We would like to immobilize live Daphnia for laser scanning microscopy (confocal microscope). Suggestion of a protocol having minimal impact on the animal physiology would be greatly appreciated.
did you try Magnesium chloride. It inhibit muscle contraction. I used to add it to the sea water at about 0.3M (not totally sure about the concentration) to immobilise small gastropod larvae.
I tested the effect of four concentrations of magnesium chloride on daphnia mobility.
0.25M killed the daphnia in 20 minutes. 0.03M had very little effect over a 95 minutes observation period. 0.13M made daphnia immobile after 15 minutes, keeping them alive for about 80 minutes. 0.06M gradually slowed down their swimming and finally immobilised the daphnia after 95 minutes.
Therefore, 0.13M MgCl2 would work fine although the impact on daphnia physiology is probably significant.
There are methods of mechanical immobilization of objects for CLSM: like methyl cellulose gel. It is usable and useless for the embryos in the early stages, but will not stop a motions of prenatal and postnatal animals.
Earlier, I have successfully used NaN3 for microsurgery on cladocerans. It is very toxic.
If you use wild cladocerans, you will see only chitin autofluorescence. All the above mentioned methods can not change the morphology of the chitin (shell). If you use any staining you strongly alter the metabolism of animals.