Reproductive Strategies and Larval Ecology

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

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

3. Metabolic energy:Individuals allocate resources among: • Maintenance • Growth • Reproduction Community and Species survival depends on successful reproduction Energy allocation in reproduction

4. Two reproductive strategies: • Iteroparous - many reproductive cycles over the course of its lifetime • 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

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

6. 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

7. 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.

8. r and K strategy and iteroparityvssemelparity • 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 • 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

10. Larval Strategies • 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) larvae crawl on bottom, generally nonfeeding larvae

11. Barnacles

12. Barnacle life cycle http://www.jst.go.jp/erato/project/fck_P/icons/grf1_1.jpg

13. Hydroid

15. Sand dollars

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

17. Gastropod

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

19. Larval Strategies • Each advantageous under certain conditions • 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)

20. Planktotrophic • 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)

21. 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

22. Lecithotropic • 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

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

24. Question: • 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)?

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

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

28. Dispersal – Vance ignores dispersalStrathmann – planktotrophs and lecithotrophs both disperse in water column • 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”

29. 4 12 1 11 13

30. Which strategy evolves? (Vance 1973)

31. Summary of Larval Strategies

32. Species inhabiting common habitats • Species inhabiting rare habitats • Are exceptions to these patterns

33. Larval Site Selection

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

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

37. Common responses of settling larvae • On any bottom type – need bacterial coating • 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

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