Alternation of Generation and Exceptions in Cnidaria
by Margaret Armstrong
BIOL/WATER 361, Fall 2012
Animals reproduce in order to give rise to the next generation. All species must do this in order to pass on genes and keep their niche filled. The process of reproduction, however, can be uniquely identified to an individual species: a combination of genes and environment can lead to an entirely species-specific form of reproduction. Depending on the combination of these factors, there are numerous types of reproduction possible. Still, some closely related groupings of animals can have similar processes. For phylum Cnidaria, the common method of species reproduction is alternation of generations. Uniquely belonging to a few plant and animal species in the world, this form of transformation is what sets apart Cnidarian reproduction from other aquatic invertebrates’ reproduction.
Alternation of generations is a type of life cycle that switches between two forms, the asexual polyp and the sexual medusa. Each reproduction, one form will give rise to the other. For example, a polyp will go through asexual reproduction to produce medusae and vice versa with sexual reproduction among medusae. In the phylum Cnidaria, class Hydrozoa has many groups that have alternation of generations. The best model of this process is Obelia, which has equally balanced forms. Although the traits of polyps and medusae may be quite different because of their lifestyles, there are similarities such as the gastrovascular cavity with tentacles around the mouth, radial symmetry and the same layers of tissue. The differences between polyps and medusae merely lie with the orientation of these parts and what they are used for.
Asexual polyps in some species are dimorphic in the fact that the “heads”, or the zooids, come in two forms. First, there is the gastrozooid. This form has both tentacles as well as stinging cnidae to capture prey. Second, there is the gonozooid. This version of the zooid is the part that reproduces medusae. Asexual polyps are often colonial. Once the larva sexually produced from medusae settles down, a polyp will form. Then, as the polyp continues to grow, budding may occur. This causes another polyp, similar to its predecessor, to grow connected to the preceding polyp through the gastrovascular cavity and outer epidermis. This is important, as there are cases in which a zooid cannot take care of itself. For example, a gonozooid has no means in which to capture food. It, therefore, depends on the gastrovascular cavity connecting it to the gastrozooids in order to get the necessary nutrition.
The sexual medusa form of a cnidarian is often the most recognizable, as it is a jellyfish. At this stage, the animal is pelagic. The body of a medusa is a hydrostatic skeleton, or a fluid filled area with contracting muscles. A gut is represented by the gastrovascular cavity, which takes in food and expels wastes through the same opening. There are other amenities such as a more developed nervous system to assist the animal with moving about and bipolar nerve net to send impulses around the body. As for reproduction, male and female medusa will produce planula larvae. The larvae are ciliated, so they can swim in the water until they settle on a substrate and become a polyp.
Cnidarian are a breakthrough in animal evolution as they were the first animals to possess tissues, such as nerves and organs. For this reason, the phylum has been called “the crossroads of metazoan evolution” (Boero et al. 2005). In addition, there are the characteristics that still set them apart today. As described by Boero et al. (2005), the “evolutionary samba”, or evolutionary process, is the coming and going of traits as permanent or temporary in a species. Unlike typical animals, Cnidaria can develop multiple characteristics at once. The polyp in a species can develop its traits while medusae in the same species develop their own. The importance of this separate development is the ability for two very different forms to continue evolving as needed.
As previously stated, Obelia has a balanced alternation of generations; both forms are well-developed. Other species or groups of Cnidaria may have one form reduced or nonexistent. In the class Scyphozoa, the medusa stage is emphasized. The polyp stage still thrives, but the reproduction is highly evolved. This imbalanced life cycle can be due to a change in genes and environment. An alteration in genes could be a form of evolution that better adapts the species for the environment, such as with species introduced to a new ecosystem. This reaction could also be where, depending on a slight difference in a species’ endemic environment, there can be an adaptation for the reproduction process. Such examples of this idea can be found everywhere.
According to Fautin (2002), the sexual reproduction in schyphozoans is important because of the emphasis on medusae. However, transverse fission, an asexual form of reproduction, is notably lacking in this class. This study went in depth to look at instances of transverse fission in the classes of Hydrozoa and Anthrozoa, as this could possibly explain as to why there is no significant evidence in Schyphozoa. The main point of the project aimed to figure out if there was a similarity in the reproductive process between the classes of Cnidaria, or if notable occurrences were a result of environmental adaptations. No solid conclusion could be drawn.
