The great Perithalassic Ocean has reached a level of productivity far exceeding the newborn ecosystem of the Muricene. Shallow areas like Perinesia support undersea prairies of plumekelp and other macroalgae, while single-celled surfdust provides a food source on the open sea. The wide mouth of the Medithalassic provides a continuous influx of organic material, spitting out large rafts of rotting plant matter for Perithalassic creatures to feast on. This inter-oceanic circulating current is also a highway of biodiversity, allowing species that originated in the balmy interior ocean to expand their range into the rest of the world. Food is abundant enough to support massive schools of nektonic organisms. These hail from many different taxa and occupy a slew of niches, but they've all independently evolved to favor gregarious behavior and rely on the rule of Safety in Numbers.
The stoutling family (Iridichthyidae) is the most common of these groups; their powerfully-built bodies make them formidable within their size class, while the seclusion of their early lives affords them a degree of safety that isn't available to other shoaling fish. The Brook Stoutling (Dimorphichthys) is one genus that maintains the lifestyle of its earliest stoutling ancestors, with semelparous females and long-lived males. Sexual dimorphism is high: males are more than double the length of their 40-centimeter-long mates, and the former has a much stockier build. Most of the time, the sexes travel together in mixed schools, which may also contain many smaller juvenile individuals. Because they're now able to rely on the protection of the powerful males, young Dimorphichthys make the journey out to sea much earlier than previous stoutling species. This instinct is promoted by adult females, who prefer to mate with the most protective males.
The great red stoutling, once the biggest fish in the world, lost that title when the Ice Age wiped out most of its breeding grounds. Though it narrowly escaped extinction, its smaller descendants would never again occupy an apex predator niche in the Perithalassic. The Lesser Red Stoutling (Rubrichthys gregaricus) forms massive schools far from the shore. Though it doesn't reach sizes much larger than its brook-spawning cousin, both sexes of this species reach the same length and mass. During the non-breeding season, it can be quite difficult to distinguish between male and female red stoutlings. Young are born in tidal streams and estuaries not far from the open sea, exposing them to more predators but shortening their journey - and, more importantly, the journey that females must take. By expending less energy to reach these coastal nurseries, mother red stoutlings can survive to breed many years in a row, often several times a year, more than making up for the increased predation rate.
The most specialized of all stoutlings are called Seastouts. Belonging to the genus Bicolorichthys, the seastouts have a range of forms adapted for pursuing different oceanic food sources. They too descend from the great red stoutling, but they belong to a different offshoot from the medithalassic and lesser red species. Faced with a decline in habitable lakes and calm, slow-moving rivers, their ancestors evolved to breed in the ocean, abandoning the safety of freshwater for the stability of the sea. Many species diverged as the Arthrocene progressed, from algivorous grazers to tideslope-dwelling generalists. The largest seastout can be found here in the Perithalassic. A dedicated epipelagic hunter, the Social Seastout (B. thunnoides) is a speedy predator of the open sea. Like Rubrichthys, this genus is smaller than its predecessor, but at 3.5 meters long, the social seastout is the largest extant stoutling. It can achieve brief bursts of speed exceeding 50 kilometers per hour, though it cruises at a leisurely 5-7. Its main diet is smaller forage fish like brook stoutlings, Pelagarthrids, and other small schooling creatures, rarely going for prey over two kilograms in weight.
The grumblefish are another vast clade with many oceangoing representatives. Two in particular are prone to shoaling, and thus have achieved success across the Perithalassic. The first of these has maintained its prosocial nature for millions of years, descending from a Late Muricene species called the Social Grumblegulper (Extendopharynx gregaricus). This coastal scavenger was the smallest of its genus, measuring about 35 centimeters long, but survived the Ice Age in far greater numbers than other grumblegulpers. As pelagic ecosystems became more productive, this lineage got bigger, including in its descendants a new species called the Grumblegrouper (Pharyngeojactus haustus). Mature grumblegroupers are four-meter-long giants, and their robust bodies can weight well over 300 kilograms. They eat anything that can fit in their mouths, which in practice means nearly anything else in the sea. Even their own offspring aren't safe, so young grumblegroupers often form all-juvenile schools for protection.
