Spectacular Favites pentagona (“War Coral”) in full feeding mode, with polyps
extended to catch passing food items and tuck them into green, hungry mouths.
PLANKTON
One of the first things that had to be done was to verify that there
actually was nothing for the corals to eat. By the late 1950s, the waters
around coral reefs had been sampled in many parts of the world, and
the plankton that researchers thought could have been coral food was
nowhere to be found. The sciences of oceanography and marine biology got their starts in the cold, rather murky waters of the north temperate regions—particularly in the North Sea, around New England,
and in the dark waters of Puget Sound. These regions are characterized
by very rich, seasonally abundant plankton populations called plankton “blooms.” To the average landlubber, the amount of life in the sea
water during one of these temperate spring plankton blooms is unimaginable: a cubic meter (about 264 gallons) can easily contain over
half a million animals that are visible to the unaided eye. The sheer
abundance of these little creatures is, in part, what reduces the water
clarity in these temperate areas, rendering it effectively opaque.
When scientists who had taken samples from such dense temperate plankton populations sampled the water around coral reefs, they
found nothing comparable. In fact, in many cases they found nothing
at all. And, of course, they were not surprised—one needed only to look
through the water to see that there was no plankton in it. Therefore,
case closed—the oceans around reefs were “biological deserts.” As a
corollary of this finding, it was assumed (but not proved) that corals
must produce all, or at least the vast majority, of their own food. And
the only way they could do this was to be mostly autotrophic. In other
words, corals didn’t need to feed; they got all of their nourishment
from their symbiotic zooxanthellae.
WHEN DINOS RULED THE REEF…
It was during the early to mid-1970s that the physiology of the coral—including algal symbiosis—was first investigated in depth. The late
Len Muscatine and his students and co-workers at the University of
California, Los Angeles led this pioneering work. They showed quite
conclusively that the dinoflagellates in most zooxanthellate corals produced a tremendous amount of photosynthate, and that, in many cases, the majority of this material leaked from the zooxanthellae into the
coral host. Photosynthesis makes only carbohydrates, basically sugars
and sugar derivatives, made of carbon, hydrogen, and oxygen. The researchers found that as a part of the coral’s nutrition, the zooxanthellae could provide all of the coral animal’s daily carbon requirements.
This was an amazing finding, and the knowledge that the zooxanthellae
contributed greatly to the coral’s nutrition soon became widespread.
This information was melded with the idea that there was little in
the way of plankton in the surrounding waters, and then transmogrified into the notion that corals could live where they did because they
didn’t need to feed at all.
Note the chain of events here: First, the plankton in the waters
around reefs appeared to be insufficient to feed all the corals. Second,
it was discovered that the zooxanthellae found in corals produced all
the basic carbon compounds that their hosts needed. Third, somehow
the statement “Zooxanthellae can produce 100 percent of the carbon,
or sugar, required by corals” morphed into “Zooxanthellae can produce 100 percent of the nutritional needs of corals.” Finally, the con-
