Recruitment to the reef
column spacer Recruitment to the reef
 

Recruitment in reef organisms can involve either sexual or asexual reproduction. The first involves copulation/spawning (animals) or gametes/spores (plants), while the second involves fragmentation of the body to produce genetically identical clones. Asexual reproduction occurs in both animals and plants.

NOTE recruitment in its broadest definition is the addition to a population of a reproductive, or potentially reproductive, individual. In comparison, settlement is simply the arrival of an organism, often in larval form, at the sea bottom. Some authors define the events based on time, with settlement lasting 1-2d, and recruitment having occurred if the organism is still around by a couple of weeks later

 
 

Sexual reproduction

  Sexual reproduction in animals is considered in this section, while ASEXUAL REPRODUCTION is dealt with elsewhere.
 

Recruitment in sexually reproducing animals involves 3 main topics: 1) spawning, larval development, and dispersal of larvae, 2) settlement and metamorphosis, and 3) survival during early life.  The first topic is considered in this section.  The other recruitment topics are accessible via the icons

NOTE lit. "later shape". An abrupt change during development of an animal from larva to juvenile. For most marine invertebrates, where larval forms differ markedly from the adult, metamorphosis can involve extreme changes in form, behaviour, and function. In most fishes, however, the transition from larva to adult is more gradual

title for recruitement to the reef part of BCCR hot-button icon for linking to the topic of spawning, larval development, & dispersal of larvae in BCCR hot-button icon for linking to the topic of survival during early life in BCCR
 
 

Sexual reproduction: spawning, larval development, & dispersal of larvae

After a brief introduction, this section is divided into smaller sections on selected invertebrates and fishes. See icons at the bottom of this page.

 
 
seahorse dive leader for BIOLOGY OF CARIBBEAN CORAL REEFS website photograph of a Caribbean patch reef taken from a video

" Ever wonder how all these organisms got to this little patch reef? Well, they were spawned elsewhere and their larvae drifted here mostly by chance. Check out the schoolmasters. One of them has just released a load of sperm" - Cayman Islands 2003  First part of video courtesy Andy Stockbridge, Belize.

NOTE Lutjanus apodus

 

 
 

drawing showing patterns of development in invertebrates and fishes
In invertebrates and fishes the pattern of development is similar, from an egg to a pelagic larval phase...then, metamorphosis to a juvenile and settlement to the sea bottom. In some forms, settlement precedes metamorphosis, so the larva may live for a short period on or just above the sea bottom.

In the coral example shown here, fertilised eggs float up from the coral towards the sea surface. They hatch to a short-lived larval stage that floats around for a few days before settling and metamorphosing. Here, a floating, non-feeding larva metamorphoses to an attached polyp that begins to feed and secrete a calcareous skeleton, and the transformation to adult form is relatively abrupt.

In fishes, eggs are released by the female, fertilised by a male releasing a cloud of sperm, and the eggs later hatch to larvae. The larvae live in the plankton for a time subsisting on yolk. In comparison with most invertebrates, metamorphosis of a larval fish to a juvenile is gradual, with relatively little change in form and behaviour.

 
 

Characteristics of planktonic larvae of invertebrates include small size, spines, and transparency, adaptations that protect the larvae from predators. Larvae of invertebrates may look quite different from the adults. For example, these goose barnacles begin life as small larvae called nauplii. They are photograph of pelagic goose barnacle Lepas anatifera less than 1mm in length, live freely in the plankton for several weeks after hatching, feed on phytoplankton, and look nothing like the adult. Photograph of nauplius courtesy Kristi Miller, DFO, Nanaimo.

NOTE pl. for nauplius and pronounced "nauply-eye". The name is from the Greek and means "a kind of mollusc", based on the early belief that the little shelled larva was a photograph of nauplius larva of a barnaclekind of pelagic mollusc. Only in the later 1800s was it discovered to be the larval form of an arthropod; specifically, a barnacle


Goose barnacle
Lepas anatifera 1X

 

Nauplius larva of a barnacle
Balanus sp. 100X

  Here is a selection of Caribbean reef invertebrates along with their larvae to emphasise the different appearances of adults and larvae. The adults are not scaled to size, but the larvae are all 20X life size. The name of each larval type is accompanied by a short description.
 
Channel clinging crab Mithrax spinosissimus selected reef invertebrates with descriptions of their larvae
Milk conch Strombus costatus
Cushion star Oreaster reticulatus
Sea egg urchin Tripneustes ventricosus
Donkey dung sea cucumber Holothuria mexicana
Brain coral Diploria strigosa
  selected adult reef fishes with their larvaeCharacteristics of planktonic larvae of coral-reef fishes include small size, spines, and transparency, adaptations similar to the ones described for reef invertebraes, and ones that similarly protect the larvae from predators. Note in the accompanying illustration that while these larval fishes do differ in appearance from their adults, they still have some basic "fishy" resemblances. Contrast this with the invertebrate array above, where adults and larvae bear little similarity with one another. Drawings of larval fishes from various authors in 1984 Ontogeny and systematics of fishes Amer Soc Ichthy Herp Vol. 1
 
 

Of what adaptive value is a pelagic larval drifting phase in the life cycle of a coral-reef fish? Consider the possibilities below, try to pick out the LEAST LIKELY one, and CLICK HERE to see explanations.

Reduces contact with reef-based predators during early development

Allows dispersal to new habitats.

Pelagic drifting minimises energy expenditure.

If the larvae drift away, there is less chance of them being smashed on the reef by waves.

Permits better exploitation of patchy food resources.

 
 
It is only possible to consider a few representative reef organisms in this section on spawning, larval development, and dispersal of larvae. Click on an icon to learn about each group hot buttons for spawning/larval development/dispersal section of BCCR website hot button for sponges section on spawning/metamorphosis/dispersal hot button for corals section on spawning/metamorphosis/dispersal hot button for snails section on spawning/metamorphosis/dispersal hot button for sea urchins & relatives section on spawning/metamorphosis/dispersal hot button for fishes section on spawning/metamorphosis/dispersal
 
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