I don't know if any of these quite fit the brief as "unique" but I found all of them remarkable when I saw them.
1) the pairing of pistol shrimp and shrimp goby is such a hardwired partnership.
alpheus randalli shrimp and stonogobiops nematodes were bought around the same time from different suppliers in different parts of the country. I forget which was added first, but despite the fact that they were each less than the size of my pinky in a 60 gallon system, and collected from different locations and never been introduced to each other - it took less than 5 minutes of them both being in the tank for them to search each other out and pair up. I was also amazed at how the shrimp takes rocks and closes up the tunnel entrance at night. I would never have guessed that a fish would be fine spending the entire night trapped in such a tiny tunnel.
2) a yellow headed jawfish had been in my system and was still skittish. Would eat spot-fed mysis near the tunnel opening but at the time wouldn't come more than half a body length outside the tunnel for anything.
One day I was harvesting chaeto out of my sump and shook out the amphipods first, before I chunked the algae. I had the amphipods in a beaker to pour into the main tank. When a big amphipod went in and swam in the water, the jawfish shot out from the tunnel, crossed half the tank and devoured it immediately. Lightning fast - If you'd blinked you'd have missed it. That natural feeding response was shockingly different from how the fish responded to the frozen stuff he was getting every day.
3) lawnmower blennies and the toxic dinoflagellate ostreopsis have a synced up daily schedule.
The blenny grazes the most in the first hours of daylight, grazes very little throughout the middle of the day and then hits the algae hard in the afternoon a a few hours before lights-out. The dinoflagellate mostly goes into the water at night, gradually during the morning daylight hours it settles out of the water onto surfaces - macroalgae being its main attachment point in the wild. And then while it's still light, several hours before dark - it leaves the surfaces and takes to the water again. This schedule means that the dinoflagellate gets fewer of its cells eaten, and the blenny avoids ingesting so many toxic algae cells. Did the fish sync its grazing to when the algae "tastes best"? did the dinoflagellate population through selection start hitting the water early because the cells that stayed later in the afternoon got grazed by herbivores? both? just coincidence? I dunno. but it's pretty neat.
