290 million years PE, the mollyminnows, a group of primitive poeciliid fishes distantly descended from mollies, are the dominant teleost group of Serina (if tribbetheres, by any metric the most derived of all Serina's fish, are excluded.) The euryhaline ancestors of these fish survived the Mid-Ultimocene ice age in productive oceanic habitats during the ocean age, and then moved inland along glacial rivers in the great thaw when most marine life succumbed to the collapse of the marine food chain. Like occurred in the Thermocene-Pangeacene boundary before them, these combined freshwater and saltwater extinction events so close to one another killed off the majority of fish species, especially large and specialized forms, leaving mostly small and primitive survivor lineages. For the first time since Serina's beginning, the early hothouse was a time where only live-bearing fish existed, for all egg-laying forms - which have evolved again from livebearers independently many times over history - did not survive. Though they are again present in the world today, they are rare, and those fish which skip the egg stage and produce living offspring are still the vast majority. Of them, the mollyminnows are roughly 85% of living fish excluding tribbets. Eelsnakes comprise around 10%; a few relict survivor groups round out the last 5%. Of those 85% of hothouse fish that are mollyminnows, around half are bannerfish.
Bannerfishes are a diverse group of poeciliids which diverged 270 MPE, which are distinctive for their specialized anal fins that branch and form all manner of flags, hooks, lures, decoys, and pouches. The ancestral form taken by a male bannerfish's anal fin was a gonopodium, a hooked, tube-shaped fin utilized to transfer sperm from male to female and facilitate internal fertilization, an ancient and successful strategy of the livebearer fishes. But unlike other mollyminnows, the ancestors of bannerfish had branched gonopodiums, with two small spurs on either side of the fin supporting additional rays and fin membranes. This produced a small display structure, which males could fold up when swimming but spread when trying to court a female. It appeared to be a modest modification to the fin, but that branching structure would prove extremely versatile, for the gonopodium could now be multipurpose. In specializing into a mating tool, the fin was less useful for balancing the fish as it swam in the water; now the gonopodium could be pulled up out of the way when not mating, with the secondary set of small fins spreading and taking the role the ancestral fin originally would have held, making them more efficient swimmers, especially at larger sizes. Over the last 20 million years, bannerfishes diversified enormously as they spread worldwide in freshwater and then back into the sea, with males of every species developing its fin into a new role. Ornaments became dramatic and unmistakable; banners trailed behind males as they swam, increasing their size and making them the center of female attention. Some became so massive that they produced significant drag; they slowly acquired fish-like shapes themselves as natural selection shaped them to have the most hydrodynamic forms, so that they now resembled other fish swimming behind the male, attached to his body by thin wiry fin rays. From here, some fish exapted their sexually-selected traits into more practical ones; decoys that predators could mistake for the real fish, letting it escape, and oppositely into lures, tricking smaller fish into coming close so the real fish could eat them.
But all this selection ongoing upon the fins of male bannerfishes had effects on females too. Hormones of either sex determined the specific specializations the anal fin would undergo; males would get their elaborate fin structures, but females would also inherit related traits useful for the male, but not immediately useful to them. In order for the male's fin to get big and branches, the female also acquired a branching fin and larger membranes. For some bannerfish females, the large, multi-faceted anal fins, with their multiple fin membranes, found another unintended use in providing shelter for their offspring, which could hide beneath the fins and so seek shelter while staying close to the parent. Bannerfish evolved to protect their young by hovering over them, keeping them close; from a metabolic perspective, doing this was more efficient than remaining pregnant for longer, for her babies would feed themselves - she only provided protection, not direct nutrition. But over time, some female bannerfishes' fins began to specialize toward a childcare function. The paired fins branching off the central fin - flags and other decorations in the male - now folded up against the flank, forming a secure tube-like hideout for her fry to live in for their first vulnerable weeks and months of life. In some species, the fins would eventually fuse into the body wall, becoming a pouch with only a small valve-like entrance. Such species would now have to feed their young, and did so by chewing food and spitting particles out their gills toward the pouch, where the fry would emerge to snatch tidbits and morsels and then take them back into their hiding place. But they would also reduce predation on their offspring dramatically, and by sealing their fins into a brood pouch, they could leave the shelter of vegetated shallows and grow larger, traveling in open water and carrying their small, fragile young with them until they were eventually big enough to survive in the open on their own.
