Transcript 3 Larval ecology jh 2009

Reproductive Strategies and
Larval Ecology
What are larvae?
•Independent
•Morphologically different
stages
•Develop from fertilized egg
•Small
•Metamorphosis to adult
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
Metabolic energy:
Individuals allocate resources among:
• Maintenance
• Growth
• Reproduction
Community and Species survival depends on
successful reproduction
Energy allocation in reproduction
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
• 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.
• 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
• 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.
r and K strategy and iteroparity vs
semelparity
• 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
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
Barnacles
Barnacle life cycle
http://www.jst.go.jp/erato/project/fck_P/icons/grf1_1.jpg
Hydroid
Sand dollars
http://www.sbg.ac.at/ipk/avstudio/pierofun/planci/images/cycle.jpg
Gastropod
http://www.fao.org/docrep/field/009/ag150e/AG150E14.gif
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)
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)
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)
• Disadvantage – fewer eggs (risk of loss or
miss), larger target, poorer dispersal
Non-pelagic
• Adv. – no planktonic predator exposure,
no unpredictable food source
• 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 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
- 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)
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
Dispersal – Vance ignores dispersal
Strathmann – 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”
4
12
1
11
13
Which strategy evolves? (Vance 1973)
Planktotrophy Lecithotrophy
Non-pelagic
Larvae
Predictable
food supply
Very low food
available
Unpredictable
food supply
Low predation Moderate
rate
predation rate
High predation
rate
Summary of Larval Strategies
Planktotrophic Lecithotrophic
Non-pelagic
Clutch size
(no. of eggs)
Large
Medium
Small
Investment by
parent
Low
Medium
High
Development
time
Slow medium
Fast
Slow - medium
Dispersal
High
Medium
Low
Survival rate
of larvae
Low
Medium
High
• Species inhabiting common habitats
• Species inhabiting rare habitats
Are exceptions to these patterns
Larval Site Selection
“Competent”
Metamorphosis – rapid – nonfeeding,
defenseless time
Settlement behavior
1. Broad exploration
2. Close exploration
3. Inspection
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
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
Conclusions about Settlement
• Most larvae have some powers of site selection
• Exact process still unknown
– “Chemotactic” response – chemoreception and
touching
• Settlement influences community composition