Grumblefish

Bottom-feeding mosquitofish were a staple of Apterra's marine ecosystems as far back as the Early Muricene, cleaning up corpses and toiling in vain to slow the inevitable accumulation of dead algae on the seafloor. The most common of them belonged to the species Gambusia bradymelanocisor and abyssalis, two closely-related minnows that found success as an integral part of the oceans' cleanup crew for millions of years. Their descendants, many of which have grown to massive sizes and adopted new niches, remain abundant, forming a large family called the Grumblefish (Abyssalichthyidae). Like in ages past, most Abyssalichthyids occupy the middle and lower levels of the water column, though some offshoots have expanded into other territories. All members of this clade have exceptional tolerance to extreme water conditions, especially the presence of algal toxins. Additionally, grumblefish have better-than-average resistance to fluctuations in salinity; while some species remain seagoing, several have moved into freshwater habitats, and others can move freely between rivers and the ocean. The family is split into the deep-sea "true grumblefish" (Abyssalichthyinae) and the more generalistic Grumbletoads (Bufognathidae), descended from G. abyssalis and G. bradymelanocisor respectively.

True grumblefish have experienced the most extreme gigantism as a result of their abyssal habitat, with most species nearing a meter in length. Many are peaceful and pose no danger to their neighbors. Abyssalichthys, the type genus of the family, subsists in this way, sifting through huge mouthfuls of slime as it patrols the seabed alongside fellow detritivores like mudeater crabs. The ten or so species of these Filthfish are outclassed in size only by giant stoutlings. They gorge themselves and can grow to several meters on their never-ending supply of food. Their close cousin, the durophagous Shellcrack Grumblefish (Ostracofatsicor) frequents the same waters, but it prefers to feed on the woodlice themselves. These mudeater-eaters have powerful jaws lined with blunt teeth to crush the exoskeletons of the aquatic isopods, spitting the hard parts out and scarfing down the soft insides. Like all members of their family, shellcracks can ingest vast amounts of food in one sitting; their stomachs can easily hold half their body mass in food at any given time. For Abyssalichthyines whose food source is less reliable, this allows them to go many weeks between meals. 

Smaller species like the Wandering Grumblegulper (Extendopharynx pererrus) use this to their advantage, taking few risks and attacking other abyssopelagic fish only at the most opportune moment. Cowardly and skittish by nature, they will abort an attack if their target notices them, for their extensive reserve of energy means they can always try again another time. Once they get their jaws around their prey, though, nearly nothing can make them let go. The skin of their underbelly can swell to twice its normal size when engulfing their food, while the thick lining of their stomach resists tears and punctures even when stretched thin. These shy carnivores pale in comparison to their larger cousin the Megamouth Grumblegulper (E. recurvidens), a two-meter-long apex predator with an appetite even greater than its wandering sister species. Other grumblefish are on its menu, as are mid-sized stoutlings and thalassaurs if they venture too deep below the surface. Being slow swimmers, all members of the grumblegulper genus are ambush hunters, relying on camouflage to help them approach their targets unseen. As a result, most are bottom-dwellers, sticking to the sand and mud to remain out of sight. Megamouths are the only exception, being found in open abyssal waters. As an adaptation to this change of habitat, they are much sleeker than their relatives, with their only non-hydrodynamic feature being their immense mouth. They hunt at night, allowing them to maintain the element of surprise even without obstacles to conceal themselves. Once in striking distance, their muscular tail kicks into action, propelling the megamouth with a burst of speed that leaves little time for their victim to make an escape. Diving back down to the seafloor to consume their prize, the grumblegulper drags the unlucky creature into the dark void of the algal wasteland, ensuring it will never again glimpse the sunlight above.

