Reproductive Strategies and Larval Ecology What are larvae? Independent Independent Morphologically different stages Morphologically different stages Develop from fertilized egg Develop from fertilized egg Small Small Metamorphosis to adult Metamorphosis to adult

Benthic invertebrate abundance Larval ecology central to understanding why communities persist Communities maintained by larvae Recruitment – benthic or aquatic communities Recruitment – benthic or aquatic communities Migration - blue crabs, marine reserves Migration - blue crabs, marine reserves Asexual reproduction – corals, sponges, ascidians Asexual reproduction – corals, sponges, ascidians Mortality – predation, competition, density independent limits (food, space), e.g pelagic sharks Mortality – predation, competition, density independent limits (food, space), e.g pelagic sharks

Metabolic energy: I ndividuals allocate resources among: Maintenance Maintenance Growth Growth Reproduction Reproduction Community and Species survival depends on successful reproduction Energy allocation in reproduction

Two reproductive strategies: Two reproductive strategies: Iteroparous - many reproductive cycles over the course of its lifetime Iteroparous - many reproductive cycles over the course of its lifetime Semelparous - "big bang" reproduction, reproduces a single time before it dies; salmon Semelparous - "big bang" reproduction, reproduces a single time before it dies; salmon limited amount of energy available, must "choose" how to use it: trade-off between fecundity, growth, and survivorship

trade-off between offspring produced (benefit) and offspring forgone (cost) trade-off between offspring produced (benefit) and offspring forgone (cost) Reproductive effort (RE) —the proportion of energy put into reproducing, as opposed to growth or fecundity— Reproductive effort (RE) —the proportion of energy put into reproducing, as opposed to growth or fecundity— Optimal RE occurs at the pt of max distance between offspring produced and offspring forgone. Optimal RE occurs at the pt of max distance between offspring produced and offspring forgone.

Iteroparous : marginal cost of offspring produced is decreasing (each less "expensive" than the average) the marginal cost of offspring foregone is increasing. Iteroparous : marginal cost of offspring produced is decreasing (each less "expensive" than the average) the marginal cost of offspring foregone is increasing. devotes only a portion of resources to reproduction, uses rest for growth and survivorship so it can reproduce in future devotes only a portion of resources to reproduction, uses rest for growth and survivorship so it can reproduce in future

Semelparous - marginal cost of offspring produced increases, and marginal cost of offspring forgone decreases. Semelparous - marginal cost of offspring produced increases, and marginal cost of offspring forgone decreases. favorable for the organism to reproduce a single time. The organism devotes all of its resources to that one episode of reproduction, so it then dies. favorable for the organism to reproduce a single time. The organism devotes all of its resources to that one episode of reproduction, so it then dies.

r and K strategy and iteroparity vs semelparity K – predictable envt. – pays to invest resources in long life, long development K – predictable envt. – pays to invest resources in long life, long development r – risky envt. - pays to produce as many offspring as soon as possible r – risky envt. - pays to produce as many offspring as soon as possible Strategies depend on: Strategies depend on: –Min amt of reproduction needed to replace –Survivorship – long enough to reproduce –Can’t max both: resources used for fast repro. not available for long life & vice versa

Larval Strategies Three possible pathways (Thorson 1946) Three possible pathways (Thorson 1946) –Planktotrophic larvae – pelagic, feeding larvae –Lecithotrophic larvae – pelagic, nonfeeding larvae, nutrition in yolk sac –Non-pelagic larvae “brooded” – larvae in egg capsule, nutrition in yolk, hatch as juvenile (also viviporous, brooding) “brooded” – larvae in egg capsule, nutrition in yolk, hatch as juvenile (also viviporous, brooding) larvae crawl on bottom, generally nonfeeding larvae larvae crawl on bottom, generally nonfeeding larvae

Barnacles

Barnacle life cycle

Hydroid

Sand dollars

Gastropod

Larval Strategies Each advantageous under certain conditions Each advantageous under certain conditions Investment Investment –Planktotrophic larvae – lots of small eggs, low per unit energy cost –Lecithotrophic larvae – larger, so fewer eggs, high per unit energy cost –Non-pelagic larvae greatest per unit repro cost (egg cost + protection) greatest per unit repro cost (egg cost + protection)

