Environment’s Role in Parasitic Relationships

INFLUENCE OF ENVIRONMENTAL FACTORS

There are many parasitic relationships in today’s environment. This is where a parasite will use an organism to its advantage by getting proper nutrition or living conditions of some sort from their chosen host. (1) There are many things that could influence how parasites choose their hosts and gain the proper abilities to do so. One of the major influences is the environment. Factors caused by the environment can greatly influence the strengths or weaknesses of a relationship between a parasite and its host. (2) Some of these environmental factors include, but are not limited to, the temperature and altering an organism’s habitat. (3) 

POSITIVE INFLUENCE

Environmental factors can greatly strengthen the relationship between a parasite and its host. For example, whenever temperature increases in the habitat of a host and parasite, hosts can obtain the ability to have a greater amount of offspring. This being said, warmer temperatures are expected to yield a stronger presence of parasitic relationships, because that will increas the amount of susceptible hosts for the parasites. (4) As well as this, there are some parasites that survive better in the warmer temperatures, thus making them a bit stronger. For the malaria parasite, Plasmodium falciparum, temperature plays one of the biggest roles in its development. As well as the proper amount of rainfall, warm climates are crucial in order for the mosquitoes, the intermediate host of P. falciparum, to survive. This being said, mosquitoes are less likely to bite in colder temperatures, thus inhibiting the spread P. falciparum. (5)

Mosquitoes less likely to bite in cold temperatures (6)

NEGATIVE INFLUENCE

Not only can the environment have a positive influence on the relationship between the host and the parasite, but also it could affect it negatively and create a disturbance or stress for the organisms. (3) For example, humans can alter organisms’ habitat by converting a forest to something like an amusement park. This changes the environment completely for the organisms that once occupied that space, and thus creates a stressor among them. This causes the relationship between any kind of host and parasite to weaken, which might ultimately lead to the parasite’s life coming to an end. In this instance, the parasite is more affected by the environmental disturbance than the host.  However, in other cases, different hosts in a single environment may start dying rapidly thus causing the parasites to eventually be removed from the population as well. (3) The malaria virus, P. falciparum, could also be used as an example to describe how the environment can influence a parasite negatively. Whereas warmer temperatures aid in strengthening the relationship between P. falciparum and its mosquito host, the colder temperatures weaken it. Another species with similar results is the Daphnia longispina and the Caullerya mesnili parasite. An experiment was conducted that resulted in more D. longispina species being infected with C. mesnili in 12°C rather than at 20°C. (7) This being said, warmer temperatures have an opposite affect on C. mesnili than how it affected the malaria parasite.

Daphnia longispina (8)

WHY DOES IT MATTER?

The environment can affect parasitic relationships in many different ways. For example, colder temperatures have a different affect on different types of organisms. This explains why some species can only survive in certain places around the world.  This also suggests that the environment has a lot to do with the selection of hosts among different parasites (9).  Environmental factors can also take a role in the explanation of why some parasitic relationships are not strong in areas that would be expected. This could occur in places where an organism’s habitat has been altered.

SOURCES

(1) http://www.cdc.gov/parasites/
(2) http://rspb.royalsocietypublishing.org/content/early/2010/12/21/rspb.2010.2359.full
(3) http://parasitology.msi.ucsb.edu/pubs/Lafferty_Kuris_05_Thomas.pdf
(4) http://www.cidd.psu.edu/research/synopses/parasites-and-climate-change
(5) http://www.nature.com/srep/2013/130218/srep01300/full/srep01300.html
(6) http://www.dailymotion.com/video/x14tju7_mosquitoes-less-likely-to-bite-in-cold- temperatures_news
(7) http://rsbl.royalsocietypublishing.org/content/early/2010/08/06/rsbl.2010.0616.full

(8) http://www.microscopy-uk.org.uk/mag/indexmag.html?http://www.microscopy-uk.org.uk/mag/artmar02/fleanatomy.html

(9) http://www.academia.edu/4922380/Environment_can_alter_selection_in_host-parasite_interactions

Pre-Adaptive Characteristics

Parasitic vs. Free Living Worms

A parasite is an organism that grows, feeds, and is sheltered on or in a different organism while contributing nothing to the survival of its host ¹.  In fact, a parasite is only successful if it has a high reproduction rate as well as finding a host that it can live off of for a long period of time. Phylum Nematoda and Platyhelminthes both contain parasitic and free-living species. From the pre-adaptive traits of the species in each of these Phyla, it is clear that a parasitic species is more successful. 

