Читать книгу The World Beneath - Richard Smith - Страница 11

We begin our story with a humble jellyfish-like creature in the Triassic period, some 240 million years ago. After the great Permian Extinction—some ten million years before the Triassic—almost wiped the Earth clean of life, marine creatures were beginning, again, to find their feet.2 In the wake of this tumultuous period, life seized the opportunity to expand and reclaim the Earth. During the Triassic, the first ancestral scleractinian corals appeared;3 today, we know their kin as hard or stony corals. They shared the seas with enormous and menacing dolphin-like dinosaurs, called ichthyosaurs; lobe-finned fish very similar to the coelacanth, a six-foot-long fish that was long-believed extinct and famously rediscovered by Marjorie Courtenay-Latimer in 1938; feathery crinoids, ancient plant-like relatives of the sea star; and many other groups that still populate coral reefs today.

Several individual coral polyps. Solomon Islands.

The domination of coral reefs in today’s shallow tropical seas is due to the symbiotic relationship between scleractinian corals and unicellular algae, called zooxanthellae. This special ecological relationship allows corals to finesse their metabolic processes of respiration, metabolism, and waste product management, improving their growth rates and allowing them to supercharge their deposition of calcium carbonate. Early stony corals were small and solitary—it took millions of years before this symbiosis enhanced them into becoming true reef builders.4 However, many of the families of stony corals we see today are very old, having originated in the middle to late Jurassic period, one hundred and fifty million years ago.

Corals are living animals, although they may not fit our preconceived notion of what defines an animal. These tiny relatives of the sea anemone and jellyfish are sessile creatures, permanently attaching themselves to the seafloor, somewhat like a plant. The living parts of the coral are very simple, soft-bodied animals called polyps. For many colonial corals, each polyp is just a few millimeters across; solitary polyps, however, such as those of mushroom corals, can sometimes be almost a foot in diameter.

Each polyp comprises a single opening surrounded by a ring of tentacles. The tentacles are covered in specialized stinging cells, called nematocysts, which help to harpoon and trap passing food particles. Internally, most of the polyp is a simple stomach, the single opening acting for both ingestion and excretion. The living polyp sits atop a protective calcium carbonate structure, the coral’s deposit, which has been key to them becoming such prominent ecosystem engineers.

The vast majority of a coral colony comprises the dead skeleton structure beneath, which is blanketed with a very thin layer of living tissue comprised of many individual polyps. A colony of individual polyp clones can have hundreds of thousands of polyps. They are connected to one another by a thin band of living tissue. Thousands of individual coral colonies, constituting many species, make up a reef.

Darwin’s Paradox

While vivid blue tropical seas may draw vacation-goers in droves, the same crystal-clear waters aren’t as inviting to marine organisms. In the ocean, truly crystal clear water lacks the nutrients that can be exploited to sustain life. Nutrients in the water column, the stretch between the surface and ocean floor, are usually highlighted by the presence of plankton, which add a noticeable hue. Plankton is the term used for a diverse array of miniscule organisms that float or drift in the open ocean and are largely transported by the whim of currents. The term “Darwin’s Paradox” refers to the conundrum that Charles Darwin himself highlighted: coral reefs are oases in the desert of the blue ocean.5

Corals are only able to flourish and grow in these nutritionally deficient waters thanks to the symbiotic relationship shared between single-celled algae, zooxanthellae, which live within their tissues. Symbiosis means that both parties benefit from the relationship; in this case there are advantages for both the coral animals and the zooxanthellae. Zooxanthellae form colonies within the safe, soft tissues and tentacles of the coral polyps, where they harness the sun’s light to photosynthesize and produce sugars. These sugars fuel the corals and allow them to sustain unparalleled growth, compared to their relatives that don’t harbor such algae. In return, the corals supply zooxanthellae with their metabolic waste products that the algae then use to fuel photosynthesis. Corals do still need to supplement the nutrition provided by the algae, so at night the polyps swell and they feed on passing plankton using their stinging tentacles.

The meeting of two distinct coral colonies. Cenderawasih Bay, West Papua, Indonesia.

