Biology Now, 2e

Fast Lizards, Slow Corals ■ 239

different light and nutrient requirements, and

organisms 15 feet below the surface receive a lot of

sunlight, while those at 30 feet receive less light,

and light of different wavelengths. Prada found

different types of algae living on the two coral

populations (Figure 13.10), so it’s possible that

the sea fans evolved to be better hosts for the most

successful algae at their depth.

In many cases in the ocean, as with coral

and algae, the interaction between two species

so strongly influences their survival that they

have evolved in tandem—a phenomenon known

as coevolution. The term “coevolution” encom-

passes a wide variety of ways in which an

adaptation in one species evolves alongside a

complementary adaptation in another species.

as Prada sampled more and more locations, the

evidence was convincing. Indeed, everywhere

he looked, Prada saw the same thing he had

observed in Puerto Rico: broad, leaflike coral at

shallow depths, and tall, sticklike coral in deeper

waters. Distance between the populations did

not matter; some shallow-water and deep-water

fans were close enough together that Prada could

reach out and bend them to touch each other. But

depth did matter. The corals were physically close

enough to easily interbreed in the water, yet the

species had somehow become specialized to two

different depths (Figure 13.9).

Prada brought his coral samples back to the

lab and performed tests to see how genetically

similar the two groups of corals were. Prada

and Hellberg found that all the shallow- water

sea fans across the Caribbean were more closely

related to each other than they were to any of

the deep-water sea fans. The same held true

for the deep-water sea fans: they were more

closely related to each other than to any of the

shallow-water fans. In the DNA data, Hellberg

had seen some genetic exchange between the

two populations in the past, yet that exchange

of genetic materials had been limited across the

two groups, and each species had bred almost

exclusively among its own populations.

Different Depths, Different Habitats

How did a few yards of depth produce habitats

so different that the corals evolved different

adaptations to each depth? The scientists are

still investigating the differences between the

shallow and deep habitats, but they have some

hypotheses. One is that the coral have adapted

their morphologies, and possibly their biochem-

istry, to suit different symbiotic algae that grow

on them at different depths.

Symbiotic algae live on coral and, through

photosynthesis, use sunlight to produce energy

and organic compounds that coral use to maintain

and grow calcium carbonate skeletons. In turn,

the coral provide the algae with a sheltered place

to live and produce carbon dioxide that the algae

use during photosynthesis. (This cooperative rela-

tionship between species is known as mutualism;

see Chapter 20.) But different species of algae have

Figure 13.9

Depth of water provides ecological isolation for sea fans

The two depths shown here differ slightly in their quantity and quality of

light, the force of the waves or current, the amount of sediment deposited on

them, the number and type of predators, and the availability and type of food.

In ecological isolation, the

two species are usually

physically close enough to

breed, but somehow do not.

Shallow

Deep

Figure 13.10

Different species

of corals and

their resident

algae

Although all these

algae (single-celled

eukaryotic organisms)

look the same under

a microscope, they

are actually different

species with different

light requirements

and photosynthetic

capacities.