Backbones Are For Losers

Deducing the Organism

This week, we were presented with an extremely alien picture.  The organism had multiple orifices along the same plane, as well as a deep groove along its ventral plane.  I could tell it was a worm, as it looked like it had a more solid body structure than the siphonophore, which I previously thought was a worm.  I perused the textbook, taking advantage of the fact that it moves in terms of least complex to most complex, and eventually I found the trematodes of phylum Platyhelminthes.

Platyhelminthes As A Whole

Platyhelminthes is one of two major phylum containing a diverse set of worms, with the other being Annelida.  Platyhelminthes are characterized by flat bodies with no segmentation, with three developed skin layers.  Some of these organisms are free-living organisms, while others are parasitic and rely on hosts to fulfill their needs (Berkeley, n.d.).  Phylum Platyhelminthes is a very diverse phylum encompassing many types of organisms.  Everything from tapeworms in cow intestines to marine flatworms are included in Platyhelminthes, and with diverse organisms comes diverse reproduction.

Sexual Reproduction:  No Saxophone Necessary

Phylum Platyhelminthes is capable of reproducing both sexually and asexually, but as with most organisms, sexual reproduction is more beneficial to the organisms as a whole.  The most basic sexual reproduction occurs in the turbellarians, which engage in the romantic process called “penis fencing”.  All turbellarians are hermaphrodites, so when they engage in this act they attempt to impregnate one another by dueling with their penises.  Once pregnant, the eggs are concealed in a protective cocoon and released.  When they hatch, the eggs will be miniature adults, and continue their life cycle (Science Alive, n.d.).

Next in terms of complexity are the monogeneans.  Monogeneans are parasitic, and reproduce away from their hosts (Wikipedia, n.d.).  The two organisms mate similar to the turbellarians, however the eggs hatch into a highly ciliated larvae called oncomiracidium that infect hosts and continue to develop (Monogeneans, n.d.).

The next most complex group is the cestodes, or tapeworms.  The hermaphroditic cestodes exchange spermatozoa with one another via their genital pore.  From there, the gravid segments (proglottids) are passed through the host’s feces and the eggs are hopefully consumed by another organism.  This new organism becomes an intermediate host and allows the cestode to advance to its juvenile stage.  If all goes well (for the worm), a new primate host will consume the infected organism containing the juvenile worms, which will then mature in the new host and repeat this process (Wikipedia, n.d.).  For a video on cestodes and their reproduction, click this link.

Finally, the trematode peaks the Platyhelminthes sex complexity.  The female slides into the male’s genital groove where fertilization occurs, and the eggs are excreted into a freshwater source.  From there, the eggs hatch and release miracidia, which find an intermediate host, usually in the form of a mollusk.  From here, the sexual reproduction ends and it becomes asexual to complete the rest of the trematode life cycle.

Asexual Reproduction:  Attack of the Clones

For most Platyhelminths, the asexual reproduction is similar to the sexual, however the offspring will be perfect clones of the parent.  Since most Platyhelminths are hermaphrodites, they are capable of self-fertilization, albeit rarely.  Most Platyhelminths try to hybridize with another worm to keep their population diverse.  The major exception to this is the trematodes, which utilize both sexual and asexual reproduction in their lifecycle.  When in their intermediate host, the worm larvae will reproduce asexually before moving onto a final host where they complete their life cycle.

Schistosoma heamotobium:  Burrowing Their Way Into Your Heart

What differentiates the organism of the week, S. haemotobium, from the rest of the trematodes is their complex phases during their life cycles.  After the miracidia are in the mollusk’s body, they develop as sporocysts which multiply and release as cercariae into a water supply (Stanford, n.d.).  From there, chemical signals allow the cercariae to find a host and burrow into their livers where they mature into adults.  A diagram of schistosoma reproduction can be found below (IARC, 1970).

References

Cestoda. (2017, August 25). Retrieved September 22, 2017, from https://en.wikipedia.org/wiki/Cestoda

E. (n.d.). Monogeneans: Monogenea – Behavior And Reproduction. Retrieved September 22, 2017, from http://animals.jrank.org/pages/1521/Monogeneans-Monogenea-BEHAVIOR-REPRODUCTION.html

IARC Working Group on the Evaluation of Carcinogenic Risk to Humans. (1970, January 01). SCHISTOSOMA HAEMATOBIUM. Retrieved September 22, 2017, from https://www.ncbi.nlm.nih.gov/books/NBK304343/

Lie, L. I. (1970, January 01). Platyhelminthes. Retrieved September 22, 2017, from http://sciencealive-flatworms.blogspot.com/2011/09/class-turbellaria_28.html