This extremely tight cycling of nutrients means that very little goes to waste and corals are able to direct significant energy into reef building. Coral polyps deposit calcium carbonate at varying rates—some of the most prolific branching species can grow seven inches in a year—although four to six inches is more common. On the other hand, the slower growing boulder-shaped forms may grow by just a few millimeters per year.6 Over thousands of years this symbiosis has been responsible for the world’s largest biogenic structure: the fourteen-hundred-mile-long Great Barrier Reef located off the coast of Queensland, Australia. Without this symbiosis, calcium carbonate deposition and coral growth in the tropical shallows would be negligible, and reefs as we know them would not exist.

Photosynthesis above and below the waves. Solomon Islands.

Zooxanthellae: Haves and Have Nots

Corals have benefitted enormously from hosting intracellular zooxanthellae, and some other creatures have followed suit. Other immobile reef organisms, like sponges, sea anemones, and certain soft corals, also benefit from a relationship with these algae, as do certain mollusks. On land, we are familiar with slugs and snails, but in the oceans, mollusks are much more diverse. In addition to the tens of thousands of species of gastropods (slugs and snails), other well-known groups of ocean mollusks include cephalopods (octopuses, squids, and nautiluses), bivalves (such as clams and oysters), and chitons (unusual plated slug-like animals). Giant clams and a number of sea slugs have zooxanthellae living within their tissues.

Waves crashing over a Red Sea reef. Egypt.

One genus of sea slugs, Phyllodesmium, has a stronger relationship with the algae than most. These slugs are covered in cerata, which are thin, finger-like protrusions that cover the back of the animal. Cerata house the digestive glands of the slug, and zooxanthellae reside within the cells of these glands. The sea slug obtains the zooxanthellae from its food: beige soft corals found in shallow water. Each species of Phyllodesmium has selective tastes, and its camouflage reflects the type of coral that it prefers to feed on, rendering them almost invisible while feeding. The slugs can sit and feed leisurely on the coral with very little risk of predation, while also supplementing their nutrition through the photosynthesis of the zooxanthellae within their cells.7 I have even spotted a particularly large species, P. longicirrum, hanging out in the open on the reef with the casual air of a sunbather soaking up the sun’s rays.

Detail of a giant clam’s mantle. Great Barrier Reef, Australia.

Not all corals contain zooxanthellae, and it tends to be simple to tell which ones do. Zooxanthellae typically have a beige coloration, so corals containing these algae tend to be beige. Non-zooxanthellate corals (those corals without intracellular algae) tend to be much brighter in color. Most of the bright colors you might associate with coral reefs come from organisms that do not have symbiotic algae. These animals obviously do not benefit from contributions of a symbiont; however, as they do not require access to sunlight (a key ingredient in Darwin’s Paradox), these corals are not as limited in where they can grow. They will often proliferate inside large overhangs or caves that receive little natural light.

Detail of non-­zooxanthellate soft coral polyps. Raja Ampat, West Papua, Indonesia.

Rather than deriving nutrition from their symbionts, non-zooxanthellate corals mostly filter feed and usually situate around parts of the reef where nutrient-­rich currents hit. Nutrient-rich water tends to be rather murky, which explains why—in that setting—the vibrant colors of soft corals, sponges, and other filter feeders truly come alive.

At 100 feet beneath the surface, filter-­feeding organisms proliferate. Komodo Island, Indonesia.

Triton Bay in West Papua, Indonesia, experiences localized seasonal upwellings of cool, nutrient-rich water that turn the existing water into a green soup. The first time I dived there, descending with a little hesitation into the eerie water, I wasn’t sure what to expect. When I reached the seafloor I was taken aback by the profusion of growth and riot of color. Without having to compete with light-loving corals, other organisms had been able to proliferate. Soft corals in reds, pinks, yellows, and purples covered every inch of the reef. In an illogical contradiction, most color exists in the gloom.

In plankton-­rich waters, invertebrate life grows in profusion. Triton Bay, West Papua, Indonesia.

