THE PELAGIC ENVIRONMENT OF THE OPEN OCEAN
45
(or photophores), or even in a few exceptional species
derive the light-producing system from the food they
eat. Bioluminescence is an almost universal characteristic of oceanic species, particularly of those inhabiting
depths below the photic zone (Herring, 1990). As the
intensity of daylight declines with depth, coloration
ceases to play a major role in animal communication
and/or camouflage, and bioluminescence takes over
many of its functions in inter- and intra-specific
signalling.
Transmission of sound through water
Water is much more “transparent” to sound than it is
to light. But as frequencies increase (and wavelengths
shorten) attenuation increases, especially as the wavelengths approach the acoustic diameter of suspended
particles. Particles (or bodies) whose sizes exceed the
wavelength of the sound will tend to backscatter it, but
dense concentrations of finer particles also attenuate
the sound through Tyndall scattering. Thus sound with
a frequency of 10 kHz can generally penetrate to full
ocean depths, being backscattered only by the larger
fishes, whereas sound of frequency 150 kHz penetrates
to a maximum depth of 400 m and is backscattered by
high concentrations of zooplankton (e.g., Urick, 1975).
Thus, sound is used by oceanographers for observing
some of the ocean’s physical characteristics, such as its
depth, density distribution and particle concentration,
and also for locating and communicating with devices
deployed in deep water to make measurements. It
is also quite widely used by fishes and marine
mammals as a means of intra-specific communication
and echolocation.
CHARACTERISTICS OF OCEAN BASINS
Knowledge of how the morphology of ocean basins and
circulation patterns in the global ocean have changed
over geological time is constantly improving (Parish
and Curtis, 1982) (Fig. 3.1). Imprints of past ocean
circulation patterns, which have resulted from changes
in the gross distribution of continents over geological
time, have been identified in present-day distributions
(Van der Spoel et al., 1990; White, 1994). These
changes continue to have an influence on current
distributions of species and on their diversity (see
p. 60–64).
The continents are asymmetrically distributed across
the Earth’s surface; in consequence, there are significant disparities between the environmental characteristics of the major oceans and basins. In the
Northern Hemisphere 60.7% of the area is covered
with ocean, compared with 80.9% in the Southern
Hemisphere. Consequently sea-surface temperatures
range more widely at temperate latitudes in the
Northern Hemisphere than in the Southern Hemisphere
(Fig. 3.2). The boundaries of four of the major oceans
are largely determined by the distribution of the
continental landmasses.
The largest and oldest ocean by far is the Pacific.
It has a total area of around 165.38×10
6 km
2 , a mean
depth of 4200 m and a maximum depth of 11 524 m
in the Mindanao Trench. Its total volume is about
695×10
6 km
3 . It is fully open to the Southern Ocean,
but has only a shallow connection with the Arctic
Ocean via the Bering Strait. At present, it is connected
to the Atlantic only by the Drake Passage to the
south of Cape Horn at the tip of South America.
However, as recently as 5 million years ago, there was a
shallow-water connection through the Panama Isthmus;
consequently, there is close similarity between the
shallow-living tropical water faunas of the two oceans,
but marked differences between the deep-living faunas.
Faunal exchanges would also have been possible between mesopelagic species whose life histories include
a shallow-living larval phase of sufficient duration for
them to be carried by currents through the connecting
channel. There are connections between the Pacific and
the Indian Ocean via the shallow sills to the north
of Australasia between the islands of the Indonesian
Archipelago (see Chapter 9), but connections at full
ocean depths are only to the south of 44ºS. Many
of the margins of the Pacific are tectonically active,
bounded by deep trenches along the margins of the
actively subducting plates. One important influence
of the trenches is that they entrap turbidity flows,
which otherwise would transport sediment from the
continental slopes out over the abyssal plains.
The Atlantic is the second largest ocean, with an
area of 82.22×10
6 km
2 (i.e., half that of the Pacific).
