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=Hydra viridissima=

Hydra viridissima; also named Hydra viridis and Chlorohydra viridissima; or commonly known as green hydra, is a freshwater polyp in the genus Hydra. They are classified under the phylum Cnidaria and class Hydrozoa. This species is abundant in regions of Europe such as the British Isles, Iceland, and Scandinavia, as well as parts of the United States and Australia. They are generally found in small stagnant bodies of freshwater with heavy vegetation. Generally, they are sessile and attach to submerged vegetation, often duckweed roots. Hydra viridissima is a predatory species of hydra feeding on insect larvae or small crustaceans including water fleas and copepods. They can be male, female, or hermaphrodites allowing them to exhibit both sexual and asexual reproductive strategies. The genus Hydra overall has been of great interest to biologists for years due to their regenerative ability, or immortality, given the proper environmental conditions, as interstitial cells found throughout the body continuously replace tissue, a process termed morphallaxis. However, Hydra viridissima specifically has drawn more and more attention from biologists and environmentalists in the past 20 years due to their use as a model organism for toxicity testing along with their being the only species within the Hydra genus to form symbiotic relationships with the photosynthetic algae, Chlorella. Hydra viridissima's characteristic green color comes from this intracellular association as chloroplast containing Chlorella cells inhabit the Hydra's endodermal epithelial cells.

Description
Hydra viridissima are very small at approximately 1 to 3 millimeters when contracted but can expand from anywhere between 5 and 20 millimeters when fully extended. They generally have 5 to 8 tentacles that extend around half the length of their body column. Each of these tentacles are coated in stinging cells called cnidocytes. These cells contain specialized structures called nematocysts that are used for catching prey. When stimulated by an organism coming into contact with the tentacles, water enters the cnidocytes increasing the pressure which causes the nematocysts to discharge and release a neurotoxin into the prey that paralyzes them. The open wounds of the prey activate chemoreceptors in the polyp prompting a feeding response to begin ingestion and digestion. Each discharge uses approximately 25% of an individuals nematocysts which can only be discharged once, but only take around 48 hours to be reproduced. Hydra are radially symmetric and diploblastic, where the outer layer is the epidermis and the inner layer lining the gut cavity is termed the gastrodermis. The two layers are separated by the non-living mesoglea made of collagen. They do not have a medusa stage or a central nervous system but rather a neural net throughout the epidermis consisting of mechanoreceptors and chemoreceptors.

The body column resembles a hollow tubular cylinder with the tentacles and an opening to the digestive cavity at one end and the basal disk at the other. As they have a single opening, it acts as both a mouth and an anus.

Hydra are generally sessile and attach themselves to vegetation or other substrates by secretions from the basal disk at the posterior end that act as an adhesive. When movement is required, they bend the body column, grab a surface with their tentacles, release the basal disk from the previous substrate, then somersault over and reattach the basal disk to the new substrate. The extension and contraction of the body column during movement is done by a combination of muscular movements and the changing of their internal hydraulic pressure which allows them to increase or decrease their length. They do this by beating the flagella of the cells lining the gut cavity which creates a current either drawing water in or pushing water out of the cavity.

Symbiosis with Chlorella Algae
One characteristic unique to Hydra viridissima is the mutualistic relationship it has with the photosynthetic algae Chlorella that gives the polyps their green color. They are the only Hydra species currently know to form these stable symbiotic relationships. Chlorella is an unicellular photosynthetic algae that lives within the endodermal epithelial cells of Hydra viridissima. Inside the cell, Chlorella is enclosed by a vacuolar membrane thats helps protect the symbionts from the Hydra's digestive enzymes.

The nature of the associate between Hydra viridissima and Chlorella is mutualistic, where the photosynthesizing ability of Chlorella provides nutrients to the polyp in the form of maltose or glucose-6-phosphate and, in return, receives shelter and by-products of feeding such a nitrogen. Along with aiding in nutrient acquisition, Chlorella have been found to play a role in the supply of oxygen to polyps when dissolved oxygen concentrations in their environment are low due to the production of oxygen being a by-product of their photosynthesis. While this relationship provides many benefits to the Hydra and aposymbiotic Hydra viridissima are not found in nature, Chlorella is not essential to their survival. Under lab conditions, they have been found to grow easily and shown normal morphology without their symbionts as long as feeding conditions are favorable.

While Hydra viridissima are the only known Hydra species to possess a symbiotic relationship such as this in nature, the evolution of how this relationship formed and how it relates to other species of Hydra is still in question. It has been found that symbiosis with Chlorella can be induced experimentally in some other species of Hydra showing that the overall genus could possibly have an affinity for the formation of symbiotic relationships. This leave the question of whether other species of Hydra lost similar symbiotic relationships through evolution or if there simply was never an association in other species to begin with unanswered.

