A ctenophore is a small marine animal, usually with two long tentacles, that preys on tiny targets in the ocean’s photic (light) zone, including plankton, fish eggs, larvae, other ctenophores, and other tiny organisms generally around 1 mm in size. Ctenophores are sometimes called comb jellies, despite their relative complexity in comparison to jellyfish. Some ctenophores are bioluminescent, but this is only visible in complete darkness. In an aquarium, ctenophores appear bioluminescent due to rows of fused cilia on its side used for locomotion, which scatter light, producing a beautiful rainbow effect. In the best-known species, Pleurobrachia, the light does not emit from the ctenophore itself, but is created by optical scattering.
The best-known genera of ctenophore is Mnemiopsis, which can be found seasonally in the brackish water off Chesapeake Bay. Like jellyfish, Mnemiopsis and other ctenophores have bodies that consist mostly of water, in this case 97%. The ctenophore is prey for many important larger animals, including sea turtles and a variety of fish.
Ctenophores have several unique cells for hunting and locomotion. The ctenophore has a balance receptor, the statocyst, which works very differently than the inner ear in mammals, but is used to maintain upright positioning in ctenophores as needed. Ctenophore tentacles contain colloblasts, or lasso cells, which are specialized cells that send out sticky threads upon contact with prey. These are different from the jellyfish’s nematocysts, which are instead used to inject toxins, but they share some structural similarities. The release times for these cells are very impressive, in the microseconds or less, and can only be captured by extremely quick-exposure cameras.
Unfortunately, we don’t know as much about ctenophore history as we’d like, because these animals fossilize poorly. Most of what we know is gleaned from observations of ctenophores in laboratories in the present. Although more complex than jellyfish, ctenophores lack a central nervous system, possessing only a decentralized neural net to guide their behavior and reactions. The simplicity of this neural net makes it a potential target for emulation in robotics.