Transcript Marine Ecology 2009, final Lecture 3 Recruitment

Reproduction and Recruitment
Reproduction
• Sexual
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–
–
–
Hermaphroditism (simultaneous) (inverts)
Dioecious (mammals, fish, inverts)
External fertilization (invertebrates, fish)
Internal fertilization (mammals, inverts, sharks)
• Asexual
–
–
–
–
Fragmenting (corals)
Rhizomal (Sea grasses)
Budding (hydroids)
Division (anemones)
To have sex or not?
• Asexual reproduction good in stable
habitats
– Easily propagate, spread, compete, waste no
time or energy on sex, local distribution
• Sexual reproduction good in unstable
habitats
– Allows for genetic variability, plasticity
Dispersal and Recruitment:
Production of Larvae
• Many marine animals release huge
numbers of eggs.
– Even so, rates of fertilizations are thought to
be <20% for a wide range of invertebrates
• Sperm are short-lived (a few hours at most)
• In most cases, sperm concentrations are rapidly
diluted by currents and waves
• Donors are sparse
Dispersal and Recruitment:
Production of Larvae (cont)
• Behavioral modifications can overcome
sperm limitation
– Mollusks can form spawning aggregations
– Barnacles use internal fertilization
Fertilisation
• Internal fertilisation
– Requires males and females to meet
– Rare in sessile organisms (but does occur: for
example barnacles)
• External fertilisation
– Release of eggs and sperm into sea
– Requires eggs and sperm to meet
Most marine invertebrates spawn eggs and sperm into
the sea
Mobile taxa may aggregate to spawn
Broadcasters vs Brooders
• Broadcast spawning
– shed eggs and sperm into
water column
• most fish, echinoderms
and algae
• Internal fertilization
– females collect sperm from
water column and fertilize
eggs internally
• Sponges, cnidarians
mollusks, ascidians
– copulation with male
placing sperm inside
female reproductive tract
• gastropods, crust.,
sharks, mammals
Sexual ReproductionHermaphroditism
• A single individual has gonads that can
produce gametes of both sexes
• A single individual can produce gametes
of either sex at different times in it life
(sequential hermaphrodites)
– protandry male to female
– protogony female to male
Protoandry
• Function as Males first
• Found amongst species in which males are
able to spawn with large female
Ex. Clown fish
– Males are small, Females large and territorial
– Removal of female causes male to switch
– A juvenile then becomes male
Protogyny
• Often Male maintains territory with harem of
females; size of males matters
• If male is removed, one female changes to male
– Behavior (immediately)
– Gonads (a few days)
• Major rearrangement of anatomy, physiology,
hormones, and behavior
(Need not be a pure strategy - gonochoristic males
and females can exist within population)
Synchronous hermaphrodite
• Gonad has sperm and eggs
• Often a monogamous pair
takes turns playing male
and female role - Ensures
no cheating
• ex. Hamlets (coral reef)
Protogynous
Male
Female
Size
Eggs produced or fertilized
Eggs produced or fertilized
WHY?
Size Advantage Hypothesis
Lifetime fitness and size
Protoandrous
Female
Male
Size
Strange reproductive practices of fish
• Hermaphrodites
• Sex change (born one sex, become the other)
Large fish in harems are often sex-change
males (protogony)
Large fish in non-harem species are often
sex-change females (protandry)
• Parasitic males
• “Sneaker” males that look like females
• Sex-role reversal (male pregnancy in seahorses)
• Males often do parental care in fish
Rainbow wrasse
Thalassoma lucasanum
Two types of males
Two types of
reproduction.
1) Females
(yellow/red lateral
stripes)
2) Primary males
(look like females)
3) Terminal males
(blueheads)
- born female, turn
into males
http://www.oceanoasis.org/fieldguide/thal-luc.html
Rainbow wrasse
T. lucasanum
Two types of reproduction
1) Broadcast spawning Many males and females
rush to surface and
release gametes
http://www.oceanoasis.org/fieldguide/thal-luc.html
2) Harems: one terminal male
guards group of females and
mates with them
individually.
Death of secondary malelarge female turns into new
terminal male
Mass spawning of
the rainbow wrasse
Thalassoma lucasanum
Barred serrano
Serranus psitticinus
Sea of Cortez
Serranus annularis Caribbean
Orange back basslet
Simultaneous
hermaphrodite
(can act as male or
female at any time)
-dominant male
in harem mates with
“females”.
http://www.qualitymarineusa.com/fish/basslets.html#top
Parental care
•
•
•
•
•
•
Preparation of nests or burrows
Egg guarding
Production of large yolky eggs
Care of young (inside or outside body
Provisioning of young (before or after birth)
Care of offspring after independence
Seasonality
• Seasonal Reproduction
– Short reproductive phases where high
percentage of individuals are reproductively
active
– Small eggs, high fecundity and synchronized
gamete development within individuals and
within the population.
