Gigadon: Terror of the Sea

Swordwhales - a diverse group of derived poeciliids that have radiated heavily since the end of the Cryocene, today include the largest animals in Serina's oceans. Named for their ancestry - they descend from the early swordtail fishes - and their generally large sizes, these fish actually lack the tail swords of earlier relatives. Ecologically diverse, these fishes have indeed lost most obvious resemblances to their earlier ancestors and relatives such as the swordsharks as they specialized toward new niches in the ecosystem, but the males can still be recognized by the presence of a specialized gonopodium fin which is remarkably unchanged and still used to internally fertilize the female.

Swordwhales fill a wide variety of roles in the global ocean by this time. With the extinction of birdwhales at the end of the Cryocene, no extant avians approach the larger swordwhales in size, with porporants - their closest extant analogues - having not evolved any mechanisms to allow them to grow nearly so large as a result of still laying their eggs on the land. This means that the Thermocene oceans again belong to the fish - and they have made the most of this new boon of opportunity.

The apex predator of the Thermocene sea is a pelagic swordwhale known as Gigadon oculopter. Growing to upwards of 25 feet in length and weighing more than five thousand pounds this highly streamlined fish sports a large and extraordinarily powerful jaw adapted not to extend and snare smaller prey but to grip and to tear flesh from the bodies of other, sometimes even larger fishes. The teeth inside are large and conical, strong enough to endure heavy use without breaking. The body is specialized for rapid movement, with a crescent-shaped tail fin and a very large collapsible dorsal; the anal fins of males are still specialized as a mating structure and absent in females, so the pelvic fins have moved backward to fill its role in stabilizing the fish as it swims. Highly adaptable and tolerant to a wide range of water temperatures, Gigadon is found worldwide in nearly all open and some coastal water environments.

Gigadon would be a formidable threat to anything else that shares the sea with it under any circumstance, but this giant poecillid is even more remarkable for its brain than its brawn. Gigadon has taken the large brains of its group to its current pinnacle; its brain is very large by any standard but especially among teleost fishes, with a high domed skull to accommodate it. This species has one of the highest brain to body ratios of any fish on Serina, and it is used for complex, cooperative problem solving. Gigadon, though not strictly social, has the capacity to be a very effective pack-hunter that works in cohesive units to hunt and kill prey, from much smaller fishes and birds to the very largest fish in the sea - its filter-feeding relatives. Social units when present are generally impermanent and social bonds loose, with any individuals that come across each other forming temporary alliances whenever necessary to take down large prey while hunting solo when conditions better suit it, but communication between individuals is very well-developed and facilitated by the bold contrast markings and reflective blue patches of scales on their bodies. The scales on these patches can be rapidly angled up or down to darken or lighten, while a huge eye spot on the dorsal fin can be raised or lowered at will; together these pattern shifts, as well as rapid alterations in the intensity of the body pigment performed at high speed in the chaos of an attack, function as an innate and effective language that allows these fish to coordinate their attacks to best cut off the escape of their prey. Fish their own size or smaller or killed with vigorous twisting bites that shear chunks of flesh off the still-living victim, killing with blood loss and shock, but larger fishes are rammed in the gills with the jaws closed to incapacitate them through internal injury.

Gigadons do not care for their young; few swordwhales do in the way some earlier relatives did. Pregnant mothers instead migrate toward equatorial shallows to give birth to their litters; two to three very large pups up to one-tenth the mother's weight each are born after a gestation of almost one year, and are left to fend for themselves in the shallower waters where they will be among the largest and most formidable predators present from infancy. As they grow, the young will move north and south again, to open waters, and join their elders. Despite no further attention being provided to the young after birth, Gigadon is notable for being non-cannibalistic; instinctive social signals presented by juveniles toward adults when they meet, similar to the communications that occur between cooperative hunters, serve as an appeasement signal and prevent adults from preying upon the young. Despite this friendly arrangement Gigadon of noticeably different size have different prey preferences, and cooperation occurs primarily in similarly aged peer-groups. Growth occurs for a long period, being most rapid in the first five years but continuing for up to twenty years, with adult lifespans around fifty years being commonplace. Sexual maturity occurs late, after fifteen years of age, and births are years apart.