Many reef fishes, such as these anthias and the chromis on pages 50–51, feed on
minute plankton. The vast number of plankton-feeding fishes on coral reefs is
indicative of the even larger number of planktonic organisms in reef waters.
clusion was arrived at: “Corals do not need to feed.” Just shine bright
sunlight on them, the conventional wisdom said, and their zooxanthellae would produce all the food the corals could use.
To the credit of the scientists working with corals or zooxanthellae,
none of them proposed this ridiculous conclusion; however, it seemed to
rise like so many bubbles of hydrogen sulfide out of the decaying sludge
of poor scientific journalism—including, I must say, numerous aquarium references whose authors, in some cases, still seem to believe it.
YOU ONLY FIND WHAT YOU LOOK FOR
Meanwhile, back on the reefs, a new group of scientists started putting
two and two together and getting three. Things did not add up. If the
waters around reefs were so plankton-poor, how come reefs were covered with plankton-feeding fish? Maybe the zooxanthellae did the trick
for corals, but there are no zooxanthellate reef fish. And what about
all the other plankton-feeding animals in the waters surrounding the
reefs? Where did all their nutrition come from?
In several brilliant research projects from the late 1970s through
the mid-1990s, researchers took a new look at the plankton puzzle.
Remember, the early workers who had sampled the waters around
reefs for plankton had done so using the same methodology and gear
that they had been trained to use in the cold northern waters, where
they had worked very well. The scientists had assumed those practices
would work just as well in the tropics—and they do. They sampled the
plankton in the tropics and, finding virtually nothing like the plankton
found in the temperate regions, concluded that there was no plankton
in these tropical waters. Unfortunately, what they should have concluded was that there was no plankton in the tropics that compared to
that found in temperate habitats; the types of organisms from the two
habitats were very different.
Temperate plankton is composed largely of small crustaceans, fish
larvae, some mollusks, and relatively large numbers of moderate-sized
jellyfish and comb jellies. For their size, these are rugged animals.
A large proportion of them can be collected simply by pulling a net
through the water. Sure, some of them are battered and beaten up, but
a good many of them survive the sampling process. A case in point:
some of the most common planktonic animals in temperate regions
are various calanoid copepods in the genus Calanus. It has been stated
that some species of these calanoid copepods are the single most abundant large animal (large is relative) on the planet; Calanus finmarchicus
has a body about the size of a grain of rice, and it is estimated that their
annual productivity is in excess of 100 billion tons. That is a LOT of
small planktonic animals.
By the early 1990s, it was evident that tropical plankton was very
different from its temperate counterpart. It comprises small gelatinous
animals, such as larvacean tunicates; tiny jellyfishes; some huge, very
complex, but exceptionally fragile jellyfishes; and medusoid animals,
larvae, and huge amounts of bacterial particulate material. Instead
of animals, most tropical plankton is made up of primarily bacterial
aggregations. These sometimes form visible particles called “marine
snow.” The huge shoals of planktonic calanoids are notable by their
absence. Tropical plankton is simply too delicate to sample with the