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Ecological and Economic Impacts of Freshwater Cnidarians

by Eric Brandt
BIOL/WATER 361, Fall 2013

Key taxa: Cnidaria, Medusozoa, Hydrozoa

The phylum Cnidaria is a large and diverse group of aquatic organisms that can be found worldwide. This phylum is broken up into five classes that include jellyfish, hydras, corals and anemones. There are many differences between members of the different classes when it comes to body plan and lifestyle. However, all cnidarians share some general characteristics including a diploblastic body plan that is made up of two tissue layers, the epidermis and gastrodermis. Most of them also exhibit an alternation of generations that switches between polyp and medusa stages. Perhaps the most unique feature of this group is the presence of cnidae, specialized stinging structures used to deliver venom and capture prey. Marine cnidarians are widely known and often studied due to their beauty and ecological importance to many marine ecosystems. Whether they are in the form of colorful coral reefs or massive blooms of jellyfish that can be beautiful or harmful, marine cnidarians attract a lot of attention. Although they may be equally as important to their ecosystems, freshwater groups tend to go unnoticed. Whether they are native to a particular system or invasive, freshwater cnidarians can have some serious ecological and economic impacts in North America.

There are many reasons why freshwater cnidarians may not attract much attention and may even go completely unnoticed in a freshwater system to the untrained eye. First, there are just not many of them worldwide. Out of the thousands of members that make up the phylum Cnidaria, only a very small fraction can be considered freshwater species. Of the five classes in this phylum, only one, class Hydrozoa, contains freshwater species. Another factor is their physical appearance and morphology. They tend to be much smaller than their marine relatives. They also tend not to come in colorful varieties or form complex structures that are seen in some marine groups. These morphological differences are the result of adaptations to life in freshwater systems. Freshwater organisms must focus more energy on dealing with drastic changes in factors such as temperature, pH, alkalinity, and salinity that are usually not present in stable marine environments. Freshwater hydrozoans are predatory animals that feed on a wide variety of smaller organisms and can be very influential to aquatic food webs. Some members of this group spend the majority of their time in the free-swimming medusa stage, actively feeding on zooplankton and other small invertebrates throughout the water column. Others are typically found in the stalked polyp stage that can attach to substrate, rocks, or aquatic plants. Some polyps remain sedentary throughout their life and may form massive colonies, whereas, some are capable of moving around. Although they may not stand out as much as their marine relatives, freshwater hydrozoans have the potential to cause some major problems in North American freshwater systems.

When it comes to our freshwater natural resources, a growing concern is the issue of aquatic invasive species. Invasives have the potential to rapidly multiply and cause serious damage as they take over an ecosystem and dominate the native species. Some of the major invasive species, such as carp and zebra mussels, are well known by all who use freshwater resources, and there are many programs in place to try to control them. Invasive hydrozoans do not receive as much attention as some of the major invasives possibly because their effects are not as easy to see. Not much is known about them, and until recently very little research has been done on them and the impacts they can cause. A recent study suggested that certain species of invasive hydrozoans have the potential to disrupt aquatic food webs and may cause serious negative impacts on the health of natural freshwater systems (Smith & Alexander, 2008). One of the major invasive species of cnidarians is the freshwater jellyfish, Craspedacusta sowerbii. This species is native to a major river system in China, but it can now be found in freshwater systems worldwide. It is believed that their spread has been caused by the import of water lilies that had the polyp stages of the jellyfish attached to them. Once in a system, these jellyfish will rapidly reproduce and consume large amounts of zooplankton. They prefer to feed on smaller invertebrates including rotifers and nauplius larvae of small crustaceans. Research has found that this species of jellyfish can drastically decrease the community of zooplankton in a freshwater ecosystem (Smith & Alexander, 2008). This population decrease is caused by several factors including direct predation by the jellyfish and also incidental death due to contact with their stinging tentacles. The researchers discovered that although C. sowerbii only ate the smaller prey species, larger organisms were killed but not consumed. This included larger invertebrate species and even some small fish. The jellyfish medusae are capable of moving throughout the water column and can kill any smaller organisms they come in contact with. Although it is rare, massive blooms of these jellyfish can cause serious damage to an ecosystem. The organisms that are killed or consumed are important members of aquatic food webs which means these invasives have the potential to disrupt the natural order and negatively impact the native species. By consuming large numbers of trophically low organisms that feed on algae, these invasive jellyfish may also be responsible for causing massive algal blooms that can negatively impact the health of a freshwater system.

