Fishes of Sea and Shore: Swordsharks and Mudwickets

25 Million Years PE

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Fish

By the middle Tempuscenic, the live-bearing fishes have diversified enormously. From tiny colonists have radiated an enormous number of new lineages. Guppies give rise to dozens of distinct freshwater lineages, ranging from small minnow-like shoaling species to truck-sized riverine herbivores that may consume more than four hundred pounds of green food a day and give birth to as many as 100,000 free-swimming larval offspring in one litter. They give no parental care and even though they do not lay eggs, they still rely entirely on chance to reproduce and play the numbers game in hopes that one or two of their tiny infants will survive to maturity, for they contend with an onslaught of countless fierce new predators. From the platies and swordtails have arisen the swordsharks, lively shark-like carnivores with frightening, toothy maws that teem across all of the ocean's salt waters. With highly elongated tail swords in both sexes, these fast-moving oceanic fishes use these to sweep through shoals of fish, stunning them, and then turning around to gobble up the casualties. With complex social behavior, swordsharks are cooperative hunters, teaming up to trap fish in bait balls with nowhere to escape, making feeding easier; to facilitate communication between individuals they can rapidly fire up or dull down a series of bold spots on their sides which reflect light and make their positions clear to their fellows. These fish have also evolved a placental tie with which they nourish their young in the womb, thereby giving birth to fewer - but larger - offspring, each better suited to survive on its own after birth. Adults move toward shore,giving birth in sheltered floating algae thickets where the young are safer than in the open sea.

The opaline swordshark, Ensificaudix opalorica, is an eight-foot-long predator of the open Serinan seas. A typical member of its grouping, these fish hang in loose packs and work together to attack shoals of smaller fish, threshing them with their tails to stun them.

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On warm, tropical mudflats, the bizarre Mudwickets, small but oddly-shaped creatures with squat toad-like bodies, bulbous eyes, huge mouths, and flattened bellies, hop from the tide and onto the shore to hunt seaside insects and crustaceans with needle-like teeth and over-sized crunching jaws. Only very distant relatives of the swordwhales, these fish descended not from the swordtails but the guppies; and their evolutionary history has veen very different as well. They have largely left the sea and crawl about on the sand with webbed hand-like pectoral fins and periodic thrusts of short tails, which function like a spring to catapult them to safety in times of danger, and survive above the water by absorbing atmospheric oxygen through a highly vascularized plate on the roof of the mouth and to a lesser extent though their skin in a manner akin to salamanders. So long as these structures remains moist, they allow the fish to survive indefinitely out of the water. Mudwickets are terrestrial from birth, able to breathe air from infancy, and will drown if trapped for a prolonged period in deep water; their gills diminished and their swim bladders vestigial, they can no longer swim. Instead of retreating to the water when danger threatens they rush to complex, communal burrows just below the high-tide line, which may extend five feet below the ground and consist of a series of flooded chambers. Every chamber in a burrow holds a large air pocket which can only be kept fresh and oxygenated through strenuous, repeated trips back and forth to the surface at every low tide, the fish bringing up a mouthful of depleted air and then back down a bubble of new, clean air with each descent to fill its reserves. These air pockets keep the fish alive when high tide returns and covers the burrow system in as much as four feet of water. The Mudwickets' burrows may be used by as many as two-hundred fish at any one time and can become quite complex. Different chambers are utilized for different purposes - the deepest chambers are for the storage of excess food, which lasts longest in relatively low-oxygen, cooler conditions, while the warmest, most oxygenated, shallow chambers near the surface are used as nurseries in which they give birth. The infants are large and well-developed when they leave the mother's body but remain underground for a considerable time before ever leaving to hunt on their own, being fed by the colony's adults. This appears to be an unintentional effort - the mudwicket having a tendency to drag its prey underground before feeding (to prevent it being stolen by a competitor or attracting a predator), thus allowing the young to gather around and gather the scraps, but it is effective nonetheless.

The young mudwickets will remain underground while growing fat on their parent's scraps for as many as thirty days before they emerge from the burrow for the first time to hunt on their own and begin to pull their weight in maintaining the burrow. Individual colonies of mudwickets are male-dominated and these males, which are large and brightly ornamented, are fiercely territorial toward their same-sex neighbors, but as a colony grows a single male cannot possibility mate with as many females as he might control, giving males on the outskirts of the colony without harems of their own a chance to mate when the females come above ground to feed. Young males are driven from the colony at maturity, the lucky few to survive long enough to start a new burrow system beginning their own new harems and colonies, the rest surviving in only small shelter burrows and relying on sneak attacks to mate with other males' unguarded females to spread their genes. When a predatory bird threatens, every fish on the shore will vanish in an instant into the sand and seal the entrances of their burrows with a plug of mud until they are certain the threat has vanished. It is not unusual for a colony to remain entirely underground for an entire day after a fright, feeding on the invertebrates they have amassed in the lower chambers of their burrow system until they can be certain the danger has gone.

Most of a mudwicket's diet consists of insects, hermit crabs, and other arthropods along with the occasional small fish trapped in a tidepool, though large species may occasionally ambush small birds. One genus specializes in bivalves, making regular forays down steep rocks into deep tide pools to pull them from submerged stones, then crushing the shells with enormous squared-off teeth to get at the meat. Conversely, another has evolved to forage quite far up the beach and away from the surf, having abandoned colonial existence for small and solitary burrows near the treeline at the end of the beach which just tap the underground water table. One particularly promising family of mudwicket has in fact left the seaside environment completely and moved far inland, following rivers and building its burrows in the muddy banks of freshwater ponds and streams. Here they prey on crayfish and aquatic insects as well as land invertebrates such as crickets and snails, that they may forage for as far as fifty meters from the water, crawling through damp leaf litter and even scaling small logs and fallen branches in the process. In rainy conditions, members of this group may travel over a mile inland to reach isolated sources of water, digging into damp soil or hiding under logs to keep damp in between open pools of water. Most solitary inland species are colored cryptically, but a small number are extremely vibrant and impossible to miss in shades of blue, red, and yellow, advertising deadly toxicity in the mucous of their skin that they engineer from their diets, which incorporate a significant amount of poisonous ants.

The seven-spotted mudwicket, Ranacephalus septulocus, is a plump, colonial species of mudwicket that thrives along the seashores and tidal estuaries of the Sierran Subcontinent (See Map), where it dwells in large and complex burrow systems just below the high tide line. In most mudwickets males are larger and brighter than females, but females too in some species can carry some bright colors, and a female carrying a litter of offspring can become quite swollen in the days before pregnancy (pictured), rivaling the weight of her mate. Most species even at their extremes are hardly what could be considered large except on a relative scale, however; the seven-spotted mudwicket is an average 6.5 centimeters in length and just 1 - 2 ounces in weight. Though a descendant of the extravagant guppy fish, their tails today, short and stocky, function for little more than an occasional boost of propulsion to dart down a burrow to safety.