Soft corals do not produce a ridged calcium carbonate skeleton like hard corals do; rather their structure is based on a mesh of tiny calcareous rods, known as “spicules.” During the parts of the day when currents aren’t flowing, soft corals shrink, forming small spiky balls. As currents begin to pick up, they swell and expose their polyps, which can then begin to trap plankton from the passing water. These corals, which live without zooxanthellae, are also found in the deep, lightless sea where individual colonies grow extremely slowly for thousands of years. Twenty thousand feet below the surface, in water of 30 degrees Fahrenheit, life continues at a slower pace. Without the volatile climatic conditions experienced in shallower waters, the corals here have created rich but fragile ecosystems that are home to their own unique fauna.

Where Corals Grow

With the symbiotic coral-algal powerhouse facilitating growth of corals in clear tropical seas, few other organisms in this habitat are able to compete. However, corals do have specific environmental requirements; where these are not met they can be outcompeted by other organisms that are more tolerant of the particular conditions. On the whole, corals have a few basic ecological requirements to maintain meaningful reef building growth.

Warm water is fundamental to coral growth. Corals generally have a tolerance for waters between 70 and almost 90 degrees Fahrenheit; in winter, they can briefly endure temperature dips to 65 degrees. Because of this, most reefs fall within the latitudes of 30 degrees north and south; however, in some parts of the ocean the reefs might extend slightly farther toward the poles. Certain globally important currents assist in extending the reach of warmer waters beyond these latitudes, for example the major Pacific equatorial currents that flow separately in the northern and southern hemispheres. In the southern hemisphere, the South Equatorial Current is driven by winds across the central Pacific and strikes the Great Barrier Reef off the coast of Queensland, Australia; it then flows from there in a southerly direction as the East Australian Current. As the East Australian Current flows southward, it carries warm waters from the equator that can ultimately reach as far as Tasmania. This was the current that Nemo used to travel from the Great Barrier Reef in the north to Sydney in the south in Finding Nemo. Without the East Australian Current, the waters off Sydney, for example, would be significantly cooler and many organisms would have their southern ranges much farther north. Over recent decades, this current has been strengthening and extending farther south, pushing the geographic ranges of some organisms in a southerly direction.8

I have seen firsthand how this extension of the warm East Australian Current has had an unfortunate impact on the historically cool waters of southern Australia. In 2011, I visited the Tasman Peninsula in southeast Tasmania to dive the giant kelp beds. Much like the famed kelp beds of California, these giant algae form huge dense and towering forests. These kelp beds were known as being similarly impressive as their northern Pacific counterparts, and it was the only time I have had the chance to experience this amazing ecosystem. The kelp beds were previously so dense and widespread as to allow commercial harvesting but have gradually dwindled over the years. I dived one of their last real strongholds, Waterfall Bay, in southeast Tasmania—where colorful weedy seadragons and a wide variety of other unique creatures exist.

Robust coral growth enduring crashing waves. Egyptian Red Sea.

Huge and elaborate coral growth can take many decades or even centuries to grow. Egyptian Red Sea.

Having been so amazed by this aquatic jungle, I returned six years later, eager to share the spectacle with some friends. To my utter dismay, I was told that kelp had disappeared almost entirely from Tasmania since my last visit, due to the extension of the East Australian Current. Although Tasmania may seem like a remote wilderness, the area is suffering some of the most extreme impacts of climate change on the planet.9 It is among the top 10 percent of places on Earth for extreme ocean warming, heating at four times the global average, with no sign of this deadly trend abating.10 The reason for the kelp’s disappearance isn’t only the influx of warm water. The long-spine urchin, a mainland native, has migrated with the warmer waters, extending its range four hundred miles south over the past forty years.11 A voracious herbivore, the urchin eats juvenile kelp and prevents it from establishing. The loss of the kelp is thought to have directly impacted at least 150 species associated with the forests.12