All living bannerfishes today exhibit some sort of modification of their anal fins, but the different roles exapted by different species are often working against each other. The showy fins of a male, for example, are operating under very different pressures than the pouch of a female, and so the two traits are in conflict; as both male and female evolve ever more specialized fins, the two different, male or female fin structures can no longer exist in a single species without resulting in both sexes having non-functional fins of a useless, intermediate shape. So now, in the late hothouse, this has led to many species going all-in on one sort of fin or another; primitive species with strong sexual dimorphism still exist, but the most extremely modified species have often had either the male or the female fin structure become dominant in both sexes, depending on that species' most pressing needs. Polygamous fish which could get by with less dedicated childcare remained dimorphic, with decorated males courting plainer females. Males of fish which put more energy into raising their young were likely to lose their ornamented fins so that females could develop more effective brood pouches. In situations where specialized male and female traits were equally needed, some bannerfishes evolved into hermaphrodites; by combining both sexes into each individual, the shape of their fins and their behavior found a middle ground between the two extremes.
Bannerfishes are so varied and numerous, with some 18,000 living species, that no single diorama could ever depict more than a handful of their species. Nonetheless, 16 species from around the hothouse world are illustrated here, covering between them the range of form and function that their specialized fins can exhibit. Each illustration depicts where most applicable either a male, a female, both, or - in one case - the single hermaphrodite sex of a species which lacks differentiated males or females, and the sex of each fish shown is labeled with a symbol.
~~~
1. Threespot mudmolly, male. The most primitive bannerfish genus, 2-4" long mudmollies typically dwell in coastal, brackish estuaries like are found the southern saltswamp and trilliontree islands. They feed on algae and small invertebrates and can tolerate months at a time in both marine and freshwater. Males are colorful and slightly decorated, while females are plain; mothers attend their offspring for several weeks after birth, protecting them from predators in areas of thick vegetation and leading them around much like a mother duck and her ducklings. Such a level of care is common among mollyminnows in general.
2. Blue flagdancer, male. A member of a large family of around 400 species endemic to freshwater, especially the Polar Basin and Centralian Sea, flagdancers are relatively underived mollyminnows and yet are intelligent, socially complex fish which usually make their homes in rocky benthic areas in the sunlit shallows of large lakes. This 8 inch long species dwells in the Centralian Sea, feeding on smaller fish, invertebrates and molluscs it finds by prying its narrow jaws into rocky crevices. Males have two red flag-shaped fins on their gonopodiums and wave them around to attract a mate as well as to proclaim territorial ownership over a rock patch and drive off other males; they are solitary and fight often over territories. Females are very different, yellow-brown and unobtrusive. They live cooperatively, rearing creches of many young and allying together to protect their offspring from predators, which they will mob and bite just as aggressively as their mates when needed.
3. Bumpy lumphead, male. A very large bannerfish native to the polar basin, the male bumpy lumphead can reach a length of 5 feet and weigh 150 lbs. An herbivore which eats abundant vegetation, the male of this fish has hook-like spurs on its gonopodium which it uses to cling to even larger females up to half again its size for days at a time, monopolizing them during their mating season and physically preventing other males from being able to mate until the female is no longer receptive and the male then detaches. The lump on the males' head is a solid bony protrusion of his skull, and is used to bludgeon other males that try to push him off of the female so that they can mate with her instead. Females lack it entirely, and are generally far less vibrant in color, often a dull brown.
