Visit the
Molecular Expressions Website

Photo Gallery
Silicon Zoo
Chip Shots
Screen Savers
Web Resources
Java Microscopy
Win Wallpaper
Mac Wallpaper
Custom Photos
Image Use
Contact Us

Pond Life Digital Movie Gallery

Hydra (Coelenterata) Movies

In a phylum dominated by marine species as diverse as the jellyfishes and corals, the hydra (class Hydrazoa) are among the few commonly observed coelenterates in freshwater ecosystems. Skipping the medusa stage typical of the jellyfish, the stalked and tentacled invertebrate polyps can reproduce sexually or asexually. They follow the basic coelenterate radially symmetric body plan of two cell layers without a true body gut, having instead a gastrovascular cavity, with the mouth as a single opening.

Hydra Video #1 - A hydra swims toward the viewer, and turns as it comes into the plane of focus, revealing the length of its tentacles; under oblique illumination with a playing time of 24 seconds. Choose a playback format that matches your connection speed:

56k modem | DSL/Cable/T1 | download the MPEG (2.28 MB).

Hydra Video #2 - A pair of hydra engage in a struggle over prey; under oblique illumination with a playing time of 66 seconds. Choose a playback format that matches your connection speed:

56k modem | DSL/Cable/T1 | download the MPEG (6.51 MB).

Hydra bear the name of the Greek mythological creature of Lerna, described as having the body of a hound and 100 serpentine heads, each bearing poisonous breath. With so hideous a visage that many died of fear just viewing the mythological beast, the freshwater hydra has much legend to live up to. Hydras are easily identified by their numerous tentacles, each bearing poisonous nematocysts that can paralyze or kill prey. The tentacles are retractable and bear up to four different types of stinging cell. For such a primitive and simple creature, the nematocysts are among the most advanced and intricate organs developed in the aquatic realm. Some nematocysts function as harpoons, with large barbs that anchor the tentacle to the prey so that it can be reeled in, delivering the prey to the hydra's other tentacles and mouth. Another small nematocyst type features a thicker and shorter corkscrew thread that wraps around and grips the prey. A sticky, bean-shaped object is found on the end of the third type of stinging cell, and functions as an aid in locomotion. Probably used in the defense of the hydra, the fourth type of nematocyst has spines running the length of the thread.

Without circulatory, respiratory, or excretory systems, the hydra's cells depend on simple diffusion to deliver respiratory gases and ingested matter for nutrition, and these freshwater coelenterates have no functional need for blood. Asexual reproduction by the hydra occurs through budding, or regeneration. A hydra can be cut entirely in half longitudinally, and it is possible for two hydras to grow, each a genetically identical clone. In the process referred to as budding, a new individual grows from the parent's body. Once it develops tentacles and its own foot, the offspring breaks off to grow as a new hydra clone. Prior to the winter, hydras develop egg and sperm cells instead of budding, and following fertilization, a developing zygote settles into the soft aquatic sediments. There, it overwinters while the parents die in the cold waters of temperate and polar regions. In many cases, the individual hydra is hermaphroditic, producing both male and female cell types.

Contributing Authors

Cynthia D. Kelly, Thomas J. Fellers and Michael W. Davidson - National High Magnetic Field Laboratory, 1800 East Paul Dirac Dr., The Florida State University, Tallahassee, Florida, 32310.



Questions or comments? Send us an email.
© 1995-2022 by Michael W. Davidson and The Florida State University. All Rights Reserved. No images, graphics, software, scripts, or applets may be reproduced or used in any manner without permission from the copyright holders. Use of this website means you agree to all of the Legal Terms and Conditions set forth by the owners.
This website is maintained by our
Graphics & Web Programming Team
in collaboration with Optical Microscopy at the
National High Magnetic Field Laboratory.
Last Modification Friday, Nov 13, 2015 at 02:19 PM
Access Count Since September 17, 2002: 68299
Visit the website of our partner in introductory microscopy education: