Settlement of Marine Organisms in Flow
Author(s): Avigdor Abelson and Mark Denny
Reviewed work(s):
Source: Annual Review of Ecology and Systematics, Vol. 28 (1997), pp. 317-339
Published by: Annual Reviews
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1997.28:317-39
Annu.Rev.Ecol. S-vst.
? 1997 bvAnnualReviewsInc. All rightsreserved
Copyright
SETTLEMENT OF MARINE
ORGANISMS IN FLOW
Abelson
Avigdor
Tel Aviv,Israel
Research,Tel AvivUniversity,
forNatureConservation
Institute
69978; e-mail:[email protected]
MarkDenny
University,
Stanford
HopkinsMarineStation,BiologicalSciencesDepartment,
93950-3094
PacificGrove,California
KEY WORDS:
larvae,spores
flow,propagules,
settlement,
ABSTRACT
A feature
commonto manybenthicmarineplantsand animalsis thereleaseof
ofdispersal.Successful
onlymechanism
thatserveas theorganism's
propagules
transient
dispersaldependsto a largeextenton theprocessof settlement-the
phasebetweenthepelagiclifeofthepropaguleandthebenthicexistenceofthe
settlement
onthreelevels: 1. Flowcanactby
adult.The flowofwatermayaffect
the
propagules.Theseforcesmayaffect
forcesonsettling
hydrodynamic
exerting
or
encounter,
itsbehaviorfollowing
withthesubstratum,
propagule'sencounter
cue thatinducesactivebehaviorofmotile
both.2. Flowmayprovidea settlement
cues (e.g. sediment
propagules.3. Flow mayactto mediatevarioussettlement
We discussthesethreelevelsof flow
of attractants).
load andtheconcentration
offlowinthesettlement
importance
thepotential
effects
as a meansofexamining
indifferprocess,andthenwe exploretheecologicalconsequencesofsettlement
factors.
offlowandflow-derived
in lightofthedirecteffects
entflow-regimes
INTRODUCTION
Evidence is accumulatingfor the significantinvolvementof water motion in
diverse biological and ecological processes in the sea (e.g. 13, 24, 39, 62).
food, and wastes to and fromorganThese effectsinclude thefluxof nutrients,
isms, externalfertilizationof gametes,disturbanceby hydrodynamicforcesof
benthicorganisms,and the abilityof organismsto detectscents. In addition,
317
17$08.00
0066-4162/97/1120-03
318
ABELSON & DENNY
flowmaycontrol
orstrongly
regulate
thedispersalofbenthic
species.A feature
commonto manybenthicplantsandanimalsis thereleaseofpropagules(e.g.
sporesandlarvae),whichare,inmostcases,theorganism's
onlymechanism
of
dispersal.Severalpossibleadvantages
accruetoa lifehistory
thatincludesdispersingpropagules:widedistribution
ofoffspring,
geneflow,andcoexistence
withdisturbance
(39,73). Fulfilment
of thesetasksdependsto a largeextent
on theprocessofsettlement-the
transient
phasebetweenthepelagicdispersal
phaseandthesessileexistenceoftheadult.Duringthisphasepropagules
must
first
encounter
thesubstratum.
Followingitsarrivalat theseabed,a propagule
mustdetermine
theadequacyofthesubstratum
foradultrequirements
byusing
a variety
ofsettlement
cues,whichmayincludesurfacecontour(17,95), substratum
type(15), chemistry
(55,65), thepresenceofa microbial
film(82), and
flowconditions
(e.g. 58, 100). Finally,thepropagulemusteffectively
attach,
thereby
allowingmetamorphosis
anddevelopment
intoa reproductive
adult.
Flowmayaffectsettlement
ofmarineorganisms
on threelevels.First,flow
mayactas anagentexerting
hydrodynamic
forcesonsettling
which
propagules,
thepropagule'sencounter
either
withthesubstratum,
mayaffect
thepropagule's
behavior(passiveor active)followingencounter,
or both. Second,flowmay
actas a settlement
cue thatinducesactivebehaviorofmotilepropagules.Only
a fewstudies(mostofwhichdeal withinvertebrate
larvae)haveexaminedthe
role of flowregimein determining
siteselectionby propagules(e.g. 4, 58).
factoraffecting
varioussettlement
Finally,flowmayact as a mediating
cues
(e.g. sedimentation,
chemicaldistribution,
andlightintensity).
Therearethreeapproachesto thestudyof floweffects
on settlement.
The
first,and probablythemostcommon,is to monitorsettlement
on substrata
in thefield(e.g. 27, 56, 57). By characterizing
theflowpattern
inducedby
and comparing
thisto thesettlement
oflarvae,one can
substrata,
distribution
relatethesettlement
to flow.The primary
indirectly
pattern
problemwiththis
is a practicalone; itis difficult
to observethebehaviorof
approach,however,
propagulesundernaturalflowconditions.The secondapproachcircumvents
thisobservational
settlement
inthelabproblembydirectly
observing
behavior
undercontrolled
flowconditions
oratory
(e.g. 4, 15,21, 58). Thedisadvantage
ofthisapproachis thatitcannotsimulatetheexactflowconditions
experienced
bypropagulesin thefield.The thirdmethodemploysnumerical
modelingof
flowconditions
in thefieldandofpropagules'behaviorduringdisprevailing
persalandsettlement
(e.g. 25, 28, 34). Thislastapproachdependslargelyon
data obtainedby theothertwoto supplyrealisticvaluesforparameters
and
modelsmustoftenbe
boundaryconditions.To be mathematically
tractable,
modelsmayrevealneglected
simplistic.
Despitethislimitation,
aspectsofempiricalstudiesand can providenewdirections
of research.A synthesis
of all
threeapproachesis likelybestto augment
offlow
oftheeffects
understanding
ofpropagules.
on thesettlement
SETTLEMENTIN FLOW
319
In thepresent
reviewwe examinethethreelevelsoffloweffects
andthevarious structural
andbehavioral
features
ofpropagules
thatmayinteract
withflow
We thenexploretheecologicalconsequencesofsettlement
duringsettlement.
as theyareaffected
bytheflowcharacteristics
ofdiffering
substratum
types.
