A leucistic green sea turtle (spotted on r/pics).
On first appearances I’m sure the majority of us would proclaim with interest and enthusiasm, “WOW! An albino green sea turtle.” However, that isn’t the case interestingly enough! It’s actually a turtle with what’s known as leucism. According to wikipedia:
Unlike albinism, it is caused by a reduction in all types of skin pigment, not just melanin.
From my understanding, the best way to tell the difference is by looking at other parts of the body like the eyes (leucism can also be patchy in appearance where only some parts of the animals skin is affected). Melanin is the only pigment that contributes to eye colour (in the iris specifically), and hence why it’s possible to detect the difference between these two genetic disorders. (However, this method isn’t necessarily fool proof as I’m sure pigmentation attributed to melanin may vary a great deal across many different species, so genetic tests would probably be the only thing that is truly fool proof when it comes down to it).
A leucistic pigeon (not the normal coloured eyes and legs, and patches of normally coloured feathers).
In the below example using the alligator, when the eyes appear as they normally do, with grey/blue pigmentation, the animal is most likely leucistic, and if it has pink eyes lacking melanin pigmentation then it’s an albino.
A leucistic alligator
An albino alligator
People may ask why traits like albinism or leucism still exist in the wild when surely it has little if any benefits for animals? Evolution via natural selection unfortunately finds it relatively impossible to weed out traits like albinism and leucism which are recessive. That is that they aren’t exhibited by the animal (in what’s called its phenotype) unless the animal has two copies of the recessive alleles (one from its mother and one from its father). If it only has one, it will exhibit the same phenotype (it will appear the same) as would an individual without any copy of that allele.
Because of this fact, individuals who carry two alleles and appear white are likely to have a lower fitness level than other normally appearing individuals, and is thus more likely to be killed/predated prior to passing on its genes. However, because individuals can carry a single recessive allele for albinism or leucism without any changes to its appearance, if it ever mates with another individual who is carrying a single (or two) recessive alleles then offspring may be produced carrying this phenotype. Similar to genetic disorders in humans such as cystic fibrosis or sickle-cell anemia.
On a side note, during my volunteer work in Queensland at Mon Repos with sea turtles, I would often find white hatchlings. Unfortunately, the majority of them normally didn’t make it out of their shells and died in the burrows as a result of other genetic defects they also carried (they often had contorted bodies, and one I found had no eyes at all). In once case I even found twin albinos sharing a single egg shell. Sometimes you’d find abnormally large eggs an these would have two yolks or embryos in them. It was rare enough that they would actually develop at all, let alone develop and both be albino or leucistic.
Looking back I wish I’d had a closer look and could’ve worked out whether they were carrying albinism or leucism! I might have to rummage through my turtle photos and see if I can find some images of them.
Green sea turtle hatchlings, clearly some can survive, but also note the distortion of the vertebral scales along the spine of the white hatchling (though this is often seen on average hatchlings too).
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