204
E. W. Koch, J. D. Ackerman, J. Verduin and M. van Keulen
convergently–multiple times in the Najadales and
functionally in the Hydrocharitales (Pettitt, 1984;
Ackerman, 1995, 2000). Pettitt (1984) reviewed the
research pertaining to seagrass pollination biology,
but that review was limited taxonomically with respect to pollen transport and capture mechanisms.
The situation has improved, but remains largely
limited to the mechanistic studies of submarine pollination in the north temperate species, Zostera marina (Ackerman, 1986, 1993, 1995, 1997a,b, and
2002). Fortunately, recent progress has been made
with respect to other species including Amphibolis
antarctica (Verduin et al., 1996), Posidonia australis
and Posidonia sinuosa (Smith and Walker, 2002).
The mechanics of pollination in Z. marina were
studied in a laboratory flow chamber using stroboscopic photography and in the field using physical
models of pollen and adhesive surfaces deployed in
the canopy (Ackerman, 1997a,b, 2002). Pollination
was found to occur under laminar and relatively viscous conditions at the scale of flowers (i.e. low Re;
see Niklas, 1992; Vogel, 1994), and was affected
by the emergence of female flowers from within the
inflorescence. The emergence of flowers (and other
reproductive organs in subsequent phenological processes) led to an increase in the fluid shear stress (τ )
in the local flow (Ackerman, 1997a), which caused
the filamentous pollen (2.7 mm × 7.5 µm diameter)
to rotate and cross streamlines toward female flowers (Ackerman, 1997b). The axial force responsible
for the pollen motion varied directly with the length
and aspect ratio of the pollen (Forgacs and Mason,
1958). Consequently, filamentous pollen need only
be close to female flowers to pollinate by: (i) direct interception on stigmas; (ii) rotation within one
half a pollen length of stigmas; and (iii) by being redirected through streamlines toward flowers
(Ackerman, 1997b).
Ancestral spherical pollen, on the other hand,
could only pollinate via direct interception due to the
limited axial force exerted on the spherical-shaped
body. Canopy flow conditions at the scale of leaves
and flowers were laminar, which indicates that observations would be similar to those in the laboratory
flow chamber (Ackerman and Okubo, 1993; Ackerman, 2002). Higher recovery of filamentous pollen
models compared to spherical ones also supports the
biomechanical model (Ackerman, 2002). Moreover,
field observations of pollination in Amphibolis were
consistent with the predictions from Z. marina (Verduin et al., 1996), as were results from Posidonia
(Smith and Walker, 2002). It is important to note that
seagrasses can maintain relatively high outcrossing
rates through hydrophily (Ruckleshaus, 1995; Waycott and Sampson, 1997; Reusch, 2000; Waycott et
al., Chapter 2). This is especially true for populations found under conditions that promote pollen
dispersal, such as exposed bays (e.g. Waycott and
Sampson, 1997). It is reasonable to conclude that
the strong convergence of filiform pollen morphologies in seagrasses indicates a similar convergence
in pollination mechanisms linked to fluid dynamics. Further research from a diversity of seagrass
taxa will be needed to determine the validity of this
statement.
Seed dispersal phenomena in seagrasses are somewhat analogous to pollination mechanisms in that
they are poorly recognized but have become better
understood in recent years (Van der Pijl, 1972; Orth
et al., Chapter 5). This is in part due to the clonal nature of seagrasses in which rhizomatous growth, in
addition to anchoring and binding of sediments, was
viewed as the principal mechanism of population
growth. However, the occurrence of annual populations (e.g. Keddy and Patriquin, 1978), colonization
of new areas (e.g. Turner, 1983), and recovery from
small and large-scale disturbances (e.g. Rasmussen,
1977; Inglis, 2000) via seeds has led to the realization that seed dispersal is important in this group.
There is considerable diversity in seed biology
and ecology related to the polyphyletic nature of
seagrasses (e.g. Les et al., 1997) and their evolutionary innovations. For example, 7 of the 12/13
genera of seagrasses have dormant seeds, with geocarpy (releasing seeds under the sediments) occurring in Halodule, Cymodocea, and Halophila (Inglis,
2000). Geocarpy appears to facilitate recovery from
disturbances in Halodule, where dispersal involves
near-bed saltational movements analogous to sediment transport, and high densities of seeds accumulate in dugong feeding depressions (Inglis, 2000).
Other species, which have reproductive organs elevated above the seafloor, such as Z. marina, have
small, negatively buoyant seeds that sink in still water, and that likely move on the order of 1–10 m
horizontally in the water column, depending on the
canopy flow (see Okubo et al., 2002). Sometimes
seeds (5–13% of seeds in Long Island Sound) are
released with a bubble of gas (presumably from the
lacunar spaces in the infructescence), which can extend dispersal distances on the order of 10–100 m
(Churchill et al., 1985). Longer distance dispersal
E. W. Koch, J. D. Ackerman, J. Verduin and M. van Keulen
convergently–multiple times in the Najadales and
functionally in the Hydrocharitales (Pettitt, 1984;
Ackerman, 1995, 2000). Pettitt (1984) reviewed the
research pertaining to seagrass pollination biology,
but that review was limited taxonomically with respect to pollen transport and capture mechanisms.
