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13 Benthic Life Habits
Notes for Marine Biology: Function, Biodiversity,
EcologyBy Jeffrey S. Levinton
©Jeffrey S. Levinton 2001
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Important benthic lifestyles
• Benthos
• Epibenthic
• Burrowers
• Borers
• Infaunal, Semi-infaunal
• Benthic swimmers
• Interstitial
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Benthos - size classification
• Macrobenthos - shortest dimension < 0.5 mm
• Meiobenthos - smaller than 0.5 mm, greater than 0.1 mm
• Microbenthos - < 0.1 mm
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Feeding Classification
• Suspension feeders
• Deposit feeders
• Herbivores (macroalgae or microalgae)
• Carnivores
• Scavengers
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Food classification ambiguities
• Herbivores could be classified as suspension feeders who feed on planktonic microalgae
• Carnivores could be classified as suspension feeders who feed on zooplankton
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Life in Mud and Sand
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Life in Mud and Sand
Particle size - important parameterCurrent strengthSorting - variation of current strength, poorversus well sorted sediment
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Life in Mud and Sand
Particle size - measures
Median grain diameter - measured usuallyby sieving and weighing size fractions
Silt-clay fraction - % of weight of sediment< 62m
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Ripple marks on an intertidal sand flat
Sedimentary Structures
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Burrowing in sediment
• Burrowers use hydromechanical and mechanical digging mechanisms to move through the sediment
• Watery sediments with high silt clay content have thixotropy - as you move through sediment it takes yet less force to continue to move
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Burrowing in sediment 2
• Hydromechanical burrowing - combines muscle contraction working against rigid, fluid filled chamber (skeleton)
• Form penetration anchor first to allow further extension of body into sediment
• Form terminal anchor to allow pulling of rest of body into the sediment
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Burrowing in sediment -bivalvefoot
Muscle contraction
PA = penetration anchorTA = terminal anchor
PA
TA
Hydromechanicalburrowing
Circularmuscles
Longitudinalmuscles
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Longitudinal musclecontracting
Circular musclecontracting
Terminalanchor
Eversion ofpharynx
Pentetrationancthor
Lugworm, Arenicola marina
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Burrowing in sediment 4
Mechanical displacementburrowing
Inarticulate brachiopod burrows by scissoring2 valves back and forth, which displaces sediment
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Burrowing in sediment 5Biogenic structures - burrowing and processingof sediment affects sedimentary structures
1. Burrowing in mud increases watercontent of sediment2. Increases grain size (pellets)3. Alters vertical and 3-D mechanical, chemicalstructure
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Burrowing in sediment 8Biogenic graded bedding
Annelid ingests small particles at depth and deposits them on surface
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Interstitial animals
• Live in the pore waters of sediment, usually sand grains
• Belong to many taxonomic groups
• All share elongate, wormlike form, in order to move through tight spaces
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Interstitial animals 2
Polychaete Gastrotrich Opisthobranch gastropod
Harpacticoid Hydroid copepod
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Soft-Sediment Microzone
• Strong vertical chemical gradients
• Gradients strongly affected by biological activity
Oxic
Anoxic
Redox Potential Discontinuity
exchange
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Soft-Sediment Microzone 4
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Soft-Sediment Microzone 5
• Microbial types also vary with depth below the sediment water interface
• Towards the surface, aerobic bacteria dominate (energetically most efficient to use oxidation)
• Deeper, bacteria present that can live in the absence of oxygen and can produce energy through various chemical means
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Soft-Sediment Microzone 6
Aerobic bacteria (use oxygen to break down organic substrates)
Fermenting bacteria (break down organic cpds. --> alcohols,fatty acids)
Sulfate reducing bacteria (reduce SO4 to H2S)
Methanogenic bacteria (break down organic cpds. --> methane)
RPDRPD
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Benthic Feeding Types
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Deposit Feeders
• Feed upon sediment, within the sediment or at sediment surface
• Head-down deposit feeders feed within the sediment at depth, usually on fine particles, defecate at surface
• Surface browsers often feed on surface microorganisms such as diatoms
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Deposit Feeders 2
Surface tentaclefeeding polychaete
Tentacle feedingbivalve
Surface siphonatefeeding bivalve
Deep feedingpolychaete Surface feeding
amphipod Deep feeding polychaete
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Surface trace of burrow of the Lugworm, Arenicola marina
(Anglesey, North Wales)
feces
Burrowopening
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Arenicola marina burrow cross section
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Deposit feeders 3
• Microbial stripping hypothesis: deposit feeders are most efficient at digesting and assimilating benthic microbes (diatoms, bacteria)
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Deposit feeders 4
• Microbial stripping hypothesis: deposit feeders are most efficient at digesting and assimilating benthic microbes (diatoms, bacteria)
• In some sediments non-living organic matter is abundant, so even a low assimilation efficiency returns some nutrition for deposit feeders
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Deposit feeders 5
• Microbial stripping hypothesis: deposit feeders are most efficient at digesting and assimilating benthic microbes (diatoms, bacteria)
• At some times of years, fresh phytoplankton sinks and is added to the bottom: another source of non-living food for deposit feeders
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Deposit feeders 6
• Microbial stripping hypothesis: deposit feeders are most efficient at digesting and assimilating benthic microbes (diatoms, bacteria)
• Detritus from seaweeds is probably more digestible than detritus from seagrasses and marsh grasses, which have much relatively indigestible cellulose
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Deposit feeders 7
• Deposit feeders feed on sedimentary grains, microbial organisms living on particulate organic particles
• Feeding activity accelerates microbial attack - grazing stimulates microbial metabolism, results in tearing apart of organic particles
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Deposit feeders 9
Ingestion of sediment
EgestionTransport to
surface of deep sediment
POM,(mechanically,
Chemically,Transformed)
Metal, sulfur,dissolved organiccarbon exchange
Metal, sulfur,dissolved organiccarbon exchange
Sedimentstirring
Free-burrowing bivalvePOM deposition
RPD
Stimulationof microbial
growthTube
Microbialconsumption
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Suspension Feeders
• Feed on small particles, low Reynolds number
• Passive versus active suspension feeders
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Suspension Feeders 2
Bivalve, x-section Polychaete Serpula Barnacle
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Suspension Feeders 3
Sea squirt Styela montereyensisPassive suspension feeding mechanism
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Suspension Feeders 4
Encounters of particles with fibers
= fiberDirectinterception
SievingInertial
impaction
Motileparticle
deposition
Gravitationaldeposition
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Suspension Feeders 4
Encounters of particles with fibers
= fiberDirectinterception
SievingInertial
impaction
Motileparticle
deposition
Gravitationaldeposition
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Carnivores
oystercatcherBivalve Cuspidaria
Crab Callinectes sapidus
PolychaeteGlycera
Gastropod Nucella
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Herbivores
Parrot fish
Chiton
radula
Polychaete Nereis vexillosa
Urchins
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Cellulose feeder
Bivalve Teredo
foot Greatly elongated mantleObtains nitrogenwith symbioticnitrogen fixing bacteria
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The End