REPRODUCTION AND LARVAL SETTLEMENT OF CORAL

VIJAYKUMAR, M.E, DFK – 1304

I PhD, Dept. of FRMCollege of Fisheries, Mangalore-02

Overview Reproduction

Types of reproduction

Asexual

Sexual

Dispersal

Settlement

PURPOSE OF REPRODUCTION

To make sure a species can continue. Definition: Reproduction is the process by which

an organism produces others of its same kind.

TYPES OF REPRODUCTION

1. Asexual2. Sexual

Corals can reproduce either asexually by budding or sexually by releasing gametes (eggs and sperm).

ASEXUAL REPRODUCTION

A new organism (sometimes more than one) is produced from one organism.

The offspring will have hereditary material uniform with the hereditary material of the parent organism. This means they will be genetically alike.

TYPES OF ASEXUAL REPRODUCTION

Budding Regeneration Fragmentation

BUDDING

Process by which a new, duplicate plant or animal begins to form at the side of the parent and enlarges until an individual is created.

Very common in plants;

The coral colony expands in size by budding.

Budding may be intratentacular, in which the new bud forms from the oral discs of the old polyp, as in Diploria, or extratentacular in which the new polyp forms from the base of the old polyp, as in Montastraea cavernosa.

In this form of asexual reproduction, new polyps bud off from parent polyps to expand or begin new colonies.

This occurs when the parent polyp reaches a certain size and divides.

This process continues throughout the animal's life and produces polyps that are genetically identical to the parent polyp

REGENERATION

The ability to restore lost or damaged tissues, organs or limbs. It is a common feature in invertebrates, like worms and starfish.

FRAGMENTATION

Corals can also reproduce asexually by fragmentation – that is, when a portion of the colony (say, a branch), is detached from the rest and falls in suitable substrate.

This can happen either - naturally, - wave action - humans purposely take

coral fragments and place them in other substrate areas

For instance, if a portion of a larger colony is broken off from the main colony during a storm or boat grounding, the separated individuals can start new coral colonies that are genetically identical to the parent colony.

However, the success of the fragments in establishing a new colony is dependent upon whether they are exposed to favorable growth conditions.

For instance, fragments exposed to strong wave action will find it difficult to settle on a substrate for continued growth.

Broken pieces of corals that land on a suitable substrate may begin growing and produce a new colony.

This type of reproduction is common in branching corals like Acropora cervicornis in which a positive correlation was found between fragment size and survival.

ACCIDENTAL FRAGMENTATION

Turtle Damage

ACCIDENTALFRAGMENTATION Wave Damage

SEXUAL REPRODUCTION

Requires two sex cells – egg and sperm

TYPES OF SEXUAL REPRODUCTION

Broadcast spawning Brooding

Gametes (eggs and sperm) develop within the mesenteries of the coral polyps. These reproductive cells are found on the mesentery membranes that radiate inward from the layer of tissue that lines the stomach cavity.

Mature gametes are fertilized either internally or externally, leading to differences in the location of the subsequent embryonic development of the planula.

About three-quarters of all stony coral species are broadcast spawners.

produce male and/or female gametes that are released into the water column in massive numbers, enabling them to distribute their offspring over a broad geographic area.

The gametes are positively buoyant and float towards the surface before the eggs and sperm join to form free-floating larvae called planulae.

An individual planula floats in the water column until it finds a suitable space to call home - usually a hard surface to which it can attach.

Broadcast Spawning

Large numbers of planulae are produced to compensate for the many hazards, such as predators, that they encounter as they are carried by water currents.

The time between planula formation and settlement is a period of exceptionally high mortality for coral larvae

Broadcast spawner

Along many reefs, coral spawning occurs as a synchronized event, when

many coral species in an area release their eggs and sperm at about the same

time.

The timing of a broadcast spawning event is very important because male

and female corals cannot move to make reproductive contact with each other.

Because colonies may be separated by wide distances, the release of sperm

and eggs must be precisely timed, and usually occurs in response to multiple

environmental cues.

There are both long-term and short-term controls that affect the timing of

spawning events.

The long-term control of spawning may be related to temperature, day length,

or rate of temperature change.

The short-term control is usually based on lunar cues. The final release, or

spawn, is usually based on the time of sunset.

Broadcast spawning coral species may spawn on only one or a few nights each

year, and though different species may spawn at different times, the spawning

events for any given species happen at the same time.

This close-up photo shows rows of individual brain coral polyps in different stages of releasing theireggs.

Whereas three quarters of stony corals are broadcast spawners, the remaining quarter of coral species are brooders.

Brooding species generally have high success in recruiting new larvae into established colonies, but many of these species reach only small colony size and thus do not contribute much to the overall growth of a reef.

In this reproduction mode, only male gametes are released into the water column.

The male gametes are negatively buoyant and are transported by waves and current before sinking to the ocean floor.

If encountered, the male gametes are then taken in by female coral polyps containing egg cells.

BroodingBrooding involves internal fertilization, and the development of the planulae takes place inside the 'parental' polyp before release into the water, a process known as planulation.

Fertilization occurs inside the female coral and produces a small planula.

This planula is released later through the mouth of the female coral at an advanced stage of development so that it is capable of settling onto hard substrate very soon after its release.

Thus, brooding species generally disperse their larvae shorter distances from the mother colony than broadcasters

planula in polyp

Pocillopora damicornis, a brooder

zygote

Sexual Reproduction

planula larvaeegg

sperm

Environmental Stresses Yielding Lower recruits in Corals

• Turbidity & sedimentation

• High temperature

• Low salinity

• Aerial exposure at low tide

• Mechanical damage

• Intraspecific competition

• Oil & fuel oil pollution

• Eutrophication

Cleavage - two cell stage

Fertilized egg

Early Blastula

Cleavage-four cell stage

Dispersal is primarily by tiny planktonic larvae called planulae produced in large numbers to compensate for the hazards of passive transportation in water currents.

Corals, similar to many benthic animals, reproduce via a larval stage, which undergoes a dispersal period in the plankton before attaching to the reef and growing to establish a new colony (recruitment to the population).

Dispersal

Settlement rates and subsequent recruitment are a measure of reproductive success and it has long been known that variation in settlement rates is a key determinate in the understanding of how the abundance and diversity of corals vary in space and time.

With increasing disturbances to coral reefs, both directly through anthropogenic activities, and indirectly via effects associated with climate change, an understanding of coral settlement and recruitment rates and the influence of disturbance on this critical stage of the life cycle of these animals is essential.

However, if larval supply is limited, or conditions are not optimal for coral larval settlement and new recruit growth, then algae and other invertebrates out-compete corals and take over these spaces.

Successful larval settlement and recruitment of corals is therefore critical to the resilience and recovery of coral reefs.

Settlement

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