River Scrambler
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The river scrambler split from its ancestor. This small shrew has moved to riparian habitats, where it hunts small swimming fauna. This is an unusual accomplishment for something with marsupial-like reproduction. One would expect it to have taken a route similar to the Terran water opossum, or to Sagan 4's own seashrog, where the pouch can close. However, the River Scrambler has taken a different, more novel route.
The skin of the River Scrambler's fetal joeys is highly vascularized and permeable, comparable to the skin covering the gills of the Terran axolotl. They are able to take in oxygen from the surrounding water while their mother swims. The pouch still envelops them and keeps them warm, though they are also more resilient when being chilled by exposure and do not need to be kept at a constant temperature; in fact, they can recover even if their core temperature drops to near freezing, though a drop that significant would also delay their growth considerably. The mother must still leave the water periodically, as the joeys can still drown if they are submerged for too long, due to skin-breathing being less efficient than lungs. The joeys develop webbed toes and a powerful swim stroke before they are old enough to leave the pouch, so they are able to surface for air on their own and swim to their mother to nurse. They retain their bright red highly vascularized skin even after they leave the pouch and their fur starts to grow in, allowing them to nurse underwater for longer without suffocating. The switch to less permeable skin is associated with weaning, as the ability to absorb oxygen from the water is no longer needed. Unweaned joeys must stay moist, as their skin loses moisture very quickly, but they still need to breathe air regularly, especially as they get older and their metabolism increases.
Weird babies aside, the River Scrambler lives mainly in the tropical rivers of Dixon. It can also disperse, traveling between them through the Dixon side of the Dixon-Darwin Boreal, mostly eating minikruggs and similar creatures during the trek. It will also consume eggs if it stumbles upon a nest. It swims with a combination of paddling and an up-down undulation of its body. It has marsupial-like reproduction, giving live birth to fetal larvae which suckle from teats contained in a pouch. It breeds 6 times a year and has 5-12 joeys at a time. It is generally solitary and sleeps in burrows, typically within riparian biomes. It has lost most of its sexual display structures, as they created too much drag while swimming, and it now has a fully furred, slightly flattened tail. It retains its ancestor's high mutation rate, though it has evolved a defense against the fatal "scrambling" mutation that occasionally afflicted its ancestor—that is, to avoid wasting resources, any embryo that undergoes a dramatic shift in tissue placement is reabsorbed.