The State of the Sea

Despite the massive portions of permanent sea ice in both caps and the complete loss of tropical biomes, the oceans have not suffered as much as the land. Water retains heat better than air, thus dulling the razor-sharp dip in temperatures to a less deadly hone. Unlike cold air and its lessened ability to hold water for rain, cold water holds more oxygen and thus is a boon to anything that breathes it. The lowered sea levels have made some places shallow, bringing light to the seafloor, and with it, life. However, the productive marine ecosystems of yesteryear are now high and dry on land, causing a loss of productive shallow-water areas overall.

The oceans did suffer a hit that the land avoided, however. With so much land covered in ice, and with lowered overall land temperatures, chemical weathering rates are at an extreme low. Chemical weathering is the process that turns silica rocks into carbonate rocks, consuming carbon dioxide from the atmosphere and burying it underground. Low chemical weathering means extra carbon dioxide in the atmosphere. Fortunately, this was one of negative feedback loops that prevented the whole world from freezing over, since the carbon dioxide help the world hold on to its heat. However, for the oceans it has a decidedly negative additional effect. Because of how soluble it is in cold water, a disproportionate amount of this extra carbon dioxide dissolved in the oceans, turning into carbonic acid and thus making the the sea more acidic. This is a common feature in Earth's past extinctions. However, not only was this one rapid, but it was permanent. Unlike in the past, there are no volcanic gases to expunge or buffers to correct: This spike in carbon dioxide concentration is here to stay. This severely impacted the many and various animals that use calcium carbonate shells for protection, causing many losses.

There is more to the story than just loss, however. While the permanently acidified oceans made calcium carbonate shells less viable, durophagous predators such as chimeras, eagle rays, and some teleost fish, as well as various decapod predators, have weathered the extinction overall, returning to their shell-cracking ways. The unfettered return of these predators was an additional challenge for the already stressed shelled animals they fed upon. This has caused, quite literally, a revolution on the ocean floor: The Second Marine Revolution.

It is named after its predecessor, the Mesozoic Marine Revolution. This event was not so much a revolution but a gentle transition, lasting about 200 million years, or almost the entirety of the Mesozoic. Caused by the evolution of new shell-cracking predators, the increased pressure to be at least partially mobile caused a transition from a seafloor covered in sessile armored animals to one with a wider variety of moving and burrowing creatures. Crinoids, filter-feeding mollusks, and brachiopods (ancient creatures that resemble mollusks but are completely evolutionarily separate) became less common. These were replaced with mollusks with at least some ability to dig, jump, or crawl, alongside sea urchins, with their ability to scuttle about, and a very wide variation of crabs. The Mesozoic Marine Revolution reduced the proportion of sessile organisms on the seafloor from 2/3 to half; this one will reduce it further.

In contrast to the first revolution, the Second Marine Revolution is not a gentle transition caused by animals pushing further and further into durophagy. Instead, it was caused by a swift, severe, external event. As such, it will be much more rapid, lasting only 5 million years, and is thus better deserving of the title "revolution".

So what is a sessile animal to do? Answering that question cost a lot of sessile diversity at the expense of motile, for not only did many shell-builders not make the cut, but many of them became more motile as a result. However, some sessile animals find that sitting on the seafloor is still possible if one has the right adaptations; it just requires some thinking outside the box. With calcium carbonate armor now out of style, many armored, sessile animals use other materials to build their shells. Others have lighter armor in exchange for the ability to get up and move, helpful for escaping predators without need for a shell. Many others find that the seafloor itself is just as present here as it was before, acid or no acid, and burrowing into it still offers protection from the outside world. Though the world has mostly moved on, there are many non-moving animals still, and in some regions they form communities not unlike those of old Earth.

