What’s Going on Down There?? Turf Wars on the Reef!

What are benthic reef creatures?

Benthic creatures live on the bottom of the ocean Often the word benthic refers to the deep ocean but in the case of coral reefs…. It means creatures that live on the bottom or “floor” of the ocean

While not as flashy and fast moving as fish, these creatures are the “heart” of the reef

How many creatures do you see in this photo?

That last question was tricky…. Benthic reef creatures consist primarily of corals, algae, sponge and… other invertebrates that shelter within reef they create In the last photo there were over a dozen species!

When looking at pictures of a reef, the first thing noticed is the diversity of life. All space on the reef is filled with different organisms suited to fill every niche

The Benthic reef environment is a place of constant, slow motion, turf war!

How do Benthic Reef Creatures Fight for Survival?

Survival in the benthic environment is not as easy as it appears! Diversity in the benthic reef environment is the result of the organisms evolving through competition. One factor underlying this competition is aggression, both subtle and more obvious. Aggression occurs in corals as a result of their constant battle for survival. Corals have to cope with currents, predation by fish and other invertebrates, as well as competition from neighboring corals for light, nutrients, and food.

Corals battle algae for space Coralline algae

Healthy reefs in Hawaii are often algae-dominated

However, alien algae species can smother corals! Reef without alien algae species Reef smothered by alien algae

Algae release sugar, fueling bacterial growth on the corals These bacteria suffocate the coral by cutting off the supply of oxygen Once the corals die, this frees space for more algae to grow Over-fishing by humans can reduce the number of fish that graze on algae, thus increasing the amount of algae on the reef Nutrients from sewage and agricultural runoff fertilize the algae

Bubble algae growing over coral

The battle for a majority of the benthic reef “real estate” involves corals, which deploy both offensive and defensive weapons in order to survive and reproduce

Corals acquire and maintain space on a reef by: Reproducing and/or growing rapidly Competing for available food Having offensive and defensive weapons Having the ability to sustain damage and still continue to reproduce

Corals can literally grow over each other Overtopping: Fast growing corals can grow over slower growing corals, blocking their sunlight. Overtopping may not always cause death of the shadowed coral

Living in the shade may limit growth! Unlike the terrestrial world, most things growing on the sea floor are animals of some sort. They don't move because they get most of their food from tiny algae growing under their skin. It is this algae which gives them their color. This animal-plant combination only works if there is enough light for the algae to grow, and feed its animal host. Benthic real estate is limited, and there is a slow-motion, but fierce competition for a place in the sun.

Corals can grow right on top of another species!

Some corals have the ability to extract nutrition from the overrun coral!

To understand other coral weapons….. You should know a little anatomy of a coral polyp Make a labeled drawing of a polyp on your worksheet using the picture on this slide

Corals have weapons! Some corals have sweeper tentacles Much longer than other ("normal") tentacles Capable of feeding, but used primarily as weapons The tips break off and stick to other corals when contacted After contact, they continue to discharge nematocysts, damaging the invader

Nematocysts are “stinging cells” They exist in many corals and jellyfish They are tiny, coiled speargun-like structures that trigger at a touch They often deliver a toxic substance into any creature they contact

Stinging Nematocysts: Are present on short tentacles for close range offense and defense. Can be fired long range, stinging any corals downstream but usually are triggered by close contact Normally result in the death of contacted tissue

When a sweeper tentacle encounters a competing coral, it may attack the competing coral and literally "burn" the offending coral to the point of either killing it or severely damaging it.

Another weapon corals use: Mesenterial filaments

These filaments come from the stomach of a coral polyp They allow one polyp to kill or devour other coral polyps through a process similar to digestion Some corals even have the capacity to produce both sweeper tentacles and mesenterial filaments, enabling them to fight a battle on several fronts

Corals can also “fight” with Extracoelenteric Digestion Corals expel digestive filaments which contain cnidocytes (digestive fluids) Cnidocytes can be expelled from the digestive track en masse (puking) onto a nearby coral, digesting it

Yet another weapon of corals: mucus

Mucus is an effective weapon! It can: be toxic or contain nematocycts be carried long distances by water currents be quite damaging as it "sticks" to corals .

Corals may use chemical weapons! The production of toxic compounds is known as allelopathy Most commonly known producers of toxins are soft corals Effective in the aggressive competition for space on the reef – toxins can kill competitors They are also excellent defense against predation and parasitism. Chemical Competition . and gorgoneans. Toxins emitted by these corals can be lethal to fish.

A few corals use movement to defend themselves and avoid the weapons of other corals Some corals have the ability to move about on the reef. Some corals can detach from the bottom and settle in another location

There is a price for doing battle on the reef: The energy spent on both offense and defense uses precious resources that could otherwise be spent on growth and reproduction

Try to explain the concept of benthic reef “turf wars” Start by taking turns trying to explain it to a partner in class. Next, write a paragraph explaining this concept as you would to someone who has not seen this slide show.

Acknowledgements Micheal Paletta, Author and Marine Biotechnology Consultant Scripps Institution of Oceanography, UC San Diego Photos by James Watt, Kevin Hahn, Dave Krupp and Sandy Webb