A Hydra(17)

Classification/Diagnostic characteristics:

Hydras are Eukaryotes belonging to the kingdom Animalia. Hydras are part of the phylum cnidarians which includes sea anemones and coral. In this phylum, hydras belong to the clade or class known as Hydrozoan. Hydras possess a cylindrical stalk which binds to a substrate and creates lateral symmetry. These organisms are incredibly small, usually requiring specialized instruments to observe in detail.
This phylogenetic tree shows how the Cnidaria relates to a differs from the other ancestral Protists. In this phylogenetic tree, the out-group is the Porifera and the Cnidaria is similar to the other phyla because of its true tissue and radial symmetry. (13) (LK)
This phylogenetic tree shows how the Cnidaria breaks down into different species and how the hydra and Clytia are under the Hydrozoa. (14) (LK)

Kingdom: Animalia
Subkingom: Eumetazoa
Phylum: Cnidaria
Subphylum: Medusozoa
Class: Hydrazoa
Order: Anthomedusae
Suborder: Capitata
Family: Hydridae
Genus: Hydra
(SL 16)

Relationship to humans:

Hydra share a domain (Eukarya) and a kingdom (Animalia) with humans. Hydra also posses the unique trait to replicate limbs, a quality which sparks interest in biologists hoping to learn how to duplicate this trait in humans.

The ability of hydra to regenerate body parts after being wounded is of particular importance to biologists because although human body parts cannot completely regenerate an entire body part, human cells can regenerate, as seen in the case of cuts to the skin. Hydra are considerably easier than other organisms with the regeneration ability to study because they are very simple as invertebrates. The cells in a hydra are constantly undergoing mitosis, after which they move to the extremities of the body and are shed. In fact, if the head or bottom of the hydra are cut off, they will both be able to grown back. In addition, the hydra has the amazing ability to rebuild itself and to regenerate even after being put into a centrifuge and having all its cells separated in a tube. This ability of hydras could provide scientists with insight into ways to save a damaged or dysfunctional organ in humans. (12) (CM)

Habitat and niche:

Hydras reside in the water in order to support their reproduction methods, feeding methods, and structural support systems. Hydrozoans have been found in both freshwater and marine environments.
Hydra usually attaches itself to submerged rocks or plants in ponds and slow-moving rivers. Below is a picture of Hydra found in pond water with an abundance of algae (6). (SP)


Hydra are typically found in slow flowing freshwater habitats; such as: ponds, spring brooks, rivers, unpolluted streams, and the close to the shore of lakes. They are mostly sessile and are usually found in the shallow water where they attach themselves to vegetation, stones, twigs, or other objects, but their tentacles are suspended in water (10). They are strictly carnivorous and eat different types of small metazoans, algae, and other small creatures (9, 10). Small metazoans include: insects, annelids, copepods, cladocerans (10). Hydra use nematocysts formed in their tentacles, which to discharged into prey, to paralyze and kill their food. Hydra can tolerate considerable eutrophication, which is unusual for a fresh water animal (8). (SM)

Many hydra actually engage in a symbiotic relationship with the algae to which they commonly attach themselves. The algae, which supply the hydra with a source of oxygen via photosynthesis, enable the hydra to exist in environments with low quantities of dissolved oxygen. Likewise, the hydra provide the algae with nutrients, source of carbon dioxide for photosynthesis, and a protected environment. In fact, it has been shown that hydra containing algae have a higher survival rate than hydra without algae. Hydra also pass on algae to subsequent generations in eggs. (S.S.-11).

Predator avoidance:
The hydra has stinging tentacles, that are used both for capturing prey and warding off predators. Each tentacle has a nematocyst, which is an organelle that can shoot a poisonous filament at prey or predator, stunning it (4). (RG)


Nutrient acquisition:
Hydras are predators and are required to consume other organisms to obtain nutrients. They do this by utilizing toxic nematocysts, or structures which fire the toxin into relatively complex prey. The food then goes through a single opening which serves as both a mouth and anus and connects to the gastrovascular cavity where the prey is digested.

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. [SM,1]

Reproduction and life cycle:

Hydras are capable of reproducing both asexually and sexually, depending on environmental conditions. While performing asexual reproduction, hydras undergo the process of budding, a procedure which involves producing an outgrowth that will eventually break off and become a new, identical organism. Sexual reproduction is induced by producing haploid gametes (gametogenesis), bringing the gametes together by releasing the gametes into the water,(spawning/mating) and fusing the gametes to form a diploid zygote which will eventually become a new organism (fertilization).

In the Hydra, the egg and the sperm are separated in different gonads of the outer body layer. Typically hydra individuals have separate sexes, although some species function as hermaphroditic. When reproducing sexually, eggs are kept in the ovaries and fertilized by sperm from neighboring individuals. Offspring are eventually released, and are considered miniature hydras until they grow. They can also reproduce asexually through budding; outpushings against the walls of the hydra develop to form tentacles and a mouth and eventually nip off at the base, which creates a separate individual produced by a singular organism (3). (AG)

Growth and development:

Hydras begin life as a larva known as a planula, which is the product of sexual reproduction. The planula will attach to a sediment and develop into the stationary polyp, a state which the hydra will remain in for the rest of its life, with small changes to its morphology during the process of budding and producing gametes.

Interestingly enough, a large body of research has shown that hydras exhibit negligible senescence, that is they do not age once they reach a certain point. The non-senescence of hydras flipped the concept of linear aging for all organisms on its head and elucidated the possibility of biological immortality. In 1998, research revealed that hydras have stem cells that can reproduce unendingly, a process modulated primarily by the transcription factor "forkhead box O" (FoxO). (2) (YR)

Hydra Anatomy (MT)(15)
Hydra Anatomy (MT)(15)


The integument of the hydra is very thin to allow for diffusion of gasses throughout the organism. It is composed of two cell layers, the ectoderm and the endoderm with a jelly-like layer in the center.

