Wednesday, April 4, 2018

Hydra Microblog

Microblog from Linda Bower MN

Green Hydra - Wikimedia
Linda Bower sent me this fascinating video she filmed of a green hydra in pond water.  Like the crinoids of old, it ingests nutrients into a dead end stomach, expelling undigested particles back out the way they came.  What is not to love about a simple system like that?

New to hydra?  Here's an excerpt from an article by Northern State University of South Dakota, USA: "Hydra are named after the nine-headed sea snake of Greek mythology and are freshwater relatives of corals, sea anemones and jellyfish. All are members of a primitive phylum, the Cnidaria. The gut of cnidarians has only one opening. Unlike more complex animals, cnidarians are designed around 2 sheets of tissue: the ectoderm, lining the exterior; and the endoderm, lining the gastrovascular cavity. The two are separated by a gelatinous partition...greatly expanded in jellyfish, but is much reduced in hydra."

Chlorohydra viridissima is a bright green species, owing to the presence of numerous algae called zoochlorellae, which live as symbionts within the endodermal cells. Their relationship is mutualistic as the zoochlorellae carry out photosynthesis and produce sugars that are used by the hydra. In return, the carnivorous diet of the hydra provides a source of nitrogen for the algae.

Hydra viridissima - Wikimedia
At the base of the tentacles is the mouth. Smaller animals which blunder into the tentacles are stung and paralyzed by structures called cnidocytes, the little bumps seen along the tentacles in Larry Wegmann's photograph below.  The tentacles draw the prey into the mouth, and body fluids leaking from puncture wounds stimulate a simple feeding response as the shortening of the tentacles shorten, the opening of the mouth expands, and the victim is engulfed.  Digestion of the prey item in the gastrovascular cavity proceeds over several hours. Cuticles and other undigested remains are subsequently expelled through the mouth. Almost any small invertebrates, up to the size of the hydra, may be consumed, including annelid worms, rotifers, insect larvae, and (especially) small crustaceans, such as Daphnia, Chydorus and Cyclops spp."

Hydra budding - Larry Wegmann
Most hydra reproduction is asexual by budding as seen above.  A new but genetically identical hydra is extending from the body and will break free from the parent.  Hydras can occasionally reproduce sexually.  Most species being hermaphrodites, they form both an ovary and testicles on the same individual.  A single fertilized egg can be held until the conditions are right for its release.

You can read more in the Northern State University article.

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