The situation has improved, but remains largely
limited to the mechanistic studies of submarine pollination in the north temperate species, Zostera marina (Ackerman, 1986, 1993, 1995, 1997a,b, and
2002). Fortunately, recent progress has been made
with respect to other species including Amphibolis
antarctica (Verduin et al., 1996), Posidonia australis
and Posidonia sinuosa (Smith and Walker, 2002).
The mechanics of pollination in Z. marina were
studied in a laboratory flow chamber using stroboscopic photography and in the field using physical
models of pollen and adhesive surfaces deployed in
the canopy (Ackerman, 1997a,b, 2002). Pollination
was found to occur under laminar and relatively viscous conditions at the scale of flowers (i.e. low Re;
see Niklas, 1992; Vogel, 1994), and was affected
by the emergence of female flowers from within the
inflorescence. The emergence of flowers (and other
reproductive organs in subsequent phenological processes) led to an increase in the fluid shear stress (τ )
in the local flow (Ackerman, 1997a), which caused
the filamentous pollen (2.7 mm × 7.5 µm diameter)
to rotate and cross streamlines toward female flowers (Ackerman, 1997b). The axial force responsible
for the pollen motion varied directly with the length
and aspect ratio of the pollen (Forgacs and Mason,
1958). Consequently, filamentous pollen need only
be close to female flowers to pollinate by: (i) direct interception on stigmas; (ii) rotation within one
half a pollen length of stigmas; and (iii) by being redirected through streamlines toward flowers
(Ackerman, 1997b).
Ancestral spherical pollen, on the other hand,
could only pollinate via direct interception due to the
limited axial force exerted on the spherical-shaped
body. Canopy flow conditions at the scale of leaves
and flowers were laminar, which indicates that observations would be similar to those in the laboratory
flow chamber (Ackerman and Okubo, 1993; Ackerman, 2002). Higher recovery of filamentous pollen
models compared to spherical ones also supports the
biomechanical model (Ackerman, 2002). Moreover,
field observations of pollination in Amphibolis were
consistent with the predictions from Z. marina (Verduin et al., 1996), as were results from Posidonia
(Smith and Walker, 2002). It is important to note that
seagrasses can maintain relatively high outcrossing
rates through hydrophily (Ruckleshaus, 1995; Waycott and Sampson, 1997; Reusch, 2000; Waycott et
al., Chapter 2). This is especially true for populations found under conditions that promote pollen
dispersal, such as exposed bays (e.g. Waycott and
Sampson, 1997). It is reasonable to conclude that
the strong convergence of filiform pollen morphologies in seagrasses indicates a similar convergence
in pollination mechanisms linked to fluid dynamics. Further research from a diversity of seagrass
taxa will be needed to determine the validity of this
statement.
Seed dispersal phenomena in seagrasses are somewhat analogous to pollination mechanisms in that
they are poorly recognized but have become better
understood in recent years (Van der Pijl, 1972; Orth
et al., Chapter 5). This is in part due to the clonal nature of seagrasses in which rhizomatous growth, in
addition to anchoring and binding of sediments, was
viewed as the principal mechanism of population
growth. However, the occurrence of annual populations (e.g. Keddy and Patriquin, 1978), colonization
of new areas (e.g. Turner, 1983), and recovery from
small and large-scale disturbances (e.g. Rasmussen,
1977; Inglis, 2000) via seeds has led to the realization that seed dispersal is important in this group.
There is considerable diversity in seed biology
and ecology related to the polyphyletic nature of
seagrasses (e.g. Les et al., 1997) and their evolutionary innovations. For example, 7 of the 12/13
genera of seagrasses have dormant seeds, with geocarpy (releasing seeds under the sediments) occurring in Halodule, Cymodocea, and Halophila (Inglis,
2000). Geocarpy appears to facilitate recovery from
disturbances in Halodule, where dispersal involves
near-bed saltational movements analogous to sediment transport, and high densities of seeds accumulate in dugong feeding depressions (Inglis, 2000).
Other species, which have reproductive organs elevated above the seafloor, such as Z. marina, have
small, negatively buoyant seeds that sink in still water, and that likely move on the order of 1–10 m
horizontally in the water column, depending on the
canopy flow (see Okubo et al., 2002). Sometimes
seeds (5–13% of seeds in Long Island Sound) are
released with a bubble of gas (presumably from the
lacunar spaces in the infructescence), which can extend dispersal distances on the order of 10–100 m
(Churchill et al., 1985). Longer distance dispersal