Hydra (Cnidarian)

A hydra
A hydra

external image I10-82-hydra.jpg
A detailed diagram of the hydra, showing important anatomical structures (Alexander Soloviev)

1. Classification/ Diagnostic Characteristics
Cnidarians are a broadly-defined group of protosomes of the kingdom animalia, which encompasses all animals. Cnidaria are distinguished from all other animals by nematocysts. Cnidarians include jellyfish, sea anemones, and hydrozoans. [1]
Hydra gets its name from its class genus which is also hydra.

2. Relationship to humans
Cnidarians include animals such as the jellyfish, which can sting and potentially kill [[#|human]] beings. Many cnidarians can also be found in [[#|coral reefs]], which are continually damaged by human interference.[1]

Example of a potentially lethal jellyfish (Prashant)
external image Ccolorata.JPG

[[#|Coral reefs]] are a massive build of Cnidaria skeletons over the course of many years. The [[#|Great Barrier reef]], the largest [[#|coral reef]] on the planet, is off the coast of Australia and contributes an annual 5 billion dollar [[#|income]] through tourism. It is also the only living thing on Earth visible from .space. (GC)[16]

Hydra have been used by humans while researching sight. By isolating the genetic pathway that codes for the ion channel that determines light intensity humans have studied hydra to better understand how the initial neural impulse that starts vision is formulated and relayed.[17]

3. Habitat and Niche
Due to their reliance on certain types of algae, many cnidarians are limited to shallow waters.[1]
Cnidarians usually occupy two niches. They may use cnidocysts to trap prey items. A cnidocyte is an explosive cell containing one giant secretory organelle that is used for prey capture and avoid predators. It fires a structure that contains a toxin, from an organelle called the cnidocyst. On the other hand, cnidarians depend on zooxanthellae (microscopic flagellates) to survive.[2]

Many hydra live their lives on or attached to an object. They wait for other fish and organisms to swim buy them, and then they trap them with their tentacles.[6]

Hydra live attached to aquatic vegetation, submerged twigs or rocks. They are often found on floating leave and suspended on roots of duckweed.[7]

4. Predator Avoidance
Hydra often act as the predator in fresh water by using their tentacles that can extend to extreme lengths and grasping their [[#|pray]]. However this predator can easily be found by humans who use a net and sweep it in the water. (Cam Somers) [5]
If attacked, they can retract their tentacles, into small buds. The body can retract into a glutenous [[#|sphere]].

Hydra are simple animals, but are carnivorous and therefore need effective mechanisms for targeting small crustaceans. They use stinging cells (nematocysts) on their tentacles to catch their prey. The coiled tubes shoot at and entangles the [[#|pray]]. A second nematocyst with toxins is fired at the prey to paralyze it. They not only use this to find food but also to defend against themselves. The toxins in hydra are too weak to effectively ward off humans, however.[8]

Although their physical characteristics seem to do a good [[#|job]] warding off predators, the hydra is capable of regenerating in the case that it is attacked and this ability is less dependent on "where it is cut" compared to some of the other regenerating organisms on this wiki. Even if it is put in first a blender and then a [[#|centrifuge]] the organism will regenerate. [9]

5. Nutrient Acquisition
Cnidarians get their primary source of nutrition from predation. They use specialized harpoon-like cells called nematocysts to capture prey. Once the prey is captured, the cnidarian's gastrovascular [[#|cavity]] digests the prey. Nutrients are absorbed by the surrounding cells.[1] They spend a large amount of time attached to other objects, waiting for fish to swim by and then they trap them with their tentacles. (3)

Many cnidarians, particularly anthozoans, rely on zooxanthellae for nutrient acquisition; zooxanthellae are symbiotic protists that reside in the tissues of some cnidarians. Zooxanthellae are able to use photosynthesis and pass on the resulting synthesized carbon compounds to their [[#|hosts]]. In this respect, because of the close relationships between zooxanthallae and their cnidarian hosts, some cnidarians can be considered photosynthetic. (4) [GW]

Hydra live attached to vegetation by the base of the tubular body or column, with their tentacles suspended free in the water. At the base of the tentacles is the mouth. Smaller animals which blunder into the tentacles are stung and paralyzed by neurotoxins released from tiny stinging organelles, called cnidae or nematocysts. The cnidae are formed within the ectodermal cells of the tubular body and especially in the tentacles, where they are packed in high densities. Each cnida is a capsule containing a long, invaginated hollow thread. When stimulated by chemical or mechanical cues, the permeability of the cnida increases and water enters, increasing the pressure and blowing the thread inside out. Cnidae are usually released from the ectodermal cells when discharged. The largest cnidae (penetrants) contain the neurotoxins that are injected into prey by the hollow thread. Smaller cnidae include the volvents , which coil spontaneously on contact, and glutinants , which are adhesive.
Once a prey item is captured and paralyzed by the tentacles, body fluids leaking from puncture wounds stimulate a simple feeding response in the hydra. This involves a shortening of the tentacles, expansion and opening of the mouth, and the engulfing of the victim. The major stimulant for this response is the common tri-peptide (a chain of 3 amino acids), glutathione. Digestion of the prey item in the gastrovascular cavity proceeds over several hours. Large molecules are taken up by the gastrovascular cells, where digestion is completed. 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 , and Chydorus.[7]