SUBSTRATUM TYPES AND FLOW PATTERNS
The vastmajority
of studiesthatdeal withsettlement,
whether
in thefield,in
thelab,orona computer,
examinebehaviorandsettlement
overplanar
patterns
substrata.In nature,
however,
thesurfacesavailableforsettlement
(e.g. coral
reefs,kelpforests,
andartificial
reefsandotherengineered
structures)
include
a diversity
ofshapes.To understand
floweffects
on settlement
andto assessthe
we first
flow-generated
forcesthata propagulemustwithstand,
mustcharacterize theflowpatterns
associatedwithcommonsubstrata
in thesea. To thisend
we briefly
describetheflowpatterns
inducedbythreemajorsubstratum
types:
planarsurfaces,
protruding
bodies,anddepressions.Ourqualitative
description
is basedon thesimplifying
assumption
thattheundisturbed,
mainstream
flow
is steady.In thefield,however,
itis notuncommon
tofindflowthatis unsteady
duetotidesand/or
waveaction(fora morethorough
ofbenthic
flow
description
see 62).
environments,
Planar Substrata
in themarineenvironment
andinExamplesofplanarsurfacesare numerous
clude substrata
of limitedarea,suchas rockfaces,as well as morespacious
such as softbottoms.If we assumethattheflowis steadyovera
substrata,
planarsubstratum
oriented
parallelto thedirection
of watermotion,theflow
nearthesubstratum
(in thebenthicboundarylayer)is similarly
steady.The
boundarylayerovera planarsubstratum
developsfroma verythin,laminar
flowjust downstream
of theleadingedge of thesurface,to a turbulent,
thick
from
boundary
layerfurther
alongthesurfaceata certaindistancedownstream
theedge(Figure1). Iftheplanarsubstratum
is spaciousandhydrodynamically
smooth,a fullydevelopedboundarylayeris characterized
by threezones: a
thinviscoussublayeradjacentto theseafloor,a turbulence-dominated
layer
adjacenttothemainstream
flow,anda buffer
layerbetweenthetwo(Figure1).
Althoughtheocean flooris seldomsmooth,viscoussublayersthatmayexist
there(16) canbe upto6 mmthick.Forpurposesofcalculating
theforcesacting
on smallpropagulesduringsettlement,
therelevant
zone is thisthinsublayer.
Bodies
Protruding
The surfacesofmarinesubstrata
arefrequently
rough,andtheflowoverrough
substrata
can be dividedintovariousflowregimes(54,97,98). Some ofthese
involve"protruding
bodies,"thoseelementsor bodiesthatprojectabovetheir
320
ABELSON & DENNY
Um
LBL
_
-'P
ofplanarsubstratum.
The
oftheflowregimesin thevicinity
FigureI Schematicrepresentation
boundary
layer(TBL)
bysolidlines.Thefullyturbulent
laminarboundary
layer(LBL) is indicated
layeris indicatedby
is characterized
by eddiesand sweeps(indicatedby curliques).The buffer
flow;thebrokenlineindicatestheouter
smalleddiesovertheviscoussublayer.Um = free-stream
ofwhichis 3; arrowsindicatetheflow,and theirlength
edge oftheboundary
layer,thethickness
oftheviscoussublayer.
proportional
to flowvelocity.3v = thickness
andthepresenceof whichalterstheflowpattern:kelpstipes,coral
neighbors
suchas pilingsand artificial
reefs.
branches,
rocks,and manmadestructures
bodiesofferless surfacearea thando planarones,they
Althoughprotruding
forman important
of thesubstratum
in manymarine
component
nonetheless
environments.
In our analysisof flowpatternin the vicinityof protruding
as a simplemodel(Figure2).
bodies,we use a circularcylinder
The presenceofa cylinder
causesflowto accelerateas watermovestoward
in Figure2), and
itswidestcross-section
(at 0 = 900 forthecircularcylinder
On the
flownearthecylindercan reachvelocitiestwicethatof mainstream.
sideofthecylinder,
flowdecelerates.Due totheadversepressure
downstream
a separation
zoneis inducedresulting
associatedwiththedeceleration,
gradient
in a downstream
wake(Figure2).
Depressions
onthesurfacesofsubmerged
Depressions(e.g. pitsandcrevices)areabundant
manmadestructures,
andbothsoftbottoms
and
substrata
including
organisms,
hardbottoms.Generallyspeaking,depressions
havetheoppositeeffectfrom
areathrough
whichwater
protruding
bodies-theyincreasethecross-sectional
slowswhenflowing
overandintoa depression
flows,andwaterconsequently
thatresults
gradient
(62). Thedecreasingvelocityleadstoan adversepressure
anda vortical
ina separation
oftheflowattheupstream
edgeofthedepression
flowwithin
thecavityitself(Figure3). Flowoftenreattaches
tothesubstratum
at thedownstream
rim(96; Figure3C).
SE1TLEMENT
-_>
Um >
u~~_L
|-
A
IN FLOW
321
Ur2Um
W Um
Rb
W0a
B
Figure2 Schematicrepresentation
of a protruding
body(A), itscrosssection(B), and theflow
regimeinitsvicinity.
Theflowpattern
inducedbyprotruding
bodies(inhighReynoldsnumbers)
is
oftwozonetypes:accelerated
thinboundary
laminar,
andretarded
layerflow(L) inupstream
turns,
turbulent
flowin downstream
wakes(W). Um = free-stream
flow(notethatthevelocityat the
boundary-layer
edgecan reach2 Um); brokenlinesindicatetheouteredgeoftheboundary
layers;
brokenarrowsindicatestreamlines,
andfullarrowsflowvelocitywherelengthis proportional
to
velocity.Rb = bodyradius,0 = anglebetweena givenpointandtheupstream
stagnation
point;
oftheboundary
a = thickness
layer.
The maximumtangential
velocitiesof vorticeswithindepressionsare apone tenththemainstream
proximately
flowvelocityand do notexceed two
tenthsmainstream
velocity(e.g. 40, 77, 91). Periodically,
thevortexwithin
thecavitybecomesunstableandis shedfromthecavity.Thisvortex-shedding
leadstobulkreplacement
offluidwithin
thedepression
and,thereby,
potentially
to thebulkdeliveryof propagulesto thecavity.The sheddingof vorticesis
enhancedbythepresenceofa distinct
strongly
downstream
edgeto thecavity
(69).
Most studiesdealingwithcavityflowhave examineddepressionsbelowa
in whichcase sharplydefinedvorticesare thedominant
planarsubstratum,
of cavityflow. Depressionsin hardsubstrata
patterns
howmaybe formed,
ever,fromwallsraisedabove thelocal topography,
and theymayproducea
different
of cavityflow(A Abelson,personalobservation).This flow
pattern
slowentrainment
includesa relatively
of fluidoutofthecavitybythe
pattern
flow"(an outwardcurrent
"mainstream
thatoccupiesmorethanthreequarters
of thecavitycrosssection),and a rapidinwardcurrent
nearthedownstream
lip (Figure3C). Deliveryofpropagulestothecavityundersuchcircumstances
322
ABELSON & DENNY
Um
A
B
Um
C
ofa shallowdepression(A) (wherethedepression's
depthis
Figure3 Schematicrepresentation
offlow)anddeepdepressions
equal toorless thanitslengthin thedirection
(B, C) (wheredepthis
pointsare
greater
thanlength)andtheflowregimesin theirvicinity.
