13 Benthic Life Habits

Notes for Marine Biology: Function, Biodiversity,

EcologyBy Jeffrey S. Levinton

©Jeffrey S. Levinton 2001

Important benthic lifestyles

• Benthos

• Epibenthic

• Burrowers

• Borers

• Infaunal, Semi-infaunal

• Benthic swimmers

• Interstitial

Benthos - size classification

• Macrobenthos - shortest dimension < 0.5 mm

• Meiobenthos - smaller than 0.5 mm, greater than 0.1 mm

• Microbenthos - < 0.1 mm

Feeding Classification

• Suspension feeders

• Deposit feeders

• Herbivores (macroalgae or microalgae)

• Carnivores

• Scavengers

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

Life in Mud and Sand

Life in Mud and Sand

Particle size - important parameterCurrent strengthSorting - variation of current strength, poorversus well sorted sediment

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

Ripple marks on an intertidal sand flat

Sedimentary Structures

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

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

Burrowing in sediment -bivalvefoot

Muscle contraction

PA = penetration anchorTA = terminal anchor

PA

TA

Hydromechanicalburrowing

Circularmuscles

Longitudinalmuscles

Longitudinal musclecontracting

Circular musclecontracting

Terminalanchor

Eversion ofpharynx

Pentetrationancthor

Lugworm, Arenicola marina

Burrowing in sediment 4

Mechanical displacementburrowing

Inarticulate brachiopod burrows by scissoring2 valves back and forth, which displaces sediment

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

Burrowing in sediment 8Biogenic 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 2

Polychaete Gastrotrich Opisthobranch gastropod

Harpacticoid Hydroid copepod

Soft-Sediment Microzone

• Strong vertical chemical gradients

• Gradients strongly affected by biological activity

Oxic

Anoxic

Redox Potential Discontinuity

exchange

Soft-Sediment Microzone 4

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

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

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 2

Surface tentaclefeeding polychaete

Tentacle feedingbivalve

Surface siphonatefeeding bivalve

Deep feedingpolychaete Surface feeding

amphipod Deep feeding polychaete

Surface trace of burrow of the Lugworm, Arenicola marina

(Anglesey, North Wales)

feces

Burrowopening

Arenicola marina burrow cross section

Deposit feeders 3

• Microbial stripping hypothesis: deposit feeders are most efficient at digesting and assimilating benthic microbes (diatoms, bacteria)

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

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

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

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

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

Suspension Feeders

• Feed on small particles, low Reynolds number

• Passive versus active suspension feeders

Suspension Feeders 2

Bivalve, x-section Polychaete Serpula Barnacle

Suspension Feeders 3

Sea squirt Styela montereyensisPassive suspension feeding mechanism

Suspension Feeders 4

Encounters of particles with fibers

= fiberDirectinterception

SievingInertial

impaction

Motileparticle

deposition

Gravitationaldeposition

Suspension Feeders 4

Encounters of particles with fibers

= fiberDirectinterception

SievingInertial

impaction

Motileparticle

deposition

Gravitationaldeposition

Carnivores

oystercatcherBivalve Cuspidaria

Crab Callinectes sapidus

PolychaeteGlycera

Gastropod Nucella

Herbivores

Parrot fish

Chiton

radula

Polychaete Nereis vexillosa

Urchins

Cellulose feeder

Bivalve Teredo

foot Greatly elongated mantleObtains nitrogenwith symbioticnitrogen fixing bacteria

The End