27
the timing of reproduction may be offset sufficiently on both sides of the front to
produce effective genetic isolation even in the face of continued dispersal. It is also
conceivable that a species could experience sufficiently strong selection on either
side of a hydrographic boundary to produce different body size, growth rates, or
skeletal shapes with consequent changes in mating recognition systems, particularly because these morphologic variables are known to be affected by changes in
food supply, temperature, and predation intensity. Hence it is possible that pelagic
speciation occurs in the face of sustained gene flow that is rendered ineffective by
changes in mating recognition cues or reproductive timing (Norris 2000).
Regarding highly mobile, nektonic organisms (e.g. fishes, squids), species in
general inhabit extended geographic regions. Populations undergo migrations, seasonally vacating and reoccupying specific sub regions of their respective ranges.
Marine fishes and other oviparous organisms select the environment in which their
eggs will be released (Roosenburg 1996), so most organisms have a tendency to
undertake extensive movements to specific breeding sites (Breder and Rosen
1966). Spawning locations and subsequent larval distributions are associated with
well-defined and geographically predictable or stable oceanographic systems
(Sinclair 1988). This probably occurs because areas suitable for adult feeding may
not necessarily be suitable for the survival of early stages (Bakun 1996).
Marine fronts have been broadly reported as preferred spawning grounds for
fishes and squids (Sinclair 1988; Bakun 1996, 2006b; Acha et al. 2004; Houde
2009). Most fronts seem to fulfill the requirements of the “fundamental triad
hypothesis” that identify suitable spawning habitats (Bakun 1996): (i) nutrient enrichment processes, (ii) concentration of food particles, and (iii) retention
of eggs and larvae within a favorable habitat. Although fronts are diverse in spatial and temporal scales, and driven by varied forcing, recurrent features of these
scenarios are: (i) nutrient pumping due to stratification weakening or disruption,
generating enrichment in the euphotic zone that enhances primary production, (ii)
convergence of water masses that aids in concentration and maintenance of food
particles for larvae, and (iii) the existence of a vertically structured dynamics that
allows for behaviorally mediated larvae retention (Largier 1993; Mann and Lazier
2006; Bakun 2006b). As such fronts can include the whole “triad”.
Since most marine animals have a pelagic larval stage, the paradigm until recently
has been to assume extensive dispersal and massive export. In combination, the widespread existence of planktonic larvae, the broad distribution of larvae in the plankton, extended planktonic periods and poor swimming abilities of most larvae suggest
that the larval exchange among populations should be the rule. Consequently, the
concept of “open populations”, with plentiful exchange of larvae, was pervasive in
the late twentieth century. However, evidences from a variety of fields indicated that
local retention may be considerably more prevalent than previously thought, even in
species with long larval durations and, thus, that populations may be less open than
originally thought (Warner and Cowen 2002; Levin 2006). This is in agreement with
recent results based on molecular phylogenetic analyses that have revealed high cryptic biodiversity in the open ocean, and that rates of speciation can also be as high for
pelagic taxa as for shallow-marine and terrestrial species (Norris 2000).
3.5 Life Histories Traits in Relation to Fronts
the timing of reproduction may be offset sufficiently on both sides of the front to
produce effective genetic isolation even in the face of continued dispersal. It is also
conceivable that a species could experience sufficiently strong selection on either
side of a hydrographic boundary to produce different body size, growth rates, or
skeletal shapes with consequent changes in mating recognition systems, particularly because these morphologic variables are known to be affected by changes in
food supply, temperature, and predation intensity. Hence it is possible that pelagic
speciation occurs in the face of sustained gene flow that is rendered ineffective by
changes in mating recognition cues or reproductive timing (Norris 2000).
Regarding highly mobile, nektonic organisms (e.g. fishes, squids), species in
general inhabit extended geographic regions. Populations undergo migrations, seasonally vacating and reoccupying specific sub regions of their respective ranges.
Marine fishes and other oviparous organisms select the environment in which their
eggs will be released (Roosenburg 1996), so most organisms have a tendency to
undertake extensive movements to specific breeding sites (Breder and Rosen
1966). Spawning locations and subsequent larval distributions are associated with
well-defined and geographically predictable or stable oceanographic systems
(Sinclair 1988). This probably occurs because areas suitable for adult feeding may
not necessarily be suitable for the survival of early stages (Bakun 1996).
Marine fronts have been broadly reported as preferred spawning grounds for
fishes and squids (Sinclair 1988; Bakun 1996, 2006b; Acha et al. 2004; Houde
2009). Most fronts seem to fulfill the requirements of the “fundamental triad
hypothesis” that identify suitable spawning habitats (Bakun 1996): (i) nutrient enrichment processes, (ii) concentration of food particles, and (iii) retention
of eggs and larvae within a favorable habitat. Although fronts are diverse in spatial and temporal scales, and driven by varied forcing, recurrent features of these
scenarios are: (i) nutrient pumping due to stratification weakening or disruption,
generating enrichment in the euphotic zone that enhances primary production, (ii)
convergence of water masses that aids in concentration and maintenance of food
particles for larvae, and (iii) the existence of a vertically structured dynamics that
allows for behaviorally mediated larvae retention (Largier 1993; Mann and Lazier
2006; Bakun 2006b). As such fronts can include the whole “triad”.
Since most marine animals have a pelagic larval stage, the paradigm until recently
has been to assume extensive dispersal and massive export. In combination, the widespread existence of planktonic larvae, the broad distribution of larvae in the plankton, extended planktonic periods and poor swimming abilities of most larvae suggest
that the larval exchange among populations should be the rule. Consequently, the
concept of “open populations”, with plentiful exchange of larvae, was pervasive in
the late twentieth century. However, evidences from a variety of fields indicated that
local retention may be considerably more prevalent than previously thought, even in
species with long larval durations and, thus, that populations may be less open than
originally thought (Warner and Cowen 2002; Levin 2006). This is in agreement with
recent results based on molecular phylogenetic analyses that have revealed high cryptic biodiversity in the open ocean, and that rates of speciation can also be as high for
pelagic taxa as for shallow-marine and terrestrial species (Norris 2000).
3.5 Life Histories Traits in Relation to Fronts