Looking at global patterns of coral reef distributions, you find coral reefs in unexpected places. Conversely, you might not find a reef in a place where you would expect one. On the west coast of the tropical Americas, an area where you might expect coral reefs to flourish, they tend to exist only in isolated patches, measuring just a few hectares in size.13 Imagine the Galápagos Islands, which, at the equator, seem to occupy the perfect coral comfort zone. However, aside from a few scattered and species-poor reefs at Darwin Island in the far north, corals in the Galápagos remain few and far between. While Indonesia, for example, boasts well over five hundred hard coral species, the Galápagos hosts just twenty-two.14 The lack of corals is largely due to unsuitably cool water that bathes the islands. Despite their equatorial location, it is not uncommon for the Galápagos Islands to experience waters between 50 and 60 degrees Fahrenheit, which is far too cold for corals to thrive. The cool, nutrient-rich water originates in the deep ocean and is brought to the surface in a process known as “upwelling.” There are various locations around the globe where similar upwellings bring cool water to the surface, in all cases inhibiting the growth of coral, thus limiting the potential extent of coral reefs.

Competition for space on a Papuan reef. Raja Ampat, West Papua, Indonesia.

For optimal growth, corals also need constant salinity, which the large bodies of the ocean provide. It follows then that in coastal areas, large river outflows of fresh water inhibit coral growth. Reefs do not occur near the mouths of the Mississippi, Amazon, and Ganges rivers; the huge volumes of fresh water decrease salinity, and the high levels of sediment loads prevent sunlight from penetrating very far through the water column. The murky sediments in the water from the surface down to the seafloor do not permit sufficient penetration of light, thereby limiting necessary photosynthesis. Conversely, water with too high salinity can also pose a threat to coral growth, if it passes a certain threshold. The Red Sea, a substantial inlet of the Indian Ocean west of Saudi Arabia, is one of the saltiest life-supporting bodies of water on Earth and lies at the upper limits of coral’s salinity tolerance. Due to the high salinity, when I dive in the Red Sea I have to wear an extra couple of kilos of weight on my equipment to be able to descend in the water column.

Coral growth proliferates into the shallowest water. Raja Ampat, West Papua, Indonesia.

Finally, corals require ample sunlight for optimum growth. They must attach to a hard surface within sufficiently shallow depths to allow for ample light penetration. New coral reefs can’t simply open up shop in the middle of the ocean: they can only grow around islands or existing atolls, a ring-shaped coral reef. The maximum depth that light can penetrate clear water for coral growth hovers at around 160 feet, and most of the open ocean is far deeper. But what of the reefs that punctuate the remote recesses and vast stretches of deep blue sea? Charles Darwin theorized that coral atolls formed in the open ocean of the Pacific through a series of stages; first, corals settled around ancient volcanoes as fringing reefs hugging the coasts. Then, as the volcanoes began to sink and erode into the seafloor, their former coastlines receded. The reefs grew farther from these shores and became barrier reefs guarding lagoons, which filled the new distance between corals and coasts. Ultimately, the volcano cones succumbed to the depths of the ocean, but the corals remained: growing toward the light and leaving remote atolls that are scattered throughout the open ocean.

Because of the rich tapestry of life on coral reefs and the different environmental conditions that shape them, each ecosystem is unique. Through my experiences of diving around the world, I have become familiar enough with the reefs that I can tell exactly where a picture was taken by the composition of sessile invertebrates growing there (that is, if the fish don’t give it away). Sessile invertebrates are those animals such as corals, sponges, bryozoans (an ancient lineage of small filter-feeding invertebrates), and tunicates (a common marine invertebrate with an important role in the evolution of back-boned animals) that permanently attach to the seafloor or reef to grow.

Huge waves crashing through an archway. Raja Ampat, West Papua, Indonesia.

With so many sessile invertebrates vying for space on the reef, there is fierce competition among them. It is clearly not in an organism’s best interests to have other species settle directly next to it and possibly overtake it, so these reef dwellers have evolved ingenious modes of self-protection. Some soft corals and sponges release chemicals into the water, which inhibit the settlement of other sessile invertebrates directly around them.15 Inhibition of one species by another to impede growth or prevent settlement through the release of chemicals into the environment is known as “allelopathy.” The larvae of other invertebrates detect the chemicals, called allelochemicals, and avoid them.16 If they settle in the proximity of an allelopathic organism, they could be killed by the chemicals.

