A new study shows that a type of free living flatworm named planaria – a model organism for ageing due to their supposed “immortality” – are indeed mortal, as they can suffer stress and death when resources are not optimal.
Humans are mortal, and as such, thinking of immortality is not something we might naturally think of as achievable, outside of science fiction. However, the concept of immortality is not new to scientists who study the biological processes of ageing (biogerontologists), who have suggested examples of biological immortality in the natural world. Some suggested candidates include the freshwater metazoa hydra (Hydra spp.; Schaible et al. 2015), prickly pear cactus (Opuntia rastrera; Roper et al. 2021), or the Bristlecone pine (Pinus longaeva; Flanary & Kletetschka 2005). Some well studied creatures in this context are planaria. In these species their assumed “immortality” is due to their lack of biological ageing or senescence rather than an absolute ability to cheat death. Senescence occurs across many – but not all – forms of life, where physiological degradation with age results in reduced performance of individuals. In the case of potentially immortal species however, senescence is either negligible or negative, which means that their mortality rate doesn’t increase as they age – or even more unusually they actually become less likely to die, the older they get.
Senescence rates in a species are not set and can be impacted by variation in environmental conditions, particularly resource availability. Yet, although the impacts of resource availability on senescence in mortal species is well studied, the same has yet to be fully investigated in “immortal” species.
This new study investigates how variation in resources, specifically the quantity, quality, and when individuals are fed impacts populations of two species of long-lived planaria that do not show biological ageing: Schmidtea mediterranea and Dugesia tahitiensis. Dr Jacques Deere, lead author of the work explains:
“It was a significant amount of work over the course of four months, each week we counted and measured each individual in 60 populations that varied in their resource.”
The researchers show that, in both species, the variation in food resource availability does not result in populations going extinct. However, for one species, S. mediterranea, the researchers found that suboptimal conditions does result in stress. Where the number of individuals in populations of D. tahitiensis either increased or stayed the same, S. mediterranea populations were declining significantly.
The contrasting responses by the two species could be attributed to different mechanisms regulated by their life histories. Professor Aziz Aboobaker, co-author of this work, stated:
“Planaria have a remarkable ability of being able to shrink and regrow depending on available resources, which is underpinned by their ability to regenerate damaged cells.”
Professor Rob Salguero-Gómez, senior author, added:
“Here both species are resorting to shrinkage and regeneration in periods of low resource availability. However, D. tahitiensis is much better at doing this as it has higher regenerative capacities and is able to repair damaged cells more effectively than S. mediterranea.”
As such, D. tahitiensis is able to navigate changing resource availability more effectively than S. mediterranea. Conditions will never be optimal at all times. This study shows that it is important to question whether a species is still immortal when they do face some stress – more realistic, natural conditions. The findings indicate that a clearer understanding of how stress can impact non-senescent species and how resilient an organism is to stress will determine if a species can be ‘truly’ immortal.
To read more about this research, published in Journal of Animal Ecology, visit: http://doi.org/10.1111/1365-2656.14184
