The different ranks of Undine are distinguished by increasing pieces of armour; ranging from nothing at all on the lowly ensigns, up to full body armour, including optical connections from equipment right into the Undine's brain, for the captains. Intermediate ranks have the same armour components as the captains, just not the whole set.
Here's how character artist Joe Jing, Sr. described the final look in Online's development blog:
The final concept carried over many of the attributes seen in the reference. The plates look almost grown, yet high tech. They also look light, but strong. Their gauntlets include control panels very reminiscent of the monitor seen used by an Undine in the show. And, finally, we wanted to promote the idea of the Undine’s psychic abilities, so the design includes optic cables attached directly to their temples as if to enhance and/or tap into utilizing their natural abilities.And here are some of the alternate concepts that lead to the new look Undine:
We also wanted to focus the Undine’s niche as masters of Fluidic Space, which we’ve tried to characterize as being ocean-like. The Cruiser can now Subspace Dive – it can open a rift into subspace, and a few seconds later it will randomly come out of another rift. The Battleship can open a Fluidic Space rift; this rift slowly pulls you in due to its pressure gradient and leaks fluidic space into normal space. The leaked fluid slows you, debuffs your accuracy, and deals damage over time if your ship collides with it. The rift itself eventually destabilizes and explodes.You can read more about the Species 8472 redesign on Star Trek Online's development blog.
In other STO news, the game has recently launched a Mac client, so Apple users can now play the game too!