Coral Reefs Explained: Cnidaria Part 6 | Zoology

[00:00:06]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.