A single individual can produce offspring asexually and large numbers of offspring can be produced quickly. In a stable or predictable environment, asexual reproduction is an effective means of reproduction because all the offspring will be adapted to that environment. In an unstable or unpredictable environment asexually-reproducing species may be at a disadvantage because all the offspring are genetically identical and may not have the genetic variation to survive in new or different conditions.
On the other hand, the rapid rates of asexual reproduction may allow for a speedy response to environmental changes if individuals have mutations. An additional advantage of asexual reproduction is that colonization of new habitats may be easier when an individual does not need to find a mate to reproduce. During sexual reproduction the genetic material of two individuals is combined to produce genetically diverse offspring that differ from their parents.
The genetic diversity of sexually produced offspring is thought to give species a better chance of surviving in an unpredictable or changing environment. Species that reproduce sexually must maintain two different types of individuals, males and females, which can limit the ability to colonize new habitats as both sexes must be present. Asexual reproduction occurs in prokaryotic microorganisms bacteria and in some eukaryotic single-celled and multi-celled organisms.
There are a number of ways that animals reproduce asexually. Fission , also called binary fission, occurs in prokaryotic microorganisms and in some invertebrate, multi-celled organisms. After a period of growth, an organism splits into two separate organisms. Some unicellular eukaryotic organisms undergo binary fission by mitosis. In other organisms, part of the individual separates and forms a second individual.
This process occurs, for example, in many asteroid echinoderms through splitting of the central disk. Some sea anemones and some coral polyps Figure 1a also reproduce through fission. Budding is a form of asexual reproduction that results from the outgrowth of a part of a cell or body region leading to a separation from the original organism into two individuals. Budding occurs commonly in some invertebrate animals such as corals and hydras.
In hydras, a bud forms that develops into an adult and breaks away from the main body, as illustrated in Figure 1b, whereas in coral budding, the bud does not detach and multiplies as part of a new colony. Figure 1. Figure 2. Sea stars can reproduce through fragmentation. The large arm, a fragment from another sea star, is developing into a new individual. Fragmentation is the breaking of the body into two parts with subsequent regeneration.
If the animal is capable of fragmentation, and the part is big enough, a separate individual will regrow. For example, in many sea stars, asexual reproduction is accomplished by fragmentation. Buds originate at the junction of the stalk and gastric regions. The bud begins as a hemispherical outpouching that eventually elongates, becomes cylindrical, and develops tentacles.
The bud then pinches off and a new individual becomes independent. Buds are produced every two to three days under favorable conditions. Following unfavorable conditions, such as injuries or periods of scarce resources, hydras occasionally reproduce through transverse and longitudinal fission. Hydra oligactis is mostly sessile.
Hydras attach to stones, twigs, vegetation, or debris. The brown hydra rarely is found at depths exceeding 1. Spontaneous movements are few.
When the hydra remains undisturbed, its body is extended and the tentacles spread. For no apparent reason, contractions and expansions of the body occur at intervals and the tentacles are constantly in motion. Free hydras can move from place to place by basal gliding. The hydras usually move by looping and somersaulting, by attaching the tentacle ends and pulling themselves along. When there is an insufficient supply of oxygen, hydras move to regions of higher oxygen content.
In general, the behavior is characterized by its mechanical nature, great independence of parts, lack of integration, and lack of exact responses. Hydra oligactis , as in all Cnidaria, are strictly carnivorous and eat many different kinds of small metazoans, including annelids, copepods, cladocerans, and insects.
Hydra capture their food by paralyzing and killing the food organism by means of nematocysts, which are discharged into the prey. The prey is brought to the mouth proctostome by the tentacles, a response that is induced glutathione.
This is considered the key mechanism in digestion. The organism is then taken in through the mouth, which is star-shaped or circular. Hydras have been known to feed on the organic material of the substrate when the food supply is insufficient. This behavior, however, is not considered normal. This presents a significant advantage to Hydra given that they can be able to synthesize their own food when environmental conditions change food is scarce.
As a result, green Hydra have a big advantage over brown Hydra which lacks the chlorophyll needed for photosynthesis. This is only possible as long as green Hydra is exposed to sunlight. Despite being carnivores, green Hydra are able to survive for about 3 months using sugars produced through photosynthesis. This allows the organism to tolerate starvation in the absence of prey. Essentially, regeneration refers to the ability of an organism to replace given lost or damaged parts.
For instance, Geckos are capable of regenerating their tails when it is lost through the activation of a group of stem cells. Although a number of other organisms can regenerate lost parts, Hydra has become one of the most studies organism due to its ability to regenerate even after being cut in half or being put in a blender and centrifuged. This ability, as well as the fact that Hydra are far less complex, has made them the ideal subject for regeneration studies.
Although the mechanism through which Hydra are able to regenerate any part of their bodies so effectively is yet to be fully understood, a number of theories have been proposed based on a number of studies. The following are two of the most widely accepted mechanisms:. Developmental organizer center located near the head pole and base of the organism. According to a study conducted at the University of Geneva in Switzerland, head regeneration in Hydra was shown to depend on the transformation of stump into the head organizing center tissue.
Here, the organizer was shown to play an important role in inducing the differentiation of stem cells into specialized head cells thus directing the construction of the head. In addition, the organizer was also shown to play a role as an inhibitor. Through its inhibitory activity, it inhibits the formation of additional heads. According to another study that was conducted at the Technion-Israel Institute of Technology in Haifa, researchers found cytoskeletons to play an important role in regeneration.
From the research study, cytoskeleton was shown to play an important role in signaling ultimately contributing to the regeneration process. The growth zone below the tentacles contains interstitial cells that produce all the other cells. As older cells are shed from either pole of the Hydra, new ones continue to replace the old ones allowing Hydra to continue living for a long period of time.
The removal of these cells growth zone cells has been shown to cause the death of Hydra within a few days. Hydra move from one point to another using a number of strategies. This allows the organism to change location in response to stimulus. These movements are made possible by the epidermal muscle fibers and include:. Here, Hydra bend so that the tentacles touch the substratum.
This is followed by the pedal disc moving closer to the tentacles allowing the organism to move to the direction it intends. By repeating this process, Hydra is able to move from one point to another. This type of movement resembles that of gymnasts. Here, the Hydra uses its tentacles as legs to move from one point to another. This is the type of movement resembling that of snails. Return to Multicellular Organisms main page. Return to Pond Water under the Microscope. Anita Kaliszewicz.
Interference of asexual and sexual reproduction in the green hydra. Nikunj Bhatt. Hydra: An outline of Life processes. International E - Publication.
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