The oceans are diverse but sparsely populated; altogether holding only half of the planet’s biota.
Of the 33 animal phyla, 30 are ocean residents; 15 exclusively so. Only 16 phyla are on land or freshwater, and only one is exclusively terrestrial.
The land is a more competitive biome. Thus, while oceanic phyla diversity is considerable, species diversity is sparse.
5 to 50 million macroscopic terrestrial species exist, mostly vascular plants and insects, which are the land’s earliest large inhabitants. In the oceans, an estimated 450,000 species are extant; a figure unlikely to break 1 million at most, microbes excluded.
The air and ocean as much different media go a long way to explaining how life differs between the two. Seawater is 800 times as dense as air, and vastly more viscous.
Sunlight does not penetrate the deep, and so most marine organisms are found in the upper waters, feeding on the plankton and those that feed on the plankton.
Phytoplankton are crucial oceanic producers. Their density varies by region, as they depend on certain nutrients, notably B vitamins. Without enough vitamins in the water a local food web is never spun.
The large algae and aquatic plants that dwell on the bottom do so only in shallow coastal waters, and so contribute little to overall bioproductivity.
The ocean lacks much plant life because it lacks vital nutrients. Nitrogen and phosphorus are only 1/10,000th of that found in fertile soil. Whence the happy evolution of land plants. The more abundant and relatively long-lived terrestrial producer base explains the grater fecundity of animal life on land.
Unlike terrestrial trees that may survive for centuries, plant life turnover in the ocean is rapid. Dead oceanic organic matter, mostly phytoplankton and zooplankton, drop into the deep and dissolve. This marine snow is part of the food web.
Before heading into the deep, let’s first glance a life near the surface of the seas.
Coral Reef Gardeners
Coral reefs are the rainforests of the sea; forming some of the most diverse ecosystems on Earth. Though they occupy less than 0.1% of the world’s ocean surface, coral reefs are home to 25% of all marine species. Coral is one of the most important keystone species on the planet.
Paradoxically, coral reefs flourish even when surrounded by otherwise nutrient-poor water. They can do so because of their little friends. Coral have beneficial viruses, bacteria, algae, and fungi as microbial symbionts (microbiome).
Colorful damselfish live in tropical coral reefs. They eat small crustaceans, plankton, and algae. But not just any algae. Damselfish lack the digestive enzymes to consume many kinds of algae. But red alga is scrumptious. So damselfish cultivate gardens of red alga on the reefs. They pluck unwanted algae varieties from their territories and dispose of them at a distance. They weed out invasive plants. They chase away troublesome invaders, such as sea urchins, which would trample their fields. Protected red alga turfs thrive, affording damselfish abundant dining on one of their favorite foods.
Punishment can help to sustain cooperation where it would otherwise fail. ~ English zoologist Nichola Raihani
Cosmopolitan coral reefs make an irresistible place for ectoparasites to feed. Fortunately, blighted fish can get cleaned free of charge. Cleaner fish, such as the cleaner wrasse, work hard at keeping their clients unsullied.
Cleaning is competitive. Cleaner fish provide a higher-quality cleaning service when competitors are around.
An individual wrasse inspects as many as 2,300 fish a day, consuming up to 1,200 parasites from clientele. This sums to 7% of a wrasse’s body weight.
Each male cleaner wrasse holds a territory which encompasses several female breeding partners. For cleaning services, cleaner wrasse often work in pairs: a male and a female.
Although the service that cleaner wrasse provide is removing skin ectoparasites, they prefer to feed on client tissue. A client won’t sit still for that.
In order to receive a good cleaning service, clients require cleaners to cooperate by feeding against their preference. Clients achieve this either by avoiding cleaners they observe cheating other clients, avoiding cleaning stations where they have received a poor service in the past, or aggressively punishing cheating cleaners. ~ Nichola Raihani
Male wrasse keep their ladies in line; punishing them for cheating by aggressively chasing after them. The larger the client, the more esteemed. To females who cheat, male cleaners mete out a level of punishment commensurate with how valuable the client was.
Female wrasse may switch sex if they become as big as their partner. This is the real incentive for male wrasse to keep females in line, beyond the immediate problem of losing clients. A female that eats too well becomes a competitor.
In contrast to wrasse, Caribbean cleaner gobies do not change sex. Male gobies do not punish females that cheat.
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The service provided by cleaner fish is invaluable to clients. Fish that fail to go to the cleaners have 5 times as many ectoparasites as those that do.
Fish cleaning is not without hazard, and not just from snappy clients or greedy workmates. Cleaner shrimp form monogamous pairs that claim exclusive service areas. Competitors are eliminated, typically during the night while shedding old skin, when they are momentarily more vulnerable.
There’s still a lot we don’t know. ~ Australian marine biologist Julie McInnes in 2018
The marine food web is little understood. Biologists long ignored the abundant jellyfish, as the gelatinous animals are 95% water and provide scant calories.
Jellyfish are ubiquitous in the world’s oceans and can occur in very high densities. Yet they have long been considered trophic dead ends that are ignored by most predators because of their low nutritional content. ~ Australian marine biologist Graeme Hays
Many sea creatures, from tuna to turtles to penguins, seek jellyfish to eat.
The more we look, the more animals are feeding on jellyfish. They’re absolutely, really important. ~ Irish marine biologist Thomas Doyle
There’s a lot more to jellyfish than jelly. Our perception has switched hugely. It’s almost a reboot of jellyfish ecology as a central part of the ocean system. ~ Irish marine biologist Jonathan Houghton in 2018
The ocean is temperature-stratified (thermoclines). The uppermost shell is warmed by the Sun, at least in the tropics.
