SMS 322 Biology of Marine Vertebrates
Spring 2001
Diversity of Habitats in the Oceans Occupied by Vertebrates
January 30
Reading assignment: Helfman et al. Pages 295-309 of Chapter 17; Lutz and Musick
Chapter 6; Berta and Sumich Chapters 2-5 (as suggested by Ione).
1.
2.
Following brief introduction to who the marine vertebrates are, let’s look at where
they live in a general sense.
There are a at least three instructive ways to slice the oceans to examine zones
or habitats occupied by marine vertebrates.
a.
The first is a general classification used by oceanographers and marine
biologists, which separates into zones the water and the bottom portions
of the oceans, based partly on depth and partly on physical features of the
oceans. Marine vertebrates occur in all the zones I’m about to define.
(See Fig. 9-1 from Pinet (2000).)
i.
ii.
iii.
The pelagic province (or zone or realm) is the ocean water from
the bottom to the surface, i.e., what oceanographers call the water
column.
The benthic province is the sea bottom.
The pelagic province is divided horizontally into the neritic and
oceanic zones.
(1)
(2)
i.
Neritic zone is the water column over the continental
shelves, defined loosely as the zone from the shore to the
continental shelf break, where the slope of the bottom
increases sharply (usually at about 150-200 m depth).
Zone may be 1000 km wide as in parts of the Arctic ocean or
only a few km wide as along the west coasts of N. and S.
America.
Oceanic zone is the water column beyond the shelf break.
The oceanic zone is divided vertically into five pelagic zones.
(1)
(2)
Epipelagic zone, arbitrarily from 0-200 m, but it roughly
corresponds to the depth of the shelf break and the depth of
significant photosynthesis in the clearest ocean waters.
Important because here is where almost all primary
productivity occurs, driving food production for marine
vertebrates.
Mesopelagic zone, arbitrarily from 200-1000 m, with the
faintest downwelling sunlight becoming undetectable in the
(3)
(4)
(5)
i.
The benthic province is also zoned vertically by depth.
(1)
(2)
(3)
(4)
(5)
(6)
i.
a.
zone. Food for marine vertebrates or their prey below the
epipelagic zone must be carried down by critters or rain
down by gravity.
Bathypelagic zone, arbitrarily from 1000-2000 m.
Abyssalpelagic zone, arbitrarily from 2000-6000 m.
Hadalpelagic zone, from 6000 m to bottom, basically the
deep-sea trenches so this definition isn’t quite so arbitrary.
(Hades=god of the netherworld and dispenser of earthly
riches in Greek mythology; underground abode of the dead;
hell.)
Supralittoral zone is above high tide line but influenced by
salt spray; not of major concern to us except that many
marine vertebrates use this zone for reproduction, basking,
or sometimes feeding.
Littoral zone is the extent of sea bottom between high and
low tide lines.
Sublittoral zone is the sea bottom from low tide line to the
continental shelf break, again usually 150-200 m deep.
Bathyal zone is sea bottom from 200-2000 m.
Abyssal zone is sea bottom from 2000-6000 m.
Hadal zone is sea bottom below 6000 m.
The abyssalpelagic zone and the abyssal benthic zones have by far
the greatest volume and area, while the epipelagic zone and
sublittoral benthic zone, where primary production occurs, have
small volume and area. (See Table 9-1 from Pinet (2000).)
We might slice the oceans from pole to pole; I just do this briefly to
illustrate some extremes. Vertebrates occur in all areas of the
hypothetical slice.
i.
ii.
iii.
iv.
Arctic Ocean, ice-covered water year-round in the polar region; no
continent; coldest water in the world’s oceans at the bottom of the
Arctic (below 0 C).
Temperate oceans north and south, usually ice-free except bergs
or near-shore winter ice; cold water deep all year; summer surface
temperatures to about 20 C.
Sub-tropical and tropical oceans; cold water deep all year; surface
temperatures greater than about 18 C all year; surface
temperatures in tropics >25 C to as much as 32 C in western
Pacific.