Separationandreattachment
indicates
illustrated
bythestreamlines
(in C). Solidarrowsindicateflowdirection;
arrowthickness
relativeflowvelocity;wavylinesindicatethezone of slow outwardcurrent;
Um = free-stream
vortex.
flow;P = primary
vortex;S = secondary
whilepassiveescapeofpropagmaybe enhancedbytheinward,
rapidcurrent,
current
is slowerthanthepropagules
sink.
ulesis unlikely
becausetheoutward
no quantitative
ratesto
Unfortunately,
studyhas yetaddressedthetransport
cavitiesunderthesecircumstances.
FLOW AS A FORCING AGENT
can be dividedintotwocriticalstages:thedeliveryofpropagules
Settlement
to thesubstratum,
and thesubsequentestablishment
of thelarvaeor spores,
SETTLEMENT IN FLOW
323
and metamorphosis.
a stagethatincludesbothattachment
Here,we describe
possibleeffects
of flowas a physicalforcing
mechanism
thatactson settling
propagulesduringthesetwostages. Transport
of propagulesto
mechanisms
each substratum
typeare listed,alongwiththerelevant
problemseach poses
forpropagules.We thenexamineproblemsthatpropagulesmightencounter
in exploringthesubstratum
and achievinga residencetimesufficient
forthe
creationofa firmattachment-aprocessthatis a function
oftime(e.g. 29).
MechanismsofPropagule TransportTo Substrata
tothesubstratum
canbe achievedthrough
activeswimDeliveryofpropagules
ming,passivetransport
bytheambientflow,or both.Passive,flow-mediated
delivery
maybe dividedintotwomajorcategoriesdependingon whether
the
flowis laminarorturbulent.
Marinepropagulescan be classifiedacthemselves.
Nonmotile
cordingto theirabilityto transport
propagulesinclude
variousbacteria,bacterialcapsules,fungaland algal spores,larvaeof some
invertebrate
species,dispersivefragments
of bothplantsand animals,and the
slow-moving
juvenilesand adultsof tinyspecies. Motilepropagulesinclude
somealgalsporesand(primarily)
invertebrate
larvae.Thesemotilepropagules
canreach5 mminlength
(10) butarefrequently
whereasthenonmuchsmaller,
motilefragments
canbe muchlarger-uptoseveralcentimeters
long(e.g. 35).
Amongpropagulesthatare nominallymotile,mostare weak swimmers,
capable of maintaining
speeds of less than 1 cm/s(19,51). This speed is
less thantheflowspeedscommonnearocean substrata
(e.g. 13), suggesting
ofmostself-propelled
thattheflowmaylimitthemaneuverability
propagules.
we assumethatundermostprevailing
flowconditions,
turbulent
Accordingly,
ofpropagulesto thesubstratum
is due to passivemotion.
transport
DELIVERY BY ACTIVE SWIMMING
The basic mechanisms
by whichparticlescan
inlaminarflowwereintroduced
be delivered
tothesubstratum
tothebiological
literature
& Koehl(72) inthecontext
ofaerosolfiltration
byRubenstein
theory.
a numberof studies(e.g. 8, 42, 75) have contributed
to our
Subsequently,
of
the
mechanisms
marine
understanding
controlling
particleentrapment
by
Fourofthesemechanisms
organisms.
mayalso operatetotransport
propagules
inwhichneutrally
tosubstrata:1. Directinterception,
follow
buoyant
particles
within
if
are
carried
oneparticle
radiusofthesubstratum,
streamlines;particles
in whichparticlesthataredenserthan
it. 2. Inertialimpaction,
theyencounter
as flowis accelerated
whilepassingarounda
thefluiddivergefromstreamlines
denseparticlesmoveinthe
protruding
body;underappropriate
circumstances,
3. Gravitational
a mechanism
direction
ofthesubstratum.
similar
deposition,
toinertial
butoneinwhichdeviation
froma streamline
is causedby
impaction,
DELIVERY BY LAMINAR FLOW
324
ABELSON & DENNY
inwhichverytinypropagules
mayexhibit
a gravity.
4. Diffusional
deposition,
causingthemto encounter
thesubstratum.
random,
Brownianmotion,thereby
The natureofturbulent
flowsnearthe
bed is characterized
sequenceof "ejections"and "sweeps,"
by an alternating
whicharerandom
inspaceandtime(84). Theejectionphaseis a slowmovement
in theflow,whichmove
of vorticesthatcreateshorseshoe-shaped
structures
The sweepphaseis characterized
by high-speed
awayfromthesubstratum.
flowthatpenetrates
theviscoussublayeras fluidmovestowardthesubstratum.
arecarried
tobe a mechanism
bywhichpropagules
Sweepshavebeensuggested
on
theviscoussublayerto thesubstratum
(18). To date,information
through
ratesof
theefficiency
mechanism,
andontheencounter
ofsweepsas a transport
particlestransported
bysweeps,is lacking,andwe mustrelyon thetraditional
by
fora qualitativeestimation
of theefficiency
theoryof turbulent
diffusion
whichturbulence
transports
propagules(e.g. 25, 28).
DELIVERY BY TURBULENT
TRANSPORT
ofpassiveproTYPES Thetransport
TRANSPORT
TO DIFFERENTSUBSTRATUM
transport,
pagulesto horizontal,
planarsubstratais dominatedby turbulent
and downwardswimming
(e.g. 24, 30, 34). Turbugravitational
deposition,
is a function
of turbulent
transport)
lence intensity
(an indexof theefficacy
of bothflowvelocity(24) and whether
theflowis accelerating
(50). At high
is
mainstream
velocitiesin decelerating
or steadyflows,turbulence
intensity
mechanism
to a planarsubstratum
transport
typically
high,and thedominant
regardless
ofpropagulesize. However,under
is likelytobe turbulent
transport
flows),
intensities
(e.g. slowandaccelerating
conditions
ofverylowturbulence
bygravitational
deposition
(especiallyfor
transport
mayinsteadbe dominated
Given
thatconsequently
sinkfast)anddownward
swimming.
largepropagules
inthesea andthevariety
thevastareaofplanarsubstrata
ofeffective
transport
ratesovera wide
mechanisms
thatcan combineto maintainhighencounter
rangeofflowvelocitiesandpropagulesizes,itcan be arguedthat,amongthe
threesubstratum
planarsubstrata
types,thechancesofa propaguleencountering
arethehighest.
