Download Coral Reef - Center for Occupational Research and Development

Student Background: Coral Reef
Like brilliant underwater architects, reef building corals lay down spectacular structures that
provide a habitat for a colorful and diverse array of marine life every year. Relatives of the
jellyfish and sea anemones, corals are a member of the phylum Cnidaria [nahy-dair-ee-uh].
From an evolutionary standpoint, they fall “between” the sponges and the coelomates. Unlike
sponges, cnidarians have true tissues but still have no cavity between the body wall and the
digestive tract like coelomates. Despite the wide variety of species in this phylum, cnidarians
come in one of two shapes, the polyp and the medusa. Both forms display a mouth, (through
which both food is ingested and undigested material is ejected), and tentacles. The polyp form
is normally tubular in shape and spends life attached to a base, while the medusa form is free to
float with the ocean currents. All cnidarians are armed with cnidocytes, specialized cells that
when stimulated by touch; inject a venomous poison into their prey. Despite the deadly venom,
cnidarians are not ruthless hunters; instead they wait for prey to happen by them. When the illfated prey touches the hungry cnidarians; venom is injected and the prey is entrapped in the
tentacles which force it though the mouth and down into the gastrovascular cavity.
Corals can reproduce both asexually and sexually. Sexual reproduction increases the genetic
variability of the population as both eggs and sperm are released into the water, a.k.a.
“spawning.” The new free-floating larvae eventually attach to a surface and become polyps.
During this free-floating larval stage, it is possible for a new coral colony to be formed at a novel
location. Asexual reproduction will not increase genetic variability but allows the coral colony to
grow in size.
All cnidarians are confined to water habitats and corals are further restricted to warm tropical
waters. They are very picky about their real estate and only live in places where the water is just
the right temperature, the right salinity, not too deep and has plenty of the right neighbors. In
this case, the corals are specifically interested in a certain photosynthetic unicellular algae
called zooxanthellae. They don’t want to just live near them either; they want the zooxanthellae
to live inside them! Reef-building corals and the zooxanthellae have a symbiotic relationship,
that is, both parties benefit. It’s kind of an “I’ll scratch your back if you’ll scratch mine” set-up
where the photosynthetic zooxanthellae provide the coral with the products of photosynthesis—
oxygen and glucose—and the corals provide the zooxanthellae a safe environment, carbon
dioxide, nitrogen and phosphorus. Zooxanthellae give the corals their brilliant color and help
produce the calcium carbonate (CaCO3) that forms the coral exoskeleton that protects the
delicate polyp and produces the reef itself.
Not all corals are reef-builders; only hard corals produce a reef. Soft corals don’t build reefs.
You can tell the difference by counting the tentacles. Soft corals have eight tentacles while hard
corals have tentacles in multiples of six. It is hard corals that form massive reef structures by
secreting an exoskeleton of CaCO3 that lies underneath the polyp. The skeletons produced by
millions of coral polyps create the framework of the reef. Over time, the living corals will die, and
their skeletons are left behind to accumulate and act as anchors for other living corals and the
cycle continues as it has been for millions of years!
Activity 3: Coral Reef Background
Page 1 of 2
Contextual Biology Integrated Projects
Created by the Center for Occupational Research and Development
http://www.cordonline.net/HiESTbiology
The reefs of the Hawaiian Islands are relatively young in “geology-speak,” they are small and
located close to the shore. The predominant species are the lobe coral (Porites lobata), finger
coral (Porites compressa) and cauliflower coral (Pocillopora meandrina). Despite their relatively
young geological age, these reefs provide homes for a diverse range of marine wildlife. Twentyfive percent of the fish that lives amongst the reefs of Hawaii exist nowhere else in the world!
Annually, five million people visit the beaches of Hawaii to enjoy the marine ecosystems that
drive the tourism industry.
Unfortunately, like all coral reef systems, the reefs of Hawaii are in decline due to sediment,
pollution run-off, boat traffic, and weather. These factors “stress-out” the reefs. We all know
what too much stress feels like, and even though live corals don’t have a brain, they feel the
effects of stress too! Coral bleaching is one effect of stress. Coral bleaching happens when for
some reason the corals eject their zooxanthellae partners. In doing so, they literally lose their
color and turn white. Eventually, they will die without their photosynthetic buddies. Why do they
turn suddenly and decide to eject the zooxanthellae? Scientists propose that stress can cause
this event, stress like a change in water temperature or salinity.
The reef off south Moloka'i is the longest fringing coral reef in the Hawaiian Islands and
although it is one of the best preserved, it too is threatened. In 1998, a team of “aquanauts” in
Key Largo, Florida spent 8 days underwater collecting coral samples in an attempt to elucidate
which coral colonies can be transplanted successfully. Coral grown in a lab could be used to
restore coral reefs in the ocean if they can be successfully transplanted.
In this lab, you will act as molecular ecologists, aiding your own Hawaiian aquanauts in a search
for why some coral can withstand the stress of a transplant better than others. In this lab, you
will use DNA microarray technology to compare the genetic profiles of stressed versus normal
live coral.
To learn more about coral reefs, take a virtual dive at www.coralfilm.com/virt.html!
Activity 3: Coral Reef Background
Page 2 of 2
Contextual Biology Integrated Projects
Created by the Center for Occupational Research and Development
http://www.cordonline.net/HiESTbiology