Antarctic or Southern Ocean surrounds continent of Antarctica;
fringed with ice of seasonally greater or lesser extent; also sea
v.
temperatures below 0 C.
The zones along this north-south slice are defined by
oceanographic features.
(1)
(2)
(3)
(4)
(5)
a.
Thirdly, we might look at specific habitat types, sometimes defined by a
bottom type, but perhaps including the overlying water column. No detail
now, but some examples.
i.
ii.
iii.
iv.
v.
vi.
1.
Polar seas occur poleward of oceanographic features called
polar convergences.
Temperate seas occur between polar convergences and
subtropical convergences.
Subtropical seas occur equatorward of subtropical
convergences.
Tropical seas occur surrounding equatorial current regions.
The separations between polar, temperate, and subtropical
areas are where surface temperatures change rapidly and
frequently demarcate faunal changes, at least in the
epipelagic zone.
In shallow tropical seas, mangroves, coral reefs, and sandy
bottoms.
From the littoral to the abyss, mud substrates.
In polar seas, the ice edge.
In the Gulf of Maine, rocky intertidal and subtidal areas.
The air over the oceans doesn’t fit neatly into any of our
classifications, but is clearly important for sea birds.
Adjacent land doesn’t fit neatly either, but is clearly important for
some marine reptiles, birds, and mammals.
Now let’s look at who lives where and why in a general sense.
a.
b.
c.
I’ve given you a lot of terms, but as I talk about where marine vertebrates
are, remember our goal of developing a perspective. We might consider
what some of the constraining factors are in considering where marine
vertebrates dwell.
We’ll consider zoogeography specifically later, but now let’s see what
types of marine vertebrates live in some of the areas mentioned in our
classification “slices.”
Consider this Gary Larson cartoon. Gary is a zoologist turned cartoonist,
but what is wrong with this picture?
i.
ii.
Polar bears and penguins both adapted to polar conditions and
inhabit the edge of the ice, but they wouldn’t be found on the same
cake of ice.
Polar bears are endemic to the Arctic and all penguins are endemic
iii.
to the southern hemisphere, with several endemic to the Antarctic.
Vertebrate faunas of the Arctic and Antarctic oceans are quite
distinct, with more endemic species in the Antarctic than in the
Arctic. I said zoogeography later, but just to finish the thought:
(1)
(2)
(3)
Antarctic Ocean has had cold climatic conditions, and has
been well separated physically and oceanographically from
other areas, for about 25 million years–lots of time to evolve
endemic species.
Arctic Ocean is more directly connected to north temperate
seas, and it has had repeated warming and cooling until cold
conditions stabilized only about 3 million years ago–not so
much time for endemism to develop.
This is not to suggest there is no overlap in Arctic and
Antarctic vertebrate faunas.
(a)
(b)
a.
Fishes are found everywhere in the marine environment; reptiles, birds,
and mammals are not. Why not? What is a major constraining factor in
habitat diversity of the latter groups?
i.
ii.
a.
Arctic terns, e.g., nest along the northern rims of the
northern hemisphere continents, i.e., at the edge of
the Arctic Ocean and on its islands.
In autumn, they migrate to the Antarctic where they
feed during austral summer at the edge of the ice
pack.
They gotta breathe air.
Thus, even though some may dive into the mesopelagic or
sometimes bathypelagic zones, all the reptiles, birds, and mammals
are epipelagic (or aerial) most of the time.
Let’s start with the mammals and work down to the fishes. Where are the
marine mammals with respect to our slices? A major habitat dichotomy is
based on another constraint, the necessity in some to give birth or perform
other necessary functions on land.
i.
Those tied to land at least at some stage in life cycle.
(1)
(2)
Sea otter (Order Carnivora, Family Mustelidae) is neritic,
seldom going more than 2 km offshore, and temperate to
subarctic along North Pacific coast of North America. Quite
marine despite coastal distribution. Usually stays at sea
except in storms, but some sleep ashore. Give birth on
shore, but soon return to sea with pup.
