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13 Benthic Life Habits
Notes for Marine Biology:
Function, Biodiversity, Ecology
By Jeffrey S. Levinton
Important Benthic Lifestyles
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Benthos
Epibenthic
Burrowers
Borers
Infaunal, semi-infaunal
Benthic swimmers
Interstitial
Benthos - Size Classification
• Macrobenthos - shortest dimension > 0.5
mm
• 0.1 mm < Meiobenthos < 0.5 mm, greater
• Microbenthos < 0.1 mm
Feeding Classification
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Suspension feeders
Deposit feeders
Herbivores (macroalgae or microalgae)
Carnivores
Scavengers
Life in Mud and Sand
Soft sediments are a mixture of inorganic
particles, organic particles, and pore water.
Mechanical measures of sediment grain
size
Particle size - measure of current strength
median, silt-clay percent (<62mm)
Sorting - variation of current strength, poor
versus well-sorted sediment
Life in Mud and Sand
Particle size - measures
Median grain diameter - measured usually
by sieving and weighing size fractions
Silt-clay fraction - percent of weight of sediment
< 62mm
Sedimentary Structures
Can result from physical processes:
Ripple marks on an intertidal sand flat
Sedimentary structures
Can result from biogenic processes
Sediment surface showing fecal casting (left) and
Burrow opening (right) of lugworm Arenicola marina
Burrowing in Sediment
• Burrowers use hydromechanical and
mechanical digging mechanisms to move
through the sediment
• Watery sediments with high silt clay content
have thixotropy - but burrowing also
propagates cracks in muddy sediments with
lower water content - particles coated with
organic polymers stick together1.
1. Dorgan et al. 2005 Nature 433:475
Burrowing in Sediment
• 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
Burrowing in Sediment - Bivalve
Foot
Lugworm, Arenicola marina
Longitudinal muscle
contracting
Terminal
anchor
Eversion of
pharynx
Circular muscle
contracting
Penetration
ancthor
Burrowing in Sediment
Inarticulate brachiopod
Mechanical displacement
burrowing
Burrowing in Sediment
Biogenic structures - burrowing and processing
of sediment affects sedimentary structures
1. Burrowing in mud increases water
content of sediment
2. Increases grain size (fecal pellets to aggregated
fine particles)
3. Alters vertical and 3-D mechanical, chemical
structure
Burrowing in Sediment
Biogenic graded bedding
Annelid ingests small particles at depth and deposits them on surface
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
Interstitial Animals
(a) polychaete, (b) harpacticoid copepod, (c) gastrotrich,
(d) hydroid, and (e) opisthobranch gastropod
Depth
Burrowed, pelletized layer
Water content
Soft-sediment microzone
Soft-Sediment Microzone
• Strong vertical chemical gradients
• Gradients strongly affected by biological
activity
Soft-Sediment Microzone
Soft-Sediment Microzone
Aerobic bacteria (use oxygen to break down organic substrates)
RPD H2S oxidizing bacteria
RPD
Fermenting bacteria (break down organic cpds. --> alcohols,fatty acids)
Sulfate reducing bacteria (reduce SO4 to H2S)
Methanogenic bacteria (break down organic cpds. --> methane)
Organic Matter in Sediment
• Particulate organic matter derives from
sedimenting phytoplankton, particulates
from seaweeds, sea grasses
• Decomposition involves leaching of
cellular constituents, fragmentation from
mechanical processes and feeding, and
decay by microbes
Benthic Feeding Types
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
Deposit Feeders
(a) the surface tentacle feeder Hobsonia, (b) the within-sediment
tentacle-feeding bivalve Yoldia limatula, (c) the surface depositfeeding siphonate bivalve Macoma, (d) the within-sediment-feeding
Atlantic polychaete Pectinaria gouldii, (e) the surface-feeding
Corophium volutator, and (f) the deep-feeding Arenicola marina
feces
Burrow
opening
Surface trace of burrow of the
Lugworm, Arenicola marina
(Anglesey, North Wales)
Arenicola marina burrow cross section
Deposit Feeders
• Microbial stripping hypothesis: deposit
feeders are most efficient at digesting and
assimilating benthic microbes (diatoms,
bacteria)
Deposit Feeders
Deposit Feeders
• 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
Deposit Feeders
• Microbial stripping hypothesis: deposit
feeders are most efficient at digesting and
assimilating benthic microbes (diatoms,
bacteria)
• At some times of year, fresh
phytoplankton sinks and is added to the
bottom: another source of nonliving food
for deposit feeders
Deposit Feeders
• 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
Deposit Feeders
• 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
• Some deposit feeder guts - surfactant activity emulsify particles
Deposit Feeders - Optimal gut
passage time
Food
digested
Richer food
Gut passage time
See Taghon, G. L. 1981, American Naturalist, 118: 202-214
Deposit Feeders - Main Points
• Ingest sediment including nonliving particulate organic
matter (POM) and microbes
• Microbes digested more efficiently than POM, but
bacteria not abundant enough to support most
macrobenthic deposit feeders
• Some deposit feeders have surfactant digestive activity
- loosen POM from sediment particles
• POM often dominates total organic matter, so poor
digestion can still bring a reward
• Surface-feeders (better food) vs. deep head-down
feeders (more refactory food)
• Spring deposition of POM from water column can
bring a nutritious source of food in a pulse
• Some environments (mangroves) have a continuous and
nutritious source of organic matter.
Suspension Feeders
• Feed on small particles, low Reynolds
number within a chamber (e.g. bivalves),
higher Re outside
• Passive versus active suspension feeders
Suspension Feeders
(a) the active suspension-feeding parchment worm Chaetopterus; (b) the suspension-feeding
polychaete Serpula, which uses ciliary currents to draw particles to tentacles; (c) cross section
of a bivalve mollusk, an active suspension feeder (arrows denote ciliated tracts transporting
particles); (d) the acorn barnacle Semibalanus balanoides with cirri protruded like a basket
Suspension Feeders
Encounters of particles with fibers,
direct interception the dominant mode
Suspension Feeders
Bivalve, x-section
Polychaete Serpula
Active suspension feeders
Barnacle
Active Suspension Feeders - Issues
• Concentration of particles - saturation and
even clogging
• Selection of high-quality particles
• Current velocity and ability to create
current, and keep siphon erect
Particle Selection by Bivalves
Use surgical telescope to look within
mantle cavity
Fiber-optic
light source
Endoscope
Video connector
Oyster
Gill of mussel Mytilus edulis, red
shows ventral tract, where particles
are gathered and transported in mucus
Oyster gill lamella
Measuring Selectivity
Flow Cytometer: Chlorophyll, Phycoerythrin, Particle
Size, Index of refraction
Experiment to study particle selectivity of Eastern
oyster Crassostrea virginica
Oysters fed particles 20 microns in size, either algae
or poor-quality cellulose-rich particles
Question: Can oysters discriminate between particles
Procedure: Endoscope used to see results, but
particles samples from two ciliated tracts, one where
particles are transported to mouth or another where
they are transported to palp and rejected