4. Convict flagfin, male and female. Named for the vertical barring of both sexes, reminiscent of old prisoner uniforms. An insect-eating fish with an upturned mouth, it feeds at the surface and is around 5 inches long. These are native to freshwater in equatorial areas, always in highly vegetated, slow-moving streams and ponds. One of the earliest genera of highly dimorphic bannerfish to evolve and ancestral to many others, convict flagfins retain very different males and females. The former have a large paired set of flag-like fins on their gonopodiums which when spread make the male appear far larger. Females, much less colorful, have upward folded anal fins which shelter small litters of offspring which leave to forage independently and return when threatened. The large fins of the male are rectangular and produce a lot of drag in the water, but this species is not highly encumbered for they are slow anyway, hide rather than run from danger, and live in areas without current to catch their fins and tear them.
5. Badger-head umbrellafish, female. This large 3-4 foot long herbivore loves to dig in the dirt like its mammal namesake, foraging in muddy sediment in murky freshwater, especially in the polar basin, using a downturned snout to snuffle through murk and mud for edible scraps of food. A slow-moving bottomfeeder in adulthood, they begin life much smaller and less specialized, with fry sticking close to their large mothers under the protection of her large, splayed fins that shelter broods of up to 900 young like an umbrella, keeping away most enemies. The fry dart forward to collect planktonic prey and scraps tossed into the water column by their mother as she feeds, rushing back into her shadow the moment any danger threatens. She will care for her young for up to 6 months, by which time the survivors - a small number of the total born, but more than would survive with no care - will be almost a foot in length.
6. Disappearing dartfish, male, a 3-4" long dweller of sunlit tropical reefs in calm ocean regions. Males of this species, which dwells in rocky areas, can extend two vaguely fish-shaped banners from their anal fins when courting a mate. The banners can be retracted almost to invisibility below the body, only to be flashed out in an instant when needed. This fish is named for its ability to hide its flashy ornaments in a moments notice and rush to safety in a rock hole when anything bigger and hungrier comes cruising along the reef. Females guard their young in their rocky hiding places, and have no need for a brood pouch; their anal fin is unspecialized.
7. Lazy lurkfish, female, a 12" long bottom-dwelling marine ambush predator of coastal marine habitats. These fish hunt other fish, waiting patiently while hidden against the background in murky, turbulent waters for smaller fish to com within range of their toothy, upturned jaws which can extend upward to engulf prey in the blink of an eye. Females protect their young within their upturned fins, forming two pouches, one on either side of their body, and the young emerge when the coast is clear to enter the female's gills to bite chunks from the carcass as she chews her food. Male closely resembles female but lacks pouch; his anal fin is a simple gonopodium with two small, vestigial flag-shaped fins along its length.
8. Winged skipper, male and female. A marvelous semi-terrestrial fish of up to 5" length which makes its home on the mudflats of the Trilliontree Islands, the winged skipper is named for the paired "wings" which grow from the male's gonopodium. Bright red on their underside, he flashes them to tell other males to get lost, while females are drawn to the vibrant color and rapid movements. Like the convict flagfin, this species remains very dimorphic; while males fins are decorative, females have a fully sealed brood pouch which is filled with water so that her small liter of 6-10 young can be kept hydrated as she leaves the water to catch shore-dwelling insects. Like much earlier and unrelated mudwickets, muscular pectoral fins and sideways flicks of their tails let these fish crawl above the water line to find food. They live in monogamous pairs, digging burrows in the mud which reach a submerged chamber that they retreat to periodically to refill water-filled chambers in their gills which let them breathe atmospheric oxygen as long they remain damp.
9. Pelagic purseperch, female. A shoaling open ocean fish of up to 3 feet in length. These plankton-eating fish which strain food through their gills are very abundant, living in shoals of many millions in the open water, and are one of only a small number of fish which are today adapted to live in this way in the open sea, otherwise dominated by snarks. Females carry their young in a sealed brood pouch fused to their sides until they can fend for themselves, and this lets them compete with snarks and avoid heavy predation in their most vulnerable early ages. Males are identical to females externally but lack brood pouches; females have evolved a fleshy adipose fin on the underside of the rod-shaped anal fin to stabilize the fish as it swims at high speed, which compensates for the real fin membranes being so modified and connected to the body wall to hold in their young.