Polar bear (Order Carnivora, Family Ursidae) is really land or
(3)
ice based, entering the sea to feed (even on small belugas).
However, most of its hunting is of marine mammals on the
ice (various seals, walrus). It is north polar as noted. Tied
to marine food chain for sure–call it a marine mammal or not.
“Pinnipeds” (Order Carnivora, sometimes Suborder
Pinnipedia) are in three families. They all haul out on ice or
land to rest, sleep, vie for mates, breed, or give birth.
(a)
(b)
(c)
(d)
(e)
i.
The walrus (Family Odobenidae) is Arctic in
distribution, and is neritic, feeding on benthic
invertebrates, esp. molluscs.
The eared seals (Family Otariidae) (8? fur seals and 5
sea lions).
The true seals (Family Phocidae) (??) haul out, but
they are clumsier on land or ice than the other
pinnipeds.
These are either neritic or associated with pack ice
over deep water; though they all haul out, some are
deep divers into the mid-mesopelagic zone, as we will
discuss later.
Most are polar or temperate, but a few species are
found in tropical regions, e.g. in the eastern Pacific
Ocean.
Those not tied to land.
(1)
(2)
Manatees and the dugong (Order Sirenia) are coastal or
estuarine (or freshwater) in tropical and subtropical waters.
Because they eat attached aquatic vegetation, they are
restricted to shallow near-shore waters, though they never
come onto land. (Steller’s sea cow was an Arctic sirenian
that humans extinguished in just a quarter-century after its
discovery by European explorers.)
The remaining mammals (Order Cetacea, the whales,
dolphins, porpoises) are still less tied to land or the ocean
margins. However, there is still considerable variability in
where in the ocean slices particular species are found.
Some are highly migratory as well.
(a)
(b)
(c)
Gray whales (Family Eschrictiidae), e.g., are highly
migratory but quite coastal in distribution along Pacific
coast of N.A.
Beaked whales (Family Ziphiidae) are poorly known,
in part, because they are oceanic pelagic over the
deep oceans away from land.
Sliced another way, there are Arctic species (narwhal
and beluga) all the way to Antarctic species (pygmy
right whale).
a.
Where are marine birds with respect to our habitat slices? Let’s let our
perspective again be on constraints to habitats occupied by sea birds.
Arguably, there are 20 or so families that contain sea birds or birds
dependent on the sea. All must lay eggs on land, so in contrast to marine
mammals, all are tied to the land at some stage.
i.
ii.
There is probably not a dichotomy, as in marine mammals, but
rather a continuum of use of the marine environment by different
groups or species of birds.
Here’s an example contrast. Two groups of tropical and
subtropical birds that spend lots of time flying or soaring over the
ocean feeding primarily on fish are the tropicbirds (Family
Phaethodontidae) and the frigatebirds (Family Fregatidae).
(1)
(2)
i.
ii.
a.
Frigatebirds cannot walk or swim. They fly above the ocean
and swoop down to catch fish at surface or steal food from
other birds. They return to land at night to roost in trees.
They are primarily above the neritic zone.
Tropicbirds fly above and then plunge into water to catch
food. Outside the breeding season they are truly pelagic,
remaining far from land for long periods.
Cormorants (Family Phalacrocoracidae) range as different species
from Arctic through temperate, tropical, regions to Antarctica. All
are neritic, needing to dry their wings periodically, as the feathers
are not completely waterproof.
Albatrosses (Family Diomeidae) range as different species from
Antarctica to north temperate (in Pacific), but the juveniles and
non-breeding adults wander far from land for long periods.
How about the marine reptiles? All tied to land by need to lay eggs.
i.
ii.
Marine iguana spends lots of time on land venturing into water near
shore to feed. Definitely neritic.
At the other extreme are some of the sea turtles.
(1)
(2)
(3)
Early juvenile loggerheads, green turtles, and probably
leatherbacks and hawksbills are found all over the North
Atlantic, e.g., and may be in the open sea for one to several
years. Definitely oceanic pelagic. (Some are in other
oceans too.)
Early juvenile Kemp’s Ridleys are all over the Gulf of Mexico.
