The Grandest Guppy: Meet the Gemnus

This is a guest entry conceptualized by Joseph Cawley, who commissioned the project author to illustrate this species to flesh out the project. His writing below has been supplemented by the project author for clarity.

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In the Mid to Late Hypostecene, the freshwater ecosystems of Serina had stabilized and diversified into rich environments full of diverse plant and animal life. Freshwater habitats presented excellent opportunities for speciation due to the constantly changing environmental conditions as rivers twisted and turned, connected and disconnected from ponds and lakes, and moved toward the sea. As with all life introduced to Serina, the evolutionary vacuum also had profound effects on the livebearing fish that were seeded in Serina’s waters, which diversified dramatically to fill the niches of an empty ecosystem.

From the Late Hypostecene to the early Tempuscene, many lineages of guppies evolved to exploit all the vegetation that formed the basis of these new aquatic food chains. Many new families evolved that were either entirely herbivorous or whose families had representatives that fed predominantly on plant life.

In the tropics, this explosion in herbivore diversity can be most plainly seen. Here, one can find fishes straining the surface for tiny leaves of water lily-like floating clovers, others that scrape low lying algae off rocks, filter-feeders that strain for the smallest of phytoplankton with gill rakers, bottom-feeding forms which scour the mud on the bottom for edible detritus, and finally leaf croppers who browse the larger macroalgae, elodea and aquatic grasses with blunt cutting teeth. Even species that fed on similar plant or algae species could avoid competition with each other by feeding at different times of the day or in the case of large upright aquatic plants, could have different species exploiting them at different depths.

When these herbivorous fishes first evolved, a veritable bonanza was available to them. Snails and crayfish among other invertebrates fed on Serina's plant life since the beginning of Serina's history, but they were not especially mobile. Fish meanwhile were able to move much further into new ranges in a single generation. They could, in most instances, also grow larger, allowing them to become the primary herbivores in aquatic environments within just a few million years. Though Serina had several entirely aquatic plants introduced at its start, terrestrial species quickly also adapted to survive and even thrive in riparian environments and eventually in fully submerged habitats. Grasses were among the first, producing several aquatic lineages in the first ten million years of evolution on Serina. With sturdy leaves protected from many grazers with silica particles in their tissues, grasses were tough to consume. Some guppies grew larger to consume them. They evolved pharyngognathy - this is when the left and right pharyngeal tooth-bearing bones in the jaw fuse together to form a more sturdy jaw structure able to chew coarse plants. Various muscles strengthened around these throat jaws to enable a wide variety of movements of both the upper and lower jaws and provide a wide cutting bite. Wide, cusped teeth evolved to process aquatic grasses, which were continuously replaced as the silica found within wore them down, while symbiotic bacteria in stomachs and guts made breaking down cellulose and other formerly indigestible elements of this diet possible.

As happens in any evolutionary race between predator and prey, plants adapted in turn ways to reduce the damage inflicted by these herbivore fishes. Some plants and algae simply grew at enormous rates, able to recover and endure even heavy grazing simply through rapid proliferation. The cell walls of some descendants of elodea became composed of ever more complex polysaccharides that made extracting energy difficult. Algae developed mineralized, coral-like forms as protection from the algal scrapers. Though life in water usually means plants are free from the need to have sturdy supporting stems, heavy grazing encouragd the development of more rigid - even woody - stems to make aquatic plants resistant to attacks by nibbling fish, while hooks, thorns and spines were also deployed on a variety of aquatic angiosperms - anything to make feeding on them more difficult. By the Tempuscene many water plants had adopted strange and bush-like growth forms, creating dense thickets of woody stems underwater which were difficult for most fishes to consume. Aquatic grasses meanwhile spread wildly with long, well-defended silica-infused leaves sharp enough to cut unwary swimmers-by. Never had Earth ever known such a viciously defensive underwater ecology, because never had Earth provided the environmental conditions to allow for the evolution of large numbers of huge, plant-eating freshwater fish.

But not even the most vicious defenses can dissuade the largest and most specialized of these grazing fishes evolution could produce. Originating in the tropical rivers of northern Karii, one group of guppy descendants known as gemnuses evolved as veritable living lawnmowers able to get past even the most vicious defenses. Bulky herbivores with well developed pharyngeal jaws for slicing and grinding up the toughest plant material, their jaws are full of large cusped teeth while their intestines are incredibly long, often 15-20 times that of the body length. These adaptations together enable them not only to extract as much nutrients from some of the most well protected plant life around, but to specialize upon it. Because they favor the harshest plant diets, they have little to no competition and can grow to quite large sizes, with the smallest being no less than ten feet long. As far as guppy clades go they are not the most diverse by any means; just 12 species exist worldwide, but their large size and continuous browsing on vegetation makes them have significant impacts on the ecosystems they inhabit. By the middle of the Tempuscene, one particular species of gemnus has evolved to grow to truly extraordinary dimensions surpassing any freshwater fish on Earth today. Native to the deep, vegetated rivers throughout the continent of Karii, it is notable for its opportunistic grazing over land when seasonal rainfall causes the rivers to overflow their banks, at which time the gemnus takes advantage of the situation and grazes these submerged grasslands much like an underwater cow.