Chlorella's Role in Growth and Reproduction
The mutualistic symbiont of Hydra viridissima, Chlorella, can aid in the organism's development and survival-hood due to the nutrients and oxygen it provides. One factor that is greatly assisted by this relationship is the growth of the polyp. One study, looking at the tissue growth rate in the Hydra genus with availability of food, found that most species in the genus had a strong correlation between the two variables where tissue growth rate was dependent on food acquisition. However, when examining Hydra viridissima in terms of this relationship, it was found that they were far less sensitive to variable nutrient conditions and could survive longer under starvation than other aposymbiotic species due to the presence of Chlorella and the symbionts provision of nutrients.

The same study referenced above also investigated the relationship between food availability and reproduction rates in Hydra. The results showed that under favorable feeding conditions aposymbiotic Hydra reproduced at relatively similar rates to Hydra viridissima. However, as the hydra's availability of food decreased to low and moderate conditions aposymbiotic Hydra species asexual production of offspring decreased significantly relative to that of Hydra viridissima inferring that Chlorella and the nutrients it provides aid in the production of asexual budding.

Chlorella may play a role in the initiation of oogenesis and sexual differentiation of polyps. During oogenesis, Chlorella transfer from epithelial cells in the endoderm to oocytes in the ectoderm in order to pass the symbionts on to the next generation of offspring.

Reproduction
Hydra viridissima, like most species in the genus, can reproduce both sexually and asexually and do so mostly during the summer season where asexual reproduction is the dominant strategy used to produce offspring. They use a process of asexual reproduction called budding where a bud complete with a digestive cavity, tentacles, and a mouth develops from the body column of the parent which eventually break off when mature. When under good conditions, these Hydra can produce multiple buds simultaneously, all at different rates of development. It has been found that along with favorable food conditions, the rate of asexual budding and the quantity of asexually reproducing polyps has a positive correlation with increasing temperature which is common among many Hydra species. While asexual accounts for a higher proportion of offspring, Hydra viridissima also undergo periods of sexual reproduction when conditions are unfavorable and offspring must be dispersed to increase the likelihood of survival. Generally, the transition from asexual to sexual reproduction begins when environmental conditions such as temperature, nutrient availability, and the daily duration of light availability change, prompting H. viridissima to begin development of the gonads. Unlike most Hydra species, Hydra viridissima can be hermaphroditic as well as male or female where the differentiation of egg and sperm production during gametogenesis is stimulated by environmental cues which are still a mystery to some extent in nature. Gametogenesis occurs when interstitial stem cells divide and collect in the intraepithelial space to procure germ cells that later form gametes. They undergo sexual reproduction via broadcast spawning where male or hermaphrodite Hydra's testis release sperm into the water column which then enter the ovaries of a female or another hermaphrodite forming a zygote. Upon fertilization, the zygote enters a dormancy stage until conditions are right, where it then develops without a larval stage.

One characteristic in terms of Hydra viridissima reproduction is how during the transition from asexual periods to sexual periods, if an individual is in the process of forming an asexual bud while beginning to develop gonads the development of the bud being produced is not interfered with. In fact, it has been shown that in males and hermaphrodites simultaneously undergoing sexual reproduction and development of an asexual bud has no effect on the budding process. In the same study it was found that while the budding process was inhibited in females undergoing sexual reproduction, the process wasn't fully discontinued as in many Hydra species. In a separate study, it was found that during gametogenesis, the production of eggs generally only occurred when Chlorella algae was present in the individual. Similar results were found for spermatogenesis however the correlation wasn't as strong. Furthermore, the energetic cost of developing sexually as a hermaphrodite has been shown to be higher than that of males, females, and asexual polyps. This inference was devised from the finding that the number of asexual buds produced by a male or a strictly asexual individual had a positive correlation with the size of the individual; however, while hermaphroditic individuals are much larger on average than their single or no sex counterparts, they never produce more than one bud at a time.

Toxicological Impacts to Hydra viridissima
In the past 20 years, Hydra viridissima has drawn the interest of many Biologists and Environmentalists for their high sensitivity to certain toxins. Many studies have been conducted examining different compound's toxicological impacts on Hydra viridissima and how they can be used as model organisms for toxicity testing in the environment.

