It's hard to imagine an organism much simpler than the hydra. It doesn't have brains, hearts, or eyes - it's basically just tentacles with a mouth attached. And yet these simple creatures pull of biological feats no other animal can.
Hydra are one of a few species - flatworms are another recently discussed example - that can regenerate their body parts, seemingly indefinitely. They also possess a surprisingly complex weapon system known as cnidocytes, which enable their tentacles to shoot powerful neurotoxins into their enemies. What's interesting is that these attacks seem to be connected to light levels, and yet hydra don't just lack eyes - they don't even have a visual system at all. And yet they hunt on a clear night-day cycle and have been observed moving in response to sudden changes in light.
To get around the minor impediment of not having any eyes, the hydra have developed a unique way to see the world. Back in 2010, genomic researchers discovered hydra contained the genetic material needed to build opsins, which are photosensitive proteins found in all animals that possess sight. Usually, of course, these opsins are connected to the eyes, but in the case of hydra, these proteins are found clustered on the creature's tentacles and around the mouth.
These proteins then form part of a surprisingly complex system that connects it to neurons and, in turn, to the cnidocytes. This means the hydra, without any brain, eyes, or any sort of central system to coordinate its actions, is able to respond to light by activating its natural weapons system.
A team led by UC Santa Barbara researcher David Plachetzki made the find, and he thinks the proteins don't just help keep the hydra on the correct night-day cycle. He suspects it's also precise enough to respond to sudden shadows and other quick light changes, allowing the hydra to detect and strike any prey that wanders into its field of non-vision. In a statement, Plachetzki explains just how complicated this system is:
"Not only did we find opsin in the sensory neurons that connect to cnidocytes in the hydra, but we also found other components of phototransduction in these cells. These included cyclic nucleotide gated ion channels (CNG) required to transfer the signal and a hydra version of arrestin, which wipes the phototransduction slate clean for a second signal. We were also able to demonstrate that cnidocyte firing itself is affected by the light environment and that these effects are reversed when components of the phototransduction cascade are turned off."
For more, check out Scientific American for a more comprehensive overview on the remarkable abilities of hydra.