HYDRAAmal Cheema

Hydra sp

Classification/Diagnostic characteristics:

Kingdom: Animalia
Phylum: Cnidaria
Class: Hydrozoa
Order: Anthomedusae
Family: Hydridae
Genus: Hydra[2]

Diagnostic Characteristics:
As part of phylum Cnidarian, Hydrae are diploblastic animals, known for having a homologous traits and morphological synapomorphies of 1) unique cell juctions, collagen and proteoglycans in extracellular matrix, 2) two embryonic cell layers, 3) distinct organ systems, and 4) radial symmetry. Diploblastic animals are characterized by two cell layers, an inner endoderm and an outer ectoderm. Hydrae lack, notably, a mesoderm, like placozoans, ctenophores, and, in some cases, sponges. There are three major groups of Cnidarians. The first is anthozoans, which include corals and sea anemones. The second is scyphozoans, which include jellyfishes. Finally, the third group is hydrozoans, which include hydra and hydroids.[3]
Hydra can be different colors depending on their age, type, and food ingested. The main body is an elongated cylinder, a column, with the distal end(the side that sticks out) having 3-12 tentacles. They usually have only 5 or 6 tentacles that are .5 to 5 times the length of the column. The
hypostome is a raised dome like strucure in the middle of the circlet of tentacles has a single opening that is both the mouth and the anus. [4] (SM)
As most hydras are small (they are a maximum length of only 30 mm), they are barely visible to the human eye, and the only way to see them is through a microscope. When disturbed, hydras contract and thus resemble blobs. (29) (MC)

They are multicellular organisms that can be found by just sweeping a net over weedy areas. Ranging from a few millimeters long, hydras are extremely fascinating due to their regenerative ability. They never grow old or die of age. (30)(Shwetha)

Relationship to humans:

Hydra Lab Image

(Students observing live hydra in the laboratory)
Hydrae are of interest to scientists due to their regenerative abilities, as they neither seem to get older or age. Nonetheless, Cnidarians as a whole pose more of a question to humans as jellyfishes are either delicacies or killers, and corals are of importance to the ecosystem of the waters in which we fish. Coral reefs are also build-up of Cnidaria skeletons. For now, Hydrae's strongest presence within the Homo Sapien world is reserved to Greek Myths and Marvel Comic Puns.

Habitat and niche:

Hydrae, via its basal disk, attach themselves to rocks, vegetation and other surfaces of shallow freshwater lakes, ponds, and streams.[5]

Hydra live in aquatic environments and are usually attached to submerged twigs, rocks, or vegetation. Oftentimes, they can be found on leaves that are floating in bodies of water or on roots of duckweed. They attach to these objects by secreting a mucous from their basal disc (25) (E.S.S.).

The Hydra is an extremely resilient organism that is capable of living in environments which most fresh water inhabitants cannot. The hydra tends to excel in bodies of water that move quickly but still have a certain amount of vegetation. The hydra can withstand waters of up to 1% salinity, which is barely considered fresh water. The hydra also are one of the few organisms capable of thriving in a body of water with excessive eutrophication. Only large amounts of metal toxins seem to be detrimental to the hydra population in water. ([6] ) (DA)

Predator avoidance:

Hydrae are equipped with toxic nematocysts that can sting and kill prey and predators. These nematocysts are embedded within the cnidoblasts that make up the tentacles of the hydrae.[7]

When alarmed or attacked, the tentacles of the hydra will retract into small buds and its body can be retracted into a small gelatinous sphere. Because the nerve net of the hydra is very simple, all hydra generally respond in the same way to a stimulus.[8] (JLau)

Hydra firing nematocysts to kill Daphnia prey

Nutrient acquisition:

Cnidarians are specialized carnivores, utilizing toxin in their nematocysts, that are shaped like harpoons, to capture larger and complex pray. Corals and anemones will also use photosynthetic endosymbionts, organisms found within another, in their tissues to supplement their nutrition. Hydrae have symbiotic relationships --a mutualism-- with algae, where protection is granted to algae and Hydrae receive nutrition from photosynthetic products of algae.[9]

When feeding on larger and more complex prey, the hydra captures and paralyzes its victim using its tentacles, its feeding response is stimulated by body fluids leaking from the puncture wounds. Its tentacles shorten, its mouth expands and opens, and it slowly engulfs its victim. Glutathione, a common tripeptide, is the major stimulant for the feeding response. The prey is digested in the gastrovascular cavity and can take several hours to be completely digested. Large food molecules are taken up by the gastrovascular cells. The hydra is capable of consuming any small invertebrates that are equal to or smaller than its own size. This includes annelid worms, rotifers, insect larvae, and crustaceans.[10] (JLev)

