Life Cycle of Freshwater Jellyfish: Craspedacusta sowerbyi

by Sarah Francart
BIOL/WATER 361, Fall 2012

Key taxa: Cnidaria, Hydrozoa, Craspedacusta

Currently there are about 10-14 known freshwater species of jellyfish, with Craspedacusta and Limnocnida being the two main genera (Arbaciauskas and Lesutiene 2005). Craspedacusta sowerbyi is one species that has been recently introduced to North America, and the first sightings of these tiny creatures were first reported in Wisconsin in 1969 (Engel 2007). This species has been found on all continents except Antarctica and is not harmful to humans (Arbaciauskas and Lesutiene 2005). The jellyfish is translucent with a whitish or green tinge and is native to the Yangztee River in China (Engel 2007). The freshwater jellyfish belongs to the phylum Cnidaria and is classified as a hydrozoan. The genus Craspedacusta contains only four species with C. sowerbyi being the only species of North American fresh water hydrozoa with a medusa stage in the life cycle (Pope 1995).

The life cycle of Craspedacusta consists of an alternating polyp and medusa stage. The freshwater jellyfish has three ways of asexual reproduction with sexual reproduction being a rare occurrence in the Craspedacusta sowerbyi. C. sowerbyi can exist as podocysts, frustules, planulae, polyps, and medusa. The life cycle of C. sowerbyi also includes a drought resistant diapause stage, which is how the species was able to spread worldwide during the 1990’s (Arbaciauskas and Lesutiene 2005). The life cycle and life stages of the freshwater cnidarian are distinctive and can be influenced by environmental conditions.

Polyps are the asexual stage of the jellyfish’s life cycle, and medusae are the sexually reproducing stage of the jellyfish. Polyps are cylindrical, have no tentacles, and are shaped like a bowling pin while the medusa is bell shaped and has hundreds of tentacles. Polyps can be divided into four sections consisting of the basal disk, stalk, neck, and capitulum (Pope 1995). Polyps are sessile and colonial and attach to the substratum by the basal disk (Pope 1995). A colony of polyps usually consists of 2-4 individuals (Arbaciauskas and Lesutiene 2005). Polyps have an apical mouth surrounded by nematocysts that form a capitulum and are typically 1 mm in height (Arbaciauskas and Lesutiene 2005). They can exist either alone or in colonies. A mature medusa is much larger than a polyp yet still very small. They can grow up to 25 mm in diameter, and can have 200-400 tentacles at maturity (Arbaciauskas and Lesutiene 2005). A medusa reproduces sexually by fertilization and polyps asexually by budding.

The gonads of the cnidarian are a greenish, fibrous triangular structure suspended from the four radial canals under the medusa bell (Xu and Wang 2009). Mature gonads of a female jellyfish are covered with fine round egg cells, while the male gonads are smoother than the females, with a small fold on the surface (Xu and Wang 2009). The sperm is divided into three parts consisting of the head, neck, and flagellum, and the zygotes of the cnidarian are globular and smooth (Xu and Wang 2009). Freshwater jellyfish start as eggs produced by female medusae. Eggs are released in batches into the water by female, and sperm are released by the male once they are mature. Fertilization of the eggs occurs externally in the water column (Engel 2007).

An experimental study on the life cycle of the Craspedacusta sowerbyi (Xu and Wang 2009) described the following detailed stages of the formation of the medusa through fertilization as follows. About 15 minutes after fertilization, cleavage of the egg into two cells begins followed by gastrulation 7 hours after fertilization. The fertilized egg then becomes a spherical planula larva. Shortly after, the spherical planula then loses its cilia and becomes motionless. The spherical planula then begins to elongate into a rod-like planula. The rod-like planula begins to form into a tiny polyp 5 days after fertilization as the gastrovascular cavity forms within the body wall of two-germ layers. Maturity of the polyp is reached about 10 days after fertilization.

In contrast, the freshwater hydra usually produces free living buds but as a rare occurrence, under harsh environmental conditions they can develop testes or single egg ovaries (Thorp and Covich 1991). This allows the hydra to engage in sexual reproduction. External fertilization by free-swimming sperm occurs while the egg is attached to the body of the female. According to Thorp and Covich (1991) the embryo may enter a resting stage of days to months before it directly develops into new polyps.

The three types of asexually exhibited by Craspedacusta sowerbyibuds are polyp buds, frustule buds, and medusa buds. Polyp buds remain attached to the colony and form new polyps. Polyps can generate a planula-like bud that can further develop into a short rod like frustule. It takes 4-13 days for a bud to grow on a polyp, and they initially appear as rounded swellings of the polyp wall. (Acker and Muskat 1976) The frustule frees itself from the parent polyp and has the ability to crawl for a period of time before forming a new sessile polyp. New polyps in a colony share a common coelenteron with the parent colony that is difficult to distinguish when the colony matures (Pope 1995).