One of my favorite places in the world to dive is Raja Ampat. Here, every single inch of the reef is covered in growth of some sort, and the growth continues to within inches of the water’s surface. I spent an entire two-week dive trip exploring these islands without straying thirty feet below the surface—pretty unusual, given that a recreational dive certification allows you to dive to one hundred feet. The shallows of Raja Ampat, which means “Four Kings” in Indonesian and refers to the four large islands that comprise the area, are full of hard coral growth, as well as the growth of various sessile invertebrates. Most of these islands were produced by ancient coral reefs; over time, ocean erosion has whittled away at their bases, leaving the islands with mushroom-like topographical forms. This erosion is so extreme that in some places the coastline of the islands is undercut by fifteen feet or more. Underneath the mushroom-like overhangs, light cannot reach the substrate, the hard base on which organisms live, so light-loving corals aren’t able to grow. In their place, the corals that would ordinarily only be found in deeper water appear in three-feet-deep water. Multihued sponges, gorgonian sea fans, soft corals, whip corals, and black corals cover the rock, while the waves bubble under the overhang above.

While the composition of coral reef communities varies hugely between locations, the characteristics of the corals themselves can be extraordinarily plastic too. The same species of coral can be variable in its growth shape depending on the local conditions. In deeper water, where light is at a premium, corals grow differently than those of the same species in shallow water. Likewise, in very protected areas, a branching coral is more likely to grow with a fragile, spindly appearance than in an exposed site, where branching coral will be more likely to have a squatter and more robust growth form that can tolerate a battery of waves and storms.

Perhaps, as land-living animals, we mistakenly believe that the vibrant, polychromatic nature of reefs likens them to the colorful but ephemeral annual flowers that we see in the garden, rather than ancient and enduring redwood forests. Giant clams and anemones may both live for more than one hundred years;17 even anemonefish could live up to ninety years at the extreme end of current estimates;18 some large hard coral colonies could be eight hundred years of age.19 These timescales are important to bear in mind when we consider the conservation of coral reefs and how long it takes for them to recover following damage.

Undescribed coral hermit crab. Solomon Islands.

The substrate that exists between the attachment site of sessile invertebrates is potentially as important as the rest of the community due to the other organisms that call it home. Herbivory, or feeding on plants, is a very important ecological process that builds communities both on land and in the water. On coral reefs, herbivores play a vital role in maintaining the status quo of the reef by grazing on the turfs of algae that grow constantly on rock surfaces. The many species of herbivorous fishes and invertebrates keep busy, dining on 90 percent of the algae produced each day. Without this constant grazing pressure, algae can grow large and tough and ultimately reach a size unpalatable for most herbivores. If the herbivores falter in their efficacy and allow the algae to mature, the entire ecosystem can irreversibly shift from coral to algal dominance. Overfishing of herbivores such as parrotfishes can have a wide-ranging impact as a result. Only one species of unicorn fish seems to have a penchant for these large macroalgae, and the balance of the reef’s ecology rests on the appetite of this humble fish.20 As this illustrates, each species plays its own unique role in the ecology of the reef. It is therefore critical to maintain and conserve the full gamut of coral reef species to ensure that all the reef’s vital ecosystem processes are accounted for, and no single part claims a monopoly.

Christmas tree worm. Fiji.

Coral Homes and Corallivores

The physical framework of corals provides three-dimensional structures on the reef for creatures to live in or hide in and can also serve as food for many species to live on; the deceased skeletons of hard corals even bequeath a foundation for the next generations of corals to grow on.

Big-­lipped damselfish. Raja Ampat, West Papua, Indonesia.

Coral hermit crabs are one of the species that exploits corals as a home. Instead of carrying around shells for protection like their non-reef counterparts, coral hermit crabs cozy up in small holes in the coral, filter feeding with specially adapted arms and never departing their safe havens. Likewise, Christmas tree worms depend entirely on corals providing them places to burrow, shelters from which to extend their filter-feeding bristles to trap passing plankton.

Dot and dash butterflyfish. Fiji.

