Serina: Snapjaw Calacarna

To reiterate, this was a sponsored commission for a crocodile-like snark found in and around Trang Island with trapjaw ant-like mandibles.

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During the hothouse era, one primary group of marine hunters dominates the warm seas of Serina. These are the long, four-flipped calacarnas, fast-moving pelagic hunters with some of the greatest intelligence yet achieved by mollusc-kind, ranging from reef-dwellers hardly larger than sardines to titans longer than a bus. Their presence is most common in the shallow seas and open ocean where vast shoals of schooling prey and great herds of large aquatic megafauna alike concentrate as suitable prey, but not all species are limited to the briny depths. A small number of calacarnas are able to tolerate less saline waters, becoming predators of murkier and more tangled regions where prey is often much different from that of the sea.

Around the southeastern coast of Serinarcta, between the mainland continent and the distant Trang Island, the branching arms of the Equinoctial Ocean cut the land into hundreds upon hundreds of islands and islets of varying size stretching for over a thousand miles into a gigantic archipelago known as the Trilliontree Islands. The region is dominated by underwater jungles of macroalgaes, seagrass meadows, and endless expanses of mangrove forests proliferating on a scale unrivalled anywhere else on Serina. One calacarna species has adapted to living in and around these islands, for it is an euryhaline organism capable of tolerate both totally freshwater and brackish conditions by retaining salt-containing urea in its body tissues to allow for periods away from the sea that can last for weeks, and obtain further salts by consuming animal matter, theoretically enabling them to theoretically live in freshwater for their entire lives, although most don't.

Snapjaw calacarnas are so named for their wide, heavily-serrated mandibles, which are set upon proportionately massive adductor muscles, so large that even at rest they noticeably bulge out from the bottom of the head. This allows for a tremendously fast and powerful bite that is useful for pulverizing the hard carapaces of large crustaceans and shelled molluscs, including armoured macebacks and scaly bottom-feeding escardines, which it coexists with. It's not quite as good for ripping away consumable portions, but by working together as a pack, the calacarnas take turns collectively pulling apart their prey into smaller pieces. The grinding of the inner teeth within the pharynx acts as an audible signal to nearby calacarnas that one has located food and an invitation for a communal feast, or perhaps to avoid being food for larger predators, such as seasaws, adolescent kraviathans, and sea dragons. The bright yellow tips of their fins allow the pack members to more easily see another in darker or muddier water, and occasional clicks of their radulal teeth when visual contact is obscured. At between eight and ten feet in length, they are formidably sized carnivores, but larger predators still haunt the waters. Young snapjaws have thick spines covering their back for defence against predators, although these shrink as they grow into adults.

When snapjaw calacarnas invade inland rivers and lakes, they move up the food chain, but have to adapt to a different diet, as the prey they're accustomed to out from the coast is much scarcer or totally absent. They modify their hunting strategy to become cunning ambush hunters, using their powerful jaws to capture terrestrial prey, either those wading through the shallows or coming down to the shore to drink. Members of a pack spread out over a stretch of river or lake to cover a wider area, changing the hues of their skin to match their surrounds and become nearly invisible; one good bite is usually enough to incapacitate their target, as a bite of such great strength causes devastating injury on impact, and the hooked serrations prevent any escape. Once grasped, the victim is quickly dragged to deeper waters to be dismembered and consumed cooperatively. Prey of many sizes are hunted, from small waterfowl-like sparrowgulls to full-grown giraffowl (although of course of deer-sized dwarf forms). They can travel over water that is quite shallow by heaving themselves along by their muscular fins; this is the same mechanism from which amphibious snarks originally evolved, although in this case it's only used for very brief transient voyages where it would be difficult to swim across without getting stranded.

Adults tend to give birth and rear young in freshwater, as there are fewer predators capable of threatening them, and those which are are much more easily fended off. Young are cared for a period lasting about a month or so, and adults will regurgitate part of meals to feed the growing brood. During hunts, at least one adult individual remains behind to guard the young collectively, and this individual cycles out with each hunt. By four or five weeks of age, the young, born only about one foot long, will have tripled in weight, and are shepherded into coastal waters before their numbers and growing appetite begin to negatively affect local prey numbers, and then left to creche together in estuarine waters. At this point, their larger bodies are better accustomed to changes in water salinity, they can hide within the root systems of mangrove trees, and they can initially feed upon smaller crabs, bivalves, and reef snails which are easy for novice hunters to catch. Adolescents begin to search out or make packs in more open waters once about two years old. Groups have no strict social hierarchy or relation, as collective actions are decided democratically, although older adults or those within the group for longer tend to have more sway. 

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(this lives 290 MYH, just in case you were wondering)