At the opposite end of the size scale is the Shoaling Grumbletoad (Acutipteryx perinesiensis), a type of shivering grumbletoad. Maxing out at just half a kilogram and 20 centimeters long, this fish spends most of its time in mid-sized groups of 100-1000, patrolling the shallows of Perinesia. Its pectoral fins each bear four long, blunt-tipped rays, which it uses for intraspecific communication. When no predators are nearby, the grumbletoads continuously touch one another with gentle taps, signaling that the area is safe. If any individual senses danger, a quick jab is all it takes to send the group fleeing into the plumekelp to hide. As they swim, each grumbletoad tries its best to remain in physical contact with another, and they keep tabs on each other even when hiding. Acutipteryx can thus orient themselves and ensure they remain within the security of the school, regardless of whether or not they can see each other.
Once every few years - most often following an unseasonably productive winter - these small groupings coalesce, with their numbers swelling into the millions. With abundant food all around them, the masses of fish abandon shallow water and venture into the vast meadows of plumekelp. The algae is slow-growing and can only support this many herbivores for a brief time, so the school quickly moves on, joining any others it meets as it travels. In less than a month, the entire adult population of shoaling grumbletoads swims together, crossing tens of kilometers daily. Over two billion of them feast on algae and small animals until food runs out, at which point they disperse to new territories. For this brief window - about two months, on average - the grumbletoads provide a feast for larger fish, seawis, and scavenging Caenarthrids that inevitably come to process the bodies of those that sink to the sea floor.
The last of the marine jaguarfish can also be found in this environment, representing the general body plan that gave rise to all of today's estuarine and inland jaguarfish species. The Dwarf Jaguarfish (Postabeliichthys pelagicus) is another small Perinesian fish, serving as a more stable diet for oarbirds and other predators. It spends most of its time in the middle-depths, creeping through the plumekelp and eating whatever it finds. Though it's the smallest member of its subfamily, the dwarf jaguarfish still packs a punch and can easily tear apart an unwary grumbletoad in seconds. It also eats vegetation and detritus, especially when young. Schools are relatively small, at most 100 strong, and typically well-defended, with dozens of powerful jaws ready to tear into any predator that threatens one of their number. However, during the late-winter mating season, males turn on one another, fighting viciously for breeding rights. It is during this time of year - a time when food is at its least abundant - that larger predators find success hunting jaguarfish injured from these fights.
The smallest shoaling animal anywhere on Apterra can only be found far from shore, spending its days in the bathypelagic layer of the open ocean. The Glerring (Lumenichthys), a distant descendant of the Early Muricene Gambusia poliops, has a maximum length of just four centimeters. Throughout the day, it feeds on "marine snow" in the form of rotting debris that gradually drifts down from the surface layers. In its twilight zone habitat, it can just make out the faint glow of sunlight above, and when night falls the glerring population begins a synchronized migration. The fish make their way to the surface, relying on the cover of night to keep them safe from predators. Even still, a sizeable fraction fall victim to larger hunters before day breaks. By delivering themselves to the surface, glerrings stop carbon from being forever sequestered on the seabed, returning biomass to the more productive sunlit zone.
Of course, the depths have dangers of their own, and glerrings have another adaptation to keep alive during the day. Each fish has a row of seven glowing spots on either side of its body, powered by colonies of bioluminescent bacteria. Like the spines of the shoaling grumbletoads, these dots are a means of intraspecific communication, helping the school remain cohesive as it flees danger. The light is also a deterrent in and of itself, dazzling and confusing other fish so the glerrings can make their escape. Invertebrate carnivores, on the other hand, are not so easily fooled.
The Pelagarthrid woodlouse Arthropinna persecutor is not the largest deep-water predator, nor does it possess the most dangerous weaponry of any sea creature. Averaging twenty centimeters long, it has its fair share of threats to deal with. Despite this, Arthropinna has one key advantage in hunting glerrings and other small nekton. Relying on scent and vibrations instead of sight, it is unfazed by the flashes of light, camouflage, and mesmerizing movements that shoaling animals so often rely on, and which are so effective at deceiving the vertebrate eye. Glerrings and tailtube worms make up most of Arthropinna's diet. It follows its targets during their diel vertical migration, so they are never truly safe from its claws. Large packs of 20-50 sniff out dense groups of prey, herding them into a baitball. Few manage to escape, and the meal is shared equally between all pack members. The Arthropinna themselves are mostly food for solitary predators, as their schools are rarely numerous enough to fill the bellies of, say, a pod of oarbirds.