Cause of premature death in turtle eggs
Context
I am a hobbyist turtle breeder and I have been keeping and breeding turtles for 15 years (I have hatched turtles in the high hundredths).
Long story short, few years back, my friend and I acquired a group of possible heterozygous for albinism Kinosternon sp.; here you can find a simplified but representative family tree. Squares represent adult animals (blue background for males, red background for females), while hexagons are individuals whose gender is unknown. Moreover, each shape gets a color based on the genetics of the specimen: grey for possible heterozygous (50% or 67%), black and yellow for heterozygous (100%) and yellow for albino. Everything started because of M1 carrying the genes for albinism and it is a wild caught specimen from many years ago.
The problem
Unfortunately, we are having issues hatching babies from this group. Many of them die at a very late stage of development, that is, turtle completely formed, small yolk sac (that would have been absorbed within one week tops). This is not always the case, however, as I have been able to hatch some without any intervention on my part. These were hatched from different females: a clutch from a female may hatch fine, while the next ones from the very same female might not, so the problem is not related to one or more individuals, but apparently every one.
This happens to me, but also to my friend, who is also an experienced turtle breeder and we live in two different locations. This happens with inbreeding, but also when mating unrelated pairs: e.g.: M1 or M2 with a female from a totally different genetic line. The only reliable method to increase hatching success is to frequently check the eggs by candling and crack them open when they are ready to hatch. Unfortunately, this is still a risky operation and it is sometimes difficult to estimate development stage, especially with albinos.
Incubation
We incubate our eggs in an artificial incubator. We put the eggs in moist vermiculite (1:1 ratio, which is pretty standard) inside small plastic boxes with holes on each side and lid. The eggs are buried halfway. 759
Temperature is controlled by a proportional thermostat that ensures precise temperature control. We tried constant temperature incubation at 28 °C, but also varying temperatures between 23 °C and 27 °C to emulate daily fluctuations that would occur in nature. The incubator is "big" compared to the number of eggs it contains. Talking with one breeder and owner of one of the biggest turtle farm in North America, he suggested lack of oxygen might be the cause. Hence, I moved a dozen eggs with turtles almost due (within a month or two) in a heated room (ambient temperature around 26/27 °C) and placed each lid on top of the plastic container, by allowing plenty of air to flow through, but the same problem occurs. So, lack of oxygen is not the cause, or, at least it is not caused by the incubator itself. I will try and incubate eggs once laid in that room, but I fear I will most likely end up with the same result.
Husbandry
I keep my animals in an indoor facility separated (each specimen gets its own tank) and I periodically introduce a female in the male's tank. I do not provide UVA/UVB, but just LED lighting. I provide top quality food to my animals, for instance:
* commercial pellets
* offal (heart, liver, lungs etc.)
* live preys: mealworms and earthworms
* fresh fish
* cuttlebone
Causes? Solutions?
What could be the cause of embryonic death at a so late stage in the incubation process? Is there anything I can do besides what I have already tried? Would a veterinarian be able to perform an autopsy on such small animals and provide valuable insights? I think I will possibly try shellless incubation like it is done with chickens to exclude lack of oxygen as the cause.
I have tried to be as detailed as possible, but please, feel free to ask for any clarification or missing information.
Update 2022/05/28
The issue is still there, but I am pretty confident the cause is not genetic (this also happens with other Kinosternon spp.. The interesting thing is that I have had some hatchlings able to break the egg shell on their own, only to die a few hours later, while I have been able to rescue a handful of hatchlings by opening their eggs 2/3 days before the complete yolk sac absoprtion. My guess is that these turtles are diying because they do not have enough oxygen during the very last incubation days. Those who succeed in cracking the egg, carry irreparable damage and die because of this shortly after. 671