Reproductive Strategies and Reproductive Strategies and Larval EcologyLarval Ecology

What are larvae?What are larvae?

•IndependentIndependent

•Morphologically different Morphologically different stagesstages

•Develop from fertilized Develop from fertilized eggegg

•SmallSmall

•Metamorphosis to adultMetamorphosis to adult

Benthic invertebrate Benthic invertebrate abundanceabundanceLarval ecology central to understanding why Larval ecology central to understanding why

communities persistcommunities persist

Communities maintained by larvaeCommunities maintained by larvae

• Recruitment – benthic or aquatic Recruitment – benthic or aquatic communitiescommunities

• Migration - blue crabs, marine reservesMigration - blue crabs, marine reserves

• Asexual reproduction – corals, sponges, Asexual reproduction – corals, sponges, ascidiansascidians

• Mortality – predation, competition, density Mortality – predation, competition, density independent limits (food, space), e.g pelagic independent limits (food, space), e.g pelagic sharkssharks

Metabolic energy:Metabolic energy:IIndividuals allocate resources ndividuals allocate resources among:among:

• MaintenanceMaintenance

• GrowthGrowth

• ReproductionReproduction

Community and Species survival Community and Species survival depends on successful reproduction depends on successful reproduction

Energy allocation in reproductionEnergy allocation in reproduction

Two reproductive Two reproductive strategies:strategies:• IteroparousIteroparous - many reproductive - many reproductive

cycles over the course of its lifetimecycles over the course of its lifetime

• SemelparousSemelparous - "big bang" - "big bang" reproduction, reproduces a single reproduction, reproduces a single time before it dies; salmontime before it dies; salmon

limited amount of energy available, limited amount of energy available, must "choose" how to use it: must "choose" how to use it: trade-trade-offoff between between fecundity, growth, and fecundity, growth, and survivorshipsurvivorship

• trade-off between offspring produced trade-off between offspring produced (benefit) and offspring forgone (cost)(benefit) and offspring forgone (cost)

• Reproductive effort (RE) Reproductive effort (RE) —the proportion —the proportion of energy put into reproducing, as of energy put into reproducing, as opposed to growth or fecundity—opposed to growth or fecundity—

• Optimal RE Optimal RE occurs at the pt of max occurs at the pt of max distance between offspring produced and distance between offspring produced and offspring forgone. offspring forgone.

• Iteroparous : marginal cost of offspring produced Iteroparous : marginal cost of offspring produced is decreasing (each less "expensive" than the is decreasing (each less "expensive" than the average) the marginal cost of offspring foregone average) the marginal cost of offspring foregone is increasing. is increasing.

• devotes only a portion of resources to devotes only a portion of resources to reproduction, uses rest for growth and reproduction, uses rest for growth and survivorship so it can reproduce in futuresurvivorship so it can reproduce in future

• Semelparous - marginal cost of offspring Semelparous - marginal cost of offspring produced produced increases,increases, and marginal cost of and marginal cost of offspring forgone decreases. offspring forgone decreases.

• favorable for the organism to reproduce a favorable for the organism to reproduce a single time. The organism devotes all of its single time. The organism devotes all of its resources to that one episode of resources to that one episode of reproduction, so it then dies. reproduction, so it then dies.

r and K strategy and iteroparity vs r and K strategy and iteroparity vs semelparitysemelparity

• K – predictable envt. – pays to invest K – predictable envt. – pays to invest resources in long life, long developmentresources in long life, long development

• r – risky envt. - pays to produce as r – risky envt. - pays to produce as many offspring as soon as possible many offspring as soon as possible

• Strategies depend on:Strategies depend on:– Min amt of reproduction Min amt of reproduction needed to replace needed to replace – SurvivorshipSurvivorship – long enough to reproduce – long enough to reproduce– Can’t max bothCan’t max both: resources used for fast : resources used for fast

repro. not available for long life & vice repro. not available for long life & vice versa versa

Larval StrategiesLarval Strategies• Three possible pathways (Thorson 1946)Three possible pathways (Thorson 1946)

– Planktotrophic larvaePlanktotrophic larvae – pelagic, – pelagic, feeding larvaefeeding larvae