Here we have an example of the state of sessile animals in the cool waters off the shore of India. Here, little columns dot the seafloor, home of a colorful, feathery creature that pops out and pulls in at its leisure. These are feather duster worms, family Sabellidae. They are annelids, related to earthworms and leeches, but unlike them they live attached to the seafloor, extending their feeding appendages out to catch plankton to eat. These worms have enjoyed a great radiation since the cold snap. Key to this success is their shell makeup: Instead of calcium carbonate, they make tubes of mucus and other exudates dotted with sand and bits of shell, granting them a place to hide. However, their need for protein to build their shells makes them more reliant on plankton-heavy waters to feed them. Not to be outshone, cold-water sponges are also present, specifically demosponges with their threefold skeletons: Calcium carbonate, silica, and a protein unique to them called spongin. These hybrid skeletons are a happy medium between efficiency and usefulness in the changed environment, earning them success in oceans worldwide. While few of the sponges and none of the worms photosynthesize like the corals that preceded them, they may still form reef-like accumulations where cold, nutrient-rich water upwells to the sunlit surface, the animals growing on top of each other in piles. These are neither so colorful nor so large as the coral reefs of old, however. Because of the lack of photosynthesis, these "reefs" don't require clear waters to allow sunlight through, and thus prefer cool waters, quite unlike coral reefs. For this reason they often coexist peacefully with kelp forests.

There is more to the sessile story than epifauna, or animals that live on the seafloor. These are relatively low in diversity, as can be seen. However, there are also infauna, or animals that live burrowed into the seafloor, and this way of life has seen much development. Though it costs a lot of energy to burrow and motility is even less viable when the animal depends so much on its hovel, it is also free protection that is completely unaffected by ocean acidity or durophagous predators. Therefore, unlike the relatively low-diversity epifaunal scene, infauna are booming in diversity. In areas with soft rock as a substrate such as in the image above, acrothoracican barnacles and piddocks — a type of bivalve mollusk — dominate, hiding in their drillholes away from peering eyes and hungry mouths. In areas with soft sedimentary seafloors, spoonworms and various annelids abound, filtering water in their U-shaped tubes.

These changes come at a loss for some, even after the original disaster is over. In particular are the ancient and peculiar brachiopods. These are very low in diversity and are trapped in a death spiral from which they will never recover.  

Mobile benthic communities have, of course, undergone great changes. Crabs and other crustaceans abound in many forms, often filter-feeding or eating detritus. Spry and lanky brittle stars dominate over their sluggish brethren, the starfish, for they move by walking on their arms as opposed to using numerous tube feet to crawl around and are thus better at catching mobile shelled prey. Mollusks now walk and flit about as a rule, moving from one feeding place to the next as necessity decrees, and it often does. In their goal of attaining speed over armor, some odd lineages have had the opportunity to build upon themselves, creating totally new kinds of creatures. Enter the file shells.

File shells (family Limidae) are strange mollusks already. Like some other bivalves, they can propel themselves by shutting their two shell halves, called valves, together, pushing water out. Unique to them, however, is that they make use of their many long, fleshy tentacles, which grow out from their mantle, to aid in locomotion. Some species can row their tentacles through the water, often at the same time as they jet propel, to move along faster. Others skitter lightly on the seafloor for short distances. These tentacles also have a secondary use: if grabbed by a predator, they will readily break away, allowing the mollusk to make an escape. They already depend less on their shells for defense than most other mollusks, allowing one descendant, the mantlecrawler, to rise among their ranks.

Mantlecrawlers are often found as a rosette of tentacles laying on the seabed, pulsating gently in the current. Though not evident from above, they do have shells which are buried in the sand or lodged in a crack when the animal is open and filter feeding. Their large, conspicuous tentacles sometimes aid them in this, especially the stubby ones on either side of their mouths, by circulating water as they filter it. When it is time to go, perhaps because of low plankton levels or a suspicious shadow, they pull themselves up and skitter away, tiptoeing across the substrate. They may instead choose to swim by using their tentacles to push through the water. Some specialization is evident in the way they use their tentacles: Most are used to walk or swim, some are primarily for circulating water for feeding, and a third kind is long, thin, and relatively stiff, forming sets of gradually lengthening "antennae" on either end. It is with one of these ends that they usually lead with when crawling, feeling their way along the ground.

Like all file shells, mantlecrawlers are blind, instead navigating the world via taste or touch. Much of their skin contains photoreceptors, however, granting the ability to detect ambient light levels and the general shape and size of any shadow that passes over them. If a shadow is determined to be a predator, they can still jet propel themselves away, clapping their valves together with their tentacles oriented perpendicular to the seafloor. They have many predators to run away from, for they range from 2 to 15 cm from fore to aft tentacle tips, and usually on the small side of that range.