Another species of concern is Cordylophora caspia, a colonial hydrozoan that was not found in freshwater until recently. It is believed to be native to North America and can be found along the east coast of the United States. Until recently, it was only found in estuarine ecosystems living in marine and brackish water conditions. A recent study has shown that it has been found living in the Connecticut River and has the potential to spread throughout North American freshwater systems (Smith et al., 2002). Differences in cell biology and morphology have been found between freshwater individuals and members of the same species living in estuarine habitats. This suggests that they have acquired the ability to change and rapidly adapt to the differences between saltwater and freshwater ecosystems. These are benthic predators that form large colonies and feed on zooplankton and small aquatic insects. Like Craspedacusta sowerbii, this species may also have the potential to disrupt native food webs and negatively impact the health of freshwater systems as they continue to spread.

Although they are a threat, invasive cnidarians are not the only problem. There are also many hydrozoans that are native to North America that can have some serious negative impacts on freshwater ecosystems. Species that feed on fish larvae are of great concern because of the damage they can cause to fish populations. One example of this is the species Hydra canadensis. This species is native to North American lakes and has been known to eat large numbers of bluegill larvae. The polyps can be found attached to rocks and substrate but seem to prefer attaching to aquatic plants more frequently. Attaching to plants allows them to spread themselves throughout more of the water column to feed. In a study by Elliott et al. (1997) of a Canadian lake, large numbers of hydra were found in weed beds surrounding bluegill nesting sites. They found that the hydras were capturing bluegill larvae as the fish left the nests searching for open water. This is an interesting case where the predation and death of larval bluegills by a native species of hydra increased due to the presence of an invasive plant. Hydra individuals were found in high concentrations on blooms of Eurasian watermilfoil. This plant grows in thick mats that spread from the surface to the bottom of the lake. By colonizing these plants, the hydras were able to feed throughout the water column. As the fish larvae swam through the weeds in search of open water, they were forced to come in contact with the large numbers of hydra and their stinging tentacles. The study found that fish around 8 days old and up to 7 mm in length were the most heavily fed upon (Elliott et al., 1997). Only the smaller fish were captured and eaten but many of the larger, older fish larvae were also killed due to contact with the stinging tentacles. In this particular case, fish mortality due to contact with H. canadensis is increasing due to a rapid increase in Eurasian watermilfoil. This could have some serious negative impacts on bluegill populations over time. In another study, hydras were found moving toward bluegill nests when eggs were laid.They were found surrounding the nests and feeding on the larvae shortly after they hatched.

Interactions between hydrozoans and fish can also have some negative economic impacts when it comes to aquaculture facilities. In a study by Eisler & Simon (2011), hydras were found in large numbers near the water inlets of trough compartments in aquaculture facilities. They may become introduced into an aquaculture facility through water transfers and may go undetected until they increase in size and number and are already causing problems with the fish. They were found feeding on the larvae of the salmon that were being raised at the facility. Many of the fish also showed signs of damage to skin and fins, and many also died from these injuries after contact with the stinging tentacles. Some hydras were also found in the incubation troughs where eggs are stored and raised. These can cause problems by damaging eggs and wrecking batches of young fish. These hydras can cause some serious problems for fish farmers and could cause massive losses of fish and therefore a loss of money. They may also become difficult to get rid of if they become too numerous. There are very few known methods of removal, and more research is needed.

Freshwater cnidarians can cause problems in many ways that may result in serious negative economic and ecological impacts. They have not received as much attention as their marine relatives for several reasons, but their ability to greatly impact an ecosystem suggests that perhaps they deserve more attention. Much more research needs to be done to understand all the ways they impact their freshwater ecosystems.

References Cited

  • Eisler, R. & R.C. Simon. 2011. Destruction of salmon larvae by Hydra oligactis. Transactions of the American Fisheries Society 90: 329-332.
  • Elliott, J.K., J.M. Elliott & W.C. Leggett. 1997. Predation by Hydra on larval fish: Field and laboratory experiments with bluegill (Lepomis macrochirus). Limnology and Oceanography 42: 1416-1423.
  • Folino-Rorem, N.C., J.A. Darling & C.A. D‘Ausilio. 2007. Genetic analysis reveals multiple cryptic invasive species of the hydrozoan genus Cordylophora. Biological Invasions 11: 1869-1882.
  • Smith, A.S. & J.E. Alexander Jr. 2008. Potential effects of the freshwater jellyfish Craspedacusta sowerbii on zooplankton community abundance. Journal of Plankton Research 30: 1323-1327.
  • Smith, D.G., S.F. Werle & E. Klekowski. 2002. The rapid colonization and emerging biology of Cordylophora caspia (Pallas, 1771) (Cnidaria: Clavidae) in the Connecticut River. Journal of Freshwater Ecology 17: 423-430.

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