The giant gemnus can grow just over thirty feet in length and is truly an awe inspiring sight to behold. Weighing over three tons, these truck-sized fish are true titans of the river. They are endless nomads, traveling up and down the waterways that they call home. They feed predominantly on a particularly vicious looking plant, spikeleaf waterweed - a spreading, fast-growing aquatic grass with knife-like leaves edged in barbs of silica deposits which it evolved in response to millions of years of grazing pressure by smaller fishes. When left to themselves, these plants quickly form impenetrable thickets along the river beds and are quick growing, covering the water column from bottom to surface. They shelter large amounts of small fish and invertebrates when present in moderation, but if left entirely undisturbed colonies of this plant accumulate sediment in their spines, causing rivers to gradually become clogged with dirt, shallow and muddy, eventually becoming shallow marshlands where little plant life but the spikeleaf can survive. Once the river fills with sediment and becomes too shallow for the few aquatic species that can eat it to reach it, virtually no terrestrial grazers exist adapted to consume it and huge stretches of wetland can become clogged with it to the exclusion of most other life.

Fortunately for the health of the ecosystem, gemnuses have evolved to browse the waterweed heavily, consuming it preferentially during the drier seasons when the rivers are lower and the grasslands above dry out, and preventing it from congesting the rivers and dominating the riparian landscape. They have large heavy scales for protection against the spines covering their face, present even on the lips. Their teeth are multi cusped and have additional serrations on each cusp to maximize the surface available to chew. Due to the intramandibular joint that it has inherited from its guppy ancestors it can gape wide and has increased bite force allowing it to take huge bites out of a thicket of these well armed plants. While the plant is being sliced up, the insides of its mouth produce copious mucus to protect it from the spines piercing its mouth. However, the mouth just removes plants from the substrate. It is the mighty pharyngeal jaw with rows of elongated, molar-like teeth with a ridged surface used for grinding the plant into a pulp that can then be swallowed. Able to consume more than 400 lbs of plant matter daily, even a small population of giant gemnuses serve well to limit the growth of the spikeleaf and maintain a healthy balance of microhabitats in the river; clearings formed by the grazers allow water to flow freely, preventing sediment from building up, while remnant patches of the plant proliferate in shallow bends and eddies that the largest grazing fishes cannot reach, sheltering fry and invertebrates from their predators.

Giant gemnuses are very long lived. They reach sexual maturity at 15 years of age and can live as long as 120, and though growth slows after the first twenty years it never entirely stops. Traveling up and down thousands of miles of rivers their entire lives, giant gemnuses are solitary and not territorial. Mating is a sporadic occurrence which can occur at any time. When two of these giants encounter each other, they will line up beside each other. The sexes are identical, save for the gonopodium of the male, so they each inspect the anal fin of the other. If this fin is the same shape as theirs, then they will continue on their path. But if the anal fin is a different shape then mating will commence. There is no elaborate courtship as for the most part, as adult giant gemnuses are rare and to even encounter another one is a good indicator that individual has lived long and has good genes for survival. If multiple males meet, they do not fight; both will mate a female in turn without aggression. Several weeks after mating a female gives birth to an incredible 100,000 or so young, which are shockingly small in comparison to itself, barely a centimeter in length. This shows very clearly that the gemnus lineage has evolved enormous adult sizes quickly and without any modifications to the ancestral guppy's manner of reproduction where small eggs hatch internally and are then birthed and left to their own devices.

The young experience overwhelming mortality in their first several years and less than one per brood can be expected just through its first two years - possibly as few only one individual in every five million born will manage to reach its maximum size. The young hide in the weeds like the fry of other livebearers and initially eat biofilm, algae and the larvae of insects; there are numerous predators ranging from bass-like grupfishes to small mantis-like fishing crayfish waiting for their chance to strike, and some gemnus live for just seconds if their mothers do not take sufficient effort to shelter in the weeds when giving birth. If she does not, other fishes will swim right up to the oblivious birthing mother, waiting to be the first in line to catch the fry as soon as they emerge.

When the survivors of the initial gauntlet reach a length of 4 inches, their jaw changes position from terminal to ventral enabling them to graze algae off the leaves of large water plants. Now they are a little safer as they are faster and more alert and can zip to shelter if danger approaches. When they reach a foot in length, their guts become increasingly well-developed and their mouth and throat teeth become more suiting for nipping off and chewing blades of grass, though it will be some time yet before they can consume the spikeleaf plants the adults primarily eat. At this age the juveniles will begin to venture into flooded meadows and crop the submerged land grasses, and while they have outgrown some enemies many large predator fishes will still take them. By five years of age however the ever smaller pool of survivors will have grown to a length of 10 feet, outgrown virtually all enemies, and begun feeding on spikeleaf plants, effectively adults. They are now big enough that little can bother them, and they start to become solitary. Their sharply forked caudal fin becomes more rounded for their more lethargic way of life. From here, the only further changes are gradually increasing size. For decades they grow, albeit slowly, the bigger they become the better they can digest their coarse diet. But eventually, in a cruel twist of fate, they all outgrow their rivers. A dry season might bring the end; the water will, if only temporarily, recede just enough to leave the very largest gemnuses beached, unable to free themselves, and eventually to starve or suffocate. It is possibly their indeterminate growth which serves to limit their numbers, for otherwise adults lack any predators, and it is is possible that were they never subjected to periodic droughts their lifespans and potential size could be larger than ever recorded.

When a gemnus dies, it serves as a great feast for a multitude of scavengers that can last for weeks, much like a whale fall in the deep sea on Earth. From birth to death, the gentle giant was a vital part of the ecology of the many rivers all over Karii where it dwelled. Vital to sustaining the health of the large rivers in life, in death it returns all it has taken back to to the environment in one enormous return of nutrients, which may be taken up ultimately by the spikeleaf waterweed and so in turn feed other, future gemnuses in the ongoing circle of life.