Hydra


 * HYDRA **
 * By:Ariel Friedman **


 * Classification/Diagnostic Characteristics **


 * There are roughly 11,000 species of cnidarians. Cnidarians are multicellular invertebrates. In addition, hydras have radial symmetry, where the organism is symmetrical around a central axis of symmetry. Hydras also have the capacity to regenerate lost body parts; if either is removed from an individual, the hydra is able to grow a new, identical section based on the head and foot activation gradients found in the cells of hydras. The polarity (orientation of head and foot) is not affected by such an amputation. **
 * Hydra are biologically immortal, they are capable of self-renewal indefinitely through their stem cells. **


 * Relationship To Humans: **


 * The hydra produces a toxin that paralyzes its prey, though is too weak to yield any effect on humans. **


 * Habitat and Nitche: **


 * Hydra are most commonly found in sunlit pools. They often attach to vegetation. Hydra tend to like bodies of water that move. They are also tolerant to wide temperature fluctuations. **


 * Predator Avoidance: **


 * Some predators of Hydra are flatworms, crustaceans, and aquatic insects. **
 * Hydra are sessile animals, meaning that they generally cannot move. Therefore, their ability to avoid predators is limited. **
 * General reaction to being attacked involves the hydra retracting its tentacles into buds and retracting the body column into a small sphere. **


 * Nutrient Acquisition: **


 * Cnidarians are carnivores. Mouths of cnidarians are connected to a sac called the gastrovascular cavity. This cavity functions in digestion, circulation, gas exchange, and also acts as hydrostatic skeleton. Hydra are specialized carnivores. They have harpoon like structures called nematocysts to capture large and complex prey.Enzymes in the gastrovasuclar cavity digest the prey, and cells lining the cavity take in small food particles by endocytosis. Endosytosis is when cells engulf proteins for nutrients. **
 * The hydra uses its tentacles, which contain neurotoxins, to paralyze the muscles of any predators,such as larval insects and small crustaceans. **


 * Reproduction and life cycle: **


 * 1.) Bud begins to form on the body of the Hydra. **
 * 2.) Bud develops a mouth and tentacles. **
 * 3.) Bud detaches from the parent. **
 * 4.) New Hydra becomes fully developed, continues this reproduction process. **


 * Hydra reproduce using budding. Budding is when new individuals grow from bodies of mature animals. A bud grows by mitotic cell devision and then eventually will break away from the parent. Sometimes, the body will become full grown before becoming dependent. **


 * The life cycle of hydra can be regulated by two factors, timing and temperature. Hydra are able to produce asexually at 24 degrees C, and sexually at 15 degrees. Sexual organs (gonads) begin to appear after the change from 24 degrees C to 15 degrees C, and are fully formed in 35 days. Testes appear before the eggs in this development.At 15 degrees C, 50% of the embryos hatch in about 60 days; but they have a low survival rate. Embryos kept at 4 degrees C hatch in about 45 days, with a high rate of survival. **
 * The density of nerve cells may also play a role in regulating the rate of bud production in hydra. **

In the life cycle of cnidarians, there are two distinct stages. In the sessile polyp stage, a cylinder stalk is attached to the substrate. The motile medusa is a free swimming stage shaped like a bell or umbrella. This typically floats with its mouth. The feeding tentacles are facing downward. Mature polyps produce medusae by asexual budding, then medusae reproduce sexually, forming eggs and sperm through mitosis. The gamete is then released into the water. The egg develops into a free swimming larva called a planula, which will eventually settle to the bottom and develop into a polyp. **
 * Growth and Development: **


 * Integument: **


 * Hydras are examples of Cnidaria with a radial symmetry, meaning the arrangement of the body parts are bound to a central axis.Hydras contain a separate outer epidermis and internal gatrodermis which are both made of mesoglea, a gel like substance. Digestion occurs in the coelentron which is located in the center of the gastrovascular cavity. Tentacles project from the main cavity and contain cnidodytes, or venomous cells which produce nematocysts that help paralyze prey. Nerve nets act as photoreceptor cells to detect light and exchange signals and are found throughout the polyp body. **