Seasonality cont’d
• Year-round reproduction
(1) Asynchronous
• Individuals have discrete gamete production
• Not synchronized within the population
(2) Continuous
• Most adults within population contain gametes
year round
Dispersal and Recruitment
• The importance of recruitment has been recognized by
marine ecologist for nearly a century, but only in the
past 15-20 yrs have marine ecologists incorporated
recruitment as a centerpiece of population and
community models.
– It is a common sense notion that an empty patch of
habitat will be uninhabited by a given species if its
propagules are unable to reach it.
– If true then the intensity of density dependent
interactions will be determined by the degree to
which settlement is successful
Definitions
• Recruitment is the addition of new
individuals to a populations or to successive
life stages within a population
Pre- and Post Settlement
Processes
Immigration
+
Recruitment
+
-
Population
Size
Emigration
Mortality
Dispersal and Recruitment:
Processes causing variation in
recruitment
•
•
•
•
•
Production of larvae
Dispersal of larvae in the plankton
Risk of mortality while dispersing
Larval settlement
Growth and survival of settlers until they
get counted as new recruits
Many marine species have ‘bipartite’ life histories
1. Planktonic dispersive early
stage
PLANKTONIC
LARVAE
2. Benthic or site attached adult
stage
*Larva:
an independent, often freeliving, developmental stage that
undergoes changes in form and size
to mature into the adult. Common
in insects and aquatic organisms.
SETTLEMENT
REPRODUCTION
BENTHIC
ADULTS
More marine-terrestrial differences:
you don’t see the bipartite lifestyle often on land
Dispersal and Recruitment:
Complex Life Cycles
Dispersal and Recruitment:
Complex Life Cycles
Marine organisms: complex life cycles
Recruitment is a multi-step process
Four major accomplishments of recruitment:
1) Dispersal & survival in water column
2) Settlement in an appropriate site
3) Successful metamorphosis into adult
body form
4) Post-settlement survival and growth
until detected by an observer
1 cm
Three basic modes of larval development
Direct -- essentially no larval stage
Larval stage encapsulated, internally brooded or
bypassed entirely
Lecithotrophic -- “yolk feeding”
Nonfeeding larval stage. Larvae do not require food
to complete development. Planktonic lifespan is
typically short (minutes to days)
Planktotrophic -- “plankton feeding”
Feeding larval stage. Larvae are incapable of
completing development without feeding Planktonic
lifespan typically long (days to months)
Thorson’s rule: a latitudinal cline in pelagic
larvae in gastropods
% Species with Pelagic Development
100
Thorson’s data for gastropods,
interpreted by Mileikovsky,
reveals a clear latitudinal cline in
the proportion of species
reproducing via a pelagic larva
80
60
40
20
0
Red data: northern hemisphere
White data: southern hemisphere
0
30
60
Latitude
90
For most marine species, we have NO idea where larvae go
Larval behavior can allow for retention
Tidal Flow – Flood tide
Tidal Flow – Ebb tide
Vertical migration can result in retention of larvae
within estuary: larvae rise on flood tide, and sink on ebb
Typical life cycle of marine organisms
Planktonic dispersal
Pelagic larvae
Roughly 80% of all marine organisms (>
90,000 currently described species of
vertebrates, invertebrates & algae) have
a biphasic life cycle and produce
planktonic propagules
Sedentary
Benthic adults
Cue detection &
metamorphosis
Typical life cycle of marine organisms
Planktonic dispersal
Pelagic larvae
Problem with swimming larvae: water
motion often carries them away from
appropriate habitat
Water flow in the ocean is complex -internal waves, longshore drift, winddriven currents and eddies can all affect
where larvae end up
Sedentary
Benthic adults
Cue detection &
metamorphosis
Typical life cycle of marine organisms
Planktonic dispersal
Pelagic larvae
Cues used to assess habitats can
be chemical or physical, and larvae
often respond to some
combination of multiple cues
Cues can be positive or negative
Sedentary
Benthic adults
Cue detection &
metamorphosis
Ecological consequences: egg size, larval type
and dispersal
• Larval type is related to egg size
– Feeding (planktotrophic) larvae hatch from small eggs
– Non-feeding (lecithotrophic) larvae hatch from larger eggs
• Egg size dictates fecundity
– Females produce more small eggs than large ones (fecundity/egg
size trade-off)
• Feeding larvae tend to spend longer in plankton, and hence have the
potential to disperse further
Dispersal scale (km)
Dispersal potential is related to gene flow and
hence speciation
100
10
1
0.1
0.01
0.1
1
10
100
Planktonic larval duration (days)
Data from Siegel et al., MEPS, 260: 83-96 (2003)
How does an aggregation begin?
