There are about 150 known species in the phylum Ctenophora, also known as comb jellies. 150 species may not seem like many compared to over 50,000 species of vertebrates alone, but comb jellies have existed for at least 550 million years. Porifera, or sea sponges, and comb jellies are the earliest known animals in evolution.
Evolutionary and developmental biologists long thought that the first animals were sea sponges, but recent genome sequencing indicates that comb jellies may actually have evolved first. This seems counter-intuitive, since sea sponges appear far less complex than comb jellies. Many scientists stand by the hypothesis that sea sponges came first, attributing some discrepancies to comb jellies’ high rates of genetic mutation, making comb jellies appear older than they are.
Whether comb jellies or sea sponges came first, there is no question that both phyla are incredibly ancient and far-removed from more recent evolutionary additions such as mammals. Comb jellies are incredibly bizarre and have often been called the “aliens of the sea.”
The strangest thing about comb jellies is that they have nervous systems unlike any other animal’s. They develop neurons in a unique way, lacking the genes that are essential for neuron development in other animals. Scientists do not yet understand how comb jelly nervous systems developed, or why they work, but it indicates a different evolutionary lineage. It seems that evolution took every other animal with a nervous system down one road, and comb jellies down another.
Although both have gelatinous bodies of 95% water, comb jellies only superficially resemble their distant relatives the jellyfish (phylum Cnidaria). Comb jellies hunt with “sticky” cells found along their two tentacles, not stinging cells. They have also evolved a middle cell layer, called a mesoderm, which later evolved separately in more complex animals. Comb jellies move with rows of tiny hair-like organelles called cilia, though they cannot swim against the currents.
Cilia are what give most comb jellies their beautiful, iridescent color: light refracts off the cilia as they move, creating the rainbow effect seen in this picture. Humans are just now beginning to understand and fully appreciate the evolutionary enigma that comb jellies represent, but we have always been able to see the simple beauty in these complex creatures.