Flatbacks on the other hand seem to be neritic from the
(4)
i.
a.
b.
start.
After greater or lesser time, they recruit back to neritic
regions, bays and estuaries and take up a more benthic
oriented style.
Slicing north to south, the marine reptiles are tropical, subtropical,
and maybe a bit temperate. (Why no polar marine reptiles? Food
for thought–ectothermy and air breathing may not be compatible for
moderate to large animals.)
Before going to the fishes, there is another major constraint on distribution
of marine vertebrates with respect to zones or habitats (although the
constraint applies to fishes, too). There are numerous species and
numerous populations around Antarctica–lotsa birds, lotsa whales, lotsa
seals. Ditto for the west coasts of North and South America. Contrast
with the Sargasso Sea in the middle of the North Atlantic gyre, where one
seldom sees a whale or bird or turtle. Most of the air breathing marine
vertebrates are predators, often top predators, and they are diverse and
abundant where oceanic productivity is greatest.
Now, the 15,000+ marine fishes. Fishes occupy the greatest diversity of
habitats by far among the marine vertebrates.
i.
Air-breathing (therefore epipelagic) constraint removed, so fishes
can be and are distributed from the surface to the deep sea, though
a given species can’t be.
ii.
Pressure, light, temperature, sheer volume, and food are increasing
constraints from the surface downward. Discuss these in detail
later, but a few key points for the oceanic (not neritic) groups.
(1)
(2)
Pelagic species are generally countershaded, usually
streamlined, good swimmers, with well developed but not
extreme eyes, often schooling and often migratory. They
have efficient respiration and food conversion capability.
Most of the bony fishes in this zone have swim bladders (gas
bladders) for buoyancy control.
Mesopelagic species generally have large swim bladders,
enlarged specialized eyes, specific patterns of
bioluminescence. Most are relatively small, and they have
good ability to regulate swim bladder volume. Many make
diurnal vertical migrations into the epipelagic at night.
(a)
(b)
Temperature decreases with depth, rapidly at any
thermocline, and is always cold in the lower
mesopelagic and deeper zones.
DVM species may cope with considerable
temperature change daily–unusual among oceanic
marine fishes.
(c)
(1)
i.
Examples from 22 families of eels.
We can slice from the shore out, too.
(1)
(2)
(3)
(4)
i.
Fishes in the deeper zones tend to have reduced or vestigial
eyes, reduced skeletons, reduced musculature, and reduced
metabolism. They have no swim bladders or swim bladders
filled with lipid instead of gas. Many have very large
mouths. Adaptations to a low sporadic food, high pressure
environment.
So we’ve got this tremendous range of vertical distribution of
species, but damn near every oceanic species has epipelagic
larvae. This means that larvae may develop under conditions
much different than those under which the juveniles and adults
dwell. The reasons I’ll leave for Ione for later. However, one
often reads about vertical distributions and characteristics of fishes,
but those almost always refer to adults.
(1)
i.
Some widespread mesopelagic species are deeper in
tropical than temperate regions, socalled tropical
submergence.
A few species can breathe air, or can supplement with air.
This opens the littoral zone to some even at low tide
(mudskippers, blennies).
In contrast to the other vertebrate groups, small body size of
many fishes allows exploitation of interstitial habitats
especially in neritic waters (coral reefs, tunnels, under rocks,
etc.).
Benthic habitats are more fully occupied in the neritic zone,
e.g. by flatfishes, skates, sculpins, etc.
Pelagic fishes in the neritic zone do or do not conform to the
characteristics of the oceanic pelagics in coloration, body
shape, etc.
Sliced from pole to pole takes us from endemic Arctic species
through the zones to a large number of endemic Antarctic species.
Many mesopelagic and deeper species have very widespread
distributions, because similar temperature conditions may be found
over large areas of the oceans.
(1)
(2)
The polar species cope with water temperatures below 0 C
by having antifreeze glycoproteins which lower the freezing
point of their tissues fluids.
Antarctic species have numerous other interesting
adaptations as well.