The North Atlantic began to open up at the beginning
of the Jurassic era about 200 million years ago when
the supercontinent of Pangea began to fragment and
the land mass of North America separated from
Afro-Eurasia. The opening of the South Atlantic was
initiated much later, about 100 million years ago. The
Atlantic has an average depth of 3600 m, reflecting
its relatively young age, and its volume is about
45
(or photophores), or even in a few exceptional species
derive the light-producing system from the food they
eat. Bioluminescence is an almost universal characteristic of oceanic species, particularly of those inhabiting
depths below the photic zone (Herring, 1990). As the
intensity of daylight declines with depth, coloration
ceases to play a major role in animal communication
and/or camouflage, and bioluminescence takes over
many of its functions in inter- and intra-specific
signalling.
Transmission of sound through water
Water is much more “transparent” to sound than it is
to light. But as frequencies increase (and wavelengths
shorten) attenuation increases, especially as the wavelengths approach the acoustic diameter of suspended
particles. Particles (or bodies) whose sizes exceed the
wavelength of the sound will tend to backscatter it, but
dense concentrations of finer particles also attenuate
the sound through Tyndall scattering. Thus sound with
a frequency of 10 kHz can generally penetrate to full
ocean depths, being backscattered only by the larger
fishes, whereas sound of frequency 150 kHz penetrates
to a maximum depth of 400 m and is backscattered by
high concentrations of zooplankton (e.g., Urick, 1975).
Thus, sound is used by oceanographers for observing
some of the ocean’s physical characteristics, such as its
depth, density distribution and particle concentration,
and also for locating and communicating with devices
deployed in deep water to make measurements. It
is also quite widely used by fishes and marine
mammals as a means of intra-specific communication
and echolocation.
CHARACTERISTICS OF OCEAN BASINS
Knowledge of how the morphology of ocean basins and
circulation patterns in the global ocean have changed
over geological time is constantly improving (Parish
and Curtis, 1982) (Fig. 3.1). Imprints of past ocean
circulation patterns, which have resulted from changes
in the gross distribution of continents over geological
time, have been identified in present-day distributions
(Van der Spoel et al., 1990; White, 1994). These
changes continue to have an influence on current
distributions of species and on their diversity (see
p. 60–64).
The continents are asymmetrically distributed across
the Earth’s surface; in consequence, there are significant disparities between the environmental characteristics of the major oceans and basins. In the
Northern Hemisphere 60.7% of the area is covered
with ocean, compared with 80.9% in the Southern
Hemisphere. Consequently sea-surface temperatures
range more widely at temperate latitudes in the
Northern Hemisphere than in the Southern Hemisphere
(Fig. 3.2). The boundaries of four of the major oceans
are largely determined by the distribution of the
continental landmasses.
The largest and oldest ocean by far is the Pacific.
It has a total area of around 165.38×10
6 km
2 , a mean
depth of 4200 m and a maximum depth of 11 524 m
in the Mindanao Trench. Its total volume is about
695×10
6 km
3 . It is fully open to the Southern Ocean,
but has only a shallow connection with the Arctic
Ocean via the Bering Strait. At present, it is connected
to the Atlantic only by the Drake Passage to the
south of Cape Horn at the tip of South America.
However, as recently as 5 million years ago, there was a
shallow-water connection through the Panama Isthmus;
consequently, there is close similarity between the
shallow-living tropical water faunas of the two oceans,
but marked differences between the deep-living faunas.
Faunal exchanges would also have been possible between mesopelagic species whose life histories include
a shallow-living larval phase of sufficient duration for
them to be carried by currents through the connecting
channel. There are connections between the Pacific and
the Indian Ocean via the shallow sills to the north
of Australasia between the islands of the Indonesian
Archipelago (see Chapter 9), but connections at full
ocean depths are only to the south of 44ºS. Many
of the margins of the Pacific are tectonically active,
bounded by deep trenches along the margins of the
actively subducting plates. One important influence
of the trenches is that they entrap turbidity flows,
which otherwise would transport sediment from the
continental slopes out over the abyssal plains.
The Atlantic is the second largest ocean, with an
area of 82.22×10
6 km
2 (i.e., half that of the Pacific).
The North Atlantic began to open up at the beginning
of the Jurassic era about 200 million years ago when
the supercontinent of Pangea began to fragment and
the land mass of North America separated from
Afro-Eurasia. The opening of the South Atlantic was
initiated much later, about 100 million years ago. The
Atlantic has an average depth of 3600 m, reflecting
its relatively young age, and its volume is about