My discovery of the Lost River biome seemed to kickstart a chain of minor events that led to the emerging of some new creatures across the continent I lived in. A new creature that resembled a cross between a squid, a crab, and a jellyfish was the first animal to come out of hibernation and take to the ocean currents. It had a large, squishy, glowing, transparent head membrane, with 2 pairs of beady eyes on it, and a harder, transluscent head inside of the squishy membrane. Instead of tentacles, it had a variety of different-sized crab-like arms with small pincers for grabbing at the end of each one. They weren't very thick or bulky, each one being made for walking on the ocean floor or propelling the "Crabsquid" forward in open waters instead of pinching or crushing predators or prey.
However, its unique physiology held a secret form of self-defense, one whose origin I've still been unable to pinpoint exactly. It has something to do with its internal organs being able to create a shockwave around the Crabsquid's body that blasts away enemies and stuns its prey. Crabsquids are mostly defensive, and switch from walking on the ocean floor to swimming around like jellyfish when necessary. They made interesting clicking and chirping sounds to communicate, meaning they were intelligent, but they preferred to be left alone by other creatures. Another odd creature that wasn't exactly triggered by my presence, but nonetheless appeared for the first time shortly after I discovered the Lost River was a fascinating creature I dubbed the "Sea Treader", due to its enormously long legs and penchant for constantly roaming the ocean floor, travelling via Sea Treader "paths" to different areas.
There was actually such a path in the Danger Reef, and it spiraled up the large mountainous ridge that dominated the biome, leading to a natural land-bridge connecting to a massive cave in the Deep Grand Reef, where the wide open sandy plains of the tunnel were covered in long-forgotten Sea Treader tracks, indicating that they used said path frequently, most likely a migratory path. It lead to the Dunes, which were perfect conditions for a super-tall walking giant with low mobility. Its legs could move a bit, but with only three segments and only three legs, it couldn't keep any of its legs off the ground for very long due to the risk of losing its balance and collapsing. Its main method of defense was to raise up on its back legs and poke at its enemies with its front leg, which was directly connected to its face. On either side of its face were long feelers meant for sensing predators. The end of its front leg was actually a mouth that could rummage in the sand of the paths they travelled, and every once in a while the trails were barren of food, so they would make the perilous journey to the Dunes to find more food.
The journey was dangerous, with Reaper Leviathans ambushing them along the way, especially on the Danger Reef mountain path, where Reapers had an obvious advantage, being able to come back for multiple attacks on the Sea Treaders as they tried to maneuver the narrow, dangerous path, prone to falling to their deaths if caught off-balance. It was fascinating to watch them, although they moved quite slowly, meaning that watching them move from place to place was a very slow process, but periodically checking in on them was very interesting, to see how far they've gotten and how may Sea Treaders have joined the pod since the last check-in. Apparently, the fall and the end of summer were months where the Sea Treaders knew there wasn't any food on their sandy trails, so they spent the end of the summer and all of the fall in the Dunes, risking attacks by Reapers in order to have enough food to support themselves. They were quite majestic creatures, and admiring them when they weren't under attack was amazing, observing their blue and orange blotching patterns and listening to their whale-like cries as separate pods called to each other.
The Crabsquids, wanting to explore outside of the Lost River biome, would jump into the early winter currents and be wisked away by them, usually landing in one of the Grand Reef areas, but I've also seen Crabsquids in the Blood Kelp Forest, the Koosh biome, the Sparse Reef, and a couple jumping around between Underwater Islands on occasion. There wasn't a huge number of Crabsquids, so it wasn't really an infestation or invasion, but more like an introduction. Crabsquids fit into most ecosystems pretty well, feeding on scraps that other creatures didn't want and only hunting down their own prey when other food was unavailable. Usually, this meant smaller fish such as Spadefish and other bottom-dwellers, sometimes even blasting and fighting Sandsharks for their prey, and Crabsquids rarely showed up in the Grassy Plateaus, meaning their exposure to Sandsharks was rare. Other than that, Crabsquids didn't conflict very much with other life forms. Sometimes, a random Boneshark in the Underwater Islands would get brave and attack a Crabsquid, but the Boneshark would always get blasted back into its place on the food web with a quick shockwave from the Crabsquid's blast organ. Stalkers didn't really see Crabsquids very often, if ever, but when they did, they knew enough to leave them alone. Fortunately, the Crabsquid was the only Lost River resident that liked to leave the biome via swimming and riding currents, but not the only one that could. There was another, mysterious denizen of the Lost River that could leave in the blink of an eye- literally.