Planktotrophic Advantage – large numbers – shotgun approach – someone will survive and make it; dispersal Advantage – large numbers – shotgun approach – someone will survive and make it; dispersal Disadvantage – food dependent (unpredictable), long exposure to predation, chance of “missing the mark” (need to time larval development) Disadvantage – food dependent (unpredictable), long exposure to predation, chance of “missing the mark” (need to time larval 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 Advantage – not dependent on unpredictable food supply, less time exposed to predators, closer to good habitat (origin) Advantage – not dependent on unpredictable food supply, less time exposed to predators, closer to good habitat (origin) Disadvantage – fewer eggs (risk of loss or miss), larger target, poorer dispersal Disadvantage – fewer eggs (risk of loss or miss), larger target, poorer dispersal

Non-pelagic Adv. – no planktonic predator exposure, no unpredictable food source Adv. – no planktonic predator exposure, no unpredictable food source Disadvantage – few eggs, poor dispersal, benthic predators Disadvantage – few eggs, poor dispersal, benthic predators

Question: According to Vance, what two factors have driven the evolution of larval reproductive strategy? Assumptions involved? According to Vance, what two factors have driven the evolution of larval reproductive strategy? Assumptions involved? According to Strathmann, what additional factor did Vance overlook (or, why does a larva swim so long)? According to Strathmann, what additional factor did Vance overlook (or, why does a larva swim so long)?

Patterns in nature – do they match these predictions? YES Latitude Latitude - tropics – 70% species planktotrophic - poles – 90% species large yolky eggs, most non-pelagic development Depth gradient Depth gradient - shallow – more planktotrophs - deep sea – almost all non-pelagic larvae (lack of food, low temp, no dispersal advantage, gigantism)

Polar waters Non-pelagic common, long time for development Non-pelagic common, long time for development Food, temperature, light limits: Small repro summer window of ocean productivity, temperature for growth Food, temperature, light limits: Small repro summer window of ocean productivity, temperature for growth Lecithotropic >> non-pelagic – why? Lecithotropic >> non-pelagic – why? Dispersal important, highest survivial for # eggs produced Dispersal important, highest survivial for # eggs produced

Dispersal – Vance ignores dispersal Strathmann – planktotrophs and lecithotrophs both disperse in water column Dispersal is advantageous: Dispersal is advantageous: –If conditions near adults are deteriorating, overcrowded –There is spatial variability in favorability of sites –Provides increased genetic exchange –Minimizes chance that population will be eliminated in local catastrophe Overdispersal – “wastage of larvae” Overdispersal – “wastage of larvae”

PlanktotrophyLecithotrophy Non-pelagic Larvae Predictable food supply Unpredictable food supply Very low food available Low predation rate Moderate predation rate High predation rate Which strategy evolves? (Vance 1973)

PlanktotrophicLecithotrophicNon-pelagic Clutch size (no. of eggs) LargeMediumSmall Investment by parent LowMediumHigh Development time Slow - medium Fast DispersalHighMediumLow Survival rate of larvae LowMediumHigh Summary of Larval Strategies

Species inhabiting common habitats Species inhabiting common habitats Species inhabiting rare habitats Species inhabiting rare habitats Are exceptions to these patterns

Larval Site Selection

“Competent” Metamorphosis – rapid – nonfeeding, defenseless time Settlement behavior 1. Broad exploration 1. Broad exploration 2. Close exploration 2. Close exploration 3. Inspection 3. Inspection

Common responses of settling larvae On any bottom type – need bacterial coating On any bottom type – need bacterial coating Gregarious settlement – selection of a site already inhabited by adults of one’s own species Gregarious settlement – selection of a site already inhabited by adults of one’s own species -For barnacles – ensures a neighbor close enough for copulation

Common responses of settling larvae On any bottom type – need bacterial coating On any bottom type – need bacterial coating Gregarious settlement – selection of a site already inhabited by adults of one’s own species Gregarious settlement – selection of a site already inhabited by adults of one’s own species -For barnacles – ensures a neighbor close enough for copulation -If habitat good for adult, good for larvae too -“Safety in numbers” – protection from predators

Conclusions about Settlement Most larvae have some powers of site selection Most larvae have some powers of site selection Exact process still unknown Exact process still unknown –“Chemotactic” response – chemoreception and touching Settlement influences community composition Settlement influences community composition