Pelagic animals have made changes as well. The trio of ex-landlubbers which typified great swathes of the ocean before — whales, penguins, and seals — have all weathered the extinction. Of whales, only the omnipresent dolphins, the suction-feeding beaked whales, and the strange and mysterious pygmy and dwarf sperm whales pulled through. No species survived that weighed over 800 kg (1760 lb). Of penguins, species of the genera Pygoscelis, Spheniscus, and Eudyptes survived, united by relatively wide ranges and of course tolerance of cold temperatures. Though penguins were cut down to just a few representatives, they are at long last found all over both hemispheres. Of seals, as luck would have it, only the true seals survived; thanks to their aquatic adaptations such as more streamlined bodies and the lack of external ears, they had a wide variety of species that ventured far into the ocean for food. This is in contrast to most others which were tied to sites of upwelling. In addition to these three, there is a fourth lineage consisting of one lucky species that defied the odds: Great auks, spared the wrath of humanity and the cold snap both.

Other pelagic animals make use of the great gluts of plankton that enjoy the world's cool, nutrient-rich waters. In particular, krill abound in the seas worldwide, omnipresent in their sheer numbers. Spurred by the extinction of many old filter-feeders such as manta rays and baleen whales, many new lineages have stepped up to filter the water for food. Key among these are various small fish which prospered in the early years of recovery, gobbling zooplankton and multiplying fruitfully. Seemingly typical, the bigmouth anchovy is one of these.

Bigmouth anchovies have big mouths, but they are also just big fish — that is, by anchovy standards, at 70 cm long. They may not be the tasty little baitfish they used to be, but they are still on the menu for many other animals. Like their ancestors, they are filter-feeders, gulping in water and straining it with their gill rakers for plankton, but unlike their ancestors, they are usually seen with their mouths tightly closed. This is because bigmouth anchovies are pickier, only feeding in seas rich enough to sustain them while avoiding competition with other filter-feeders, and thus trading the sheer numbers of their predecessor for a larger size.

See, these fish are built for endurance, despite that their food source is all around them. Swimming in small schools, they often frequent coastal shallows, but individuals never linger for long, always on the move from one feeding spot to another. What they seek they do not literally look for, for they are even more wall-eyed than most anchovies and can barely see what's in front of their snouts. Instead, they make use of their rostral organ. All anchovies have these organs, a system of fluid channels dotted with sensory neurons that take up much of their snout space. These are very sensitive to the water in front of them and are strongly suspected of granting them the sense of electroreception, presumably to detect the presence of quarry such as copepods and larval fish in front of them. Bigmouth anchovies have developed this sensory organ to the point that it is easily visible. With it they read the water in front of them, choosing wisely when to open their jaws which are normally held streamlined under the head when swimming. They will only open their mouths to feed if the amount of zooplankton in front of them exceeds a certain threshold, a mathematical optimum that balances the resources gained from feeding with the energy lost from the drag of their open maws. This is the reason they wander, feeding on rich patches of growth before moving on, taking their cream of the crop while staying clear of their fellow filter-feeders. In this way they serve to distribute resources across the ocean.

Of the predators to avoid in the deep blue expanse, hiding in crevices or in the dark of the abyss, there is one fish which stands out as a threat around the world: The grand lancetfish. As a lancetfish, it brings with it a long list of strange traits. It is a non-discriminatory predator, living at many depths and in many places, eating anything its snaggletoothed jaws can catch. Their stomachs are hardly used for digestion, instead swelling enormously to store whatever the glutton has caught. Strangest of all, these fish are simultaneous hermaphrodites - they posses both male and female sex organs! This last fact has allowed them to bounce back from the extinction with celerity. From them, the grand lancetfish has grown to a whopping 4 meters (13 ft) long, and has become so hungry that its stomach is completely non-functional save as for a pouch to store its many kills. These are attained via ambush, like its ancestor, for the fish moves little most of the time. When prey is spotted, it lunges using its forked tail and its tall, trajectory-correcting dorsal fin. It will then slowly digest its meal, whatever animal it may be, until the day comes that it hungers again.