MB5400 Life History and Evolution of Coral Reefs

Difference between unitary and modular organisms

Unitary organisms - size is determinate, governed by genetics, determinate lifespan, individuals sexually derived

Modular organisms - growth is indeterminate as colonies can increase in size (through replication) or decrease in size (through fission, partial mortality)

What is a module

A morphological individual

What are polymorphisms in corals?

Polyps can undertake different and specialised forms. For example sea pens have both axial and secondary polyps - the axial polyp provides support and anchors colony, secondary polyps have different functions, some feed and reproduce, others pump water through colony

Modular growth

Module is replicated and remains attached so colony grows larger

Modular reproduction

A new, separate individual is produced, can be sexually or asexually derived

Consequences of modular organisation

• connected modules allows for the sharing of resources, communication, collaboration and enhances colony survival

• modules have their own birth and death rates: size and age decoupled

• placement of modules determines colony form - potential for morphological diversity

Colony vs aggregation

Colony - replicated modules that remain attached

Aggregation - replicated modules that detach

Physiological implications for colony shape

• Biomechanical constraints of skeleton govern optimal environments

• SA:V affects prey and light capture

Ecological implications of colony shape

Shape determines how the colony interacts with:

• competitors

• predators

• the environment

Key decisions that govern the evolution of an organism’s life history

• size

• reproductive age

• fecundity

• longevity

• parental responsibilities

• number of reproductive events

Ideal life history

Fecundity - high

Size at birth - large

Age at maturity - young

Size at maturity - large

Early mortality - low

Late mortality - low

Longevity - high

Implication of the ideal life history

Ideal life history requires significant effort and energy expenditure - tradeoffs must be made

Reasons for the evolution of so many different life history strategies

1. Genetic variation

2. Natural selection

What is life history

The analysis of what causes differences in fitness among life history variants

What is life history evolution

The evolution of major features of the life cycle, mainly age distribution of birth and death rates, growth rates, and size of offspring

Importance of studying life history

• increases understanding of how the phenotype is designed for reproduction and survival

• key to understanding population dynamics

• makes sense of all biology, the diversity of organisms and their life cycles

What key aspects of corals have shaped their life histories?

1. Corals are sessile - can’t seek refuge from environmental events

2. Corals are modular - success related to size

Principle life history traits of organisms

• size at birth

• growth rate

• growth form

• age at maturity

• size at maturity

• number, size and sex ratio of offspring

• age and size-specific reproductive investment

• age and size-specific mortality schedule

• lifespan

Define fitness

Relative contribution of a genotype to succeeding generations

What life history traits are commonly involved in tradeoffs?

• reproduction vs survival

• number vs size of offspring

• growth rate vs growth form

• growth rate vs competitive ability

Example of a life history tradeoff

Pocillopora - has small polyps which limits the number of eggs per polyp

What determines a species’ schedule of growth, reproduction and mortality?

Energetic tradeoffs - compromises on how best to invest its resources

What is the r-k selection theory?

Theory relating to how populations increase (r) relative to the capacity of their environment (K)

Traits of r-selected species

adapted to unstable, unpredictable environments

• invade new habitats quickly

• grow and reproduce quickly

• rely on high reproduction of use short-lived ephemeral resources

• population boom and bust cycles

Traits of K-selected species

adapted to stable, predictable environments

• good competitors

• ability to acquire and persist within a space

• rely on competitive ability to persist at carrying capacity

• maintain steady population rate

Inconsistencies and problems with r-K selection theory

Corals persist a mixture of r and K-selected traits.

Porites would be considered a K-strategist as it is a good persister, grows large and is long lived, however has a high reproductive output and is a poor competitor, which are r-selected traits

Major problem: r is a population parameter but K is a function of the environment - doesn’t account for interactions

Key features of Grime’s C-S-R selection?

Competitors - low stress, low disturbance - growth focused

Stress-tolerators - high stress, low disturbance - lifespan focused

Ruderals - low stress, high disturbance - reproduction focused

How does Grime’s model compare with r-K selection?

Grime’s model extends the r-K selection model by addressing two distinct environmental pressures - stress and disturbance - rather than relying solely on population density or stability

What inconsistencies and problems are there with C-S-R selection theory?

Defines life history strategies in terms of the environment and focuses on the adult stage, where in corals, there are lots of energy tradeoffs that occur early in the life stage

Semelparous vs iteroparous

Semelparous breed once and die (annuals)

Iteroparoud breed repeatedly throughout lifetime

Key details of Charnov and Shaffer’s model

Compares survival probabilities - when you die determines when you should reproduce and how much to reproduce:

• adults likely to survive spread reproduction over time

• adults unlikely to survive put everything into one big reproductive effort

Model incorporates adult vs juvenile mortality and early vs late reproduction

Population growth rate of annuals

Birth rate x probability of surviving 1st year of life

Population growth rate of perennials

Birth rate x probability of surviving 1st year of life + adult survival rate

How does Charnov and Schaffer’s model improve on previous theories?

