105
4 Spawning Aggregations in Reef Fishes; Ecological and Evolutionary Processes
different patterns of diversifi cation among two of the main lineages. Wrasses, which
appear to be exclusively resident spawners, have a relatively small mean size,
although a minority of basal species are very large (Choat et al. 2006 ) and show a
great deal of morphological diversifi cation (Westneat and Alfaro 2005 ) . Groupers,
which contain most of the recorded transient spawners, are more uniform morphologically with the primary axis of morphological diversifi cation being an increase in
size (Wainwright and Bellwood 2002 ; Craig and Hastings 2007 ) .
4.5 Evolutionary Processes and the Development
of Aggregative Spawning
The factors that drive the evolution of spawning aggregations remain unresolved.
However important processes will be those that enhance survival of offspring relative
to those produced by other individuals. These include two fundamental aspects of
reef fi sh reproduction. The fi rst is that spawning occurs at sites which result in
advection of propagules to open water (Colin 2010a, b ) . The second is that anisogamy dictates that ova are the limiting reproductive resource and this will profoundly
infl uence the reproductive behaviour of both sexes (Trivers 1972 ; Munday et al.
2006a ) . Given the disproportionate investment of resources in eggs relative to sperm,
individual females will seek to maximize zygote survival. In this context egg release
in a hydrodynamic and biological environment favouring zygote and larval survival
is critical. If aggregation sites represent such environments and females migrate
there to spawn, anisogamy ensures that males will follow.
This section discusses advection of eggs and zygotes from the reef following
gamete release with the following sequence. (1) The issue of advection with respect
to the scale of movement of propagules and the separate but related concepts of
larval retention and dispersal. (2) The case for local advection as a process with
general evolutionary implications including the signifi cance of mortality early in
teleost life histories and the ecological and evolutionary consequences of dispersal.
(3) The factors that may restrict transient spawning to particular reef fi sh lineages
and to a limited number of sites and spawning times (Chap. 6 ).
Debates concerning the capacity of currents to move passive propagules away
from the reef usually focus on the extent to which they are retained in the natal area
or are dispersed into the oceanic pelagic environment (Colin 2010a ) . Both scenarios
involve the movement of propagules tens to thousands of kilometres. Advection as
argued here involves a much smaller scale, tens to hundreds of metres. The high
densities of benthic fi lter-feeding organisms on reef edge habitats and the numerous
planktivorous fi shes in the adjacent waters form a narrow predatory fi eld where eggs
and larvae are subject to high mortalities (Hobson and Chess 1978 ; Holt et al. 1985 ) .
Advection refers to transition through this narrow but intense inner predatory fi eld
to the pelagic environment (Hamner et al. 2007 , Chap. 6 ).
The case for advection driving aggregate spawning behaviour is based on three
observations. Firstly, most aggregation sites are adjacent to open water on reef
4 Spawning Aggregations in Reef Fishes; Ecological and Evolutionary Processes
different patterns of diversifi cation among two of the main lineages. Wrasses, which
appear to be exclusively resident spawners, have a relatively small mean size,
although a minority of basal species are very large (Choat et al. 2006 ) and show a
great deal of morphological diversifi cation (Westneat and Alfaro 2005 ) . Groupers,
which contain most of the recorded transient spawners, are more uniform morphologically with the primary axis of morphological diversifi cation being an increase in
size (Wainwright and Bellwood 2002 ; Craig and Hastings 2007 ) .
4.5 Evolutionary Processes and the Development
of Aggregative Spawning
The factors that drive the evolution of spawning aggregations remain unresolved.
However important processes will be those that enhance survival of offspring relative
to those produced by other individuals. These include two fundamental aspects of
reef fi sh reproduction. The fi rst is that spawning occurs at sites which result in
advection of propagules to open water (Colin 2010a, b ) . The second is that anisogamy dictates that ova are the limiting reproductive resource and this will profoundly
infl uence the reproductive behaviour of both sexes (Trivers 1972 ; Munday et al.
2006a ) . Given the disproportionate investment of resources in eggs relative to sperm,
individual females will seek to maximize zygote survival. In this context egg release
in a hydrodynamic and biological environment favouring zygote and larval survival
is critical. If aggregation sites represent such environments and females migrate
there to spawn, anisogamy ensures that males will follow.
This section discusses advection of eggs and zygotes from the reef following
gamete release with the following sequence. (1) The issue of advection with respect
to the scale of movement of propagules and the separate but related concepts of
larval retention and dispersal. (2) The case for local advection as a process with
general evolutionary implications including the signifi cance of mortality early in
teleost life histories and the ecological and evolutionary consequences of dispersal.
(3) The factors that may restrict transient spawning to particular reef fi sh lineages
and to a limited number of sites and spawning times (Chap. 6 ).
Debates concerning the capacity of currents to move passive propagules away
from the reef usually focus on the extent to which they are retained in the natal area
or are dispersed into the oceanic pelagic environment (Colin 2010a ) . Both scenarios
involve the movement of propagules tens to thousands of kilometres. Advection as
argued here involves a much smaller scale, tens to hundreds of metres. The high
densities of benthic fi lter-feeding organisms on reef edge habitats and the numerous
planktivorous fi shes in the adjacent waters form a narrow predatory fi eld where eggs
and larvae are subject to high mortalities (Hobson and Chess 1978 ; Holt et al. 1985 ) .
Advection refers to transition through this narrow but intense inner predatory fi eld
to the pelagic environment (Hamner et al. 2007 , Chap. 6 ).
The case for advection driving aggregate spawning behaviour is based on three
observations. Firstly, most aggregation sites are adjacent to open water on reef