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3 Biology of Fronts
The most striking difference between aquatic and terrestrial mating is that
aquatic organisms commonly shed female gametes as well as male gametes.
Though water is a benign medium for gametes, external fertilization is by no
means easy, especially for sedentary or completely sessile organisms (Strathmann
1990). Fertilization by distant males and females is limited by the life span of
active sperm, predation on gametes, and dispersion of gametes with dispersion
probably the greatest obstacle: classical diffusion principles imply that particulates (e.g. gametes) should disperse widely over a vast area, somewhat like a cloud
of ever-increasing dimension (Wolanski and Hamner 1988). All these problems
diminish with reduced distance among individuals (i.e. increasing organisms’ spatial density). It can therefore be argued that for free-spawners the high fertilization success in crowded populations offsets the reduced fecundity from resource
limitation due to increased competition (Strathmann 1990). For benthic animals,
there is adaptive advantage in settling out from the plankton in optimal conditions
for adulthood. Hence, mechanisms permitting some settlement near to a sustained
parent population might reasonably also be expected to have been selected during
evolution (Naylor 2006). Moreover, if reduced larval dispersal resulted in reduced
genetic exchange among populations, it could increase possibilities for local adaptation (Strathmann 1990). Though settlement near parent population could increase
intra-specific competition; benefits from retention could surpass its disadvantages.
Because the ocean is a highly dispersive environment, drifting and mobile
organisms are continuously dispersed with the consequence that the distance to the
nearest mate persistently increases; thus the chance that one individual encounters
another with similar genetic material decreases monotonically as time after birth
increases. Diffusion itself from a point source for nonmobile drifting organisms,
or random movement for mobile organisms, minimizes the frequency of sexual
encounter that is necessary to allow persistence of the population. Survival itself
is not the only issue; finding a mate in a diffuse environment at low concentrations
becomes the additional, perhaps more critical, challenge. Thus the very existence
of a population may depend on the ability of larvae to remain aggregated during
the first few weeks/months of life (Sinclair 1988).
Water dynamics of marine fronts offer opportunities for planktonic organisms
(including larvae of fish and benthic animals) to be retained. There are frequently
steep gradients in flow velocity and even reversals in flow direction associated with
fronts (McManus and Woodson 2012). Plankton is a highly diverse group whose
components display a wide range of behavioral capabilities that bridge the transition from being a passive particle to being able to determine vertical and horizontal
position in the ocean (McManus and Woodson 2012). If an organism is not a passive particle (i.e. has the ability to float, sink or swim) then the potential exists for
the organism to become concentrated in certain types of flow (Franks 1992). Tiny,
weakly swimming organisms that may be unable to resist being passively swept
along in the horizontal ocean flow may well be able to control their depth level in
the much less energetic field of vertical motion in the ocean. Estimates of vertical velocities at fronts from field studies and modeling indicate maximum speeds
of ca. 0.2 mm s −1 . This is the same order of magnitude as the swimming speed
3 Biology of Fronts
The most striking difference between aquatic and terrestrial mating is that
aquatic organisms commonly shed female gametes as well as male gametes.
Though water is a benign medium for gametes, external fertilization is by no
means easy, especially for sedentary or completely sessile organisms (Strathmann
1990). Fertilization by distant males and females is limited by the life span of
active sperm, predation on gametes, and dispersion of gametes with dispersion
probably the greatest obstacle: classical diffusion principles imply that particulates (e.g. gametes) should disperse widely over a vast area, somewhat like a cloud
of ever-increasing dimension (Wolanski and Hamner 1988). All these problems
diminish with reduced distance among individuals (i.e. increasing organisms’ spatial density). It can therefore be argued that for free-spawners the high fertilization success in crowded populations offsets the reduced fecundity from resource
limitation due to increased competition (Strathmann 1990). For benthic animals,
there is adaptive advantage in settling out from the plankton in optimal conditions
for adulthood. Hence, mechanisms permitting some settlement near to a sustained
parent population might reasonably also be expected to have been selected during
evolution (Naylor 2006). Moreover, if reduced larval dispersal resulted in reduced
genetic exchange among populations, it could increase possibilities for local adaptation (Strathmann 1990). Though settlement near parent population could increase
intra-specific competition; benefits from retention could surpass its disadvantages.
Because the ocean is a highly dispersive environment, drifting and mobile
organisms are continuously dispersed with the consequence that the distance to the
nearest mate persistently increases; thus the chance that one individual encounters
another with similar genetic material decreases monotonically as time after birth
increases. Diffusion itself from a point source for nonmobile drifting organisms,
or random movement for mobile organisms, minimizes the frequency of sexual
encounter that is necessary to allow persistence of the population. Survival itself
is not the only issue; finding a mate in a diffuse environment at low concentrations
becomes the additional, perhaps more critical, challenge. Thus the very existence
of a population may depend on the ability of larvae to remain aggregated during
the first few weeks/months of life (Sinclair 1988).
Water dynamics of marine fronts offer opportunities for planktonic organisms
(including larvae of fish and benthic animals) to be retained. There are frequently
steep gradients in flow velocity and even reversals in flow direction associated with
fronts (McManus and Woodson 2012). Plankton is a highly diverse group whose
components display a wide range of behavioral capabilities that bridge the transition from being a passive particle to being able to determine vertical and horizontal
position in the ocean (McManus and Woodson 2012). If an organism is not a passive particle (i.e. has the ability to float, sink or swim) then the potential exists for
the organism to become concentrated in certain types of flow (Franks 1992). Tiny,
weakly swimming organisms that may be unable to resist being passively swept
along in the horizontal ocean flow may well be able to control their depth level in
the much less energetic field of vertical motion in the ocean. Estimates of vertical velocities at fronts from field studies and modeling indicate maximum speeds
of ca. 0.2 mm s −1 . This is the same order of magnitude as the swimming speed