Although Hydra are fairly simple organisms, the stinging cells that they use to capture prey are quite complex. They are multiple types of stinging cells embedded on a Hydra's tentacles. First, coiled tubes are shot out which paralyzes passing small crustaceans (the Hydra's main source of food). Then, the hydra releases a second type of nematocysts, which act like barbs and penetrate their prey. These barbs release toxins that further paralyze the creature. Digestion is left to enzymes secreted into the Hydra's cavity. This system allows the Hydra to eat relatively large crustaceans. This is important because when catching their prey, Hydra may use up to 25% of the nematocysts. These toxins can take up to 48 hours to rejuvenate. ([11] ) (SJ)

Reproduction and life cycle:

In regards to reproduction, Hydra, including Hydra viridissma,are characterized by asexual reproduction. Simple animals will asexually reproduce through a process called budding, where a bud, an outgrowth, will form through mitotic cell division off a mature animal, until the bud is either size of the parent or mature enough to be independent, and subsequently break off.[12]

These buds are produced every two or three days when conditions are favorable. However, following unfavorable conditions like injury or a lack of resources, hydras may reproduce in other ways such as transverse and longitudinal fission. [13] (WSS)

Although most Hydra reproduce asexually through budding during the summer, some can reproduce sexually but usually only after the summer. Most species are hermaphrodites, meaning the testes and ovaries developing on the same animal. Once a hydra's egg is fertilized by another hydra's sperm, the egg develops a protective ornamented shell and remains dormant until favorable conditions, where it then develops into a hydra without a larva stage. (7) (KG)

Sexual reproduction amongst hydra occurs under unfavorable conditions such as lack of food extreme temperature variations etc. Special cells germ cells perform sexual reproduction. Most of the species of Hydra are dioecious.(HSC)

Life Cycle:
Many cnidarians have two unique stages in their life cycles: a sessile, immobile, and a motile, moving. After an egg is formed, it will become a planula larva, which then forms into a polyp that has tentacles and a moth-anus. Through asexual budding of a mature polyp, medusae, or free swimming motile versions of the animal, are formed. Medusae often become jellyfish, and thus are not distinctive to hydrae. Nonetheless, Hydrae are bottom-dwellers that will remain in the polyp state for the entirety of their lives. If living conditions are poor, Hydrae will, in some circumstances, sexually reproduce as to releases gametes into the water and allow the zygote exist in open water until conditions improve. This zygote will then develop into a planula larva until it anchors itself to mature into a polyp. [14]

Growth and development:

The Hydra will begin as a bud in favorable circumstances off the parent, using its nutrition and resources to mature. Once it reaches the size of the parent or is mature enough, it will break off and become a polyp, anchoring to a nearby surface. See Life Cycle for more information.[15]

Hydra have been carefully studied after a discovery that they appear to be non-senescent, that is, they do not age and do not reproduce at a slower rate over time. The transcription factor FoxO has been found to be responsible for indefinite self-renewal. There are still no implications that this research could be transferred to slow human aging.[16] (DM)


The integument system is defined as system that protects the organism from damage, where in humans it is the skin. For Hydrae, the integument system is made of the ectoderm and endoderm, a two cell thick 'skin,' through which gas exchange can occur.
Hydra’s are diploblastic, or have two body layers. This consists of two layers of cells, with a sheet of mesogloea in between. The outside layer is called the epidermis, and the inner layer is called the gastrodermis. The mesogloea is responsible for the jelly like characteristics of the hydra. (LC) 27

Mesoglea is a special feature of the hydra's integumentary system. The mesoglea layer lies between the two epithelial layers and is composed of mostly water with a few other substances including fibrous proteins including collagen. Although the layer is mostly acellular (not composed of cells), it does contain nerve and muscle cells that play a role in contracting the body of the hydra. It also contains mobile cells which play a role in phagocytosis (encompassing and disposal of pathogens and material) of debris and bacteria and also fight infections by releasing antibacterial chemicals. It serves as a skeleton, supporting the body, but at the same time is very elastic and helps the hydra return to its original shape after it has contracted its muscle cells. (BH)[17]


The epithelial cells with muscle fivers contract and enable to movement of hydrae. Another aspect of movement are the nerve nets integrated into the animal. The nerve net is present in the absence of a brain or any cephalization. Commonly found in organisms with radial symmetry, the nerve net is the simplest of nervous systems, allowing the hydra to respond with toxins to a stimulus, but not pin point its source. [18]

