Importance and Life History of Palaemonetes spp.
by Ryan Gibson
BIOL/WATER 361, Fall 2013
Palaemonetes is a genus within a group called caridean shrimp this group falls under the order Decapoda. Palaemonetes are found on every continent around the world, excluding Antarctica. Members of the genus are found in various habitats, from freshwater rivers and caves to coastal salt marshes. Within this paper the life history of Palaemonetes spp. will be discussed along with how these “grass shrimp” play a great role in the environments in which they are found. The life history of the genus Palaemonetes and its occurrence in many different aquatic habitats around the world and around North America, along with many other characteristics, make them a very important bio-indicator species, and a very good model organism for the environments in which they are found.
The genus Palaemonetes contains about 32 known species and counting. This genus has both fresh and saltwater shrimps. In North America the two groups, fresh and salt/brackish water, can be separated in a number of ways. These two groups were at first thought to be of polyphyletic origin. However, upon further investigation there is substantial evidence to disregard the possibility of multiple invasions of freshwater by saltwater Palaemonetes. In the brackish grouping all members exhibit a smaller egg size with increased number of eggs. In freshwater the eggs are generally larger and less numerous. Clutch size, which is extremely variable between species, is also affected by a number of other factors, making it variable even within species. Palaemonetes pugio exhibited a varying clutch size at four different locations across North and Central America. In the southwestern Gulf of Mexico, the average clutch size was 186 eggs, whereas in Rhode Island it was 486, in Texas it was 372, and 247 in South Carolina (Chazaro-Olvera, 2009). This variance could be due to a number of environmental factors, such water temperature or salinity (Matocec et al., 2006). For Palaemonetes antennarius, a brackish water species, optimal fecundity occurs at about 3% salinityas indicated by the high number of egg bearing females found. In a brackish environment, it is possible for yields to be as high as 100 eggs per clutch. However in freshwater conditions the yield is down to about 50 or 60 eggs.
The two groups, freshwater and brackish water shrimp, can be distinguished in a couple other ways. All Palaemonetes species have larval stages. Within the first larval stage you can identify the differences between the two shrimp groups based on the antennal scale morphology. Freshwater Palaemonetes have an unsegmented antennal scale, whereas the brackish water varieties have a segmented antennal scale. In the adults, upper antennal flagellum morphology is used to distinguish the groups as described by Strenth (1976). These characters are satisfactory evidence for a monophyletic grouping of the freshwater Palaemonetes of North America. These differences in freshwater/saltwater Palaemonetes are however not shown by all freshwater Palaemonetes across the world. P. argentinus finds its home in Argentina, Southern Brazil and Uruguay. This species exhibits many characteristics of the marine Palaemonetes, such as the segmented antennal scale and a large clutch sizes with small eggs. However it is mostly found in freshwater systems. This species represents an invasion of freshwater that is independent of the invasion by the North and Central American Palaemonetes.
Interestingly enough and yet unexplained, is the relationship between a few other Palaemonetes species. P. varians is a marine species found from the Baltic Sea to the Mediterranean Sea that exhibits similar characteristics such as segmentation of the antennal scale in the first larval stage, and large clutch size. This shows that these two varieties are closely related to eachother. However, P. antennarius, the freshwater grass shrimp around the Mediterranean Sea shows the same morphologies as the North American varieties. P. antennarius has an unsegmented antennal scale, small clutch size and large eggs. All of this coincides with the same freshwater invasion event as the North American varieties of Palaemonetes.
The sex ratio in nature for Palaemonetes is variable depending on the time of year and a number of other factors. Throughout most of the year, a sex ratio of about 1:1 is exhibited. For one species, Palaemonetes pugio, the highest population density was evident in the spring and summer seasons. This population density was correlated with the amount of vegetation present in the ecosystem. In winter and spring the sex ratio heavily favored females. This is a reproductive strategy exhibited by Palaemonetes, these shrimps find mates by searching for them, and the more females there are the greater chance that males will happen upon numerous females. The large number of females increases reproductive success for the species as a whole. There are a few other possibilities for this skewing of sex ratio. These could be different habitat preferences, different mortality rates, and differences in other behaviors such as migrations (Chazaro-Olvera, 2009).
Female Palaemonetes exhibit higher fecundity rates in habitats with optimal salinity (3%) however the larvae of these Palaemonetes seem to lack the ability to cope with any changes in salinity (Matocec et al., 2006). These shrimp go through three zoeal stages and two post larval stages. In the study by Matocec et al. (2006), no juvenile or larval-stage P. antennarius were found in any body of water with the possibility of salinity fluctuations. However, the abundance of larvae collected in the freshwater lakes was directly influenced by water temperature.