Focuses on natural selection driving survival and reproduction tradeoffs

A = mouth

B = oral disc

C = tentacle

D = mesenterial filament

E = eggs/spermaries

F = septum

G = basal plate

H = basal disc

I = wall

J = column

K = coelenteron

L = mesentery

M = stomadeum (oesophagus)

Key characteristics of Cnidarians

• specialised cells grouped into tissues which perform functions, but tissues not grouped into organisms

• diploblastic tissues - 2 epithelial cell layers separated by a connective acellular layer (mesoglea)

• epidermis contained cnidae

• widespread algal symbiosis

• radial/biradial symmetry

• two polyp forms - polyp and medusa

Red = epidermis

Blue = mesoglea

Green = gastrodermis

Roles of mesentaries

• digestion

• defence

Steps to form coral skeleton

1. Larva attaches to substratum

2. Mucoid layer

3. Basal plate

4. vertical structures

5. Complete juvenile skeleton

A = plocoid - separate walls

B = phaceloid - separate, extended walls

C = Cerioid - walls shared

D = Meandroid - walls shared, enclose many mouths

E = Flabello-meandroid - walls separate, many mouths

Why does competition come about?

Competition is an interaction between individuals due to a shared requirement for a resource in limited supply, leads to a reduction in survivorship, growth and reproduction of individuals

Define histocompatibility

Ability for a coral to recognise itself

Isografts

The same genotype

Allografts

Different genotypes, but the same species

Xenografts

Different species

Responses to histocompatibility

• fusion of tissue along point of contact

• rejection

What is a chimera

A mix of tissues from 2 or more genetically different individuals that have fused

Advantages of chimeras

• juveniles increase in size more rapidly

• acquire a life long mate

• greater genetic variability

Disadvantages of chimeras

• genotypes may compete for resources

• parasitism of one genotype by the other

Competitive mechanisms in corals

Direct interactions: mesentarial filaments, overgrowth, sweeper tentacles

Indirect interactions: overtopping, allelochemicals

Standoffs: expending energy to inhibit growth of neighbour

Most effective competition mechanism for slow growers

Direct injury

Most effective competition mechanism for fast growers

Overtopping

Role of competition in structuring communities

• creates dominance hierarchies or competitive networks

• retards rate of resource monopolisation

• enables coexistence of competing species

3 levels of organisation in modular organisms

1. module (polyp) - morphological individual

2. colony - physiological individual

3. genet (clone) - genetic individual

Ecological/demographic consequences of asexual reproduction

• replication of polyps (growth) and colonies (asexual reproduction)

• genotype dispersed over a wider area, greater range of habitats/resources

• spreads the risk of mortality over 3 levels

3 levels of mortality

1. Polyp death - partial colony mortality - reduces colony fitness

2. Colony death - affects abundance/distribution of genotype - reduces genet fitness

3. Genet death - reduces genetic diversity - reduces population fitness

Evolutionary implications of cloning

• Preservation of successful genomes

• don’t need a mate to reproduce

• cloning may circumvent senescence of a genotype

Growth vs reproduction tradeoff interacting with reproduction

Corals channel resources into growth rather than reproduction until the colony attains an appropriate size

Spawners vs brooders egg size/egg number

Spawners - more eggs, smaller eggs

Brooders - less eggs, larger eggs

Brooder vs spawners reproductive tradeoffs

• Brooders require increased energy for parental care

• Most brooders pass down zooxanthellae

• Spawners are able to disperse more widely

• Brooders have greater chance of fertilisation success and lower juvenile mortality

Physiological vs evolutionary tradeoffs

Physiological - responses of individuals to extrinsic factors (e.g. growth rate vs competition)

Evolutionary - responses of populations or species (e.g. egg size vs egg number)

3 levels of synchrony for mass spawning, their proximal and ultimate factors

1. Hour - P=daily photo-period cycle, U=avoid visual predators

2. Night, P=lunar cycle, U=neap tides to maximise dispersal and fertilisation

3. Month, P=temps?, U=optimise juvenile survival and growth

3 hypotheses about ultimate factors that could’ve led to the evolution of synchronous spawning

1. Predator satiation hypothesis - implies that different species have co-evolved short, synchronous breeding periods and suggests that mass spawning should occur in all regions where planktivorous fish are abundant and that mass spaning should not be tied to any particular season

2. Genetic legacy hypothesis - assumes that current spawning patterns are a result of a historical, genetic legacy

3. Environmental constraints hypothesis - assumes that each species has independently responded to environmental, ecological or physiological factors that constrain the optimal time of breeding, and suggests that synchrony occurs because species respond similarly to these factors and that the timing of spawning should vary regionally in response to regional variation in these factors

Links between physiology and life history

• physiology controls responses of an organism to the environment

• physiological mechanisms link performance of organism and life table of population to environment