On the bottom of the tube-like body, a structure called a basal disc secretes a sticky substance that helps hydras attach to a solid base. Hydras can move either by gliding along the basal disc or by somersaulting along. When somersaulting, hydras detach the basal disc, bend over, and place their tentacles on the substrate. Then, they reattach the disc further along and repeat the process. Hydras can also float in the water upside down because the basal disc produces a gas bubble which carries the hydra up to the water surface.[19] (JF)


When the Hydra feels that it going to be attacked by an organism, its tentacles become small and they wrap themselves into a small jelly like ball.(28)(NC)

Sensing the environment:

The nerve net is responsible for the response of the Hydrae to a stimulus. Although it cannot realize the source of a stimulus, it can detect physical contact, including food and chemicals, permitting a response from the hydra. As a result, the same motor output will be exacted by the hydra.[21]

Gas exchange:

Hydrae use their epithelial cells that make up theirs skins for gas exchange and material exchange. Hydrae do not have lungs and heart, and the absence of these organs reduces the complex actions they can take and thus account for their small sizes.[22]

Exchanges oxygen gas and carbon dioxide gas by direct diffusion, two gases are close to each other, causing an intertwine between the two and exchange of both.[23] (TM)

Waste removal:

Hydrae remove wastes through the flow of fluid in its gastrovascular cavity through the opening of its mouth-anus. [24]
Simply put, because the hydra does not have a separate anus for excreting waste, any material that is not digested will be regurgitated out of its mouth.[25] (SJ)

Environmental physiology (temperature, water and salt regulation):

Hydrae are osmoregulators, meaning they constantly monitor and control their water and salt concentration of their extracellular fluid, the fluid in the organism excluding the fluid inside the individual cells. Hydrae expel excess water by forming contractile vacuoles, membranous sacs, around the fluid and removing them from the organism by fusing the vacuoles with the organism's outer membrane, releasing the water from the organism. [26] (AY)

Internal circulation:

Hydrae are rather simple animals, and, as represented by their millimeter size, circulatory systems are unnecessary by its body plan. Made of a body wall that is only two cells thick, the center of the hydra, its gastrovascular cavity, serves for both digestion and for diffusion. This cavity supplies all nutrients, obtains oxygen from water within the cavity, and releases other wastes into the cavity, from which the fluid can leave through its mouth-anus opening.[27]

Chemical control (i.e. endocrine system):

Hydrae are not complex organisms. However, there is reason to believe of a neuropeptide that stimulates regenerative abilities or the source of toxins within the tentacles.[28]
The hydra does have an endocrine system, but the details of it are yet to be determined. Several hormones found in vertebrates are also found in hydra, such as steroids and neuropeptides. Sex steroids have definitely been identified. More research is being conducted on the environment's effect on the hydra endocrine system. [29] (AA)
The endocrine system is in charge of reproduction and feeding. Thyroxine release promotes asexual reproduction through budding while the feeding behavior is regulated through chemical signaling. [30] (BB-V)

Review Questions!
  1. How do Hyrda exchange gas if they do not have lungs or gills?(BS)
  2. What method do hydras use in order to excrete waste? (CC)
  3. How doe hydras reproduce? Explain this process briefly. (PS)
  4. Why don't hydra need a circulatory system? (MDS)

Made famous by:

In Greek Mythology Hydra:"The Hydra which lived in the swamps near to the ancient city of Lerna in Argolis, was a terrifying monster which like the Nemean lion was the offspring of Echidna (half maiden - half serpent), and Typhon (had 100 heads), other versions think that the Hydra was the offspring of Styx and the Titan Pallas. The Hydra had the body of a serpent and many heads (the number of heads deviates from five up to one hundred there are many versions but generally nine is accepted as standard), of which one could never be harmed by any weapon, and if any of the other heads were severed another would grow in its place (in some versions two would grow). Also the stench from the Hydra's breath was enough to kill man or beast (in other versions it was a deadly venom). When it emerged from the swamp it would attack herds of cattle and local villagers, devouring them with its numerous heads. It totally terrorized the vicinity for many years. "external image img_hydra.jpg"Heracles journeyed to Lake Lerna in a speedy chariot, and with him he took his nephew and charioteer Iolaus, in search of the dreaded Hydra. When they finally reached the Hydras' hiding place, Heracles told Iolaus to stay with the horses while he drew the monster from its hole with flaming arrows. This brought out the hideous beast. Heracles courageously attacked the beast, flaying at each head with his sword, (in some versions a scythe) but he soon realized that as one head was severed another grew in its place. Heracles called for help from Iolaus, telling him to bring a flaming torch, and as Heracles cut off the heads one by one from the Hydra, Iolaus cauterized the open wounds with the torch preventing them from growing again. As Heracles fought the writhing monster he was almost stifled by its obnoxious breath, but eventually, with the help of Iolaus, Heracles removed all but one of the Hydras' heads. The one remaining could not be harmed by any weapon, so, picking up his hefty club Heracles crushed it with one mighty blow, he then tore off the head with his bare hands and quickly buried it deep in the ground, placing a huge boulder on the top. After he had killed the Hydra, Heracles dipped the tips of his arrows into the Hydras' blood, which was extremely poisonous, making them deadly. Other versions say that while Heracles fought the Hydra the goddess Hera sent down a giant crab which attacked his feet). This legend comes from a marble relief dating from the 2nd century BCE found at ancient Lerna, showing Heracles attacking the Hydra, and near his feet is a huge crab. Also other legends say that a stray arrow set alight the forest, and it was the burning trunks which Heracles ripped up and used to cauterize the open wounds."(Taken from The Encyclopedia Mythica)

