part in the middle is the actual statoblast valve.

Cristatella is special because it has spikes extending from the statoblast (Pectinatella also has them).
Cristatella statoblast with annulus

Statoblasts develop within the zooid on a strand of tissue running from the underside of the moving part of the zooid to the bottom of the zooidal space, the funiculus. The drawing below, again from [Allman] showing P. fungosa shows them at the bottom of the drawing marked 'Z'. The structure above them marked 'X' (small granules with hairs coming out of them) is intended to show testis with spermatozoa being released and are marked 'E' when free swimming.
The bundle lines running from the bottom of the zooid to the bottom of the drawing are the retractor muscles that retract the zooid. Allman calls this species Alcyonella fungosa.

On the right hand side there is a photo that provides a view inside a P. magnifica zooid, presenting the funiculus with the developing statoblasts (in a fairly early stage of development). Also the muscles are clearly shown and are a bit more complex than shown in Allman's drawing.

Statoblast production Statoblast production in Pectinatella
Allman figure 7 P. magnifica underside of a zooid

The statoblasts are entombed in the zooid cavity until the zooid dies and disintegrates. In some species statoblasts are expelled trough a special pore below the anus while the zooid lives.
Some species' statoblasts have a gas-filled annulus and will float to the water surface on release. Other statoblasts have no gas in the annulus or a marginally developed annulus and will stay put or drift away in a current.
Statoblasts with a gas filled annulus that drift to the water surface are called floatoblasts. Statoblast without or with a minimal annulus that stay put or drift away are called sessoblasts.

Soort Annulus Floatoblast Sessoblast
Paludicella articulata No No No
Victorella pavida No No No
Cristatella mucedo Yes Yes No
Fredericella sultana Yes No Yes
Lophopus crystallinus Yes Yes No
Pectinatella magnifica Yes Yes No
Hyalinella punctata Yes Yes Yes
Plumatella casmiana Yes Yes Yes
Plumatella emarginata Yes Yes Yes
Plumatella fruticosa Yes Yes Yes
Plumatella fungosa Yes Yes Yes
Plumatella geimermassardi Yes Yes Yes
Plumatella repens Yes Yes Yes