Someone had to be the first one to settle,
and they didn’t respond to other adults!
Desperate Larva Hypothesis: larvae
search for suitable site until they run out
of energy and then take whatever they can
find rather than die in the water column
Founders & Aggregators: Some species produce two distinct types
of larvae: one type seeks out adults of their own species, the other is
specifically a ‘pioneer’ larva that seeks new uninhabited, bare
surfaces to colonize.
If the ‘pioneer’ larva survives and grows into an adult, it can form
the nucleus of a new aggregation.
Gregarious settlement
Gregarious settlement
• Larvae settle on (or very near)
adults of the same species
•Identifying settlement cues is
difficult and not many larval
inducers have been conclusively
identified
•Many species cannot move after
settlement & even those that can
need to feed soon
•Larvae settling with adults can
obviously tell that site will be able
to support them after they settle
Settlement choice
• Bacteria probably play an important role, but exact effects are
unknown for all but a couple of species
• Physical cues associated with flow conditions at site of
settlement are frequently important
• Chemical cues (e.g., from food source or conspecifics)
frequently play a role, also
Do numbers of settlers reflect number that
eventually recruit to the assemblage?
•
Barnacles (Bertness et al., 1996)
– Yearly differences in number and
distribution of larval settlers,
reflecting wind effect on larval
populations
•
Scallops (Peterson & Summerson, 1992)
– Variability in spat explained 71%
variability in recruitment in 1988,
but only 4% in 1989
•
Lobsters (Hernkind & Butler, 1994)
– No relation between settlers and
subsequent recruits over 3 years in
Florida
The answer is sometimes
Post-settlement mortality
Organisms only recruit to population (establish) if they survive after settlement
Taxon
Weekly
Annual
Polar
survival % mortality % survival %
Ascidians
Barnacles
Bryozoans
Bivalves
Gastropods
Decapods
Echinoids
Octocorals
Polychaetes
~71
~88
~94
~86
~91
~93
~86
~92
~93
75-100
92-100
99-100
90-99
10-100
~99
90-99
75-95
90-99
ND
95-100
89-100
90-100
ND
ND
ND
ND
90-100
Little suggestion that mortality higher in polar regions
Causes of post-settlement mortality
•
•
•
•
•
•
Delay of metamorphosis
Biological disturbance
Physical disturbance
Physiological stress
Predation
Competition for space or
food
Physical and chemical defenses of larvae
Physical -- spines & bristles
• make it difficult for small fish & invertebrate predators
to swallow them
Chemical -- chemical defenses make larvae distasteful
• some chemical defenses simply taste bad
• others have more dramatic effects – e.g., some coral
and tunicate larvae make fish vomit immediately
after ingesting a larva
Other -- larvae may have behavioral or physical adaptations
to avoid detection
• larvae may be transparent, or only active at night when it
is difficult to see them
So Why Disperse?
•
High probability of local extinction
–
•
Spread your young (siblings) over a variety of
habitats
–
•
evens out the probability of mortality
Maybe it has nothing to do with dispersal at all
–
•
-- best to export juveniles
just a feeding ground in the plankton for larvae?
Life history theory predicts species in marine
environments do best when they ‘hedge their
bets’
–
some larvae recruit to adult habitats and others
disperse to try new habitats
Dispersal: Metapopulations
How do we Design an
What is
the
optimal
network
design?
Effective Network?
We need a much better understanding of larval dispersal
Migratory patterns
• Anadromous - breed in freshwater and living in
seawater
– salmon, shad, sea lamprey
• Catadromous - Adults live in in freshwater
then migrate to seawater to spawn
– eels Anguilla
• Oceanodromous- live totally in seawater
– herring cod and plaice
Catadromous - breed at sea, migrate into rivers to grow
(16 spp freshwater eels)
adults spawn and die in Sargasso Sea / larvae in plankton 1
yr+/ metamorphose into juveniles / grow and mature in rivers
Figure 8.22
Salmon
(Anadromous)
Spend lives at
sea feeding, return
to rivers to breed:
Magnetic field and smell
of home rivers
Skipjack tuna
(Oceanodromous)
Tropical species
that travels to
temperate water to
feed. Halfway across
globe each year.
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