Protozoa and Parazoa

Protozoa & Parazoa (and other tidbits)

Most phyla you see today have humble origins residing in a mere 70 to 80 million year gap known as the Cambrian Explosion. This event that occurred approximately 550 million years ago and was a time of rapid evolution, providing the earth with diversity in the form of perhaps over 100 phyla. Today we compartmentalise the extant multicellular animals into 32 phyla. 

Protozoa

This unicellular group of organisms that can be found in the Protista kingdom are said to give rise to all animalia. How they came to be is still riddled with uncertainty, but what is clear is that a form of endosymbiosis occurred between large bacteria and aerobic bacteria with the required enzymes for aerobic respiration. The bactera engulfed are now known as mitochondria, with most of their genetic material in the nucleus of the cell. 

Margulis, L., 1981. Symbiosis in cell evolution: Life and its environment on the early earth. , p.419.

Animal-like Characteristics of Protozoa

• Lack a cell wall
• Have at least one motile stage in the life cycle
• Ingest food

Originally Protozoa was its own phylum, but after analysis using the array of biological techniques to determine genetic classification, it is now considered to have at least 7 phyla (some say up to 30). It's important to mention that Protozoa is an informal paraphyletic taxon, not to be mistaken as a monophyletic group.

Cilia and Flagella (undulipodia) + Pseudopodium

Motility is a key characteristic of Animalia, it allows the organism to find food sources, reduce the boundary layer effect,  escape prey and other undesirable conditions. Ciliary movement is not just for movement, but also for feeding and respiration. It's distinction from flagella is simply defined by the direction water is exerted although cilia more often than not occur in groups covering a large surface. 

Their structure and movement is explained best with the sliding microtubule hypothesis 

http://www.ncbi.nlm.nih.gov/books/NBK21698/ - Cilia and Flagella: Structure and Movement

Isokont - two flagella w/ equal length

Heterokont - two flagella w/ unequal length

Amoeba and Sarcodina have an alternative form of locomotion in the form of pseudopodia. These are hyphae-looking bulges in the cytoplasm. The specifics of how this actually propels the organism remains to be on the fringes of scientific discovery.

At the apical end of the pseudopod exists a hyaline cap with endoplasm containing actin sub-units flowing towards it. As it flows into the cap, it fountains out to the periphery. This subsequently interacts with lipids in the cell membrane releasing actin subunits to polymerase into filaments.

Newell, P. C. (1986), The role of actin polymerization in Amoebal Chemotaxis. Bioessays, 5: 208–211. doi: 10.1002/bies.950050505 

Amoeba's star appearance on Star Trek

As a single-celled organism suspended in water, osmoregulation is key to a long and fruitful life. Without the vital organelles to deal with hypertonic conditions (where solute concentration is higher in the external solution) or hypotonic conditions (vice versa). Contractile vacuoles concern themselves with this inconvenience. It's purpose is to expel excess water from the cell, altering the solute concentration and thus osmotic pressure of the cell.


Protozoans are often parasitic, one of the most infamous being the Plasmodium genus, guilty of 655,000 human deaths in 2010. Of course I am talking about malaria, with the most effective Plasmodium being falciparum which uses Anopheles gambiae (Mosquito species) found in sub-saharan Africa.


C.J Thomas & S.W Lindsay (2000),, Local-scale variation in malaria infection amongst rural Gambian children estimated by satellite remote sensing, Transactions of the Royal Society of Tropical Medicine and Hygiene Volume 94, Issue 2, Pages 159-163


(C.J Thomas being my tutor)


What is really fascinating about protozoa, is the aggregation and differentiation that can occur resembling early signs of a metazoan body plan. With all of taxonomy the organisms we separate into groups are never capsuled by clear bold lines. This conveniently leads me to Porifera, the first and simplest phylum of the Animal Kingdom. 

Porifera 

Porifera (otherwise known as sponges) have an asymmetrical shape, something exclusive to them and a few adult flatfish. This means their shape is indeterminate which is not a problem for the sponge because it leads one of the most mundane lifestyles you can find from animals. They also popped up before the cambrian explosion.
Their morphology is termed diploblastic although their layers are not homologous to higher metazoans. It comprises of an endoderm, ectoderm with a layer of mesohyl in the middle (confusing i know but it doesn't count as a layer of itself)
   They filter feed with the use of choanocytes found all over their body (fun fact: possibly origin from choanoflagellate protozoa) These are also known as collar cells and each have a swirling flagella that generates a current of water carrying food particles into the frilly net that they each have. I suppose in that sense the process is far from passive, because as they are sessile without creating small eddies and what not the boundary layer effect would take hold and starve the sponge. The water that has entered the sponge is released via the osculum (the equivalent of a primordial arse). This is found at the top of the animal because no animal wants to sit in their own crap.


Despite having a potential infinite designs their are a few distinguishable characteristics differentiating one species of sponge to another. The basic levels of complexity are as follows:


  

                                  Far left: Asconoid, Centre: Sycanoid, Far Right: Leuconoid


Then you get forms such as the Poterion, Callyspongia, Leucosolenia and more.




As you'd expect, these animals don't have skeletons, instead they have spicules and spongin,  two structures dedicated to giving sponges strength in form. If this fails and the sponge is somehow broken into parts, they can bind together again or start anew. This interestingly enough does not occur if the sponge is mixed with another species' tissue. 


Reproduction


There are two over simplified types of reproduction; Sexual and Asexual production. I say over simplified because there is an array of ways to procreate asexually for various species. Porifera can reproduce in both ways, commonly using budding  (internal and external) or gemmules (commonly found in freshwater sponges). Gemmules are the collection of totipotent archaeocytes surrounded in a mesh of spongin and spicules created in the mesohyl. 
These gemmules and internal buds do not germinate as long as they are held inside the 'mother'. 


As sexual reproduction is concerned, sperm is released by the sponge and taken into the canal system of another. Fertilisation occurs within and a parenchymula is released and floats freely in water until settling on hard ground.


Usher, K.M., Sutton, D.C., Toze, S., Kuo, J., and Fromont, J. (2004). Sexual reproduction in Chondrilla australiensis (Porifera : Demospongiae). Marine and Freshwater Research 55, 123–134 




This blog has Integrated Principles of Zoology by Hickman, Roberts & Larson to thank for much of the information.