160
W.M. Hamner and J.L. Largier
6.1 Introduction
A pelagic larval stage is found in all species of reef fi shes known to aggregate for
spawning (Appendix ). These larvae are the main dispersal stage, transported via
water movement and active larval swimming (Leis 1991, 2006 ) . The distance
pelagic larvae can travel typically exceeds the migration of adults to/from aggregation sites (although adults can migrate tens to hundreds of kilometres). Pelagic larvae
are the only life history stage during which most resident reef fi shes can or will
cross water barriers, often including deep channels through the reef, and therefore
dispersion of larvae rather than of adults is primarily responsible for population
structure of many reef fi shes. Strathman et al. ( 2002 ) suggest that an early planktonic life history is particularly advantageous because planktonic larvae can feed
readily in productive inshore waters, avoid reef predators, and break cycles of parasitism. They note that the benthic parental habitat is clearly a favourable location for
recruitment of juveniles, with natural selection favouring recruitment to natal reefs.
The early life history (ELH) of aggregating reef fi shes from spawning to settlement requires from about 2 weeks to a few months. It starts with passive pelagic
eggs that take 24 h or less to hatch. After hatching from pelagic eggs, larvae go
through a relatively non-motile yolk-sac stage of a few days, then transition to
larvae which can actively feed but still have limited motility due to incomplete fi n
development (pre-fl exion larvae). Near the midpoint of the planktonic phase the
larvae have a greater sensory capacity (see Chap. 7 ) and develop supporting
elements for the caudal fi n (post-fl exion) as well as well-defi ned medial fi ns with
supporting elements, resulting in a major increase in horizontal swimming ability.
Thus pelagic larvae of reef fi shes have both an initial planktonic (pre-fl exion) stage
and a subsequent nektonic (post-fl exion) phase. As plankton, the subject of this
chapter, eggs and larvae are initially passive drifters, but they soon develop limited
motility that permits them to swim suffi ciently well to feed and control their vertical
location in the water column. As nekton, larval fi shes can swim well and hold their
own against weaker horizontal ocean currents, as well as potentially sensing the
presence of distant reefs and navigating directionally in the pelagic waters around
coral reefs in order to return to and settle on the coral reef.
The release of millions to billions of propagules within a short time and limited
volume of water during aggregation-spawning poses questions about the potential
disadvantages or advantages (costs versus benefi ts) of this life history strategy
compared with other methods of spawning. In this chapter we examine aspects of
the physical and biological ocean environment that infl uence the survival and distribution of the planktonic phase of those reef fi shes that originate from spawning
aggregations. Physical mechanisms such as tidal currents and eddies are important
in initial transport and in the short term (a few days) a signifi cant portion of eggs and
early larvae can be returned back to their initial spawning sites (Hamner et al. 2007 ) .
After these initial days, behavioural mechanisms, such as vertical migration and
reaction to frontal conditions, are also important in determining where larvae will
go during their fi rst several weeks of life. Later, after the larvae become more active,
W.M. Hamner and J.L. Largier
6.1 Introduction
A pelagic larval stage is found in all species of reef fi shes known to aggregate for
spawning (Appendix ). These larvae are the main dispersal stage, transported via
water movement and active larval swimming (Leis 1991, 2006 ) . The distance
pelagic larvae can travel typically exceeds the migration of adults to/from aggregation sites (although adults can migrate tens to hundreds of kilometres). Pelagic larvae
are the only life history stage during which most resident reef fi shes can or will
cross water barriers, often including deep channels through the reef, and therefore
dispersion of larvae rather than of adults is primarily responsible for population
structure of many reef fi shes. Strathman et al. ( 2002 ) suggest that an early planktonic life history is particularly advantageous because planktonic larvae can feed
readily in productive inshore waters, avoid reef predators, and break cycles of parasitism. They note that the benthic parental habitat is clearly a favourable location for
recruitment of juveniles, with natural selection favouring recruitment to natal reefs.
The early life history (ELH) of aggregating reef fi shes from spawning to settlement requires from about 2 weeks to a few months. It starts with passive pelagic
eggs that take 24 h or less to hatch. After hatching from pelagic eggs, larvae go
through a relatively non-motile yolk-sac stage of a few days, then transition to
larvae which can actively feed but still have limited motility due to incomplete fi n
development (pre-fl exion larvae). Near the midpoint of the planktonic phase the
larvae have a greater sensory capacity (see Chap. 7 ) and develop supporting
elements for the caudal fi n (post-fl exion) as well as well-defi ned medial fi ns with
supporting elements, resulting in a major increase in horizontal swimming ability.
Thus pelagic larvae of reef fi shes have both an initial planktonic (pre-fl exion) stage
and a subsequent nektonic (post-fl exion) phase. As plankton, the subject of this
chapter, eggs and larvae are initially passive drifters, but they soon develop limited
motility that permits them to swim suffi ciently well to feed and control their vertical
location in the water column. As nekton, larval fi shes can swim well and hold their
own against weaker horizontal ocean currents, as well as potentially sensing the
presence of distant reefs and navigating directionally in the pelagic waters around
coral reefs in order to return to and settle on the coral reef.
The release of millions to billions of propagules within a short time and limited
volume of water during aggregation-spawning poses questions about the potential
disadvantages or advantages (costs versus benefi ts) of this life history strategy
compared with other methods of spawning. In this chapter we examine aspects of
the physical and biological ocean environment that infl uence the survival and distribution of the planktonic phase of those reef fi shes that originate from spawning
aggregations. Physical mechanisms such as tidal currents and eddies are important
in initial transport and in the short term (a few days) a signifi cant portion of eggs and
early larvae can be returned back to their initial spawning sites (Hamner et al. 2007 ) .
After these initial days, behavioural mechanisms, such as vertical migration and
reaction to frontal conditions, are also important in determining where larvae will
go during their fi rst several weeks of life. Later, after the larvae become more active,