Cristatella mucedo statoblasts

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The classical image of a Cristatella mucedo statoblast with apologies for the slight unsharpness
A floating statoblast is also called a floatoblast. The photos show a top view (dorsal vriew).

Cristatella mucedo statoblast

Key elements for C. mucedo statoblasts are:

Lighted in a different way it becomes clear that the center part of the statoblast is not a smooth surface but has a ring shaped structure along the edge. This is called the annulus or float-ring. This is a gas-filled ring that provides floating capacity to the statoblast. Also it becomes clear that the spikes extend from under the annulus and do not run over the top surface.
To create this picture i mainly used sideway topside lighting.

Cristatella mucedo statoblast

Using more extreme lighting techniques a skirt can be shown to exist around the spikes and extending beyond them.

Cristatella mucedo statoblast from [Allman]

As far as i know this is the first instance describing this skirt. Nor [Mundy], nor [Wood_II], mentions it.
[Allman] states "Statoblasts orbicular, with an annulus and marginal spines" (page 77). On page 79 a more extensive description of the statoblasts can be found: "The statoblasts are very characteristic. They are about 1/33 of an inch in diameter, exclusive of the marginal spines, and, with the exception of the statoblasts of Pectinatella, which they closely resemble, are larger than any other fresh-water Polyzoon. They are also, with the same exception, the only ones having an orbicular shape. One face is a little more convex than the other. The annulus is wide, very distinctly cellular, and of a light yellow color. The disk is deep reddish-brown, and elegantly mamillated. The spines spring from both faces of the disk, just within the annulus, and thence radiate outwards, extending some distance beyond the margin. The spines springing from the more convex face are somewhat longer and more numerous than the others, and alternate with them. All spines are terminated by two, three, or four hooks resembling grappling-irons." No notice of a skirt at all. Allman goes on to describe statoblasts attaching themselves to aquatic plants by means of the grappling-iron like hooks.
However the color plates dos show a skirt. Apparently this information has been lost.

A key question is what the purpose of this skirt is. Possibly it enhances the statoblasts' floating ability by using the surface tension of the water surface. Possibly it extends the ability to float beyond the point the gas has escaped from the annulus reducing its upward effect.
Another key question is what the purpose of the spikes is. The usual argument is that the anchor the statoblast to for example aquatic plants or bird feathers (e.g. ducks). Another possibility is that the real role they play is to support and extend the skirt to optimize the contact with the water surface. As the skirt extends over the spikes the anchor role should not be that strong.

The fact that this is the first time the skirt has been described is likely te result of the standard approach to microscopy: make a slide and use standard microscope lighting. The skirt is likely to be too fragile to be undamaged by the transfer to the slide. Also it requires special lighting techniques to make the skirt visible. My approach to photograph free floating statoblasts just emerged out of the colony envelope prevents the skirt becoming damaged (and makes photographing statoblasts much more complicated).
To make this photo I used one sided off-axis oblique lighting by placing a dustcap over the field aperture and positioning it in such a way that all direct light was blocked and a small 'new moon' of light emerged from one side. The photo was made using a Zeiss standard microscope with a 2-12,5 mm biotar or a 25 mm luminar (sorry i forgot which one) lens as oculair (and a 10 x photo objective in my trinoculair tubus projecting in to a Canon A640 digital camera).

Cristatella mucedo statoblast

I once accidentally photographed a free floating statoblast in situ. This statoblast had just been disloged from the colony envelope (see below).
The annulus seems white because it has been overly lighted by my flash. Now we know there is a skirt surrounding the statoblast we can also consiously see it on this photo. Also the spikes are visible if barely. The central part - the actual statoblast capsule - is shown orange - brownish, a more representative color than the almost black of the photos above.

Cristatella mucedo free floating statoblast

Statoblasts form inside of the Cristatella colonial sac. After around six weeks the zooids die and leave the statoblasts in a sac - the remainder of the colony envelope.
The photo below is made in-situ. Some colonies have a few living zooids that have been forced to the side/underside by the statoblasts that fill the colony.
A small disruption is sufficient to tear open the sac freeing the statoblasts. After a few days the sac tissue dies and the statoblasts emerge spontaneously.
Interesting enough the combined upward force generated by the statoblasts' annuli is insufficient to make the colony sac float. If an ex-colony becomes disloged (a small inattentive move by a photographer is sufficient) it sinks to the bottom.
This photo does not show free statoblasts. The round objects in the upper half are oxygen bubbles produced by the algae. Other round objects are free floating particles. This photo is made using natural light only.

Cristatella mucedo statoblasts in colony sac

This in-situ photo shows how statoblasts escape from a colony-sac. The sac has ruptured. Earlier photos made just before this one (and not shown as they are not sharp enough) show that it is a fairly rapid movement. Please notice all statoblasts move with the upper side first (and upward). From this it can be derived that the upper side of the annulus (floatation ring) provides most floatation ability.

Cristatella mucedo statoblasts escape from colony sac

Under a microscope at low magnification it can be clearly seen how the statoblasts are bundled together by the colony sac.
The haze over the statoblasts is caused by the colony sac tissue. Through it the statoblasts and their annulus can be clearly seen. At the upper left side a statoblast is visible from the under (ventral) side. It is clear the spikes originate from inside the annulus at the underside of the statoblast, not from the side.
At the righthand side of the photo a statoblast is shown from the side. Statoblasts are obviously disk shaped and the annulus extends upwards and downwards from the statoblast capsule (the inner part) itself.

Cristatella mucedo statoblasts in colony sac detail

This in situ macro photo of statoblasts in a died-off colony-sac provides a few interesting examples. There are statoblasts with a dark centre - we see them on the back / upper (dorsal) side. A few of them (barely) show the spikes that run across the annulus. There are statoblasts with a light brown center - we see them on the belly / under (ventral) side. A few statoblasts clearly show the skirt, specially those we seen on the underside. This teaches us that the skirt attaches on the underside of the annulus, together with the spikes.
Lower left we see some statoblasts on the side. Here we can see that the upper side is slightly convex and the under side more convex.

Cristatella mucedo statoblasten in koloniezak in situ

This in situ photo by Silvia Waajen shows a number of statoblasts still together while the colony sac has fallen apart. Apparently there is insufficient water movement to disperse the statoblasts. The 2nd photo shows a detail (cutout).
click on the photo for a large version

Cristatella mucedo statoblasts after colony sac has fallen apart
Cristatella mucedo statoblasts after colony sac has fallen apart - detail

The next photo shows a single statoblast as seen under a microscope and through the colony wall. I suspect the statoblast is immature and the annulus has not yet completely formed, or have the spikes. The skirt can be clearly seen. The shadows are the underside of surrounding zooids.
Click on the photo for an enlarged version.

Cristatella mucedo statoblast detail through a microscope in colony sac

While making these photos i noticed a large number of small animals moving rapidly across the colony sac surface. This photo is an attempt to make them visible, which was far from easy.

Cristatella mucedo colony sac surface inhabitants

One of these animals got captured in more detail. I am not yet sure what this species is.

Cristatella mucedo bewoners colony sac surface inhabitants