In the previous tutorial we learned a lot about the class Anthozoa, so we know a few things about
coral. But now it’s time to look more closely at entire coral reefs. Coral reefs are some
of the most productive ecosystems in the world. However, unlike most highly productive ecosystems,
their primary producers are not plants, but the zooxanthellae that live within
the tissues of hermatypic coral polyps, and the coralline algae that create calcareous deposits.
Coralline algae are red algae in the order Corallinales. Their importance is sometimes
overlooked, but their excretion of calcium carbonate not only adds to the total mass of
the reef, but also aids in holding everything together. Their calcium deposits are usually
pink or red but can also vary in color from purple to blue, and yellow to white or green.
In some parts of the world, most notably the Mediterranean, coralline algae are
primary reef builders since they can survive in areas inhospitable to the hermatypic corals.
In addition to coralline algae, many other Cnidarians, like the octocorallian corals
and the hydrozoan fire corals, also contribute to the calcareous
structure of the reef. An enormous variety of other animals also aid in reef-building.
However, they are all often seen more as “supplemental” to the hermatypic corals,
which are generally considered to be the primary reef builders of tropical coral reefs. Most
hermatypic corals require warmth, sunlight, and the salinity of undiluted saltwater. As a result,
they are limited to shallow waters between 30 degrees north and 30 degrees south latitude.
They are also very sensitive to pollution and sedimentation
since hermatypic corals rely on their mutualistic dinoflagellates, known as zooxanthellae.
It’s important to note that not all hermatypic corals contain zooxanthellae. Some, like Lophelia,
are cold-water corals that survive in deep waters throughout the North Atlantic. These
deep-water stony corals, like black corals, soft corals, and sea fans,
provide habitats for other organisms, but rarely form large coral reefs like the tropical corals.
Stony corals, like all anthozoans, can reproduce sexually and asexually. When an egg cell is
fertilized by a sperm cell, the resulting zygote develops into a free-swimming planula. If this
planula successfully settles into the substrate, it will metamorphosize into a primary polyp.
This primary polyp will then form the basis of its calcareous skeleton through excretions of
the lower epidermis. This initially forms a tiny cup for the young polyp. The polyp
then secretes septa in the characteristic six-fold pattern of all hexacorallians.
Then, the polyp will repeatedly reproduce asexually to give rise to an entire colony.
A colony of coral polyps are connected by horizontal sheets of tissue known as coenosarcs
that extend over the outer surface of the calcium carbonate skeleton and completely cover it. These
sheets not only connect polyp bodies, but are also extensions of the gastrovascular cavity,
meaning that nutrients and water can be exchanged between members of the colony.
These colonies undergo sexual reproduction once annually. Cued in intricate ways by
the lunar cycle and water temperature, they simultaneously release eggs and sperm into
the water. If an egg cell is fertilized by a sperm cell of the same species,
they form a free-swimming planula which can start a new colony elsewhere. Once in a while the sperm
can fertilize eggs of other coral species, but the zygote usually dies before forming a planula.
Less frequently such a hybrid zygote will form a planula, but this typically dies very quickly.
Even less frequently the hybrid planula can settle to form a polyp and even a juvenile coral,
but there are no recorded cases of these hybrid corals living past the juvenile stage.
Getting back on track, before a colony of coral polyps can begin to form, the polyp needs to
obtain their photosynthetic zooxanthellae. This can happen through either a direct or indirect
transfer. In direct transfer, the mother coral polyp allows her own zooxanthellae
to enter developing eggs, or the eggs themselves absorb zooxanthellae before they are released.
In indirect transfer, developing planula and polyps absorb their zooxanthellae
from the fecal matter of fish or by capturing free-swimming dinoflagellates.
Most individual coral polyps are tiny, about 1-3 millimeters in size,
but the skeletons they construct can be massive. Not only that, but since
corals grow at a steady rate and are extremely sensitive to conditions in the world’s oceans,
coral reefs, like tree rings, provide a type of climatological proxy data. Like coring a tree,
something we discussed at length in the botany series, taking a slice of
a coral skeleton allows researchers access to deposits that were excreted slowly over time.
Though corals don’t generate rings like trees do, there are still numerous ways to date coral reefs.
One common protocol for dating deep sea corals involves a combination of radiocarbon dating,
a method of dating comparing the current carbon-14 to carbon-12 ratio with that of a past atmosphere,
and uranium-thorium dating, a method of radiometric dating that determines
the age of calcium carbonate, the dominant material in coral deposits.
A second method is a combination of magnetostratigraphy,
which is a geophysical correlation technique, and strontium isotope analysis,
which are used in conjunction to determine the earliest evidence of the great barrier reef
at around 600,000 years, though its most recent form is only about 6,000 – 8,000 years old.
Despite the incredible diversity of reefs, the most common are fringing reefs, barrier reefs, and
atolls. Fringing reefs form close to land masses, while barrier reefs run parallel to the shore and
form wide and deep lagoons. Atolls are ring-shaped reefs that form lagoons without land masses.
The Great Barrier Reef is, contrary to its name, actually a complex of many different reef types.
Large reefs generally have distinguishable zones. The side facing the sea is known as the reef
front, which runs parallel to the shore and slopes downward into the water. In many regions around
the globe, coral reef ecosystems exhibit a similar depth zonation of dominant organisms. That is,
shallow waters, less than 50 meters in depth, are dominated by coralline algae and zooxanthellae
containing hermatypic corals. These organisms then give way to black corals, false corals,
and other Cnidarians that do not contain zooxanthellae and dominate from below the
transition zones to areas of the abyssal plains. Back close to shore, in shallow water, the reef
forms a reef crest that gently slopes into a reef flat, which slopes down further into a lagoon
that is protected from the waves. An astounding number of species make their homes in different
areas of the reef and form unique and fragile habitats with strong ecological connections.
Globally, these connections, and hermatypic corals in particular, are under stress due to
climate change and ocean acidification. Global warming has been correlated to coral bleaching
which occurs when corals lose their zooxanthellae and become white and brittle. As the water warms,
it damages part of the photosynthetic mechanism in the zooxanthellae,
which leads to a buildup of harmful oxidants which diffuse into the polyp tissues and begin
to harm or kill the corals. In an attempt to survive, many corals expel the zooxanthellae.
In some cases, bleached corals can then uptake new zooxanthellae later,
but in others the corals become too weak. These brittle corals are then colonized by algae
that outcompete the weakened polyps and colonize the skeleton, killing the coral.
Corals are also under threat due to ocean acidification, where carbon dioxide entering
seawater reacts to form carbonic acid, causing an increase in acidity that makes it harder,
and sometimes impossible, for corals to form their calcium carbonate structure. In many parts
of the world, corals are also under threat due to physical damage, sedimentation from urban run-off,
nutrient pollution from farms, pathogen infection from slugs, toxic substances from sunscreen,
microplastics from stormwater run-off, destructive fishing practices, and coral harvesting.
Though all these problems add up to major issues for the corals,
it’s worth noting the increase in local and global efforts to save coral reefs. Sanctuary
reefs have been increasingly protected in Central and South America, destructive fishing practices
over cold-water reefs have been prohibited in many parts of Europe,
funding to save local reefs has increased in many parts of the world,
and regulations to control sedimentation have become stricter. That isn’t to say coral reefs
aren’t still threatened, but the increase in public awareness is at least promising.
And with corals covered, we can move forward and visit just a few other types of Cnidarians.