Just below is a mixed surface layer. Winds and tides mix the water, but the biota there also do their part, including the copious congregations of jellyfish. Even zooplankton are substantial contributors to the mixing. All told, life in the near-surface layer contributes as much to mixing as the wind and waves. This mixing provides for both oxygenation and distribution of warmth to a greater depth.
Below the thermally mixed layer is a narrow thermocline that separates the warm surface water from the colder and heavier water beneath. At the poles, much of the ocean is equally cold.
Though life concentrates near the surface, it is the cold, heavy water in the deep that revitalizes the food web. The marine snow of organic detritus drops down in prodigious volumes. Most marine snow is consumed within the top 1,000 meters, but a considerable quantity reaches the deep. Upwelling cold-water currents return that nutrient-rich supply toward the surface.
The richest concentrations of sea life occur where these cold currents come up. Near the coast, where steady winds sweep warm surface waters offshore, deep-water rises to fill the displacement. In temperate regions, winter storms churn the water, delivering cold comfort to life there.
In the tropics, the separation of warmth at the surface and cold at depth is so great the even hurricanes and typhoons cannot thoroughly mix the two. Tropical seas stay crystal clear, bereft of the microscopic clouds that bring fine dining from below.
On land, oxygen is available at a fairly constant level: 210 milliliters per liter. At sea, usable oxygen enters only at the surface.
Because most of the water in the deep ocean originated on the surface, in or near the polar regions where it sank, it holds the most dissolved oxygen. These down-welled water masses may spend centuries in the deep before rising again. As life is sparse there, oxygen is rarely depleted.
The intermediate depths are where oxygen is at a minimum. In the Pacific Ocean, the oxygen minimum zone (OMZ) is between 500 and 1,000 meters down. Oxygen may be only 1/30th of its concentration on the surface.
Only a few organisms are adapted to living in this band of oxygen-poor water. The vampire squid is one; the only cephalopod to tolerate the OMZ.
The vampire squid is something of a hybrid of squid and octopus; a living relic; the only modern representative of cephalopods before they split into 2 groups: one with 8 limbs, the other 10.
A vampire squid has 8 arms that are webbed together. It propels itself through the water by flapping 2 small fins, 1 on each side of the mantle.
Inside webbed limbs are 2 tactile filaments, which can extend well past their arms. These filaments let vampire squid forage.
Vampire squid are saprovores: feeding on marine snow and other detrital matter that comes around.
This relic is admirably adapted. Vampire squid can breathe normally when oxygen is just 3%. Their metabolic rate is the lowest of all deep-sea cephalopods. Their gills cover an especially large surface area. Their blue blood’s hemocyanin binds and transports oxygen most efficiently.
To help minimize physical requirements, the gelatinous tissues of vampire squid closely match the density of the surrounding seawater. Vampire squid have weak musculature but are able to stay agile and maintain buoyancy with minimal effort, thanks to balancing organs (statocysts) which are similar to the human inner ear.
Life in the slow lane has its advantages. Whereas all other soft-bodied creatures like it (coleoids) have a single reproductive cycle, vampire squid have multiple reproductive cycles.
Vampire squid are covered in photophores: tiny light-producing organs. They can exercise exquisite control over this multitude of luminescent spots: able to produce disorienting flashes of light for up to several minutes.
If threatened, the vampire squid releases a dazzling luminescent mucus from its arm tips that lets it disappear into the blackness without having to swim far.
This self-limited lifestyle has a large payoff. The only predators that vampire squid have are those transiently passing through the OMZ.
Pressure is another challenge in the deep, though less than lacking oxygen. Many invertebrates and some fish can tolerate trips between the surface and a kilometer down.
In the deep sea, life’s diversity is high, but density is quite low. Marine snow can only feed so many. The food pyramid is small.
Those creatures that live in deep waters have adapted in various ways. They tend to be sluggish, and often gelatinous.
Calcium carbonate is hard to come by, and so skeletons are lightweight if they exist at all. Fish that live midwater tend to be small; typically, no more than 20 centimeters.
Invertebrates are not so restricted. Comb jellies become the size of basketballs. Giant squid may reach 20 meters. Siphonophores can be twice that. With a body length of up to 50 meters, the giant siphonophore is the longest sea life.
These marine invertebrates that resemble jellyfish are colonial. Each is comprised of innumerable, tiny, connected individuals – zooids – each with a specific function (feeding, defense, et cetera), energetically and communicatively coordinated into an acting organism.
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The dark is the final frontier of the deep, and so 90% of midwater creatures provide their own light. Bioluminescence offers several advantages beyond being able to see, including communicating to one’s own kind, luring prey, and startling predators. Some can even put an “eat me” sign on an advancing predator by releasing a sticky, glowing tissue that coats the attacker, making it more vulnerable to its predator.
At a few hundred meters depth, dim sunlight still penetrates. A bit of bioluminescence lets a creature blend in so as to be invisible from below.
Although able to flash light, fishes tend to be black, crustaceans red. Large comb jellies and jellyfish prefer purple or red as well. Long wavelength red light does not penetrate the deep, and so reddish hues are effective camouflage.
Top carnivores have no need to hide, and so seldom take such solemn tones of appearance. Tuna, seals, sea lions, dolphins, and whales come and go as they please.
Almost nothing is known of predation in the deep ocean – such as how a sperm whale can dive a kilometer and snag a giant squid.