Download Benthos – Chapter 14

Benthos
• 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
Benthos
• 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
plants!
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
time)!
• 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
Mostly
shelled
organisms
Intertidal Zone
Mostly
softbodied
organisms
and algae
http://www.humboldt.edu/~intertid/zones.html
• 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
tides
• Many organisms in the high tide zone are
sessile, and remain attached to bottom,
competing with others for limited space
www.armofthesea.info/flora_faunaff_speciespp/moll.htm
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!
http://en.wikipedia.org/wiki/File:Bride-Brook-Salt-Marsh-s.jpg
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
www.stripersonline.com/surftalkshowthread.php?t=417951
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
www2.watertown.k12.wi.us/pagesfifth_grade_websites.cfm
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
www.cgrer.uiowa.edu/peoplecarmichael/atmos_course/ATMOS_PROJ_99/jlmichfin/main.html
Corals are stressed by environmental
change
• 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
itself
• 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!