The World Beneath. Richard Smith
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Название: The World Beneath
Автор: Richard Smith
Издательство: Ingram
Жанр: Биология
isbn: 9781948062237
isbn:
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 СКАЧАТЬ