Corallivores, those organisms that feed on corals, are easily spotted on healthy coral reefs. I became fixated with one corallivore in particular: a damselfish called the big-lipped damsel. I had only ever seen pictures of the fish and was having a hard time finding them in the wild. Finally, while diving a small marine reserve in the southern Philippines, I spotted one of these superficially unspectacular fish by chance in a rich coral garden. I identified it by the quick glimpse I caught of its preposterously large lips as it fed on a branching coral. These lips are so large that they’re surely the true inspiration for the inflated Hollywood “trout pout.” However, these lips serve a purpose, as does everything in nature. After finding more of these fish a year later in West Papua, I spent an hour or so watching their antics and confirmed the function of the lips. These fish are obligate corallivores, and rather than utilize the approach of the dainty butterflyfish that pluck one polyp at a time, the damselfish instead completely denudes the surface of the coral of tissue, their sensitive flesh meeting the sharp coral skeleton beneath. Their voluptuous lips protect them from cutting and tearing their skin against these edges.

The most well-known of specialist corallivorous fishes must be the butterflyfish, which are ubiquitous and conspicuous members of coral reef communities around the world. Because most butterflyfish completely rely on healthy and diverse coral assemblages, they quickly respond to changes in coral health and abundance by moving location; in extreme cases of ecosystem degradation, they can die. In the early 1980s it was suggested that butterflyfish could act as indicators of coral health, as they are easy to survey and changes in their populations are easy to document.21 It would be much more difficult to survey corals directly as they are so numerous and can be very hard to identify. Butterflyfish can be quite picky about their favored food sources, so their population fluctuations also help to highlight changes within specific coral groups. They are also territorial, and the density at which they will tolerate other butterflyfish is directly related to local food supplies. Therefore, these coral reef staples can tell us how many species of coral are present in an area, as well as their density.

Spawning

One evening in mid-November, after spending the twilight hour making observations on the social behavior vswam up into a cut in the reef full of corals, sponges, and crinoids. There was an excitable energy in the water, and before long I started to notice a few little white specks lifting off a coral. Very quickly, several corals and even a crinoid began spawning, releasing great clouds of sperm and eggs. It was magical to witness this event firsthand. The crinoid appeared almost on fire as a smoke of gametes were released. The coral took a subtler approach, releasing visible individual large gametes into the water.

Spawning barrel sponge. Tubbataha Reef, Sulu Sea, Philippines.

This type of reproduction is known as “broadcast spawning,” or “mass spawning.” Most corals and other animals that are permanently stuck to the reef release their sperm and eggs into the water in this way, where fertilization and development then occur externally.22 Mass broadcast spawning is one of the greatest natural spectacles on Earth since most of a given reef’s organisms are synchronized to spawn on one or a few nights per year, turning the water surface into a noticeable slick due to the density of the spawn. Most species in a given area tend to spawn on the same evening, although there may be other evenings throughout the year where conditions suit the spawning needs of other species. Mass spawning of different species may be an adaptation to overwhelm predators and prevent them from taking too much of a toll on the reproductive potential of a single species. Interestingly, researchers have found that spawning just several hours apart on the same day is enough to prevent coral colonies from cross-fertilizing, and this may lead to reproductive isolation and the evolution of different species.

Spawning Acropora coral. Wakatobi, Sulawesi, Indonesia.

Mass coral spawning typically occurs a few nights after a full moon. Environmental cues remain vital in enabling one hundred-plus species of corals and other organisms to orchestrate their spawning on the exact same evening of the year, with each species typically utilizing just a four-hour window in which to spawn. Various environmental cues such as day length and luminance from the moon appear to allow the creatures to synchronize with other members of their own species.23 Other organisms plan their schedules around the same yearly cycle to exploit the coral spawning. Whale sharks make great migrations from across the Indian Ocean to coincide with the spawning at Ningaloo Reef in Western Australia, as the spawn significantly contributes to the sharks’ annual dietary intake. The tourism industry has also developed around the regularity of the spawning and the shark’s arrival.