A preliminary
modelof transport
to protruding
bodiessuggeststhatdirect
is thedominant
data). The
mechanism
(A Abelson,unpublished
interception
is proportional
andpropagofdirectinterception
tobothflowvelocity
efficiency
thetransport
ratesofpropagules(whicharetypiule size (42,72). Therefore,
are
velocitiescommonin themarineenvironment
callysmall)in thelow-flow
expectedtobe low.
inhibits
mostofthetransport
mechanisms
Theseparated
flowina depression
as suggestedby Yageret al
thatprevailoutsidecavities.The sole exception,
which,in thiscase, is coupledto theprocess
(96), is gravitational
deposition,
SETTLEMENT IN FLOW
325
Particlescan effectively
ofvortex-shedding.
settleonlyin theinterim
between
withwhichvorticesareshedis determined
sheddingevents.The frequency
by
theflowvelocity,
cavitydimension,
and a dimensionless
index(theStrouhal
whichis a function
number),
ofthecavityshapeandReynoldsnumber(92).
is likelytobe thedominant
Gravitational
mechanism
deposition
transporting
propagulesintodepressions.One shouldbearin mind,however,
thatparticle
dynamicsin vorticescan be complicatedand are notwell understood.For
example,submicron
particlescan segregate
fromtheflowduetotherotational
motionoftheflow(33), a processforwhicha physicalexplanation
is lacking.In
withlow flow-velocities,
deepdepressions
diffusional
deposition
anddirected
swimming
mayalso be important.
TheLimitstoExploration
andAttachment
Once a propaguleencounters
the substratum,
thenextstepin settlement
is
eitherexploration
of thesurfaceor immediate
attachment.
In eithercase, the
propagulemustbe able to controlitspositionin space,whether
to stayin one
toestablisha permanent
spotfortimesufficient
attachment
ortomoveinsearch
ofan optimalsite.
The abilityof a propaguleto controlitspositiondependson whether
itcan
resistthehydrodynamic
forcesimposedon it. Understeadyflowconditions,
a propaguleon thesubstratum
is exposedto dragand liftforces,and if the
flowaccelerateseitherspatiallyor temporally,
an acceleration
reactionis also
thevelocitygradient
imposed.In addition,
inthebenthicboundary
layerexerts
a torqueon thepropagule,a "rotary
force"thatrollsthepropagulealongthe
substratum
(A Abelson,P Sanjines,S Monismith,
submitted).
In theabsenceof adhesion,even theslightest
forcesare sufficient
to dislodgepassiveparticlesfromthesubstratum,
withtheresultthatnonadhesive
propagulesarelikelyto settleonlywheretheflowis exceedingly
slow(e.g. in
depressions).Undersuchbenignflowconditions,
directed
swimming
mayalso
be effective
in allowinga propaguletomaintain
itsposition.
Assumingthatthesurfacesofsubstrata
areequallyrough,therelativeforces
ongivenpropagule
sizesatthesubstratum
surface
maydependonthesubstratum
type.Protruding
bodiesinducethelargest
hydrodynamic
forcesamongthethree
substratum
types.Theserelatively
to thesteep
strongforcesaredue primarily
overtheprotruding-body
velocitygradients
as wellas totheincreased
surfaces,
flowvelocityalongthesidesofthebody(which,in cases ofcircularcylinders,
mayreachvaluestwicethemainstream
flowvelocity;Figure2). An additional
roleindetermining
the
force,theacceleration
reaction,
mayplayan important
resultant
forceon relatively
largepropagules.Planarsubstrata
presentintermediatevaluesof meanforcesexertedon settlingpropagules.These forces
are lowerthanthoseexertedby theflowin thevicinityof protruding
bodies
326
ABELSON& DENNY
on planarsubstrata,
buthigherthanthesein depressions.Propagulessettling
shearstressesdue to sweepsthatmay
can experienceinstantaneous
however,
exceed30 timestheaverage(26).
Problems
ProposedSolutionsToForce-Related
barrithatcan actas potential
twofactors
In thediscussionabove,we identified
manner
inwhichthey
ofmarinepropagules-theinefficient
erstothesettlement
forces
bodiesand theneedto resisthydrodynamic
aredeliveredto protruding
totheseproblems
Thesolutions
whilemaintaining
a positiononthesubstratum.
with
thatincreasetheprobability
ofencounter
are of twokinds:mechanisms
(or
instantaneous
attachment
and mechanisms
of temporary,
thesubstratum,
adhesion),whichenablethepropaguleto achievetheresidencetimerequired
fora permanent
attachment.
bodiesis posiof transport
to protruding
As has beennoted,theefficiency
toincrease
withpropagulesize; hence,an obviousmechanism
tivelycorrelated
a protruding
bodyis to increase
of a propaguleencountering
theprobability
thepropagule'ssize. However,largepropagulesexperiencelargeflow-related
Likewise,largepropagules
forcesthatcan detachthemfromthesubstratum.
theoptimalsolutionshouldbe one thatinare costlyto produce.Therefore,
butdoes notincrease
size priorto encounter
creasesthepropagule'seffective
theforceexertedafterencounter.
The evolvedsolutionsto thisproblemare based on theuse of flexibleapdevicesthatincreasethepropagule'svolume.For
pendagesorothertemporary
example,mucussheathsenvelopethepropagulesofredalgae (12) anddiatoms
thesheathof a red-algalsporemayin(E Gaiser,personalcommunication);
(Figure4A). Another
creaseitsactualvolumebynearlyan orderofmagnitude
fromthepropagule'sbody(Figure4B), a sois to extendprojections
strategy
lutionfoundinbacterialcapsules(20) andpili(60), fungalspores(38), andthe
of redalgae (47) and softcorals(Y Benayahu,personal
dispersivefragments
Thesefragments
producestickycrownsofrhizoids.
communication).
devicesthatcoverthewholesurfaceareaofthepropagulebody
Attachment
whichinmany
beimpractical
formotilepropagules,
ofitmight
ora largeportion
(bycilia,forinstance).Motile
cases utilizetheirbodysurfaceforlocomotion
are
juvenilesoradultsofinvertebrates
propagulessuchas larvaeanddispersive
appendagewhich,due tothecontinuous
morelikelytouse a single,projecting
occupya muchlarger
reorientation
of thepropagulein flow,maynonetheless
without
an appendage(4) (Figure4C). Projecteffective
spacethana propagule
ingappendagesincludethemucousthreadssecretedby larvaeof hydrozoans
sea anemones(76), corals(4, 88),
information),
(37; A Abelson,unpublished
and polychaetes(32), and byjuvenilesand smalldispersiveadultsof snails
(49,90) andbivalves(23, 87).