10. Papillon puppeteer, male. Among the most incredible of all bannerfish are the puppeteers, 10" long, long-snouted grazers of algae whose paired flags are relatively enormous and shaped similarly to the fish itself. Male pappilon puppeteers, endemic to the Meridian Seamount, get their name from the French word for butterfly, which their flags resemble as they "flutter" through the water behind their owner, flapping together and coming apart in the current. Males use these fins to attract mates in lek-like systems; schools of males travel across the seagrass meadows of the seamount to impress drab females hiding in the foliage. As hundreds of males in the shoal extend their fins and the flock of "butterflies" takes flight, the sight is extraordinary and overwhelming to predators, making the school appear to contain many more fish than it really does. The females are discerning, making their choice of just one male in the group, which she leads away to a private place to mate. The female provides only very brief protection to her young, which scatter into the seagrass within a few hours of births; there has been little selection for more maternal care, for they can hide here from most enemies - a privilege not available to species of more open waters without such hiding places.
Puppeteers have unusual fin arrangements even outside their fish-shaped anal fins; as this fin is so unusually shaped as to be useless to actually stabilize the fish as it swims, the tail has fused with the dorsal fin, and the lower lobe of the tail migrated down to take the role the anal fin formerly held in keeping the fish upright as it swims.
11. Tricksterfish, male and female alike. A 4" long saltwater "minnow" common to near-shore waters, these minnows live in large shoals for protection and eat mostly phytoplankton. When targeted individually by a predator, both the male and the female tricksterfish can release paired minnow-shaped flags attached to its rod-shaped anal fin by a thin cord of tissue. The fins wiggle violently and flush bright colors, distracting the fish's enemy so that it is likely to snap at the fin and sever it instead of biting the fish's body, letting it escape to live another day. These decoy fins quickly regenerate, to save the day again at a later time. As both sexes have decoy fins, there is no brood pouch and no parental care; the range of the adult is limited to coastal seas as the young are washed closer to shore to develop in sheltered rocky alcoves and seagrass meadows before joining the adults in open water when mature.
12. Bewildering twinfish, male and female alike. A 12-16" long shoaling reef fish in the puppeteer family. These fish feed on small invertebrates and soft algae, picking it from between rocks in sunlit coastal oceans. Their paired fin flags are among the most incredible of any bannerfish, up to half their total body size each and with patterns closely mirroring the vibrant colors of the body. Both sexes exhibit them, and their role is to confuse predators, making schools of small numbers of fish appear three times as populous. It is hard for enemies to single out a real fish to bite, for two out of three times they will hit only a fin, which can be severed and regenerate later as in the related tricksterfish. Both sexes exhibit these fins, which have acquired a practical survival function from their origins as a sexual signal. The splitting of the tail fin is complete in this species, which is more derived than the papillon puppeteer, and the lower half of the former tail now replaces the original anal fin.
13. Luring lurkfish, male and female alike. A 10" long marine predator of northern coastal oceans. Related to other lurkfishes, they intriguingly have settled onto the opposite spectrum of fin function from most of their relatives, with both sexes retaining flag-like structures emerging paired from their anal fins and females lacking a brood pouch. The fins are now used to lure in smaller fish toward the mouth, resembling injured minnows struggling in the water - an irresistible stimulus for a hungry fish. When prey comes close, the lurkfish devours it. Parental care still exists, with a burrow now substituting for the lost pouch of the female. Both sexes live together, and each will fend off enemies and provide chewed up food to the offspring for up to four months.