Free-Living Species

 Free-living species are considered to not be dependent on another organism to survive. In phylum Platyhelminthes, Class Turbellaria (flatworms) is considered to the only subdivision that contains free-living species. These species have bilateral symmetrical bodies and vary in length from as little as ¼ inch to many inches⁶.  They also do not contain a body cavity and therefore do not have their three layers separated¹⁰.  Because these species lack a respiratory and circulatory system, they need to live in moist habitats. Turbellarians can be hermaphroditic and have fertilization or reproduce asexually by fission⁶.  An example of a Turbellaria would be Pseudoceros dimidiatus. This type of flatworm has a display of distinctive color patterns. The body is always black with an orange margin that acts as a warning to predators to not eat them⁹. This can still be a disadvantage to the flatworm because it is not protected inside a host as a roundworm would be.

Pseudoceros dimidiatus¹¹
http://www.rzuser.uni-heidelberg.de/~bu6/flat0446.html

Parasitic Species

A parasitic lifestyle has a few traits that are extremely beneficial for them. These include their surface to volume ratio that allows them to have more options to a wider range of hosts, the mobility to find their hosts, the ability to use their resources efficiently, possibly being hermaphroditic, a low metabolism, a shorter generation time, a higher mutation rate, higher fecundity, and the ability to penetrate.

An example would be Phylum Platyhelminthes that contains flatworms that are acoelomate, triploblastic (three embryonic cell layers: ectoderm, mesoderm, and endoderm), and bilaterally symmetrical⁴.  These flatworms have a digestive cavity of only one opening and do not have a circulatory system. Out of the four Classes in the Phylum, Monogenea, Trematoda and Cestoda are parasitic.  

Relationships among parasitic Platyhelminthes¹²

    http://www.eplantscience.com/index/general_zoology/phylogeny_and_adaptive_radiation03.php

Both Parasitic and Free-Living Species
Phylum Nematoda contains species that can affect humans both directly and indirectly through domestic animals, and also free-living species that can be decomposers and predators on microorganisms³.  Although there are two types, both species contain similar characteristics that make them parasitic and overall more successful. Just like Phylum Platyhelminthes, the characteristics that make Nematodes special include having a triploblastic and bilateral symmetrical body that is covered with a secreted, flexible, non-living cuticle, and muscles that run longitudinally in the body wall⁵.    Nematodes also lack features such as cilia and a well-defined head⁶.  The sizes of most of the species in this phylum are under 5 cm long, while others can be microscopic⁵.  Parasitic Nematodes, on the other hand, can be as long as 1 meter. Their bodies are long and narrow and have an epidermis that is made of mass of cellular material and nuclei without separate membranes⁷.  Moreover, parasitic nematodes have more than one host to distribute themselves and finish their lifecycle.   

Caenorhabditis elegans (C. elegans) is a prime example of a free-living roundworm that primarily lives in soil where it can feed off bacteria and fungi. Advantages to this free-living nematode include its microscopic size, ease of propagation, compact genome, short life cycle, and high fecundity rate².  C. elegans, like the other nematodes, has an unsegmented, cylindrical body shape that is tapered at the end².  These roundworms contain hermaphrodites and male worms. These characteristics thus make C.elegans more successful than typical free-living species.

http://www.youtube.com/watch?v=zjqLwPgLnV0

Sources

1. http://www.thefreedictionary.com/parasite 
2. http://www.wormatlas.org/ver1/handbook/anatomyintro/anatomyintro.htm 
3.  http://animaldiversity.ummz.umich.edu/accounts/Nematoda/
4.  http://www.infusion.allconet.org/webquest/PhylumPlatyhelminthes.html
5.  http://www.reefed.edu.au/home/explorer/animals/marine_invertebrates/nematode_worms
6.  http://mdc.mo.gov/discover-nature/field-guide/turbellarians-planarians-free-living-flatworms
7. http://www.ucmp.berkeley.edu/phyla/ecdysozoa/nematoda.html 
8. http://www.youtube.com/watch?v=zjqLwPgLnV0
9.  http://oceana.org/en/explore/marine-wildlife/divided-flatworm
10.  http://www.sciences360.com/index.php/what-are-the-differences-between-flatworms-roundworms-and-segmented-worms-6865/
11. http://www.rzuser.uni-heidelberg.de/~bu6/flat0446.html
12. http://www.eplantscience.com/index/general_zoology/phylogeny_and_adaptive_radiation03.php