The Estuarine Grumbletoad (Bufognathus abeliensis) is perhaps the best representative of its side of the Grumblefish family tree. It has a non-specialized diet, feeding happily on grasses, algae, crustaceans, smaller fish, and the occasional tailtube worm. Like its progenitor G. bradymelanocsior, it has a particular fondness for carrion, scraping clean any carcasses that drift its way. Living at the mouth of a great river, therefore, suits this species well, as rotting land animals arrive regularly from upstream after meeting all manner of unpleasant fates. When it needs to, though, the estuarine grumbletoad can migrate upriver or out to sea, using a large gland on its snout to either excrete or retain salt as needed. This structure does not develop until several months of age, so females can only give birth in brackish backwaters where salinity doesn't fluctuate too much. The fry of other Bufognathus species may have different preferences, so some grumbletoads spend more of their time in the ocean, while other members of the genus are more bound to inland regions.

The Snapping Grumbletoad (Bufognathus lacustris) is one species that has become permanently landlocked, living its entire life in shallow lakes and slow-moving streams. It occupies a similar niche to benthic grumblegulpers, though it also adds aggressive mimicry to the basic Abyssalichthyid toolkit. As the snapper never ventures into the ocean anymore, its salt gland has been repurposed, now secreting chemicals into the water that, from the perspective of other fish, smell exactly like a helpless mosquito larva or gillywog. Lying still on the lakebed and relying on this scent (plus the vaguely wormlike shape of the gland itself) to draw in unsuspecting prey, it then lunges forward once the smaller fish is in range, pushing off the ground with its powerful pectoral fins. 

In the tropical sea, the Panther Grumbletoads (Cryptospinus) are the dominant Bufognathine genus. They can be found in huge numbers within seabasket forests, where some - like the eighty-centimeter C. canistrophagus - have turned to complete herbivory, digging the basket-grasses out by their roots. By distending their stomach to a degree unseen in any other grumblefish, they can proceed to eat the entire plant, working their way along the often fifteen-meter stems like a mouthful of spaghetti. They then bury themselves in the sand up to their eyes, sitting and digesting their immense meal for up to a week before they move again.

The grass-eating panthers are, however, the exception among a genus mostly composed of hypercarnivores. The Leopard Grumbletoad (C. punctatus), a small but efficient predator that rests among the swaying leaves, blends into its surroundings with tall rays on its dorsal fin that mimic the blades of a newly-germinated seabasket. This draws in small herbivores (siren gecko hatchlings, for instance) that prefer these seedlings, finding the full-grown plants too large to easily graze. When it tries to bite into this fake grass, the lizard is in for a surprise. The tips of these spines are barbed, remaining embedded in the gecko's flesh as it tries to remove itself from the now-awakened grumblefish. The spines quickly fold forward and down, placing the flailing reptile within reach of the grumbletoad's jaws. The fish then unceremoniously rips the thalassaur free and shoves it down its gullet.

Colder shallows are home to yet another unique spin on grumbletoad morphology, this time coming in the form of the Shivering Grumbletoad (Amicospinus gregaricus). Occupying a genus all its own, this species is distributed across the northern half of the Perithalassic Ocean, patrolling coastal waters in groups of 20-50. While never more than 30 centimeters long, they tend to favor prey far larger than themselves, going after other fish, seawis, and any land creature that wades a bit too far from the beach. Upon selecting a target, they attack together, puncturing its flesh hundreds of times with the spiny rays that jut out of their pectoral, pelvic, and dorsal fins. Unlike panther grumbletoads, they have no barbs on their spines. Thus, they can be removed with ease from a struggling animal. With their food dying from a thousand cuts, the shoal quickly begins to feast, folding their fins flush with their bodies to avoid injuring one another. Working together, they tear the meat into chunks small enough to swallow whole, and all members of the group swim off with a full belly. Such social behavior is likely to suit this species well, granting it increased adaptability to whatever the future holds. The grumblefish in general are well-positioned to make it through whatever Apterra throws their way; with a wide range of habitats and niches, their strategy is one that makes them quite resilient to all sorts of challenges.