14. Spectral spiderfish, male and female alike, up to 24". An unusual benthic specialist of the polar basin, adapted to life in total darkness in deep, murky waters in summer and in surface waters during the polar winter. These mollyminnows have entirely lost their external eyes and get around through a combination of electrosensory input, echolocation with clicks of their jaws, and tactile information received through two feeler-like extensions of the anal fin, which long ago supported display structures now lost to time. They are intelligent, social carnivores that hunt other fish. The spiderfish can coordinate hunts in lightless environments by producing weak electrical signals to tell others their intentions when surrounding prey, and bounces clicking vocalizations off of the rocky substrate to triangulate exactly where its food is hiding. Ancestors and relatives often used their feeler-like fins to poke around and find hiding prey, but now these fins are used more for social communication and reassurance within the pack. With many large sharp teeth, spiderfish quickly dismember cornered prey. Spoils are shared within the group, a family unit usually comprised of an alpha female, her mate, and their young of several broods, but sometimes multi-generational families will stay together and include aunts, uncles, grandparents and cousins. Young are reared in rocky caves or, lacking such, in muddy burrows dug into the shore, and young are dependent for a very longtime, not begining to join adults in hunting for up to 18 months. Cryptic and hard to study due to their habitat, the spectral spiderfish gives every indication of being one of, if not the most intelligent of all the non-tribbethere fishes.
15. Seakeet, hermaphrodites without differentiated sexes. Seakeets are small 6-8", brilliantly colorful fish native to the clear, near-shore waters of the Centrallian Sea where they live in very large "flocks" feeding on algae, bacteria, and small invertebrates scraped from abundant rocks. These fish are all both male and female, combining the sexes of other bannerfish into a single composite which can both produce sperm and give birth; this condition evolved through an intermediate step of sequential hermaphroditism, where fish began life as female and became male with age, as is seen in some Earth fish like the Anthias, a reef fish. Lifelong monogamous, seakeets form pairs early in life which are mostly up to personal compatibility. Two individuals form their bond by forming a unique and individualized sequence of movements which they will then practice together, forming an elaborate courtship dance specific to their bond. Capable of rapid and dramatic color change, seakeets adopt their vibrant "male" patterns of blue, gold and violet when fighting rivals, courting, and reaffirming their bonds, but become very pale grey-blue when brooding their young so as not to attract unwanted attention. Both partners in a bond fertilize the other, reaching their gonopodiums into the others pouch, and a few weeks later both give birth to several dozen fry which they then carry, kangaroo-style, for several more weeks, feeding them pre-chewed food fragments before they eventually outgrow the pouch and follow their parents as a school. Seakeets are boisterous, nippy fish and their flocks are often cantankerous gatherings. Large numbers provide protection from predators, and so the pairs work together to spot danger and protect their young, but broods never mix and lost young are unlikely to be adopted by other pairs, for each parent recognizes its own young by scent. Pairs have up to four broods per year; young are independent at 90 days of age and sexually mature around 7 months.
16. Streamstilt, male and female alike. A strange freshwater fish of fast-flowing inland rivers in Serinarcta, up to 14" long. A relative of the spiderfish, streamstilts have not only elongated rods on their anal fins, but equally long pelvic and pectoral fins. With wide eyes and sharp, narrow jaws, they are adapted to lying in wait against the current to snatch passing fish. They angle themselves at a 45 degree angle, wedging their anal and pelvic fins into crannies in the rocky sediment and then lying in wait without expending any energy as food inevitably flows right toward their mouth. Their pectoral fins spread out when feeding, detecting the movement of any fish to either side of the jaw; if either is bumped, the fish will lurch to that side to intercept its target, capable of an extreme burst of speed by flicking its muscular tail. Both sexes have a small pouch on their anal fins, which opens backwards rather than forwards, unlike any other bannerfish, and keeps the current from opening the pouch and washing away its offspring. Males and females are separate sexes despite sharing their pouches; pairs form until parturition, when females give birth and the male immediately collects half of the brood to care for. The couple then splits, increasing the survival odds of their young by dividing the brood between them. Adults directly feed their brood tidbits of meat, maneuvering it into the pouch with their pectoral fins; they become independent in several months.