Mechanisms of Larval Dispersal and Settlement of Reef Organisms

This topic submitted by Bridget Hohner ( Bridgethohner@yahoo.com) at 9:26 PM on 3/30/06.

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My topic is Larval Dispersal and Settlement in Coral Reefs. I want to discuss the mechanisms by which reef fish, invertebrates and coral larvae locate a settlement site and the use of sensory cues.
Coral reef community structure
High productivity, high species diversity (species from 32 phyla represented)
Complex biological associations
Reefs are topographically complex
Many strata many different species of fish and invertebrates in different strata
Depth zonation
Corals are framework of reef- they precipitate calcium carbonate moving living
surface upward
Zooxanthellae-symbiotic algae providing nutrients to coral
Herbivores exert strong effects on species composition of reef
Space limited- interspecific competition (overgrowth, shading, aggressive
interactions)
New recruits settle in community to replenish reef populations
New recruits must survive a planktonic larval phase, and disperse by active or
passive means
Larva are preyed upon heavily
Reef fish life cycle and larval dispersal
Life history
High fecundity, iteroparous
Sexual reproduction
Fertilization is external, many species forming aggregations to spawn, sometimes
preferring to spawn in pairs
Majority of reef fish take little or no care of young
*NOTE: some variation among life histories, this suggests adaptation
Temporal variation in spawning
Bipartite with dispersal larval phase
Most species of reef fish larvae are advected from the reef, forcing them to
find a way to return to the reef
Pelagic larval phase to juvenile to sedentary reef phase
Larval phase of most species 3-6 weeks
Size- 1-20mm long
Active dispersal (behavioral and physiological studies present evidence
of active dispersal)
It was initially thought that pelagic fish larvae were dispersed only through
Advection, however advection alone cannot explain recruitment success
Strong swimming abilities
Only useful if larvae are able to detect suitable settlement
Save energy by locating currents using vertical migration
Return of larvae to natal reef
Some species may imprint to natal reef as embryos using reef odor
(mouth brooding may facilitate this)
Sensory mechanisms used in active dispersal (species may use a
combination of sensory mechanisms or just one)
Chemical cues
Ebb tide plumes of lagoon water extending many km from reef
provide cues for swimming larva to orient themselves
Multiple sources provide chemical cues
Respond to changes in water chemistry
Differences in concentrations of amino acids, fatty acids,
alcohols, salinity and temperature (temperature can affect the
dispersal and activity of the chemical stimuli)
Chemical stimuli of biotic origin
Metabolites from reef organisms
Chemical stimuli of abiotic origin
Changes in salinity, temperature, concentrations of calcium
carbonate from reefs
**combinations of temperature and salinity define water
masses, are excellent cues, and found on vertical and horizontal
planes
Odor cues
Apogonids, pomacentrids, blennies, and gobies able to sense
reef odors
Amino acids detectable by olfaction-useful cues for
locating reef (concentrated source of amino acids from high
density of living organisms)
Auditory cues
Sound travels in water regardless of current flow
Otic capsule
Functional in a number of species shortly after hatching
Can hear frequencies within 50 and 5000 Hz
Physical sources
Breaking waves (can vary daily and seasonally)
Biological sources
Species specific sounds known in some sp. telling larva to
approach or avoid
Capable of localizing source
Visual cues
Well developed sense in reef fish
Functions at short distances
Reef Invertebrate Life Cycle and Larval Dispersal
Bipartite with dispersal larval phase
Swimming abilities
Partial navigation-vertical migration
Sensory abilities
Some invertebrate larvae able to orient themselves toward settlement site
Using sensory cues and swim toward the site
No evidence of hearing in invertebrate larvae
Chemical cues
Nudibranch retract their velum and stop swimming when they sense coral
metabolites
Use of chemical cues in sessile invertebrates (barnacles)
Cues bind to chemosensory receptors
Visual Cues
Use visual cues of celestial bodes
Used when target is large
Used in partial navigation
Coral Life Cycle and Larval Dispersal
Colonial
Reproduce asexually (budding) or sexually (releasing gametes)
Reproduce both internally (brooding) and externally (broadcasting) (majority of
sp. externally)
Coordinate release of gametes to ensure fertilization (mass spawning)
Based on lunar cycles
Adult stage is sessile
Planula larval dispersal
Planula is planktonic, ciliated and free-swimming
Dispersed by advection (passive dispersal)
Potential for long distance dispersal, however often settle near origin due
to cyclonic currents

References


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