Hi r24111,
I still remember the time i went to japan for a holiday and saw axolotls at the aquarium shop. I have seen the giant japanese salamander at the zoo as well!
You have raised an interesting question and frankly i have been researching fervently since then to see if i can find anything of use.
Blueberlin (eva) has given good advice as usual, especially regarding the use of salt baths. I agree with her that it should only be used to treat fungal infections and not unnecessarily.
The axolotl skin consists of two main layers. The epidermis with its outer horny layer or stratum corneum and the underlying dermis. The stratum corneum prevents dessication (especially when metamorphosized) and is periodically shed and renewed.
The epidermis possesses various mucous and granular glands which extends into the dermis. The mucous glands are distributed all over the body and produce a slimy secretion that keeps the skin moist and facilitates respiration.
The granular glands are located in particular parts of the body such as the parotid glands at the sides of the neck. They produce specialised secretions such as digestive enzymes and in some other species of salamanders, can even be toxic.
The dermis is well supplied by blood vessels which is also essential for respiration and also contains pigment cells which gives the axolotl its unique colouration.
You are also right that amphibian skin has ion channels. However the skin has selective permeability to electrolytes and micronutrients so you cannot guarantee the axie will absorb whatever you place it in, in the right amount you want. Transdermal route of fluid therapy is not unheard of but i would go with an intracoelomic or subcutaneous injection instead. These routes ensure that the fluids/electrolytes/glucose/vitamins administered is in the right volume and concentration, and delivered to the animal.
Vets often use a special amphibian ringer solution or saline solution composed of some electrolytes (and even vitamins/glucose) to hydrate dehydrated and emaciated amphibians. We can administer an isotonic solution (or hypertonic then isotonic) solution to the salamander. Calcium gluconate can be used in emergencies to correct hypoglycaemia and hypocalcaemia. That has definitely been performed in vet practice.
However, there is still a lot of space for further research to fully understand the unique physiology of amphibians and even to titrate the right dosage of treatments. The above methods are quite honestly emergency protocols to try salvage. They are not methods to replace proper nutrition in your axolotl.
For an inappetant/ poor body condition axolotl, you have correctly identified stress as one of the reasons. Always find the underlying cause and treat it first. Ensure good environmental conditions such as good water chemistry (0 ammonia, nitrite, <60 nitrate, pH neutral), ideal water temp of around 18 degree celsius, low water currents, dim lighting conditions, no other aggressive conspecifics or cross species tank companions, no gravel substrate and proper hiding places.
Next check that is is not suffering from a concurrent illness or disease. Monitor for signs of illness like lesions and gill pathology. Check if there is unusual behaviour such as writhing. Parasites (both ectoparasites and intestinal helminths) can cause inappetance or loss in body condition. A vet can obtain faecal samples for faecal float, microscopy, culture and staining.
Axolotls obtain most of their nutrition via the intestinal tract. Therefore, offering a large variety of nutritious food, including live earthworms, bloodworms etc. should be the first treatment. Live food can stimulate feeding. Also check that the water temperature is not too high (stress) or not to cold (low digestibility) which causes regurgitation.
Lastly, there is a difference in treatment of fully aquatic axolotls versus terrestrial axolotls. Their skin structure is slightly different, so is their physiology. For a fully aquatic species. i would be most hesitant to administer something directly into their water, be it glucose or salt. Unlike terrestrial axolotls, they cannot escape or move away from the water environment and are hence much more vulnerable to the effects. Even if you add something 'harmless' like glucose into the water, you are essentially changing the concentration gradient. The axolotl's skin and gills are sensitive to such changes and can be damaged as a result. It is much safer for a vet to give it the necessary nutrients and fluids via a subcutanous or intracoelomic injection.
Cheers.
. I'm not good at English, so I needed many time to understand..., but it is really instructive for me.