Uniqueness of Comb Jellies' Nervous System Sets Them Apart from Other Animals

A recent study published in Science reveals that the nerve system of comb jellies, also known as ctenophores, is unlike any other known animal. While electrical impulses pass between nerve cells in everything from anemones to aardvarks, ctenophores have a fused nerve net, lacking the distinct connection spots, known as synapses. A new 3D map of its structure generated by researchers reveals that the nerve net is made up of long, stringy neurons sharing a cell membrane.

Ctenophores, which are often called comb jellies because they swim using rows of beating, hairlike combs, are considered to be one of the earliest species to branch off the animal tree of life, and their possession of a simple nervous system has been an area of particular interest to scientists. The uniqueness of the ctenophore nervous system’s architecture and their ancient position in the animal kingdom raises the possibility that nerve cells actually evolved twice, according to Pawel Burkhardt, a comparative neurobiologist at the University of Bergen in Norway, and his colleagues. Further research is needed to verify their evolutionary origin.

If ctenophores are the most ancient animal group rather than sponges, as some evidence suggests, it “implies that either sponges have lost a massive number of features or that the ctenophores effectively evolved them all independently,” says Graham Budd, a paleobiologist at Uppsala University in Sweden who was not involved in the research. Even if sponges emerged first, it is still possible that ctenophores evolved their nerve net independently rather than inheriting it from a neuron-bearing ancestor. Ctenophores have other neurons outside the nerve net, such as mesogleal neurons embedded in a ctenophore’s gelatinous body layer and sensory cells.

There are still many unknowns about this unique synapse-less nervous system, such as how it works, with reports of potentially similar systems in other animals. Studying them in detail, along with nerve nets in other ctenophore species, could determine just how unusual it is. The findings provide a significant piece of the puzzle for understanding the origins of the animal nervous system.