SETTLEMENT
IN FLOW
327
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toehnedecutraeflavewtpordnbde4.
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(.g
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(e.g..ucous.an.byssus)
After
thepropagulehasreachedthesubstratum,ithastwopotentialremainin
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it
t
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328
ABELSON & DENNY
in searchof a moreappropriate
explorethesubstratum
site. In bothcases,
someformofadhesionis required.
Oftenan adhesiveis secretedpriorto theorganism'sencounterwiththe
substratum,
examplesbeingthemucousthreadsand sheathsdescribedabove.
Theseallowthepropaguletoadhereinstantly
uponcontactwiththesubstratum.
Ifthepointofcontactis inappropriate,
somepropagules
can cutthemselves
off
thesubstratum
andreturn
to theflow(4). Thistrial-and-error
strategy
maybe
an inefficient
meansof searchingfora favorablesiteto settle.Alternatively,
adhesioncan be effected
by devicesthatenablethepropaguleto movewhile
keepingits attachment.
The mostprominent
exampleis thebarnaclecypris
larva,whichusesitsantennular
attachment
organstoholditselfon thesubstratumwhileit explores(e.g. 21, 99). The exactadhesivemechanismof these
antennules
is notclear,butitseemsthatthelarvacan controltheadhesiveness
ofitsantennules.
A similarroleis playedbythepyriform
organofthecyphonauteslarvaofbryozoans.Thisorganis reported
tosecretea thinmucussheet
thataidslocomotion
andhelpstofastenthelarvatemporarily
tothesubstratum
(83); larvaeofthebryozoanMembranipora
membranacea
havebeenobserved
to exploresubstrata
in flow(1), althoughtheexactmechanism
oflocomotion
in cyphonautes
larvaeis notyetclear.
The morphology
of a larvacan affectitsexploratory
locomotionwhenthe
is exposedto flow.This effecthas beenexaminedusingthreemororganism
phologicalmodels(spheres,symmetrical
elongatedmodels,andasymmetrical
subelongatedmodels)in a flowtank(A Abelson,P Sanjines,S Monismith,
mitted).The threemorphologies
behavedifferently
whenexposedto theshear
flowofa benthic
modelsweresubjected
boundary
layer.Specifically,
spherical
to torquelevelsstronger
thantheforcesmostlarvaecan generate;theyrolled
alongthesubstratum.
their
Thus,sphericallarvaemayhavetroublecontrolling
in a pattern
orientation.
The symmetrical
that
elongatedmodelsautorotated
includedtwoweakequilibrium
so larvaeofthisshapemayalso have
positions,
theirorientation.
The asymmetrically
problemscontrolling
elongatedmodels
kepta stablepositionparallelto theflowdirectionand seemedto have the
bestsuitedforlocomotion.
morphology
FLOW AS A SETTLEMENT CUE
It is reasonabletoassumethatthevariouseffects
offlowon settling
organisms
cue.
of thelocal flowregimeas a settlement
mayhaveled to theutilization
Indeed,some fieldstudiesshow activesite selectionby larvaethatmaybe
attributed
to larvalresponseto different
flows(e.g. 57, 93). The roleof flow
itseffecton otherenvironmental
maybe indirect,
actingthrough
parameters
thefewstudieson activebehaviorof
(see thefollowing
section).Nevertheless,
SETTLEMENT IN FLOW
329
settling
propagulesin responseto flowshowthatlarvaecan selectsettlement
sitesbysomehowsensingthelocal flowregime(e.g. 4, 43, 58).
theflowregimein
thatmaybe used by larvaeto characterize
Parameters
pressure
gradient
(adverseor
include:flowdirection,
shearstress,
theirvicinity
or viscoussublayer
turbulence
intensity,
boundary-layer
favorablegradients),
areofteninterrelated
(e.g.
Theseflowparameters
andacceleration.
thickness,
and turbulence
inof theboundary
layerthickness,
shearstressis a function
tensity
maybe linkedto acceleration).As a consequence,it can be difficult
behavior.For example,Crisp(21)
is affecting
to determine
whichparameter
plates. He
studiedthebehaviorof barnaclelarvaein tubesand on rotating
thatlarvaetendtomoveactivelyagainsttheflowbothbyswimdemonstrated
mingand by usingtheiradhesiveantennules.Crisparguedthatthevelocity
theattachment
thataffects
is theflowparameter
gradient
closetothesubstratum
only
oflarvaeofsessilespecies,andthenominalspeedofthewateris important
when
forits influence
on thevelocitygradient.In thesame study,however,
Crisprelated
behavioroflarvaebeforeattachment,
referring
totheexploratory
alongthe
rather
thanto thevelocitygradient
toflowdirection
larvalmigration
designin Crisp'sstudydid not
theexperimental
Unfortunately,
substratum.
and velocity
betweenlarvalresponseto flowdirection
enablediscrimination
gradient.
In another
corallarvaewereshowntoactivelyselectsetstudyofsettlement,
tlement
fromothersitesonlyintheirflowregime(4). Although
sitesthatdiffer
accelerations
bydifferent
thecorallarvaeselectsitesthatcan be characterized
used to
thepreciseflowparameters
steadyflow,deceleration),
(acceleration,
selectpreferred
siteswerenotclear.
responThe studiescitedabovefailedtoisolatethespecificflowparameters
to isolatethe
sible fortheobservedlarvalbehavior.It is possible,however,
flowregimes.
thatinducedistinct
effects
offlowparameters
byusingsubstrata
betweenlarvalresponsesto flow
For example,it is possibleto discriminate
andflatplatesin a flowtank.
andshearstressusingcircularcylinders
direction
of
A cylinderaffects
theflowsuchthatshearstressincreasesin thedirection
ona flatplatethesheardecreases
flowbecausetheflowaccelerates.In contrast,
in thedirection
of flowbecausetheboundarylayergrows.It was foundthat
rather
thanto changingtrendsin
larvaerespondto flowdirection
cyphonautes
shearstress(1).
FLOW AS A MEDIATING FACTOR AFFECTING
SETTLEMENT CUES
someofwhichareusedas setFlow affects
variousenvironmental
parameters,
Despitethelikelihood
ofbenthic
marineorganisms.
tlement
cuesbypropagules
330
ABELSON& DENNY
thisis probablytheleaststudied
ofsuchindirect
effects
offlowon settlement,
The mostprominent
environmental
aspectof therole of flowin settlement.
cues aretheconcentrafactors
thatareaffected
byflowandusedas settlement
inducers,
themagnitude
ofsediment
tionofchemicalattractants
orsettlement
andthecharacteristics
ofthe
load,thegrainsize distribution
ofsoft-substrata,
microfilm.
