Benthos – Chapter 14

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Transcript Benthos – Chapter 14

• Unlike the drifting plankton and swimming
nekton, benthic organisms – commonly
referred to as benthos – live on or near the
ocean bottom
• A benthic habitat may be shallow or deep
• Most benthic organisms are sessile (immobile)
and anchored to the benthic environment;
others crawl or swim over the ocean bottom
• Of the 250,000 known species that inhabit the
marine environment, >98% are benthos!
• The majority of benthic organisms live on the
continental shelf; sunlit areas of relatively high
primary productivity
• Benthos include animals,
protists (algae) and even
Intertidal Zones
• Rocky shorelines team with organisms that
live on the ocean floor (epifauna)
• Typical rocky shores are divided into distinct
zones, characterized by the height of the
water (which is itself determined by the tides)
• So called intertidal zones are among the most
densely-populated areas on Earth; hundreds
of species crowd this junction of land and sea
Life in the intertidal zone is harsh!
• Adverse conditions require special adaptations
of organisms to live both underwater (some of
the time) and exposed to air (the rest of the
• Strong wave activity, desiccation (drying out),
limited space, rapid changes in temperature,
salinity, pH, and oxygen content, and
predation are just some of the challenges
found here
Spray zone –
region above
the spring high
tide line;
covered by
water only
during storms
Intertidal zone
– region
between the
high and low
tidal extremes
Intertidal Zone
and algae
• High tide zone:
relatively dry
• Middle tide zone:
alternatively wet
and dry
• Low tide zone:
usually wet, but
exposed during
low tides
Life in the intertidal: High tide zone
• Most animals living in the high tide zone have
protective coverings to prevent desiccation
• Seaweeds living in the high tide zone have
thick cell walls to reduce water loss during low
• Many organisms in the high tide zone are
sessile, and remain attached to bottom,
competing with others for limited space
Life in the Intertidal: Middle tide zone
• Seawater constantly bathes the middle tide
zone, so a greater variety of seaweeds and
soft-bodied organisms live here
• Greater biomass in middle tide zone, and so
greater competition for space!
• Mussels and barnacles are common here –
hard-bodied, filter-feeding organisms which
require seawater to feed and to
support planktonic larval stages
Crying cockles and mussels, alive, alive-O!
• The middle tide zone is also home to
carnivorous snails and sea stars
• You, too, can be a sea sleuth….
• Sea stars pry open clams and mussels with
hundreds of tube feet; Predatory snails bore
holes with scraping tongues and suck out prey
• Hermit crabs, sea urchins, and sea anemones
are also found here
Life in the Intertidal: Low tide zone
• The low tide zone is almost always
submerged, so an abundance of algae
(seaweed) is typically present
• Seaweeds are multi-cellular algae (protists)
• Seaweeds attach themselves with a structure
known as a holdfast and use gas bladders to
reach upward to sunlit surface water;
photosynthetic; important source of habitat
Life in the Intertidal: Low tide zone
• Numerous crabs and shellfish live in the low
tide zone
• Benthic fish swim through the low tide zone,
along with larval nektonic forms which seek
shelter and habitat in this protected area
Salt marshes are sediment-covered shores
• Salt marshes and estuaries (regions where
freshwater and saltwater meet) are highly
productive benthic habitats
• Much of this productivity comes from sea
grasses, mangroves and other vascular plants
• Salt marshes form in estuaries and are
characterized by specialized plants capable of
surviving in (and then out of) salt water
Salt marshes: Home Sweet Home
• Salt marshes – via their specialized plants –
form protective barriers against erosion, and
promote new land formation as plant roots
trap sediments on each tidal cycle, and filter
out excess nutrients and pollutants
• Salt marshes also provide protective habitat
for larval fish species, and provide food and
shelter for migratory waterfowl; marsh plants
also form the base of the food web via decay
Salt Marshes are Vital Habitats!
Sand and Cobble Beach Communities
• Not all intertidal areas are rocky or muddy;
some are sandy or consist of gravel or cobbles
• As benign and peaceful as sandy beaches look,
they are among the most hostile
environments for small organisms
• Sand grains are abrasive and many organisms
must have protective coatings and/or be able
to burrow below the surface for protection
Sand and Cobble Beach Communities
• In fact, very few organisms survive in waveswept sandy beaches
• Some larger crabs can outrun the crashing
waves and locate food within sand grains
• Coquina shells and mole crabs are common
along Long Island sandy beaches
Coral Reef Communities
• Corals are animals (Cnidarians) related to
anemones and jellyfish
• Most corals secrete hard skeletons of calcium
carbonate and produce coral reefs
• An individual coral – known as a polyp – feeds
by capturing and eating plankton that drift
within reach of their tentacles
• Corals produce sexually and asexually
Coral Reef Communities
• Corals form symbiotic relationships with
dinoflagellates, known as zooxanthellae
• Zooxanthellae receive nutrients and shelter
from the coral, and photosynthesize, providing
the coral with organic compounds
• Zooxanthellae provide corals
(otherwise translucent)
with their brilliant colors
Tropical coral reefs support large
numbers of species
• Reef-building corals provide substrate for
other organisms to attach and hide
• Corals also provide a source of food in
otherwise weakly-productive regions
• Coral bleaching (the loss of the symbiotic
zooxanthellae in response to environmental
stress) may kill the coral, and have devastating
impacts on the coral reef community
Coral Bleaching
Corals are stressed by environmental
• A water temperature change of only 1°C
above the normal summer high temperature
for a few weeks leads to coral bleaching
– Coral expels zooxanthellae or zooxanthellae expels
• El Niño events can drive coral bleaching
• May be reversible – corals can re-aquire new
zooxanthellae if the stress is not too severe
Want to learn more?
• Take our Marine Biology and/or Marine
Habitat Ecology courses!
Until then, so long and thanks for all
the fish!