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FALL 2016
LIFE
AQUATIC
M
ore marine organisms live
on Indo-West Pacific (IWP)
reefs than anywhere else on
Earth. The IWP region covers
a significant portion of the
planet’s surface, ranging from
the shores of eastern Africa across the Indian Ocean,
the Andaman Sea, the Philippine Sea and much of
the tropical Pacific. Within this immense zone lies a
unique hub called the Indo-Malayan Triangle, also
known as the Coral Triangle. Covering 2 million
square miles of ocean, the Coral Triangle includes
the diving utopias of the Philippines, Indonesia,
Malaysia, New Guinea and the Solomon Islands,
and it’s where underwater biological diversity
reaches its zenith.
Divers travel the globe to submerge in places such as
the clear and colorful Caribbean, New England’s shark-
filled seas, the eastern Pacific’s sinuous kelp forests and
the dazzling reefs of the Coral Triangle. Each body of
water, each coast and each island, no matter where
in the world it lies, has its own particular community
of life. This diversity keeps divers roaming from one
destination to the next. But what underlies regional
differences in marine biodiversity? And why does
the Coral Triangle support so much underwater life
compared to any other place on Earth?
SOMEWHERE WARM
For millennia, warm waters have bathed the
Coral Triangle’s flourishing reefs with virtually
no seasonal temperature variation. Warmth and
just the right amount of nutrients allow corals to
effectively outcompete algae for space and sunlight,
thus providing shelter for innumerable fish and
invertebrates. Sea temperature is a major reason for
latitudinal variation in diversity of marine life. There
is longitudinal variation as well; marine life is generally
more concentrated on the western sides of both the
Atlantic and Pacific Oceans. In a nutshell, this has to
do with oceanic current circulation. Cold, nutrient-
filled upwellings generally occur on the eastern sides
of these bodies of water, while the western sides have
fairly clear, balmy waters.
In temperate seas, where algae outgrow corals,
organisms must be able to handle seasonal swings
in temperature and food sources. Temperate and
polar species tend to be ecological generalists, while
tropical species can become specialists due to constant
temperatures, primary production and plentiful food.
Specialization in the tropics leads to a profusion of
symbioses. A dive on any healthy IWP reef will reveal
dozens of commensal associations: anemonefish living
with host anemones, shrimp and gobies, squat lobsters
dependent on crinoids for food and shelter, cleaner
wrasse ridding fishes of parasites, remoras hitching
rides on sharks, and the list goes on.
A CHANGING SEASCAPE
Another factor underpinning the mind-boggling
diversity of life in the Coral Triangle lies deep in the
past, when sharks were much larger and humans were
just a twinkle in the eyes of our primate ancestors.
The land and seascape of the Coral Triangle looked
very different 50 million years ago, when what are now
islands of the southern Philippines, Indonesia, Malaysia
and New Guinea were more widely spread across four
different tectonic plates. Since that time the Pacific,
Indian-Australian, Philippine and Eurasian plates
have been thrust together, forming the convoluted
conglomeration of thousands of islands and reefs
that is the Coral Triangle. Fish and invertebrates
that evolved in separate habitats, originally far from
one another, have amassed in what is known as a
biogeographic sink. The great number and diversity of
marine habitats and environmental conditions make
for an area that attracts species.
The Coral Triangle can be viewed not only as a place
where species have aggregated but also as a source
of marine biodiversity. Some marine biologists point
to evidence that the Coral Triangle is where many
reef species originated before proliferating across the
planet’s seas.
“Concepts of the Indo-Malayan area as a cradle of
diversification or as a museum for species that originated
elsewhere are not mutually exclusive propositions,” said
Gustav Paulay, Ph.D., of the Florida Museum of Natural
History, “nor are they the only ones.”
In regions where temperatures remain warm
and nutrients are plentiful year-round,
species can become specialists, which leads
to the abundance of symbiosis that can be
observed on tropical reefs. In addition to water
temperature and nutrient availability, geography,
climate, plate tectonics, habitat variety and
oceanic current circulation also influence the
wild array and distribution of underwater life.
Text and photo by Ethan Daniels
MARINE BIOGEOGRAPHY