Hydras have a basal disc on the bottom of their tube-like body that secretes an adhesive substance which helps to attach themselves to a solid base. While attached, hydras move their bodies in the water, extending and contracting by a mixture of muscle movement and water pressure. that is created inside their digestive cavity. The cells lining the digestive cavity have little flagellae (minute hair-like structures) on their surface. By beating these flagellae currents are created to draw water into the digestive cavity, raising the water pressure inside, which causes the hydra's body to increase in length. The body of a single hydra can change from being 20mm long when it is relaxed and extended, to being as little 0.5mm long when it has contracted (using muscular movement) after being disturbed.

Hydra are not always attached to the substrate and can move from one spot to another, either by gliding along on the basal disc. When moving, they detach the basal disc and bend over and place the tentacles down on the substrate. This is followed by reattaching the basal disc further along and repeating the whole process again. Hydras may also float about in the water upside down with the aid of the basal disc which produces a gas bubble to carry the animal up to the water surface. (5) (NU)

Hydra's induce movement with specialized epithelial cells that contain muscle fibers which contract and propel the organism. They can also utilize their gastrovascular cavity, which serves as a hydrostatic skeleton, or fluid filled cavity surrounded by muscles, to create motion.

Hydras move by a process of looping and somersaulting. When they somersault, they detach their basal disc from the substrate and then bend over to put their tentacles down on the substrate. Hydras repeat this process over and over to move along the substrate. They can also move by gliding on their basal disc. (JM 7, 8)

Sensing the environment:

Hydras can be classified as protostomes and contain traits shared among all protostomes such as a ventral nervous consisting of paired nerve cords. These nerve cords form a nerve net which allow the hydra to responds to and process any physical stimuli with its anterior brain located around the entrance to the digestive tract.

Hydra also receive information about the environment through chemoreceptors, or receptor proteins which bind to certain molecules (a trait shared among all animals). These messages prompt signals which allow the hydra to respond to any chemical stimuli introduced into its environment.

Gas exchange:
Hydras perform gas exchanging using the gastrovascular cavity, the same area used for digestion. Due to the small size ,each cell is able to adequately respire through diffusion. Any gases which the organisms needs removed diffuse directly back into the environment.

Waste removal:

Hydras utilize the same opening for the mouth and the anus. This means that any nutrients or particulars which cannot be digested by the gastrovascular cavity are released through the same opening which they originally entered, ridding the hydra of unnecessary and useless nutrients and avoiding the complexities of a large digestive track.

Environmental physiology(temperature, water, and salt regulation):

Hydra must spend their lives immersed in water due to their reproductive system (releasing gametes is only effective if water is present to carry the gametes) and their specialized structure. Hydras can be found in wide range of different fresh water systems, signifying a varying range of salt tolerance.

Internal circulation:

Due to the small stature of the hydra, this organisms does not utilize an established circulatory system. Instead, the gastrovascular cavity performs the function of circulation by allowing nutrients to travel from the gastrovascular cavity directly into the destination cells.

Chemical control:

Hydras do not possess an endocrine system such as is commonly found in vertebrates. Instead, these organisms make use of several neuropeptides which induce response to the environment and processes such as budding

Review Questions:

1. What kind of symbiotic relationship do algae and hydra have? What do algae and hydra each contribute to this relationship? (AC)
2. What body part do hydra use to capture prey and ward off predators? What organelle on this body part shoots poisonous filaments? (AC)
3. Which types of reproduction does hydra use to reproduce? Why are the multiple ways to reproduce beneficial to the hydra? (RS)
4. Which characteristics of hydra would be beneficial for further research? Why? (RS)
5. Explain why Hydra must spend their entire lives submerged in water. (SL)
6. Explain how Hydra are able to move, and what that means to their genus. (SL)
7. List organisms which are similar to hydras and explain how these organisms differ in traits and classification(SF)


1. http://www3.northern.edu/natsource/INVERT1/Hydra1.htm (SM)
2. http://www.pnas.org/content/109/48/19697
3. www.britannica.com/EBchecked/topic/278116/Hydra
4. "Hydra Nematocyst." Hydra Nematocyst. N.p., n.d. Web. 18 Nov. 2013.
5. http://www.offwell.free-online.co.uk/hydra.htm
6. "Information on Hydra." Information on Hydra. The Offwell Woodland & Wildlife Trust, n.d. Web. 17 Nov. 2013.
7. "Information on Hydra." //Countrysideinfo.co.uk//. Countrysideinfo.co.uk, n.d. Web. 18 Nov. 2013. <http://www.countrysideinfo.co.uk/hydra.htm>.
8. "Hydra." Hydra. N.p., n.d. Web. 23 Nov. 2013.
9. "Green Hydra." Green Hydra. N.p., n.d. Web. 24 Nov. 2013.
10. "Animal Diversity Web." ADW: Hydra Oligactis: INFORMATION. N.p., n.d. Web. 24 Nov. 2013.
11. http://www.offwell.free-online.co.uk/hydra.htm
12. "Regeneration."Developmental Biology Interactive. N.p., n.d. Web. 25 Nov. 2013.
13. Ancestral Protist Phylogenetic Tree
14. Cnidaria Phylogenetic Tree
15. "BIOLOGICAL DIVERSITY: ANIMALS I." //Biological Diversity 7//. N.p., n.d. Web. 25 Nov. 2013.
17."BioTechniques - Surprising Genetic Connections between Humans and Hydras."BioTechniques - Surprising Genetic Connections between Humans and Hydras. N.p., n.d. Web. 01 Dec. 2013.