recycling process, and the abundant sunlight for
photosynthesis in the tropics, that coral reefs are among the
most productive ecosystems on earth.
Phytoplankton is the
basis of the food chains in the pelagic ecosystem. These microscopic
floating plants can only survive and produce food in the shallower,
sunlit waters of the open ocean. The phytoplankton is fed
upon by zooplankton, and the zooplankton, in turn, provides food
for small fishes, shrimps, and squids. Increasingly larger
fishes make up a food chain which culminates with the top predators,
the tunas, marlins, and sharks. Nutrient recycling is incomplete
in the pelagic ecosystem, because much plant and animal material
falls into the deeper, sunless zones before it decomposes, and
so the nutrients are not available to the phytoplankton which
is confined to near-surface waters.
This rain of decomposing
detritus is, however, utilized by the animals of the deep benthic
ecosystem. Because there is not enough light at these depths
to support plant life, the food chains of the deep benthic ecosystem
are based upon consumption of detritus by invertebrates (crabs,
shrimps, mollusks, and worms), which are fed on, in turn, by deep-dwelling
fishes.
In addition to having
their own characteristic food chains, the three marine ecosystems
also exchange energy and materials with each other. Plankton-feeding
reef fish feed on pelagic zooplankton which is carried onto the
reef by ocean currents. Pelagic tunas, billfish, and sharks
pass near the margin of reefs and feed on young reef fishes swimming
in these waters. Mesopelagic fishes which migrate up and down
in the waters of the open ocean are eaten by tunas and billfish
in the shallow pelagic zones and by deep benthic fishes when they
approach the sea bottom. These interrelationships and interdependencies
among marine ecosystems are the reasons that impacts on one ecosystem,
such as pollution or overharvesting, can very rapidly influence
the organisms in other, quite distant, parts of the ocean.
BIOLOGY AND BEHAVIOR
The fishes of Guam can
be divided into two major categories, the cartilaginous fishes
(sharks and rays) and the bony fishes (all the rest). The
only other living fish group is the jawless fishes (lampreys and
hagfishes). Until recently, none of these primitive fishes have
been found around Guam, but in the last year, some hagfish specimens
have been collected in deep waters near Guam and Saipan.
The cartilaginous fishes
(Class Chondrichthyes) have a skeleton made of cartilage rather
than bone. The scales of these fishes are of a characteristic
type known as placoid, and they resemble very tiny teeth. These
scales give the skin a rough, sandpaperlike texture. Five
to seven gill openings are present on each side of the head of
sharks; these gill openings are located on the under surface of
the head on the flattened rays. Most sharks and rays give
birth to free-swimming young, although some of them lay eggs from
which the young subsequently hatch.
Sharks and rays have
a wide array of sense organs, some of which are sensitive to things
which humans, with their traditional “five senses,”
cannot