Use as a Model Organism for Toxicity Testing
In terms of their use as a model organism, it was found that Hydra viridissima has a very high potential as a model to test how both acute and chronic inorganic toxins, specifically xenobiotics, affect freshwater invertebrates. Their potential for this use stems from how the introduction of inorganic toxins, such as Endosulfan and particularly copper of anthropogenic origin, cause changes in population growth rates, impairment of bud separation and high levels of deformities during asexual reproduction, and even death in just four to six days. While they have high potential for investigating inorganic compounds, the same study found their sensitivity to organic toxins to be much lower than most invertebrates, therefore limiting their application for use as a model in studies examining the effects of organic compounds.

Magnesium Toxicity
One toxin that is very detrimental to many species overall is magnesium, where elevated levels are common in mine waters and areas that observe high levels of mineral weathering. Magnesium toxicity in Hydra is know to lead to impaired growth, tentacle functioning, and nematocyst discharge. It is believed that magnesium toxicity is a function of the element acting as a physiological blocker of calcium that inhibits its uptake and distorts the biological processes that require calcium. However, it was found that the toxicity of magnesium was correlated with calcium levels where an increase in calcium concentration led to a reduction in the observed toxicity of magnesium in a study observing the effects of magnesium on the functioning of six different freshwater species found in Australia's Kakadu National Park. Of the six, Hydra viridissima was found quantitatively to be the second most sensitive to magnesium toxicity with an extreme reduction in growth rate as levels of magnesium increased; however, this study also found that the effect of increasing calcium on magnesium toxicity reduction was highest in Hydra viridissima.

Toxicity of Oil Spill Compounds
Freshwater oil spills are much more detrimental to their ecosystem than those occurring in the ocean due to their confinement to a small body of water. To determine the effects of oil spills on freshwater organisms, a study investigated how crude oil and two common dispersants used in oil spills effect Hydra viridissima's growth rate. It was found that over a seven day period, as the concentration of the two dispersants increased the growth rate of polyps decreased significantly indicating that the use of dispersants for oil spills have their own negative environmental effects. In addition to testing of the dispersants, the effect crude oil had on Hydra viridissima was also examined with an interesting result. While it is known that crude oil is toxic to Hydra, in low concentrations it was found to cause a phenomenon in Hydra viridissima termed hormesis. Hormesis is an effect where the introduction of toxins in low doses leads to a stimulation of a biological process, in this case growth rate, rather than an inhibition of it. While this result was not expected, other species of Hydra have been previously known to undergo hormesis when introduced to low concentrations of lead and copper so it is not completely surprising.

Relationship Between Uranium Toxicity and Environmental Variables
Uranium compounds are currently extremely concerning toxins due to the increasingly growing mining of such for the production of nuclear energy, specifically in Australia, and the negative effects they have on freshwater organisms. In an attempt to uncover how the environmental effects of Uranium entering freshwater bodies could be negated, one study looked into the effects of isolating water hardness and alkalinity on Uranium toxicity to Hydra viridissima as they are extremely sensitive to these compounds where toxicity was measured by observing changes in growth rate over a 96 hour period. They found that when water hardness was constant, a 25-fold increase in alkalinity did not have any significant effect on decreasing the toxicity of Uranium. However, it was found that when alkalinity was held constant a 2-fold increase in water hardness reduced Uranium toxicity by 24%, a 25-fold increase decreased toxicity by 55%, and a 50-fold increase in hardness reduced the toxicity by 92%. These results were found to be statistically significant indicating that there is a strong correlation between increasing water hardness and the reduction of Uranium's toxic effects in Hydra viridissima.

Phylogenetics
One important factor that should be known about Hydra genus is their taxonomic classification and phylogenetic origin. Given the genus' developmental and regeneration abilities, along with Hydra viridissima's symbiosis abilities, learning their place in evolution is important in fundamentally understanding these organisms and their role in the environment. In order to better understand the evolutionary relationship of Hydra, a group of researchers used molecular data from the nuclear and mitochondrial genes of eight different Hydra species to map their phylogenetic tree. From the molecular DNA and RNA samples, it was deduced that Hydra viridissima is the most basal species of Hydra as well as the sister group to every other species whose genes were tested. This conclusion arose due to Hydra viridissima possessing the smallest genome of the Hydra genus at only 380 million base pairs where the size of their chromosomes were very small, resembling that of their genome size relative to other species of Hydra. This indicates much less complexity in their evolutionary biology compared to other Hydra. Along with that, Hydra viridissima being the only species to have natural photosynthetic symbionts also points towards its basal location in phylogenetics as symbiotic relationships like this commonly contribute to success in many cnidarians indicating that Hydra viridissima may have evolved as the first Hydra species from some other symbiotic polyp over time.