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The Asari were the first species to arrive on the citadel. Upon the arrival of the salarians, it was the Asari's idea to form the council and to achieve peace in the galaxy. Asari have the ability to live for over 1,000 years, through these years they pass through 3 stages;

• Maiden Stage
  
• Matron Stage
  
• Matriach Stage

Each stage plays a different part in the Asari's Life;
Maiden Stage: they wander restlessly, seeking new knowledge and experience.
Matron Stage: When the matron stage begins, they meld with interesting partners to produce their offspring.
Matriach Stage: The final stage where they assume the roles of Leaders and/or Councillors

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The salarians are an amphibian race, and the second species to join the citadel, they also have a high metabolic rate which is the reason for them speaking, talking and thinking fast. Compared to the Asari's, though, Salarians are very lucky to bypass the age of 40.

The salarians are responsible for advancing the development of the Krogan Species (info below) to fight for them during the Rachni Wars. They also created the 'Genophage', the Genophage is still a major problem and allows only 1 in 1,000 Krogan to survive birth.

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Around 1,200 Years ago the Turians were asked to join the Citadel to help Galactic Peace, the Turians have the biggest military presence on the citadel and rely on The Salarians for intelligence and Asari's for diplomacy. The Turians are mostly known for their Military Battles, The first contact war which was a small three-month war between humans and Turians due to ten humans activating Relay 314 which was forbidden under "Citadel Regulations", instead of negotiating, the Turians opened fire and the war began. The citadel intervened and quickly brand peace between the two races.

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Drell are a reptile-like race who were saved from their homeworld Rakhana by the Hanar, since then; The Drell have remained loyal to them and have become and every day species on the citadel. As stated, they are reptile-like species who prefer dry, rocky conditions such as their home world; Rakhana.
The Drell current home planet is Kahje, though, they are treated as Second-class citizens to the Hanar and are gain respect by leaving their home planet; they are then labelled as "Adventurers".

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The Hanar have a resemblance to Jellyfish, they are known for their politeness and religious beliefs regarding the protheans who they refer to as "The Enkindlers", the hanars can be found all over the citadel and are very polite when speaking.

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The Elcor are a citadel species ascended from their home world Dekuuna, they usually prefer to stay on one colony and is why not many Hanar are seen across other colonies.They are huge, muscular creatures who stand on 4 legs. The Elcor came from a high-gravity world (Dakuuna) and this is why they move slowly, they are still adapting to the gravity of the citadel.

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Humans, from the planet earth (as most of you should know )are the newest species in the galaxy but are almost the rapidly expanding species so far. They solely discovered a prothean artefact on mars in 2148 and the mass relay networks soon after. Humans have The ability to live up to 150 years with the help of salarian scientists.
Eden Prime
Humans were caught off-guard by the geth on Eden Prime, Eden Prime was humans most wealthy and blooming colony. The likes if the geth prompted Commander Shepard to arrive on Eden prime and take it back. From there on in, the battle with sovereign and saren began.

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The Keepers are a bio-engineered, insect-like race found only on the citadel. They are harmless and pose no threat to any other races. When the protheans created the citadel they created the Keepers to maintain the citadel and keep it running. The Keepers were the first True species to land on the citadel (aside from the protheans). When the first Asari arrived on the citadel the Keepers were already there. They quickly helped the Asari race settle on to the Citadel.

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The Volus are another race on the citadel with their own embassy, their home planet is Irune which contains high-pressure greenhouse gases and must wear a pressure suit and mask to keep-up with the ammonia-based atmosphere. The Volus have been present on the citadel for many years but have never been asked to join the council which is a sore point for many Volus Individuals.

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A Distinctive race with 4 eyes, the Batarians home planet is Khar'shan. The Batarians decided to isolate themselves from many other species, they are often found lurking around the terminus systems as Batarian Pirates and Slaving rings further stereo-typing the 'Batarian Thug'. They have a reputation for being shrewd business man and being brutal with their 'clients', they often prefer doing business at gun-point or with weaponry around them.

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The collectors are an enigmatic race that live behind the Omega 4 Relay, a Mass relay in the same system as Omega. They possess High-technology weapons and often participate in trades within the terminus systems. These trades often involve Races for weaponry, such as; 16 Batarian Twins, 2 dozen Left handed Salarians and 2 dozen "Pure" Quarians.
The collectors are most known for their role in ME2; (Possible ME2 Spoilers Below!)
They abducted human colonists, and "digested" them to make a human reaper. The collectors were figured out to be a genetically modified version of the Protheans.

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The Geth are a race of networked artificial intelligence, they were created by the Quarians to be used as labourers and to fight with them in wars. When the geth started to question their creators the Quarians attempted to exterminate the geth resulting in a war; which the geth won.
Role in Mass Effect 1 - Spoilers!
Sovereign programmed the geth to fight for them and aid saren to take over the citadel and release the reapers from dark space.

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The Krogans are a big reptile-like race, their home world is Tuchanka. After years and years of fighting the Krogans became apart and split up into warring 'Clans' until they were discovered by the Salarians 2,000 years later. With the help of the Salarians, the Krogan fought in the Rachni Wars. After the Rachni wars, the Krogan became an ever expanding threat to the galaxy forcing the Turians to release the Genophage onto them. The Genophage causes a severe drop in births for the Krogan and ultimately, lowering the population of the Krogan Race.

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Very little is known about the Raloi species, the above image is what they are thought to look like.

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The reapers are a highly advanced machine race of synthetic starships. They "Hibernate" in dark space, often starless galaxies until given the signal every 50,000 years to come and wipe out all living life forms. The first known reaper to communicate with organic life forms was known as sovereign (From ME1) claiming that 'there is no beginning nor an end'.
Sovereign also states that the reapers created the mass relays and the citadel.

Virtual Aliens are a race of some one billion individuals who downloaded their minds on to a virtual world aboard a starship long ago.

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Vorcha are known for their aggressive behaviour, the Vorcha are a special race that hang out in the darker and dangerous locations, such as Omega. The Vorcha are often spotted in the blood pack mercenaries group because of their ability to adapt to any environment and learn quickly.

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The Yahg are known for their violent and aggressive behaviour. The Yahg were discovered by the citadel council in 2125, but they were barred from interacting with citadel space shortly after for massacring the councils delegation.

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The protheans are an extinct race which mysteriously vanished 50,000 years ago. The protheans rose from a single planet and created an immense galaxy-wide empire, not much else is known about them other than that they helped build the citadel and mass relays. But many of their artefacts have been discovered throughout the galaxy.

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The Rachni are an extinct insect-like race that threatened citadel space roughly 2,000 years ago causing the Rachni Wars, they posed a threat to nearly every race on the galaxy. They were defeated by the Krogan during the Rachni wars who were called upon by the Salarians because of their brute strength.

Ok, so I found this on the BioWare Forum, So credit goes to the guy who found this; his thread can be located [ Register or Signin to view external links. ]
There are a lot of choices to make so it's not possible to fit into a picture, the download link to the .txt file is below.
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