These grass shrimps also play a great ecological role. The ability of these small shrimp in large numbers to turn over substrate and release nutrients that are “locked up” in the substrate is astounding. In their environment these shrimp do a great service in regulating, and enhancing certain aspects. In a controlled experiment it was evident that Palaemonetes species play a great role in the regulation of meiofauna in an environment. Nematode, copepod, polychaete, and oligochaete populations were all kept in check by the predation and manual disturbance of the grass shrimp, causing an overall increase in environment health and a reversion back to semi-normal levels. The grass shrimp did not exhibit an affinity for any type of meiofauna and instead just decreased the level of all meiofauna as a whole (Bell & Coull, 1978). These caridean shrimps are, however, not the top of the food chain and also play the unfortunate role of food source for many organisms such as fish and other crustaceans. Palaemonetes species also play a very large role in energy transfer in a salt marsh environments. Grass shrimps are proven to graze and disturb epiphytic algae. This causes a direct increase in the absorption of sunlight by very important aquatic plants such as Ruppia maritima. In two experiments it was shown that the grass shrimp were able to aid R. maritima by inhibiting dieback in the mid and latter stages of the Ruppia life cycle (McCall & Rakocinski, 2007). Grass shrimp also aid aquatic plants by the excretion of nutrients and a very localized distribution of fecal pellets. This interaction is very useful for ecosystems in the reduction of algal blooms and increase in plant growth. These shrimp, namely Palaemonetes paludosus have been introduced by various government agencies to multiple locations as important additions to the food chain in areas such as the lower Colorado River.
Grass shrimps are very interesting little organisms as they are able to tolerate varying temperatures and salinity levels. One species has the ability to tolerate a range of 0-55 ppt as adults. This is an intense range of variance, all the way from fresh to brine water. However, these little organisms are very susceptible to other environmental contaminants, such as PCB’s and other chemicals, and these shrimp are not found in areas with contamination. Thermal pollution is something that could also be a large factor. These shrimp are known to breed about two times per year when the water reaches a certain temperature, about 20°C for one species. It was shown that in areas near one another with varying water temperatures, the shrimp in the warmer body of water will often have a longer breeding period. Even though these shrimp are able to tolerate considerable fluctuations in temperature and salinity, in one case, with the alteration of the Neretva River in Croatia, the decrease in seasonal water fluctuation and loss of many wetland plant communities has taken its toll on the invertebrate well-being in the area. Palaemonetes antennarius has become very rare in this area and has been directly affected by the actions of the humans around them (Matocec et al., 2006). The decrease in these grass shrimp will lead to a change in the food chain. This will have a direct effect on all of the other organisms in the area eventually including the humans. Palaemonetes spp. are a very important model estuarine organism in many laboratories. Much is known about the life history of these individuals so they are easily bred and maintained in captivity. Palaemonetes can be used for a number of experiments including effects of pollution on estuaries for example.
Palaemonetes spp. are very interesting and important organisms, although I would argue that many people are unaware of the existence of the phylum as a whole. They hold a very important niche in many ecosystems, sifting through detritus and upturning otherwise buried nutrients to the environment, aiding in the reduction of epiphytic algae, and holding an important spot in the food chain. For these reasons they have even been purposefully introduced into new ecosystems. These delicate shrimp are very important as proverbial “canaries in a coal mine” when it comes to pollution in their environment. However they can call many different environments home, with various temperatures and salinities. The life history of the genus Palaemonetes and its occurrence in many different aquatic habitats around the world and around North America, along with many other unique characteristics, make it a very important bio-indicator and a very good model organism for the environments in which they are found.
- Bell S.S. & B.C. Coull. 1978. Field evidence that shrimp predation regulates meiofauna. Oecologia 35(2): 141-148.
- Chazaro-Olvera S. 2009. Growth, mortality, and fecundity of Palaemonetes pugio from a lagoon system inlet in the southwestern Gulf of Mexico. Journal of Crustacean Biology 29(2): 201-207.
- Matocec S.G., A. Kuzman & M. Kerovec. 2006. Life history traits of the grass shrimp Palaemonetes antennarius (Decapoda, Palaemonidae) in the delta of the Neretva River, Croatia. Limnologica 36: 42-53.
- McCall D.D. & C.F. Rakocinski. 2007. Grass Shrimp (Palaemonetes spp.) play a pivotal trophic role in enhancing Ruppia maritima. Ecology 88(3): 618-624.
- Strenth N.E. 1976. A review of the systematics and zoogeography of the freshwater species of Palaemonetes Heller of North America (Crustacea: Decapoda). Smithsonian Contributions to Zoology 228: 1-27