“ The Weak will perish.”

The above statement is to be treated as their ideology such as the Borg’s “ resistance is futile.”

Species 8472 is a Telepathic race. Very little is known about their culture but they are known to be a devious and malevolently cold race. Being the only species to inhabit their Universe known as Fluidic space they have no concept of diversity.

The first encounter with other life forms was with the Borg who attempted to assimilate them. Because of their incredibly efficient immune system; any form of biological or technological intrusion into their tissue is instantly rejected and destroyed. This renders them immune to Borg nanoprobes, and hence incapable of assimilation. In addition, their dense structure is highly resistant to many forms of energy such as transporters and weapons. However, species 8472 is vulnerable to the modified nanoprobes developed by the Voyager crew. Species 8472 is made of the same material as their ships.

Physically, species 8472 has a roughly humanoid head and torso on a tripedal base. They are capable of living in the vacuum of space with no life support. They can also crawl along the hull of a starship without additional support. They are also capable of regeneration.

Their combat technique is to attack with their claws whenever they can. This will transfer their cells to the victim. These cells, once in the victim's blood, will begin to consume the body from the inside out, essentially eating the victim alive. The victim will be conscious during this process because all sedatives and other treatments to the infection are neutralized immediately.

Species 8472 use organic biomass ships, which appear to be actual living life form as opposed to just organic components as evidenced by the vertebrae seen upon entering the ship. Each biomass ship has a crew complement of one in most cases.

The ships seem to be impervious to Federation (or Borg, for that matter) technology: ship sensors can't penetrate the surface of the biomass ship, transporters can't lock onto it, and the tractor beam has no affect on it. It has been demonstrated that the biomass can dissolve through the hulls of Borg ships and therefore can probably dissolve through the hull of the Voyager. Wherever Species 8472 came from, their ships use warp propulsion with anti-matter technology. Inside the ship, there are organic conduits which carry an electro- dynamic fluid. This quite possibly could be an energy source. There is also a binary matrix laced with neural peptides, which could be their idea of a computer core. The biomass ships have the ability to regenerate themselves.

Tactically the Biomass ships can form a cartwheel formation to combine their fire power which is thought to multiply into a force big enough to destroy any planet. It is thought that Based on their experience with the USS Voyager and its alliance with the Borg collective, they believe that Starfleet is an invasion force, similar to the Borg.

Additional:

Species 8472 has developed a shape shifting technique using Isomorphic injections, it is thought that they have begun to infiltrate the federation and Star fleet and this is evidence with regards the contacts within Gamma fleet. They have constructed huge Terra-spheres for the use of simulating a human environment for training purposes as discovered by the Voyager crew.
Its possible that under the right circumstances that they can be negotiated with but up until; now all the contacts have mainly been violent.

When in human form they are known to be a frail as humans also. Species 8472 don't particularly like being in humanoid form and having to do things such as sleep, bi-pedal movement, and breathing oxygen.