Hydra is also the name of a powerful nemesis organization in Marvel Comics
external image tumblr_lotmddLCLP1qatl12o1_400.jpg

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  2. ^ Hillis, David M., David Sadava, H. C. Heller, and Mary V. Price. Principles of Life. Sunderland: Sinauer Associates, 2012. 639. Print.
  3. ^ Hillis, David M., David Sadava, H. C. Heller, and Mary V. Price. Principles of Life. Sunderland: Sinauer Associates, 2012. 639. Print.
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  7. ^ Hillis, David M., David Sadava, H. C. Heller, and Mary V. Price. Principles of Life. Sunderland: Sinauer Associates, 2012. 639. Print.
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  9. ^ Hillis, David M., David Sadava, H. C. Heller, and Mary V. Price. Principles of Life. Sunderland: Sinauer Associates, 2012. 639. Print.
  10. ^ http://www3.northern.edu/natsource/INVERT1/Hydra1.htm
  11. ^ http://www.offwell.free-online.co.uk/hydra.htm
  12. ^ Hillis, David M., David Sadava, H. C. Heller, and Mary V. Price. Principles of Life. Sunderland: Sinauer Associates, 2012. 639. Print.
  13. ^ http://animaldiversity.ummz.umich.edu/accounts/Hydra_oligactis/
  14. ^ Hillis, David M., David Sadava, H. C. Heller, and Mary V. Price. Principles of Life. Sunderland: Sinauer Associates, 2012. 464. Print.
  15. ^ Hillis, David M., David Sadava, H. C. Heller, and Mary V. Price. Principles of Life. Sunderland: Sinauer Associates, 2012. 464. Print.
  16. ^ http://www.fightaging.org/archives/2012/11/investigating-the-agelessness-of-hydra.php
  17. ^ http://www.britannica.com/EBchecked/topic/376740/mesoglea
  18. ^ Hillis, David M., David Sadava, H. C. Heller, and Mary V. Price. Principles of Life. Sunderland: Sinauer Associates, 2012. 464. Print.
  19. ^ http://www.offwell.free-online.co.uk/hydra.htm
  20. ^ http://www.youtube.com/watch?v=-UI531GMRTM
  21. ^ Hillis, David M., David Sadava, H. C. Heller, and Mary V. Price. Principles of Life. Sunderland: Sinauer Associates, 2012. 464. Print.
  22. ^ Hillis, David M., David Sadava, H. C. Heller, and Mary V. Price. Principles of Life. Sunderland: Sinauer Associates, 2012. 464. Print.
  23. ^ http://www.pcsd.k12.ny.us/bwoods/Regents%20Biology/Chapter%2011%20Gas%20Exchange/Chpter%2011%20Gas%20Exchange.htm
  24. ^ Hillis, David M., David Sadava, H. C. Heller, and Mary V. Price. Principles of Life. Sunderland: Sinauer Associates, 2012. 464. Print.
  25. ^ http://www.biologycorner.com/worksheets/articles/hydra.html
  26. ^ http://www.ilng.in/pdf/mtg_bio_final.pdf
  27. ^ Hillis, David M., David Sadava, H. C. Heller, and Mary V. Price. Principles of Life. Sunderland: Sinauer Associates, 2012. 639. Print.
  28. ^

    Hillis, David M., David Sadava, H. C. Heller, and Mary V. Price. Principles of Life. Sunderland: Sinauer Associates, 2012. 464. Print.
  29. ^

  30. ^


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