Reproduction and Recruitment

Reproduction

• Sexual– 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 Reproduction-Hermaphroditism

• 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)

WHY? Size Advantage Hypothesis

Lifetime fitness and size

Size

Egg

s pr

oduc

ed o

r fe

rtili

zed

Size

Egg

s pr

oduc

ed o

r fe

rtili

zed

Female

Female

Male

Male

Protogynous Protoandrous

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

http://www.oceanoasis.org/fieldguide/thal-luc.html

Rainbow wrasseThalassoma lucasanum

Two types of malesTwo types of reproduction.

1) Females(yellow/red lateral stripes)2) Primary males(look like females)3) Terminal males(blueheads) - born female, turninto males

http://www.oceanoasis.org/fieldguide/thal-luc.html

Rainbow wrasseT. lucasanum

Two types of reproduction

1) Broadcast spawning -Many males and females rush to surface and release gametes

2) Harems: one terminal male guards group of females and mates with them individually.

Death of secondary male-large female turns into new

terminal male

Mass spawning of the rainbow wrasseThalassoma lucasanum

Barred serranoSerranus psitticinusSea of Cortez

Simultaneoushermaphrodite(can act as male orfemale at any time)-dominant male in harem mates with “females”.

Serranus annularis CaribbeanOrange back basslet

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

Emigration

Population Size

Recruitment 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

2. Benthic or site attached adult stage

BENTHIC ADULTS

REPRODUCTION

SETTLEMENT

PLANKTONICLARVAE

*Larva: an independent, often free-living, developmental stage that undergoes changes in form and size to mature into the adult. Common in insects and aquatic organisms.

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 column2) Settlement in an appropriate site3) Successful metamorphosis into adult

body form4) Post-settlement survival and growth

until detected by an observer

1 cm

Three basic modes of larval development

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)

Direct -- essentially no larval stageLarval stage encapsulated, internally brooded or bypassed entirely

Thorson’s rule: a latitudinal cline in pelagic larvae in gastropods

Thorson’s data for gastropods, interpreted by Mileikovsky, reveals a clear latitudinal cline in the proportion of species reproducing via a pelagic larva

Latitude

0 30 60 90

% S

peci

es w

ith P

elag

ic D

evel

opm

ent

0

20

40

60

80

100

Red data: northern hemisphereWhite data: southern hemisphere

For most marine species, we have NO idea where larvae go

Vertical migration can result in retention of larvae within estuary: larvae rise on flood tide, and sink on ebb

Larval behavior can allow for retention

Tidal Flow – Ebb tide

Tidal Flow – Flood tide

Typical life cycle of marine organisms

Pelagic larvaeCue detection & metamorphosis

Sedentary Benthic adults

Planktonic dispersal

Roughly 80% of all marine organisms (> 90,000 currently described species of

vertebrates, invertebrates & algae) have a biphasic life cycle and produce

planktonic propagules

Typical life cycle of marine organisms

Pelagic larvaeCue detection & metamorphosis

Sedentary Benthic adults

Planktonic dispersal

Problem with swimming larvae: water motion often carries them away from

appropriate habitat

Water flow in the ocean is complex -- internal waves, longshore drift, wind-

driven currents and eddies can all affect where larvae end up

Typical life cycle of marine organisms

Pelagic larvaeCue detection & metamorphosis

Sedentary Benthic adults

Planktonic dispersal

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

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 potential is related to gene flow and hence speciation

Planktonic larval duration (days)

0.1 1 10 100

Dis

pers

al s

cale

(km

)

0.01

0.1

1

10

100

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

Little suggestion that mortality higher in polar regions

Taxon Weekly survival %

Annual mortality %

Polar survival %

Ascidians ~71 75-100 ND Barnacles ~88 92-100 95-100 Bryozoans ~94 99-100 89-100 Bivalves ~86 90-99 90-100 Gastropods ~91 10-100 ND Decapods ~93 ~99 ND Echinoids ~86 90-99 ND Octocorals ~92 75-95 ND Polychaetes ~93 90-99 90-100

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

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

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

So Why Disperse?

• High probability of local extinction– -- best to export juveniles

• 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

– 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

What is the optimal network design?How do we Design anEffective 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

Skipjack tuna (Oceanodromous)Tropical speciesthat travels to temperate water tofeed. Halfway acrossglobe each year.

Salmon(Anadromous)Spend lives atsea feeding, returnto rivers to breed:Magnetic field and smellof home rivers

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