Sessile Invertebrates

A huge variety of sessile organisms, in addition to corals, occupy the hard substrates of reefs, including sponges, bryozoans, tunicates, gorgonians, sea anemones, and corallimorphs (large disc-shaped animals, superficially similar to sea anemones). Some of these organisms are so ancient that they haven’t shared a common ancestor since the times of the very earliest animal life on Earth.

Sponges, one example, are very simple animals that draw water in through their sides using tiny whip-like cells to create a current. Recent research on sponges has found that they may be the missing link that helps to transfer organic matter in the water column into a form that the reef’s inhabitants are able to consume, filling in another missing part of Darwin’s Paradox.24 The cells in a sponge’s filtering system have a very high turnover rate, producing a consumable detritus for reef creatures. In the Caribbean, sponges are more abundant and species-rich than corals and have a huge variety of important roles on the reef, even physically cementing the whole structure together. Some sponges have zooxanthellae, like corals, whilst others have bacteria that help to fix nitrogen into a form that they can use in growth.

Many other organisms attached to the reef contribute to the diversity of coral reefs. The Anthozoa, which includes not just the corals, but also a wide variety of their relatives such as gorgonians, sea anemones, and corallimorphs are very diverse. Some of these are solitary and consist of a single polyp, such as sea anemones and corallimorphs, which are both known for their noxious stinging tentacles. Gorgonians are considered to be a type of soft coral, and like their closest relatives, do not produce a calcium carbonate skeleton. Gorgonians are largely defined by their fan-like shape, which gives them their common name, the sea fan. Some gorgonians grow to be the size of a car; others form bush-like shapes. Often, the only way to identify them at a species level is to view them under microscopic magnification.

Sea anemone tentacle detail. Wakatobi, Sulawesi, Indonesia.

Tunicates, sometimes also known as “ascidians” but more commonly known as “sea squirts,” are prevalent on most coral reefs though they are generally small and largely ignored. Sea squirts are simple animals that have a single inhalant and single exhalent opening, where water flows in and out to be filtered of nutriment. One of the most interesting aspects of their biology is that they are the most foundational living relative of the vertebrates. Vertebrates are one of the great branches in the tree of life and include those animals—humans, fish, amphibians, birds, and other mammals—that have a backbone. The link between tunicates and vertebrates has been shown through genetic analysis as well as identification of some very early key features of backboned animals seen in tunicate’s tadpole-like larval forms.25 It’s amazing to consider that these tiny gelatinous sea squirts could have given rise to all backboned life on Earth.

Bobbit worm waiting to ambush its prey. Anilao, Luzon Island, Philippines.

A playful Maori octopus, the world’s third largest species of octopus, attaining 22 lbs in weight. South Australia.

Unsexy Beasts

In addition to the many sessile invertebrates that are fundamental for coral reef ecosystems that remain rooted to the reef, there are of course many active and mobile invertebrates too. Mollusks, crustaceans (a group of arthropods that includes many familiar species such as lobsters, crabs, barnacles, and terrestrial pill bugs), echinoderms (an evolutionarily distinct group of animals that includes sea stars, urchins, and crinoids), and polychaete worms (a branch of worms with noticeable bristles) are some of the most easily spotted, but they rarely come up when people think about the biological diversity of coral reefs.

Nudibranch (Aegires villosus). Anilao, Luzon Island, Philippines.

Nudibranch (Favorinus mirabilis). Sangeang Island, Indonesia.

Nudibranch (Caloria indica). Raja Ampat, West Papua, Indonesia.

Nudibranch (Glossodoris stellatus). Raja Ampat, West Papua, Indonesia.

Nudibranch (Nembrotha kubaryana). Solomon Islands.

Nudibranch (Tambja tentaculata). Raja Ampat, West Papua, Indonesia.

Worms might be the least appreciated of reef animals, but their role in breaking down coral rock is important for the background functioning of the reef. Feather duster worms filter food from the water and tend not to leave their burrows, but other polychaete worms are much more active. Fireworms scurry about, confident that their irritating bristles will ward off predators. Another worm that appears fearless is the Bobbit worm. Named in honor of the infamous incident in which Lorena Bobbitt cut off her husband’s manhood as he slept and then threw it out of the car window into a field, these terrifying worms emerge from the sand during the night and stand upright with a pair of enormous pincers held open so they’re ready to pounce on any unsuspecting fish. They can reach ten feet in length and are a nightmarish predator for small reef fish. You certainly wouldn’t want to complete your one hundredth dive at night where these worms are found, since the one hundredth dive is traditionally done in the nude and the worm may get confused.