In the class Hydrozoa, there are many species that have equal alternation of generations. Family Hydractiniidae is an exception. Miglietta et al. (2009) reported that animals in this family have a reproduction pattern that emphasizes the polyp side of alternation of generations. The main focus of this study was on three specific genera and how they are related to other cnidarians. To diagnose which species they are more closely related to, the specific alternation of generations development stage of medusa and polyps in each genus was compared. This may not accurately discern the genera as the species in class Hydrozoa are known to have “extreme morphological plasticity depending on the substratum or environmental conditions”. The classification of the Hydrozoa clearly displays how species can be related yet be very different. The simple difference between form equality in Obelia and the polyp emphasis in Hydractiniidae is an example of this. It is a combination of genes and the environment that make an animal have to change or adapt.
An environment specific alteration can be related to the temperature of the water. As researched by Slobodov et al. (2004), there is a species of Obelia found in the White Sea near Russia. This species can thrive at a minimum temperature of 15°C. The White Sea, however, is not within the typical range for Obelia geniculata. The main goal of this study was to find what affects this unfavorable environment has on the species. O. geniculata is small in size but still maintains the typical Obelia life pattern: sexual medusa, planula larvae, asexual polyp. The end result of this study showed that the polyp form thrived just fine in the cold water. The planula larvae were able to give rise to the polyps successfully. The only environmental adaptation with these stages was the use of a frustule formation in the larvae, which characteristically grows into a polyp when the planula touches a substrate. These forms do well in the White Sea, but the main concern for this species in cooler water is the impaired formation of gonads in medusae. Researchers found the temperature was “too low for medusa of Obelia geniculata to attain maturity”. As an international species, Obelia geniculata is found in many places where environmental conditions meet the minimum temperature requirement. This instance in the White Sea displays how animals from the same species can be found in various places but have to adapt by altering their life cycle. This species in Obelia may not be best suited near Russia, but it still manages to survive season after season.
Another environment specific alteration can be related to the pH, or potential of hydrogen, of the water. This trait measures the concentration of hydrogen ions in the water, which in turn makes the water acidic or basic. This acidity can cause issues with the development of Cnidaria. An experiment done by Winans and Purcell (2010) measured the development of both polyps and medusa of Aurelia labiata, a species from class Scyphozoa, at different levels of pH and at two different temperatures. The results showed polyps easily surviving, and the ephyra, or juvenile medusa, was minutely affected. The polyps were able to undergo asexual reproduction at both temperatures but more readily at the higher temperature. Also, the tissues in the polyp helped support the diffusion of water in order to deal with the increased amounts of carbon dioxide in the water, a side affect of increased pH. The ephyra were not able to thrive as easily. There was a strong correlation between the size of ephyra statoliths, parts of the gravity-detecting organ, and the rate of metabolism due to the low pH. Class Scyphozoa is a medusa-oriented group, which would make any difficulties to medusa development a major impact. Still, the asexual reproduction of the polyps that produce medusa held steady. Also, these are not freshwater cnidarians. The animals were taken from Dyes Inlet, Washington and were placed in salt water. Theoretically, the effects of pH change could alter similar freshwater species, so it is therefore important to know how this type of environmental condition change would modify species.
Environmental factors can cause a species to adapt. Whether this transformation is permanent or temporary in the genes, the adjustment of the animals will allow the species to continue reproducing and filling its niche. Cnidaria have more of a challenge with this as they have not one, but two forms to adapt under certain conditions. There is the process of alternation of generations, which by itself sets phylum Cnidaria apart from other animals. Each class of Cnidaria has their own features, but this common development is what knits them together. Overall, the continuation of these invertebrates to evolve and adapt today make them diverse and universal. There are many variations of cnidarian reproduction that can be labeled alternation of generation and the combination of genes and environment will continue to evolve as this phylum spreads across waters.
- Boero, F., J. Boullin, S. Piraino. 2005. The role of cnidaria in evolution and ecology. Italian Journal of Zoology. 72: 65-71.
- Fautin, G. 2002. Reproduction of cnidaria. Canadian Journal of Zoology. 80: 1735-1755.
- Miglietta, M. P., P. Schuchert, C. W. Cunningham. 2009. Reconciling genealogical and morphological species in a worldwide study of the family hydractiniidae (Cnidaria, Hydrozoa). Zoologica Scripta. 38: 403-430.
- Slobodov, S. A., N. N. Marfenin. Reproduction of the colonial hydroid Obelia geniculata (L., 1758) (Cnidaria, Hydrozoa) in the White Sea. 2004. Hydrobiologia 530: 383-388.
- Winans, A. K., J. E. Purcell. 2010. Effects of pH on asexual reproduction and statolith formation of the scyphozoan, Aurelia labiata. Hydrobiologia 645: 39-52.