Flow and ChemicalCue Distribution
Numerousstudies(forreviews,see 55, 65) describetheinduction
and/or
enlarvaein responseto chemicalfactors
hancement
ofsettlement
in invertebrate
producedby conspecificadults,hostand preyspecies,and microbialfilms.
On theotherhand,veryfewstudieshave examinedtheeffectof chemicals
on settling
propagulesunderflowconditions(67, 89), and,to thebestof our
ofwater-soluble
knowledge,
onlyonestudyhasexaminedthecombinedeffects
chemicalcues andflowon settling
propagules(89).
chemicalsunDespiteourpoorknowledgeoflarvalresponseto waterborne
derflowconditions,
itis generally
acceptedthatchemotaxis
towarda preferred
in flowis unlikelyto playa rolein settlement
substratum
(e.g. 22, 65). This
flowovera
conclusionis based on threelinesof reasoning.First,turbulent
substanceis expectedto dilutethesubstanceto
bodyreleasinga waterborne
a shortdistancefromthesource(65). Second,
concentrations
within
negligible
and navigationin
thesmallsizes of larvaewouldseem to makeorientation
a concentration
gradient
unlikely(22). Finally,giventhelimitedlocomotory
oflarvae,ithasbeenassumedthattheydo notmoveupstream
(see,
capabilities
forexample,7). Because waterborne
chemicalscannotbe trackedby downstreamexploration,
thepossibility
oftracking
settlement
sitesalonggradients
ofwaterborne
chemicalswouldappeartobe impractical.
It is alwaysdangerousto underestimate
theabilitiesoforganisms,
however,
chemicalswouldseem
and theadvantageof beingable to trackwaterborne
inhabitspatiallylimited,specific
to be large. For example,manyorganisms
of
and forthemtheprobability
substrata
(e.g. thebodiesof hostorganisms),
is likely
substratum
without
theappropriate
anyremoteindicator
encountering
low. Theabilityofdispersed
chemicalstoindicateata distance
tobe extremely
thepresenceof therequiredsubstratum
could be used by a capablelarvato
movetowarditshost.Indeed,we believethatlarvaethatsettlein association
withotherorganisms
possessthecapacityto
may(whileon thesubstratum)
under
substratum
trackwaterborne
settlement
inducerstowardtheirpreferred
considerations.
We base ourviewon thefollowing
flowconditions.
mixeschemicalplumes,
is thatturbulence
rapidly
First,whiletheexpectation
it mustbe remembered
thatmixingnearthewall is relatively
weak,so that
distances(52). Additionally,
plumesmightremaincoherentforsubstantial
SETTLEMENT IN FLOW
331
flow,
whilelow concentrations
mightbe measuredin themeanin turbulent
rather
thesearegenerally
theresultofintermittent
pulsesofhighconcentration,
thanof a steadysupplyof mixedfluid(64). Relativelystrongchemicalcues
Crabstrackwaterborne
maythusbe availableoccasionallytoguidesettlement.
flow(94).
odorsfromdistancesof0.5 m and 1 m in a turbulent
Second,thearguments
againsttheabilityoflarvaeto orientandnavigatein
inexperimental
results.Mooreet al
a concentration
gradient
arenotgrounded
inchemicalsignalstructure
thatifsignificant
(53) suggested
spatialdifferences
occuratspatialscalesofa fewhundred
mm,as theyfoundintheir
consistently
and distanceinformation
fromodorplumescould be
study,thendirectional
derivedviadifferential
sensory
inputbetweenpairedsensorsinsmallorganisms.
inlarvaeis atoddswitha largebody
Furthermore,
thecase againstchemotaxis
of evidenceof chemotaxisin other,muchsmallerorganisms
(e.g. 5, 8). For
cells (a typeofphagocytic
whitebloodcells) are able to
example,neutrophil
and
detectverylow concentrations
(ca 10-10M) ofa specificchemoattractant
can detecta 1% difference
in theconcentration
acrossthecell,allowingthem
tomigrate
up a chemicalgradient
(5).
direction.Crisp
Finally,somelarvaecan exploresubstrata
in theupstream
(21) foundthatbarnaclelarvaecanactively
moveagainsttheflowbyswimming
fast(4-5 cm/s)andusingtheiradhesiveantennules.
Crisp'sresults
extremely
have beencriticizedfortheexperimental
design,whichdid notsimulatethe
naturalconditionsof larvae(58). However,it is reasonableto assumethat
barnaclelarvaearecapableoflocomoting
againstthestream.Thecyphonautes
larvaeofMembranipora
membranacea
can exploresubstrata
againstcurrents
thataremuchfaster
thantheirlocomotion
overmucussheets
speedsbycrawling
thatattachthemto thesubstratum
(1).
Insummary,
marineorganisms
maybe trackwesuggestthatlarvaeofbenthic
study
ingwaterborne
settlement
cuesunderflowconditions.However,a future
willhavetoresolvethisissuebyexamining
whether
plumescarrying
chemical
that
settlement
cuescaninduceupstream
substratum
exploration
bypropagules
habitats.
utilizethesecues totracktheirpreferred
Flow and Sediment
Load
exertsharmful
effects
on hard-bottom
dwellersin diversemanSedimentation
ners(e.g. 70). Sedimentgrainsin flowmayabradetheorganism's
tissue;suswithfilter
cancover
pendedgrainscaninterfere
feeding;anddepositedparticles
themfrom
outvariousvitalprocesses.Inlightof
organisms,
preventing
carrying
ofsediment
hardsubstrata,
sedthedeleterious
effects
on organisms
inhabiting
onthehabitatchoiceofsettling
larvae.
imentload is likelytohavean influence
rate
Thisissuehas notbeenexploredin depth,butin certaincases settlement
loads (e.g. 36, 86).
has beenshownto be reducedin areaswithhighsediment
332
ABELSON& DENNY
Sedimentload is theoutcomeoftwocounteracting
processes,sediment
erosion(orentrainment)
anddeposition.Flowis a dominant
agentgoverning
these
sedimentological
processesand therefore
is likelyto be an important
indirect
factoraffecting
propagulesiteselectionandsettlement.
Flow and Soft-Substratum
Type
Size ofsediment
grainsis an important
inthebiologyandecologyof
parameter
soft-bottom
dwellers(e.g.79). Grainsizemayaffect
selectionbydeposit
particle
feeders(74), andgrainsize maydetermine
theamountoforganicmatter
inthe
sediment
andconsequently
thefeedingefficiency
ofdepositfeeders(e.g. 85).
