Shimmering sea creatures known as comb jellies possess a remarkable ability: when injured, they can merge into one organism without exhibiting the tissue rejection reactions typical in other animals. This fusion allows their nervous systems to integrate and their digestive tracts to connect, enabling both to share food.
This groundbreaking finding has the potential to shed light on the evolution of immune systems, specifically how organisms differentiate between their own tissues and those of others. Furthermore, it offers insights into the evolutionary development of nervous systems.
Comb jellies, or ctenophores, are distinct from jellyfish, featuring fundamentally different biological structures. They represent some of the earliest living creatures that branched from the common ancestor of all animals. Their unique biology provides valuable opportunities for scientists studying early animal evolution. Notably, ctenophores possess a distinctive nervous system comprised of interconnected nerve cells, forming a net-like network rather than existing as individual cells like in other animals.
During a study on the iridescent combs of a ctenophore species, researchers observed an unusually large individual with dual rear ends and two sensory organs, indicating a potential merger of two organisms. To investigate, scientists conducted experiments by pairing segments from unrelated individuals, resulting in nine successful fusions out of ten trials, all occurring seamlessly within hours. This lack of immune rejection suggests that ctenophores may not have the ability to distinguish between “self” and “non-self” tissues.
Moreover, the fused bodies reacted in unison, indicating a complete integration of both nervous systems. The digestive tracts also conjoined, allowing food to pass through the shared system when fed to just one mouth.
These findings pose numerous intriguing questions about the evolution of allorecognition in animals and the mechanisms behind the formation and processing of nerve nets. Ctenophores may also provide insights into additional biological mysteries. Recent research revealed that individuals of this species can revert to a larval-like stage when starved or injured, only to develop back into adults, a phenomenon previously documented in a limited number of jellyfish and a particular tapeworm.
The implications of this discovery suggest that such developmental plasticity may trace back to the last common ancestor of all animals, possibly indicating a broader prevalence of this trait across various species. This adaptability of early animals could be crucial for understanding fundamental biological processes, some of which may even relate to human health issues like tissue rejection, regeneration, and aging. Ctenophores are proving to be invaluable models in tackling these significant biological questions.