Monogenea. (2017, September 04). Retrieved September 22, 2017, from https://en.wikipedia.org/wiki/Monogenea

(n.d.). Retrieved September 22, 2017, from http://www.ucmp.berkeley.edu/platyhelminthes/platyhelminthes.html

Schistosoma haematobium (blood flukes). (n.d.). Retrieved September 22, 2017, from https://web.stanford.edu/class/humbio103/ParaSites2004/Schisto/website.html

Trematode Infection. (2017, September 19). Retrieved September 22, 2017, from http://emedicine.medscape.com/article/230112-overview

Deducing the Organism

After being shown the initial picture of the organism, my first thought was that it was a feathery worm.  After finding the section on phylum ctenophora in the textbook, my thoughts then went there.  The flat shape of the organism matched the picture of a cestid ctenophore.  It wasn’t until I saw the picture of a cell with a hook, the tell-tale sign of my personal favorite animal phylum: cnidaria.  The nematocyst, or cnidocyte, is unique to cnidarians, acting as a harpoon that spears prey and injects it with venom, immobilizing the prey and allowing the cnidarian who ensnared the prey to digest it.  The diagram below shows a diagram of the anatomy of a nematocyst, including the barbed spear and capsule that conceals the weapon.  Cnidarians are relatively primitive organisms, lacking brains or neural ganglia.  They were the first organisms to have nerves, muscles, or mouths, making cnidarians the pioneers in ancient life.

Cnidarian Reproduction:  Corals and Jellies

Cnidarians are capable of sexual and asexual reproduction.  In the case of corals, they can do both.  For sexual reproduction, corals can produce gametes and release them in an event synchronized with all the other corals on a reef.  All the corals will at one time release all their gametes, which are rich in lipids causing them to float to the surface where they fertilize with other gametes.  Most corals are sexual hermaphrodites, so they produce both male and female gametes.  The gametes fertilize and develop in the form of small plankton called planulae.  The planulae will eventually settle into the ocean substrate and develop into polyps.  Once the polyps settle, they will begin dividing, differentiating, and eventually becoming colonial corals.  After this point, the coral’s skeleton can be snapped off at certain points and planted to grow a new colony  (NOAA, n.d.).  Two gametes will eventually become an entire colony of organisms acting as one, and this is true for almost all cnidarians.  The image above shows a stony brain coral releasing gametes during a mass spawning event.

Jellies reproduce in a manner similar to corals, but with more free swimming.  A mature jelly, or medusa, can release sperm and eggs into the water.  These will fertilize and become a free swimming planula like with coral.  The planula will settle, but instead of becoming a coral, it will root and bud into multiple organisms.  These organisms are called ephyra, and they will mature into the medusas  (How Do Jellyfish Reproduce?, n.d.).  For more information on jelly reproduction, follow this link.

Siphonophores:  Cnidarians With No Strings Attached

A siphonophore colony begins with a single egg.  This egg becomes a protozooid, which develops and buds the systems that will eventually become the differentiated parts of the siphonophore colony.  From here, the three major body plans of the siphonophore order develop fairly similarly.  In physonects, the protozooid elongates and thins, and one end designates for locomotion, and the other for reproduction and feeding.  Not much is known about the life cycle of the cystonects or the calcyphorans, but is known that calcyphorans shed parts of the colony that live free from the original organism.  These free-swimming eudoxids are very mysterious and it is not known how long they survive independent of the main colony.  These fragments can reproduce medusa, but not an entirely new colony  (Siphonophores, n.d.).  For an image of the three body plans of siphonophores, click here.  For a diagram of the complete life cycle of a calcyphoran siphonophore, click here.

Revised October 9, 2017

References

Cnidocyte. (2017, August 24). Retrieved September 08, 2017, from https://en.wikipedia.org/wiki/Cnidocyte

How do jellyfish reproduce? What effect does their sting have on humans? What’s the difference between red and translucent jellyfish? (n.d.). Retrieved September 08, 2017, from https://www.scientificamerican.com/article/how-do-jellyfish-reproduc/

Siphonophores. (n.d.). Retrieved September 08, 2017, from http://www.siphonophores.org/SiphLifeCycle.php

US Department of Commerce, National Oceanic and Atmospheric Administration. (n.d.). NOAA’s National Ocean Service. Retrieved September 08, 2017, from https://oceanservice.noaa.gov/education/kits/corals/coral06_reproduhttps://

oceanservice.noaa.gov/education/kits/corals/coral06_reproduction.htmlction.html

https://commons.wikimedia.org/wiki/File:Pseudodiploria_strigosa,_desovando.jpg