Hydra are small, tentacled beasties that spends their time floating around freshwater, stinging and eating small shrimp. To eat, a hydra has to rip open its own skin. This behavior is well known, but it hadn’t been captured on video—until now.

As described in a new paper in Biophysical Journal, scientists from the University of California, San Diego, captured the first ever image of a hydra opening its mouth. The hydra, named after the mythological monster, is not fearsome to anyone human-sized, but it’s a nasty predator to shrimp.

A member of the phylum Cnidaria, it’s a relative of jellyfish. And just like most jellies it paralyzes prey with barbs on its tentacles and eats them. But the hydra has no visible mouth. When it decides to eat—this happens:


Fibers in the tissue covering the opening to the mouth contract, stretching the cells and splitting open the tissue. Exposing the hydra to magnesium chloride, a muscle relaxant, stopped the process. The paralyzed prey is sucked inside the mouth and digested, at which point the hydra opens is mouth again and spits out whatever it couldn’t eat. After the whole process is done, the hydra reseals its mouth, covering it with a sheet of tissue.


[Biophysical Journal]

Video Credit: Carter and Hyland et al/Biophysical Journal 2016