– Lecithotrophic larvaeLecithotrophic larvae – pelagic, – pelagic, nonfeeding larvae, nutrition in yolk sacnonfeeding larvae, nutrition in yolk sac

– Non-pelagic larvaeNon-pelagic larvae “ “brooded” – larvae in egg capsule, brooded” – larvae in egg capsule,

nutrition nutrition in yolk, hatch as juvenile in yolk, hatch as juvenile (also viviporous, brooding)(also viviporous, brooding)

larvae crawl on bottom, generally larvae crawl on bottom, generally nonfeeding larvae nonfeeding larvae

BarnaclesBarnacles

Barnacle life cycleBarnacle life cycle

http://www.jst.go.jp/erato/project/fck_P/icons/grf1_1.jpg

HydroidHydroid

Sand dollarsSand dollars

http://www.sbg.ac.at/ipk/avstudio/pierofun/planci/images/cycle.jpg

GastropodGastropod

http://www.fao.org/docrep/field/009/ag150e/AG150E14.gif

Larval StrategiesLarval Strategies• Each advantageous under certain Each advantageous under certain

conditionsconditions

• InvestmentInvestment

– Planktotrophic larvaePlanktotrophic larvae – lots of small – lots of small eggs, low per unit energy costeggs, low per unit energy cost

– Lecithotrophic larvaeLecithotrophic larvae – larger, so – larger, so fewer eggs, high per unit energy cost fewer eggs, high per unit energy cost

– Non-pelagic larvaeNon-pelagic larvae greatest per unit repro cost (egg cost + greatest per unit repro cost (egg cost +

protection)protection)

Planktotrophic Planktotrophic

• Advantage – large numbers – Advantage – large numbers – shotgun approach – someone will shotgun approach – someone will survive and make it; dispersalsurvive and make it; dispersal

• Disadvantage – food dependent Disadvantage – food dependent (unpredictable), long exposure to (unpredictable), long exposure to predation, chance of “missing the predation, chance of “missing the mark” (need to time larval mark” (need to time larval development)development)

Larvae of the blue crab Callinectes sapidus develop on the continental shelf. The postlarval stage (megalopa) occurs near the surface and is transported shoreward by wind-driven surface currents. It then uses selective tidal stream transport for migration up an estuary. megalopae enter estuaries with a solar day rhythm in swimming activity. This rhythm inhibited by light in low salinity -light inhibits swimming during the day in estuarine water. No light inhibition occurs in offshore waters, stays at surface Stop swimming at surface in tidal outflow (fresh) and sink to higher salt, incoming tide bottom water. Out of light at bottom, start swimming upward, catch tidal inflow until tides change

Lecithotropic Lecithotropic • Advantage – not dependent on Advantage – not dependent on

unpredictable food supply, less time unpredictable food supply, less time exposed to predators, closer to good exposed to predators, closer to good habitat (origin) habitat (origin)

• Disadvantage – fewer eggs (risk of Disadvantage – fewer eggs (risk of loss or miss), larger target, poorer loss or miss), larger target, poorer dispersaldispersal

Non-pelagicNon-pelagic

• Adv. – no planktonic predator Adv. – no planktonic predator exposure, no unpredictable food exposure, no unpredictable food sourcesource

• Disadvantage – few eggs, poor Disadvantage – few eggs, poor dispersal, benthic predatorsdispersal, benthic predators

Question:Question:

• According to Vance, what two factors According to Vance, what two factors have driven the evolution of larval have driven the evolution of larval reproductive strategy? Assumptions reproductive strategy? Assumptions involved?involved?

• According to Strathmann, what According to Strathmann, what additional factor did Vance overlook additional factor did Vance overlook (or, why does a larva swim so long)?(or, why does a larva swim so long)?

Patterns in nature – do they Patterns in nature – do they match these predictions? YESmatch these predictions? YES• LatitudeLatitude

- tropics – 70% species planktotrophic- tropics – 70% species planktotrophic

- poles – 90% species large yolky eggs, - poles – 90% species large yolky eggs, most non-pelagic developmentmost non-pelagic development

• Depth gradientDepth gradient

- shallow – more planktotrophs- shallow – more planktotrophs

- deep sea – almost all non-pelagic larvae - deep sea – almost all non-pelagic larvae (lack of food, low temp, no dispersal (lack of food, low temp, no dispersal advantage, gigantism)advantage, gigantism)

Polar watersPolar waters

• Non-pelagic common, long time for Non-pelagic common, long time for developmentdevelopment

• Food, temperature, light limits: Small Food, temperature, light limits: Small repro summer window of ocean repro summer window of ocean productivity, temperature for growthproductivity, temperature for growth

• Lecithotropic >> non-pelagic – why?Lecithotropic >> non-pelagic – why?