6. Reproduction and Life Cycle
Cnidarians have two stages to their lives. The first stage, coming right out of an egg, is the polyp. The second stage is the medusa. In the polyp stage, a cylindrical stalk is attached to the ground with it's mouth facing upward. Polyps reproduce via asexual budding to form medusa. The medusa stage is a free-swimming stage shaped like an umbrella, often with tentacles. It floats with it's mouth facing downward. Medusa reproduce sexually, forming a polyp.[1]
Spawning of cnidarians is generally driven by environmental factors such as changes in water temperature, and their release is triggered by lighting conditions sunrise, sunset, or phase of the moon.[10]



7. Growth and Development
A cnidarian begins life as a ciliated larva called a plantula, which eventually reaches the bottom of the ocean and develops a polyp. After it matures, it created medusa by budding.[1]
Hydra reach maturity within 5-10 days of their birth and can live up to 4 years.[11]

images-1.jpeg development of a Hydra (evan) (http://ars.els-cdn.com/content/image/1-s2.0-S0959437X12001098-gr1.jpg)
8. Integument
Cnidarians have an epithelial membrane made up of two small layers of epithelial cells and a jelly-like substance between them. [1]

9. Movement
[[#|Polyps]], like the hydra, are rooted to the ground and do not move. Medusa move by using muscle fibers embedded in the epithelial cells. [1] Hydra can move, however, as a response to stimuli. It can retract its tentacles if it feels threatened, or in response to a variety of sensations. The hydra also moves when hunting; it can bend over to attack a prey, or even move laterally through a process called looping.[12]

10. Sensing the Environment
Only a small number of cnidarians have sensory organs.

Though without a brain, the hydra has a network of nerves which can sense and respond to environmental stimuli, which is almost exclusively coordinating movement. (Colin Gray)[13]

Some have sensory photoreceptors and touch-sensitive nerve cells. they are connected, not with a true brain, nor with the primitive brains of slugs. Instead, it has a kind of neural net.

11. Gas Exchange
Gas exchange is handled either directly through cell membranes to the external environment or through the gastrovascular cavity.[12]

Hydra have a fluid [[#|filled]] [[#|cavity]] called the gastrovascular cavity. This cavity supplies nutrients to all cells lining the cavity as well as performing most of the Hydra's gas exchange.[14]

external image img036.gif&sa=X&ei=mI7TUMHRB-io0AGhg4HADQ&ved=0CAkQ8wc&usg=AFQjCNF16G00NNUQeeAQu1G4huj0LfW2uA

12. Waste Removal
Waste removal is handled either directly through cell membranes to the external environment or through the gastrovascular cavity. The hydra does not have a separate opening for waste. Any material that cannot be digested is regurgitated out of its mouth. (3)

13. Environmental Physiology (temperature, water, salt regulation)
Cnidarians can survive in a multitude of condition, from warm and shallow water to deep and cold water.[1]

14. Internal Circulation
Cnidarians do not have circulatory organs.[1]

15. Chemical Control (i.e. endocrine system)
Cnidarians do not have an endocrine system.

Although Hydra do not have an endocrine system, they do still contain some of the hormones that organisms with an endocrine system secrete. Known to have a nerve net with typical neurons and synapses, hydras also have tissues that secrete hormones such as sex steroids, neuropeptides and gonadotrophins.[15]
Review Questions:
1. Explain why you would most likely find hydra in shallow waters.

2. The hydra uses these specialized harpoon-like cells to capture a predator to gain some nutrition?
3. As a simple life form with a lack of complex systems, what method do hydras use in order to excrete waste?

4. How do Hydra reproduce? (Be sure to mention the stages)

1. Hillis, David M., David Sadava, H. C. Heller, and Mary V. Price.
Principles of Life. Sudnerland, MA: Sinauer [[#|Associates]], 2012. Page 452, 458, 462, 463-465
6. http://www.biologycorner.com/worksheets/articles/hydra.html
7. http://www3.northern.edu/natsource/INVERT1/Hydra1.htm
8. http://www.offwell.free-online.co.uk/hydra.htm (Bhu)
9 (http://www.devbio.biology.gatech.edu/?page_id=683)
10 (http://en.wikipedia.org/wiki/Cnidaria#cite_note-Hinde2001CnidariaAndCtenophoraInAnderson-5)
11 http://genomics.senescence.info/species/entry.php?taxon=Hydra
12 http://en.wikipedia.org/wiki/Hydra_(genus)
13 http://www.biologycorner.com/worksheets/articles/hydra.html
14 (http://www.snow.edu/allans/biology1320/circulatorysystem.html)
15 http://intl-icb.oxfordjournals.org/content/45/1/201.full
16 http://www.wildlife.org.au/conservation/issues/2007/gbrmp07.html
17 http://www.edhat.com/site/tidbit.cfm?nid=28032