In some years polyp colonies produce medusa buds. Medusa buds develop as a spherical process on the mature polyp. According to Xu and Wang (2009) a four-arm radial canal develops, followed by a vertical tube in the spherical medusa buds. A velum at the bud margin and eight tentacles are then formed, and the medusa then departs from the polyp to become free living. The immature medusa bud is very small with a diameter between 0.5 mm to 1.0 mm and becomes fully mature about 5 weeks after being released (Pope 1995; Acker and Muskat 1976). The medusa then matures into a nearly transparent body called a bell, which is commonly referred to as a jellyfish. After several weeks of growth gonads develop in the radial canals. The medusa lives only for a few weeks, releases eggs, and then dies (Engel 2007).

It has been suggested that the sex of the medusa is dependent on the sex of the polyp since male and female medusa are rarely found together (Acker and Muskat 1976). Both male and female medusas have only appeared together on rare occasions only in North America and Europe (Engel 2007). In a 50 year study done in Serbia the polyp stage of the Craspedacusta sowerbyi has not been found, and the medusa stage has only been found rarely (Dikanovic et.al. 2010). The polyp stage is believed to more common than the medusa stage. Therefore, the distribution of the freshwater jellyfish may be much wider than what current records suggest since the presence of jellyfish in a water body is usually determined by the medusa, which has an irregular and sporadic occurrence as compared to the polyp (Arbaciauskas and Lesutiene 2005).

The occurrence of Craspedacusta sowerbyi is dependent on environmental conditions. Any change in ionic constitution, oxygen tension, pH, osmotic pressure, or temperature is transmitted to the environment of the growing cells of the polyp (Loomis 2005). Chemical changes, temperature fluctuations, and amount of available food can induce sexual reproduction (Thorp and Covich 1991). In observational studies, body size and budding rate were proportional to feeding rate up to a point of food saturation (Thorp and Covich 1991). According to Dikanovic et al. (2010) hydromedusae blooms are most common in summer and fall in still or slow moving water bodies. The cnidarian does not advance from the polyp to the easily identified hydromedusa stage in the absence of warm temperaturea. Polyps seem to grow better at lower temperatures than the medusae do (Acker and Muskat 1976). The minimum temperature for polyps is 6-10°C and the maximum is 30°C, with the optimum temperature between 19-25°C (Acker and Muskat 1976). The minimum for medusa budding is 15°C with a maximum of 30°C, with a high occurrence of medusae in the mid-20s°C range (Acker and Muskat 1976).

In a study of the ecology of Craspedacusta sowerbyi, Acker and Muscat (1976) found that polyps flourish in any moderate temperature with adequate food, and frustules develop in response to high feeding levels, which promotes rapid colonization during times of abundant food. The study also concluded that medusa buds form in response to sharply rising temperatures which are higher temperatures than those favored by polyps.

The freshwater jelly fish has the ability to survive severe environmental conditions by forming resistant resting stages which are produced asexually. Polyps can encyst as resting bodies called podocysts or produce resting spherical frustules (Pope 1995). Podocysts are dormant cellular balls surrounded by a protective chitin like membrane (Thorp and Covich 1991). In Wisconsin these cysts can survive on the bottoms of ice covered ponds, lakes, and river pools (Engel 2007). The frustules then form polyps when conditions improve (Arbaciauskas and Lesutiene 2005). These resting stages are cold, drought, and salinity resistant, which aids in the dispersal of Craspedacusta sowerbyi (Pope 1995).

In conclusion, the freshwater jelly fish has a varied life cycle with asexual reproduction being the most common way of reproducing. The jellyfish is a versatile creature that can exist in many stages with the ability to withstand harsh environmental conditions. The presence of the medusa, type of reproduction, and life stage of Craspedacusta may be dependent on several environmental conditions.

References Cited

• Acker, T. and Muscat, A. 1976. The Ecology of Craspedacusta sowerbyii Lankester, a Freshwater Hydrozoan. American Midland Naturalist 95(2): 334-336.
• Arbaciauskas, K. and Lesutiene, J. 2005. The Freshwater Jellyfish (Craspedacusta sowerbii) in Lithuanian Waters. Acta. Zoologica Lituanica 15(1): 54-56
• Dikanovic, V., Jakovcev-Todorovic, D., Skoric, S., and Cakic, P. 2010. Freshwater Jellyfish Craspedacusta sowerbyi Lankester, 1880 (Hydorozoa, Oliniidae) -- 50 Years Observations in Serbia. Arch. Biol. Sci, Belgrade 62(1): 123-127.
• Engel, Sandy. 2007. Small Floaters Before Your Eyes. Wisconsin DNR.
• Loomis, W.F. 2005. Environmental Factors Controlling Growth in Hydra. Journal of Experimental Zoology 126(2):224.
• Pope, L.G. 1995. Sexual Dimorphism and Symmetry Variation in the Freshwater Jellyfish, Craspedacusta Sowerbii (Lankester). Department of Zoology. Southern Illinois University Carbondale: 2-4.
• Thorp, J.H. and Covich, A. 1991. The Freshwater Cnidaria or Coelenterates. Ecology and Classification of North American Freshwater Invertabrates. San Diego: Academic Press.
• Xu, S., and Wang, D. 2009. Life cycle of Craspedacusta sowerbyi xinyangensis. Current Zoology 55(3):227-234