• Dispersal important, highest survivial Dispersal important, highest survivial for # eggs produced for # eggs produced

Dispersal – Vance ignores Dispersal – Vance ignores dispersaldispersal

Strathmann – Strathmann – planktotrophs and planktotrophs and lecithotrophs both disperse in water columnlecithotrophs both disperse in water column• Dispersal is advantageous:Dispersal is advantageous:

– If conditions near adults are deteriorating, If conditions near adults are deteriorating, overcrowdedovercrowded

– There is spatial variability in favorability of sitesThere is spatial variability in favorability of sites

– Provides increased genetic exchangeProvides increased genetic exchange

– Minimizes chance that population will be Minimizes chance that population will be eliminated in local catastropheeliminated in local catastrophe

• Overdispersal – “wastage of larvae”Overdispersal – “wastage of larvae”

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PlanktotrophPlanktotrophyy

LecithotrophyLecithotrophy Non-pelagic Non-pelagic LarvaeLarvae

Predictable Predictable food supplyfood supply

UnpredictablUnpredictable food supplye food supply

Very low Very low food food availableavailable

Low Low predation predation raterate

Moderate Moderate predation predation raterate

High High predation predation raterate

Which strategy evolves? (Vance 1973)Which strategy evolves? (Vance 1973)

PlanktotrophPlanktotrophicic

LecithotrophiLecithotrophicc Non-pelagicNon-pelagic

Clutch sizeClutch size

(no. of eggs)(no. of eggs) LargeLarge MediumMedium SmallSmall

Investment Investment by parentby parent LowLow MediumMedium HighHigh

Development Development timetime

Slow - Slow - mediummedium FastFast Slow - mediumSlow - medium

DispersalDispersal HighHigh MediumMedium LowLow

Survival rateSurvival rate

of larvaeof larvae LowLow MediumMedium HighHigh

Summary of Larval StrategiesSummary of Larval Strategies

• Species inhabiting common habitatsSpecies inhabiting common habitats

• Species inhabiting rare habitatsSpecies inhabiting rare habitats

Are exceptions to these patternsAre exceptions to these patterns

Larval Site SelectionLarval Site Selection

““Competent”Competent”

MetamorphosisMetamorphosis – rapid – – rapid – nonfeeding, defenseless timenonfeeding, defenseless time

Settlement behaviorSettlement behavior 1. Broad 1. Broad

explorationexploration 2. Close 2. Close

explorationexploration 3. Inspection3. Inspection

Common responses of settling Common responses of settling larvaelarvae

• On any bottom type – need bacterial coatingOn any bottom type – need bacterial coating

• Gregarious settlement – selection of a site Gregarious settlement – selection of a site already inhabited by adults of one’s own already inhabited by adults of one’s own speciesspecies- For barnacles – ensures a neighbor close enough For barnacles – ensures a neighbor close enough

for copulationfor copulation

Common responses of settling Common responses of settling larvaelarvae• On any bottom type – need bacterial coatingOn any bottom type – need bacterial coating

• Gregarious settlement – selection of a site Gregarious settlement – selection of a site already inhabited by adults of one’s own already inhabited by adults of one’s own speciesspecies- For barnacles – ensures a neighbor close enough For barnacles – ensures a neighbor close enough

for copulationfor copulation

- If habitat good for adult, good for larvae tooIf habitat good for adult, good for larvae too

- ““Safety in numbers” – protection from predatorsSafety in numbers” – protection from predators

Conclusions about Conclusions about SettlementSettlement

• Most larvae have some powers of site Most larvae have some powers of site selectionselection

• Exact process still unknownExact process still unknown– ““Chemotactic” response – chemoreception and Chemotactic” response – chemoreception and

touchingtouching

• Settlement influences community Settlement influences community compositioncomposition