 * Movement: **
 * Hydra have epithelial cells with mussel fibers on their tentacles. They attach their tentacles onto objects and drag their bodies forward in order to move. Some hydras use their foot to slide through the environment and some are able to ride on air bubbles! The simple nerve nets control body activities related to locomotion. **
 * Some hydras are capable of "somersaults", they bend over and attach their head and tentacles to the substrate, detach their foot, reattach it somewhere new, and then detach their head and reattach it somewhere. This continues as a loop, providing a form of locomotion. **


 * Sensing the environment: **


 * Cnidarians process information using nerve nets. Nerve nets are neutral networks that provide direct lines of communication from sensory cells to effectors. The nerve net is one neuron wide, is located in between the outer and inner body walls, and is made up of axons or nerve fibers. The hydra lacks eyes, though it can sense and react to light stimuli. An ion channel that is regulated by the protein opsin grants the organism the ability to be receptive to light and adapt a rudimentary visual system. **


 * Gas exchange: **


 * Hydra do not have a respiratory system, so gas exchange is done by diffusion across the cell membrane. Diffusion across a cell membrane is drive by concentration gradients. Hydra have a thin body structure in order to maximize surface area, an ideal surface area to volume ratio allows for fast gas exchange. They also have a central cavity through which water circulates. **


 * Waste removal: **


 * Waste is removed through the gastrovascular cavity. This single opening serves as both the mouth and anus. In the hydra, food and water go into the digestive system and solid waste, amino acids, monosaccharides, and water come out. The food begins to be digested by enzymes released by the gland cells. Cells can engulf the broken down food particles with phagocytosis. **


 * Environmental Physiology: **


 * Due to the fact that all of hydra is in contact with its environment, it is able to use simple diffusion to exchange materials with its environment. **


 * Internal circulation: **


 * Cnidarians do not have a circulatory system. The circulatory system is performed by surrounding water. Oxygen and nutrient are able to diffuse into the cinidarian, and metabolic wastes and carbon dioxide are diffused from the cells into the water. **


 * Chemical control: **
 * u **
 * Cnidarians do not have an endocrine system, but several hormones such as sex steroids and gonadotropin-relasing hormone that are usually found in vertebrates have been traced in cnidarians. It is still unclear- due to lack of research- what mechanisms cnidarians use these hormones for. **
 * Research from the Oxford Journals shows that because cnidarians are less compartmentalized, have poorly understood physiologies and regulation systems, and disruption potential is unknown, cnidarians tend to use endocrine-like signaling en lieu of an actual endocrine system that need a lot more complexity to one's anatomy. Scientists have made progress when it comes to identifying bioregulatory molecules and testing the effects of these molecules. These kinds of signaling systems in the cnidarians could lead to drastically improving the field of evolution related to hormone signaling.It will also improve understanding of how cnidarians adapt to environmental changes and provides a foundation for the development of physical and chemical stressors and how this disrupts certain physiological processes. **


 * Review Questions: **
 * 1. How does the hydra capture and digest foods to intake nutrients? **
 * 2. What anatomical structures allow the hydra to move? **
 * 3. How does the hydra exchange gasses with its environment? What properties allow it to do so? **
 * 4.What steroids do hydra have? **
 * 5.What kind of diffusion does hydra use? **


 * http://www.dailykos.com/story/2013/06/15/1213604/-Cnidarian-Nerve-Nets **
 * http://www.animalbehavioronline.com/simplenets.html **
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 * http://www.ncbi.nlm.nih.gov/pubmed/21676763 **
 * http://www.cedarville.edu/personal/jwf/bio100/lecturequiz21b.swf **
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 * http://www.transtutors.com/Uploadfile/CMS_Images/1082_LOOPING.JPG **
 * http://intl-icb.oxfordjournals.org/content/45/1/201.full **
 * http://www.devbio.biology.gatech.edu/?page_id=484 **
 * http://mdc.mo.gov/discover-nature/field-guide/hydras **
 * http://www3.northern.edu/natsource/INVERT1/Hydra1.htm **