Nudibranch (Chromodoris sp.). Hachijō-­jima, Japan.

Nudibranch (Miamira alleni). Anilao, Luzon Island, Philippines.

Nudibranch (Halgerda willeyi). Hachijō-­jima, Japan.

Nudibranch (Sakuraeolis nungunoides). Sangeang Island, Indonesia.

Nudibranch (Okenia kendi). Lembeh Strait, Sulawesi, Indonesia.

Mollusks are another group that features prominently on coral reefs. There are many sessile forms, such as giant clams and oysters, but also many mobile groups including thousands of species of sea slugs, snails, little-­known groups such as the many-­­plated chitons, and, of course, the cephalopods. Mollusks are an extremely species-­rich group, with 80 percent of mollusks belonging to the class Gastropoda, which includes all snails and slugs. Clearly, the cephalopods, a group of around eight hundred species containing squids, octopuses, and nautiluses, have evolved in a different direction than the slugs. Nautiluses are a fascinating group of spiral-­shell-dwelling free-­swimming cephalopods. Of the several thousand known fossil nautiloid species, which were once one of the ocean’s dominant marine predators, there are now just a few marginal extant species. Cephalopods have evolved to be some of the most intelligent animals on the reef; certainly, the most intelligent invertebrates. They have highly developed nervous systems and brains as large as some equivalent-­sized vertebrates, which have afforded them unparalleled learning and memory.

Peacock mantis. Dumaguete, Negros Island, Philippines.

Nectria saoria sea star. Tasmania, Australia.

Blue sea star in the shallows. Raja Ampat, West Papua, Indonesia.

Pink-­eared mantis. Dumaguete, Negros Island, Philippines.

Also among the reef’s active invertebrates are the arthropods, segmented animals whose more familiar members include insects, spiders, and centipedes. In the oceans, arthropods are equally diverse and include crustaceans such as crabs and lobsters, as well as the little-known pycnogonids, known as “sea spiders,” and horseshoe crabs. The sea spiders and horseshoe crabs are unusual and ancient branches of the group, rarely seen and minimal actors in coral reef ecology. The crustaceans, however, are a very important group, both in their free-living forms and as parasites, as we’ll learn in a later chapter. Barnacles, the familiar white mottling seen within the splash zone of the rocky shore or on the chins of whales, are nonetheless unexpected members of the crustaceans, and more recognizable as crustaceans in their larval form; as adults they remain cemented to the rock and rely on filter feeding. Shrimps, lobsters, mantis shrimps, and many types of crabs are very common on coral reefs and range in size from tiny species almost invisible to the naked eye to huge lobsters.

Pearlfish. Lembeh Strait, Sulawesi, Indonesia.

Echinoderms are the final group of dominant invertebrates and are entirely marine. They are simple and ancient animals with five main classifications: the crinoids, sea stars, brittle stars, urchins, and sea cucumbers. Among these various groups, echinoderms play a very significant role on coral reefs. Urchins furiously graze away algae; crown-of-thorns sea stars form plague proportions that can wipe out entire coral reefs; and sea cucumbers feed on the bacteria that cover sand grains. Echinoderms also provide homes for a huge array of other organisms. One of the most shocking examples is the pearlfish that lives inside the body cavity of sea cucumbers. They are a rod-like transparent fish that emerge from the cucumber’s anus at night to feed, and they pop back in during the day to hide inside the rarely predated echinoderm.

Coral reefs are composed of organisms from some of the most disparate branches from across the tree of life, all wrapped up in one ecological hodgepodge. It is due to these long-diverged branches of life that coral reefs are considered to have higher levels of diversity than tropical rain forests, with many of a reef’s organisms dating back hundreds of millions of years.

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