Flashkrill

Krill, especially Antarctic krill descendants, are absolutely everywhere in the sea. Even when the Earth was warm, they breathed life into the depths, their swarms converting huge volumes of phytoplankton into bite-sized creatures to feed the food chain. Further, their constant movement mixed the ocean waters and distributed nutrients through the ocean's layers and across its surface. Now that the Earth is cold and their numbers have exploded even past those of the old days, krill are absolutely vital to anything marine.

This humble order of crustaceans is now diversifying widely. Most have maintained the traditional krill ways, using their thoracopods, or the legs attached to their thoraxes, as baskets to filter feed. Many others have diverged, either acquiring traits anew or building on some ancestral trail. Some, like Antarctic krill, use ice-scrapers on their thoracic legs to remove algae from ice sheets. Others, like the Northern krill, have smaller baskets and instead actively pursue large zooplankton. Niches explored are various and widespread, from fish-like active hunters to algae gardeners to sedentary animals taking in water as it goes by to mutualistic cleaners —  though nearly all of these are in their infancy. They have even made headway into brackish environments, but they are blocked from going further by their relatives, the shrimp, many of which already have adaptions for this purpose.

Speaking of shrimp, what is the difference between them and krill? First, the small details. Shrimp are decapods, alongside crabs, lobsters, and others, while krill are in their own order, though they happen to be their closest relatives. Decapods are so-named for their ten walking legs or thoracopods attached to their thoraxes. Krill, meanwhile, have a varying number of walking legs, usually six to eight pairs. However, both groups have ten pleopods or swimmerets, the abdominal legs used for swimming. Krill also never evolved claws, while decapods have and usually bear at least two of them.

The final difference between krill and shrimp is that krill have ten bioluminescent organs called photophores distributed along their bodies. The first pair are on the back of either eyestalk, the second pair on either side of the thorax, and the last four run along the underside of the abdomen. Krill photophores are extremely complex marvels of evolution. They, like flashlights, have a reflective back and a lens on the front to produce a coherent beam of light. They can swivel to aim this beam in many directions, though they are by default oriented downward. They can glow at any dimness between maximum and off, and can flash on and off with little latency. Their blueish or greenish glow is due to a luciferin, or a pigment that releases light when enzymes break them down, and is a modified version of that found in dinoflagellates, a plankton that is part of their diet and can also glow. Altogether, their photophores reveal krill to be complicated and elusive creatures, for despite their complexity, what exactly these animals use the organs for is an open question. It's plausible that they conceal the animal's silhouette against light filtering in from above. They may play some role in deterring predators, glowing when in their vicinity. They also are used to some extent to communicate with other krill while swarming or courting, the organs rotating to point toward the intended target. What they are trying to communicate and why is a mystery.

This sophisticated glowing ability has formed the basis of a new genus of krill, a slight modification with major repercussions, the flashkrill.

Flashkrill, forming the genus Polyhelios, are descended from Antarctic krill. They are immediately recognized by their robust builds, much chunkier than their slender, sleek ancestors, though not necessarily any larger. Their carapace can be colored reddish or yellowish in places, but overall it is transparent. They float in the ocean as translucent glimmering shapes or dart through it in rapid bursts.

Flashkrill are filter feeders like their ancestors. Unlike them, they have no ice rakes for removing algae from ice sheets. The animal's digestion can be traced through its translucent body: It first swallows its caught algae, then grinds it into a pulp in its gastric mill, a toothy, gizzard-like organ. This produces a processed mass of plankton often visible in their stomachs. Krill generally are not very efficient at digestion, with only a simple, straight gut following the stomach. This is not a liability, but a boon, for krill populations can process massive amounts of matter this way, enriching the ocean with their feces and bodies. There is simply no point to be efficient when mortality is high and fecundity is higher. The flashkrill's transparent shell, however, belies a winding, complex gut within its abdomen, fit for squeezing more nutrients out of their food. This implies an increased importance on the individual animal's survival instead of the proliferation of many. Its stocky, muscular build and large eyes also point to this conclusion.

Indeed, such investments ought to be defended, and a defense is what flashkrill have. Though the ocular photophores are occluded by the globular eyes, the other photophores are clearly seen along the animal's sides. Their black, pigmented backsides and cup-shaped structure are very obvious, a far cry from the little pinpoints typical of other krill. Indeed, flashkrill have taken their ancestors' ability and, provided the opportunity of radiation, found a specialized use for it.

Flashkrill are not content with simply flickering their lights to imitate sunlight, or flashing them at their fellows, or doing whatever it is that krill like to do to justify their photophores existing. Instead, their photophores are devoted defenses. They have incorporated them into an ancient danger response they and decapods both share called the caridoid escape reaction. Typically, the startled creature faces the source of the stimulus and contracts its abdomen rhythmically, propelling the animal back and away from danger. Instead, when the flashkrill is startled, it first faces the predator, but at the same time uses its mighty eyes to spot the eyes of its attacker, if present. Second, it straightens its abdomen out as if it were about to flee to safety, but in an exaggerated fashion. Its photophores have by this time rotated toward the eyes of the predator, and as the abdomen straightens, all ten photophores flash brightly, their beams directly in the predator's eyes. It then pulls back and retreats as usual.

This flashing display serves primarily to startle and distract predators, allowing for a quick escape. However, a properly aimed flash can ruin a predator's night vision, making it nearly impossible to follow the krill visually. Flashkrill also engage in a sneaky use of their molted shells, as their ancestors did: A flashkrill nearing molting time can rapidly shed it during its escape reaction, leaving an empty shell as a decoy. Because of the scare, predators are more wary around small swarms of chubby krill emitting a certain crustacean odor. Still, the deterrent can only do so much, and they are still prey for innumerable other animals.

The relatively low population densities of flashkrill as opposed to other krill generally prevents a disruption large enough to produce dedicated predator countermeasures. Instead, it is an unwelcome surprise that most predators have to deal with. However, because they are successful and widespread in the Pacific, some countermeasures have inevitably appeared. The most specialized of these is the black-handed mackerel, a pelagic fish with a round, black spot on either pectoral fin. These are flared when the predator approaches a group of flashkrill. The flashkrill, when spooked, are much more likely to lock on to the splotches than the fish's eyes, which are smaller than the splotches. This spares the fish its night vision and allows it to pursue the confused crustaceans. A miscellany of less specialized countermeasures exist, such as two species of octopus that have learned to make false eyes on their tentacles and to splay them out when hunting for the same reason.