Theeffects
ofgrainsize onsoft-bottom
dwellersexplainswhathasbeenknown
fordecades-thatsediment
grainsize can determine
siteselection,settlement,
andmetamorphosis
ofsoft-bottom
dwellers(reviewsin 13,79).
Flow characteristics
(e.g. turbulence
and shearstress)are crucial
intensity
in determining
thegradeand structure
of depositedsediments
at a givensite
(withinthelimiting
frameofthesediment-grain
ofthepotential
coomposition
sources;e.g. 6). Combiningtheroleof flowin sediment
and the
distribution
roleof grainsize in siteselection,we can arguethatflowprobablyplaysan
important
rolemediating
larvalsettlement
inresponseto grainsize.
Flow andBiofilmType
Thereis mounting
evidencefortherole of flowin bacterialattachment,
removal,growth,
andspatialdensity,
as wellas foritseffect
onmetabolicactivity
ofbiofilm
communities
(1 1,45, 100). Flowcausingdifferent
shearstresses
may
inducebacterialfilmsofdifferent
andstructure.
morphology
Biofilmcommunitiesfunction
as pioneercommunities
developedduringthefirststagesof
successionon a newlyclearedsubstratum,
and theyact bothas an attractant
andas theactualsubstrate
uponwhichalgae andanimalssubsequently
settle.
Forexample,barnaclelarvaeoftwospeciesresponddifferently
toeach oftwo
films(61). Flow can thusindirectly
affect
latesettlers
byitseffecton pioneer
ofmicroorganisms.
communities
CONCLUSIONS AND SPECULATIONS
ActiveVersusPassiveBehaviorinSettlement
Mostmodelsofpropagular
settlement
thatwhena larva
assume,forsimplicity,
encounters
thesubstratum,
it immediately
adheresand settles(e.g. 25, 78).
modelsof benthicinvertebrate
recruitment
Similarly,
treatlarvaeas passive
particles
(e.g. 71). Butmanandcoworkers
(58,67) pointedoutthatsuchstudies
theimportance
disregard
ofbehaviorin settlement,
theirargument
supporting
withresultsclearlyshowingthatactivebehaviormaygreatly
affect
recruitment
SETTLEMENT IN FLOW
333
theacceptedparadigmis thatsettlement
patterns.Currently,
is a combined
resultofbothfloweffects
andlarvalbehavior
(e.g. 30). Itseemsthatthedelivery
of larvaeto potentialhabitatsis governedmainlyby flow-related
processes,
larvalbehaviormaybe important
whereasexploratory
oversmallspatialscales
aftertheinitialencounter
withthesubstratum
(e.g. 13,67, 93).
Nonmotileandmotilepropagulesexhibitmajordifference
in behaviorduring settlement.
Nonmotilepropagules,by definition,
do notactivelyexplore
thesubstratum
andcan selectan optimalhabitatonlybyinitiating
permanent
adhesionwhentheycome acrossa suitablesite. Althoughmotilepropagules
are able to exploreand selectan appropriate
settlement
site,a widerangeof
flowregimesexistunderwhichtheseorganisms
areunabletoexplorethesurface. Supportforthisassumption
is given,forexample,byPawlik& Butman
(66), whohaveobserved"erosion"of larvaefromthebed at shearvelocities
andcorrelation
(shearvelocityis a measureofthemagnitude
ofturbulent
flucin velocitynearthesubstratum)
tuations
higherthan1.03cm s-1. In suchflow
conditions,
swimming
ofmotilepropagulescan be considered
ineffective.
We therefore
suggestthatdifferential
settlement
(i.e. deviationfromthesettlement
distribution
fromthedistribution
predicted
ofinitialcontactsofpropaguleswiththesubstratum)
underconditions
ofhydrodynamic
forceshigherthan
thepropaguleswimming
forcesis due to desertion
ofunfavorable
sitesrather
thantoexploration
andactiveselectionofan appropriate
site.Invertebrate
larvae havebeenobservedto desertsubstrates
following
encounter
in flowtanks
theirsettlement
(e.g. 4, 14,58). Moreover,
larvaereportedly
havedeserted
site
aftersettlement
andmetamorphosis
due to unfavorable
conditions
(e.g. 68).
An alternative
in flowis provided
explanationfordifferential
settlement
by the"adhereandexplore"mechanisms
ofbarnaclesand bryozoans.These
mechanisms
mayexplaindeviationsofsettlement
distribution
fromencounter
ofbarnaclesand bryozoansunderflowconditions
distributions
(as was found
byWalters,
93). Thepotential
of"adhereandexplore"mechanisms
importance
is thattheysave thepropaguletheneed to return
to theflowif it encounters
unfavorable
sites.
EcologicalSignificance
ofSettlement
inEnvironments
of
Flow Conditions
Different
Flow causes foodparticlesto be distributed
in sitesthat
nonhomogeneously
differ
inheight,
orovermorphologically
ofbenthichabitats
different
substrata
(3, 59), andorganisms
inaccordancewiththeir
maybedistributed
food
potential
distribution
(2). Larvaeof sessileparticle-feeders
maysettlein flowpatterns
thatenhancetheconcentration
oftheir"preferred"
food.
In thissense,larvaeof suspension-,
bedload-and deposit-feeding
species
see 3, 42, 48) areexpectedtoselectdistinct
(fordefinitions,
microhabitats
that
334
ABELSON & DENNY
differin theirflowpatterns
and, as a consequence,theirprevailing
particle
compositions.Specifically,
larvaethatfeedon fine,suspendedparticlesmay
haveevolvedmechanisms
(e.g. adhesiveappendages)thatallowthemtosettle
onprotruding
bodieswherethefluxesoffine,suspended
particles
arethehighest
and wherethepotentially
disturbing
coarsebedloadparticlesarein relatively
low quantities.On theotherhand,propagulesofbedload-anddeposit-feeders
wouldbe expectedto have evolvedwithoutthesemechanisms
because,for
in terms
example,adhesiveappendagesmightstickthemto sitesunfavorable
of foodsupply.The larvaeof bedload-feeders
are expectedto favorsiteson
in whichthebedloadfluxesare thehighest,
planarsubstrata
whereasdepositfeedersaremorelikelytoprefer
depressions.
in flow
Larvaethattendto settlein depressions
mayrecognizedifferences
conditions
thatresultfromdifferent
pitsizes (80). Likewise,larvaeofdifferent
intheirresponsesto
species,orofthesamespeciesindifferent
seas,maydiffer
thesamepitunderthesameflowconditions
(17, 80). Alternatively,
depressions
maybe chosennotonlyfortheflowpattern
theyinduce.For instance,small
depressions,
equal in size to or smallerthanmanypropagulesand withlittle
effecton flow,maybe chosenas preferred
settlement
sites,possiblybecause
theyprovidefirmattachment
(46).
