- Senescence is a term used to indicate gradual deterioration of the body with age. In simpler words, you could call it ‘aging’. Specifically, weakening of muscles, lowering mobility, poor sensory acuity and age-related diseases are signs of an animal showing senescence. Most animals exhibit Senescence. So, as we get older our deteriorated life parameters increase our risk of dying – Humans exhibit Senescence; Crocodiles do not.
But, here on Earth, living with us, are a few species that exhibit Negligible Senescence. That means, they show almost no signs of aging, or they are ‘biologically immortal’. Animals like these only die due to diseases, accidents or predators.
In animals, sea urchins, lobsters, clams and hydras are some examples. Vertebrates like a few Tortoises, Turtles, Crocodiles, Alligators, Rougheye rock fish and Flounders have been not observed to have aged biologically. That is the reason we had a 255 year old tortoise in the Kolkata zoo till the year 2006.
Among trees, probably the best example for an individual would be one Methuselah tree, which has been living for 4800 years. Its exact whereabouts are kept a secret to save it from us. On the other hand, a colony of a single tree has been estimated to be around 80,000 years old. It is also the heaviest known organism.
Side note: Tardigrades survive extreme conditions using a technique called cryptobiosis. They can die and literally come back to life.
Here's a visualization and audio detail of Pando, the heaviest organism mentioned in the article. Also, this scratches the surface of what "limitless Telomere regenerative capacity" means and looks like in other animals - not a microscope, just another visualization for educational purposes.