Ethical Axolotls

Based on information compiled from nutritional analysis of several feeder animal options as compared to urodele dietary needs.

Earthworms are considered a nutritionally complete ‘staple’ food for most salamanders, and nightcrawlers (european, canadian, or african) are the most suitable earthworm feed for amphibians. This article will discuss the nutritional value of earthworms and briefly identify each species, and explain the biomechanics of lysenin.

Earthworms in general are known for their excellent calcium : phosphorus ratio, exceeding 1.31, whereas NO other feeders except for crustaceans, fly larvae, and the agave caterpillar were found to have a Ca:P ratio suitable for urodeles, per the 1989 thesis of Mary Eleanor Allen, PhD.

All amphibians require a certain amount of calcium in their diet and water, to prevent metabolic bone disorder – feeding earthworms can directly counter MBD if the cause was poor diet (such as pellets or bloodworms). Metabolic bone disorder, if left untreated, may result in bone density loss, inappetence, and comorbid scoliosis / fractures, as stated by the Merck Veterinary Manual.

The total protein content of earthworms exceeds 75%<, and they have a crude fat content of ~12%, as determined by the International Journal of Environmental Research and Public Health. This indicates earthworms as a potent source of nutrition for salamanders and newts, while not providing an excess of dietary fat. Urodele appear to benefit from low-fat diets, per the Journal of Agro Productividad.

Author note: This is why many axolotl breeders free-feed earthworms to their cohabitated breeding stock. An axolotl will become physically full and satiated before they could possibly consume enough fat from earthworms. This likely varies by species – consult your exotic veterinarian or species-specific breeder about the appropriate feeding schedule for your species.

‘Nightcrawler’ is a generalized term for various Lumbricus, Dendrobaena, etc. earthworm species that are commonly used for fishing. This is due to their durability in low-oxygen environments; many nightcrawler species can survive extended periods underwater if the water is clean and decently oxygenated. Nightcrawlers are prone to digging very deep underground, countering the red wigglers’ tendency to remain surface level. Nightcrawlers consume more neutral, phosphorus-rich matter, such as decaying wood and soil, whereas red wigglers tend to subsist on manure or decaying foods, and overall more acidic materials.

‘Red Wigglers’ and ‘Indian Blues’ are both small earthworm species that have bio-defensive coelomic fluids containing lysenin, which is a pore forming protein that has been found to cause GI distress in amphibians, as well as induce smooth muscle contractions in tissues exposed to the protein (such as a rat’s aorta, as found by a 2004 study by the Research Laboratory of Tokyo, Japan.), likely by inducing hemolysis (rupture of RBCs) – in fact, the etymology of the word ‘lysenin’ is ‘lysis’ + ‘eisenia’.

Lysenin binds specifically to sphingomyelin, which is a bioactive lipid noted in most vertebrate organisms, often found surrounding nerve cells. Direct exposure to lysenin has been shown to result in excessive defecation, as well as sloughing epidermal cells in multiple species of amphibians, per a 2006 study by the Research Laboratory of Tokyo, Japan.

Nightcrawlers are the most beneficial staple food that is readily available for salamander feed, and they are recommended by most amphibian breeders.