Postsettlement
also existthatenableorganisms
topersistandgrow
strategies
in theoron a widerangeof substratum
types.Forexample,theflowpattern
canbe altered.Thiscan be accomplished
eitherbychanging
ganisms'vicinity
thesubstratum
morphology
(an optionopen mainlyto soft-bottom
dwellers,
e.g. theampharetid
polychaete
Amphicteis
scaphobranchiata;
63), orbybuildinga bodythatchangestheflowpattern
experienced
bytheorganism's
feeding
devices(e.g. diversecoralspecies;2).
CONCLUDING REMARKS
theeffectsof flowon settling
We have presentedseveralthemesregarding
in studiesof benthicecology.
propagulesthatshouldreceivemoreattention
is ofgreat
areplanar.Themorphology
ofthesubstratum
First,notall substrata
indetermining
initsvicinity
theflowpattern
importance
andconsequently
may
thespeciescomposition
ofsettling
playa pivotalrolein affecting
propagules.
The studiesof Mullineaux& Butman(57,58), forinstance,clearlyshowthe
on thedistribution
distinctive
effectsof inducedflowpatterns
oversubstrata
These studieshaveconcentrated,
on planar
of larvaeat settlement.
however,
substrataand do notdeal withothersubstrataof greatimportance
(butsee
and itsresultant
flowregimeis
80). The neglectof substratum
morphology
toexplainthedistribution
oforganisms
exemplified
byfieldstudiesthatattempt
differential
on cylindrical
bodies(suchas mangrove
proproots)byexamining
SETTLEMENTIN FLOW
335
(e.g. ceramictiles; 9). Studieswithdifferent
settlement
on planarsubstrata
to assesstheexactroleofmorphology
substratum
morphologies
arenecessary
ofthedevelopingrecruits.
on siteselectionofpropagulesandsurvivorship
cue (whichmay
playa roleas a settlement
Second,flowcan simultaneously
or
influence
activebehaviorof motilepropagules)and as a factorpreventing
thesettlement
betweenthesetworolesis
disturbing
process. Distinguishing
has
attempt
forunderstanding
thesettlement
process.No significant
important
inwhichactiveexploration
is possible
beenmadetocharacterize
environments
andthosein whichactivelocomotion
is prevented
byexcessivehydrodynamic
forces.
flow
settlement
is correlated
withflowconditions,
Third,wheredifferential
thedistribution
of
is oftenconsideredto be thesole factordirectlyaffecting
cues. Pamayaffectdiversesettlement
settling
propagules.Flow,however,
thedifferential
otherthanflowbutinfluenced
byflowmaydetermine
rameters
load,substratum
settlement.
Thesefactorsincludebiofilmstructure,
sediment
ofchemicals,andlightintensity
(due to turbidity).
type,distribution
thatprotoexaminethehypothesis
Finally,further
workshouldbe conducted
of
jectingappendagesorotheradhesivedevicesareassociatedwithpropagules
characterized
byhighvelocitiesofflow(e.g. prospeciesinhabiting
substrata
hereis thatspecieslackingtheseappendagesare
truding
bodies).Thecorollary
in
with
Some support
low flowvelocity(e.g. depressions).
likelytosettle sites
coralspecies
forthishypothesis
is providedby settlement
of
several
patterns
thatmatchtheirfoodthatwerefoundtobe correlated
to theadultdistribution
insettlement
ofthesespecies
patterns
particledistribution
(4). The differences
withand attachment
can be relatedto mechanisms
affecting
larvalencounter
to thesubstratum.
settlement
on protruding
thatfacilitate
Most mechanisms
inflow
otherthansettlement
bodieshavebeenpreviously
describedincontexts
andhaveelicitedvariousexplanations
as totheirroleinthebiologyandecology
fortheroleofprojecting
ofpropagules.Themostwidelyacceptedexplanations
anddispersion
(e.g. 44, 90),
appendageson larvaeconcerntheirroleindrifting
(81) argues
aggregations
(20). Strathmann
in feeding(e.g. 31), andin creating
thatthereis no evidencethatprojecting
appendagesare used to maintainlarvae in suspension,
as is oftensuggested.Likewise,thepresenceofprojecting
appendagesamongtinypropagulesthatexhibitverylow settlingvelocities
is notlikelyto affectdramatically
theirdispersalrange,especiallyformotile
propagulesthatcan maintaintheirverticalpositionby swimming.We do not
fortherolesofprojecting
appendages.However,
denyalternative
explanations
able to settleon protruding
manyorganisms
possessingthemare apparently
bodiesandin otherenvironments
ofstrong,
flow.
accelerating
In summary,
theinformation
ofpreviousstudiessuggeststhatflowmayact
on settling
oneormoreofthreelevels:as an agentexerting
propagulesthrough
336
ABELSON & DENNY
factoraffecting
cue, and as a mediating
hydrodynamic
forces,as a settlement
theexactrole of
cues. A limitation
to interpreting
variousothersettlement
amongthethreelevelsrequiresa complicated
each levelis thatdiscrimination
technicalprobexperimental
setupthatin manycases maypose "unresolved"
varioussettlement
factor
affecting
lems.Ofthethreelevels,flowas a mediating
hycues seemsto be theleaststudied.The roleof flowas an agentexerting
propaguleshas been clearlyshownin various
drodynamic
forceson settling
studies.Likewise,thereis increasing
evidencefortheuse offlowbypropagules (mainlylarvae)in selectingpreferred
sitesthatfittheadultrequirements.
to
To use theflowas a settlement
cue,propagulesshouldpossessmechanisms
cope withforce-related
problems.Here we haveexploredsuchmechanisms
whoseeffectiveness
remainsto be examined.Takingintoaccountthesignificantinvolvement
of flowin diversebiologicalandecologicalprocessesin the
therelativeweightofeachlevel,the
sea, itis reasonabletoarguethatwhatever
overalleffect
offlowmaybe crucialformanybenthicspecies.
ACKNOWLEDGMENTS
fortheir
We arethankful
andR Strathmann
to JEckman,D Fautin,P Jumars,
constructive
comments
on previousversionsof thispaper.Thisresearchwas
FellowoftheUSIEF andtheBen-Gurion
scholarship
supported
byFulbright
ofScienceto A.A.
shipoftheIsraelMinistry
VisittheAnnualReviewshomepage at
http://www.annurev.org.
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