Chapter 4 Seagrass Sexual Reproduction
101
contacts the slender stigmas where fertilization takes
place (Black, 1913). It is also possible for some inflorescences and anthers to detach and float to the surface where they release pollen, which disperse downward through the water column (Ducker et al., 1978;
McConchie and Knox, 1989b). More recent direct
observations of pollination in the field indicates that
male plants mature earlier than female plants with
anthers shedding pollen while still attached to the
plant (Verduin, 1996; Verduin et al., 1996). Pollen
remained in the water column (it did not float to the
surface) and pollination was assumed to be threedimensional in which interfloral distances were on
the order of 20–500 cm (Verduin et al., 1996) leading
to pollination success of ∼67% based on scanning
electron microscope examination of pollen on stigmas (Verduin, 1996).
E. Cymodocea
Cymodocea nodosa (Ucria) Ascherson was found to
flower in equal sex ratios, but the density of flowers
was clumped in distribution on small-spatial scales
and tended to increase in deeper locations away from
the coast (Buia and Mazella, 1991). No information
is available regarding pollen transport and capture,
but presumably it involves within and below canopy
flow and interactions of filiform pollen with the local
flow environment around female shoots and stigmas
(e.g. Okubo et al., 2002).
F. Halodule
Immature anthers are white and exist within the foliage leaf; they change to green (red and cream have
also been reported) on maturation with the growth
of the filament that extends the anther above the leaf
axil (Johnson and Williams, 1982). No information
is available regarding pollen transport and capture,
but presumably it involves within and below canopy
flow and interactions of filiform pollen with the local
flow environment around female shoots and stigmas
(e.g. Okubo et al., 2002).
G. Syringodium
Immature anthers are green and extend above the
floral bracts on a filament once they mature (Johnson and Williams, 1982). No information is available
regarding pollen transport and capture, but presumably it involves within the canopy flow through the
interactions of filiform pollen with the local flow environment around female inflorescences (e.g. Okubo
et al., 2002). Laboratory studies involving flow visualization of female inflorescences of Syringodium
filiforme K¨ utzing in a flow chamber (Ackerman,
unpublished) indicate that bracts redirect flow and
generate zones of recirculation immediately downstream at moderate velocities (Fig. 4A). In windpollinated plants, these recirculation zones provide
opportunities for pollen to be captured on the downstream side of stigmas if they have not intercepted
or impacted on the upstream side of stigmas (Niklas,
1992). Whether this occurs in Syringodium and
other seagrasses remains to be determined, especially since the response of filiform particles in
downstream recirculation zones is not known.
H. Thalassodendron
There was an equal sex ratio in Thalassodendron
pachyrhizum den Hartog, but only ∼10% of shoots
in the population possessed inflorescences at any
time, and further examination indicated that inflorescences were produced every 3–4 years (Kuo and
Kirkman, 1987). The male inflorescences of Thalassodendron ciliatum (Forssk˚ al) are bright red and
consist of two flowers that mature at different times
and extend on a filament at maturity (Ducker et al.,
1978). Flowering in submerged T. ciliatum beds was
also variable in space and time leading to variability
in the occurrence of seedlings (Kamermans et al.,
2001). Pollen dehiscence occurs on the plant or immediately after anthers detach (Kuo and Kirkman,
1987), and presumably pollen transport and capture
involves within and below canopy flow and the interaction of filiform pollen with the local flow environment around female shoots and extended stigmas
(e.g. Okubo et al., 2002).
I. Posidonia
There can be significant differences among sympatric Posidonia species in terms of the positioning
of flowers (e.g. at the top of the canopy vs. within
the canopy), canopy structure (e.g. random vs. rowlike arrangements) and the phenology (e.g. length
and duration of flowering), which probably ensure
outcrossing and limit self pollination (Smith and
101
contacts the slender stigmas where fertilization takes
place (Black, 1913). It is also possible for some inflorescences and anthers to detach and float to the surface where they release pollen, which disperse downward through the water column (Ducker et al., 1978;
McConchie and Knox, 1989b). More recent direct
observations of pollination in the field indicates that
male plants mature earlier than female plants with
anthers shedding pollen while still attached to the
plant (Verduin, 1996; Verduin et al., 1996). Pollen
remained in the water column (it did not float to the
surface) and pollination was assumed to be threedimensional in which interfloral distances were on
the order of 20–500 cm (Verduin et al., 1996) leading
to pollination success of ∼67% based on scanning
electron microscope examination of pollen on stigmas (Verduin, 1996).
E. Cymodocea
Cymodocea nodosa (Ucria) Ascherson was found to
flower in equal sex ratios, but the density of flowers
was clumped in distribution on small-spatial scales
and tended to increase in deeper locations away from
the coast (Buia and Mazella, 1991). No information
is available regarding pollen transport and capture,
but presumably it involves within and below canopy
flow and interactions of filiform pollen with the local
flow environment around female shoots and stigmas
(e.g. Okubo et al., 2002).
F. Halodule
Immature anthers are white and exist within the foliage leaf; they change to green (red and cream have
also been reported) on maturation with the growth
of the filament that extends the anther above the leaf
axil (Johnson and Williams, 1982). No information
is available regarding pollen transport and capture,
but presumably it involves within and below canopy
flow and interactions of filiform pollen with the local
flow environment around female shoots and stigmas
(e.g. Okubo et al., 2002).
G. Syringodium
Immature anthers are green and extend above the
floral bracts on a filament once they mature (Johnson and Williams, 1982). No information is available
regarding pollen transport and capture, but presumably it involves within the canopy flow through the
interactions of filiform pollen with the local flow environment around female inflorescences (e.g. Okubo
et al., 2002). Laboratory studies involving flow visualization of female inflorescences of Syringodium
filiforme K¨ utzing in a flow chamber (Ackerman,
unpublished) indicate that bracts redirect flow and
generate zones of recirculation immediately downstream at moderate velocities (Fig. 4A). In windpollinated plants, these recirculation zones provide
opportunities for pollen to be captured on the downstream side of stigmas if they have not intercepted
or impacted on the upstream side of stigmas (Niklas,
1992). Whether this occurs in Syringodium and
other seagrasses remains to be determined, especially since the response of filiform particles in
downstream recirculation zones is not known.
H. Thalassodendron
There was an equal sex ratio in Thalassodendron
pachyrhizum den Hartog, but only ∼10% of shoots
in the population possessed inflorescences at any
time, and further examination indicated that inflorescences were produced every 3–4 years (Kuo and
Kirkman, 1987). The male inflorescences of Thalassodendron ciliatum (Forssk˚ al) are bright red and
consist of two flowers that mature at different times
and extend on a filament at maturity (Ducker et al.,
1978). Flowering in submerged T. ciliatum beds was
also variable in space and time leading to variability
in the occurrence of seedlings (Kamermans et al.,
2001). Pollen dehiscence occurs on the plant or immediately after anthers detach (Kuo and Kirkman,
1987), and presumably pollen transport and capture
involves within and below canopy flow and the interaction of filiform pollen with the local flow environment around female shoots and extended stigmas
(e.g. Okubo et al., 2002).
I. Posidonia
There can be significant differences among sympatric Posidonia species in terms of the positioning
of flowers (e.g. at the top of the canopy vs. within
the canopy), canopy structure (e.g. random vs. rowlike arrangements) and the phenology (e.g. length
and duration of flowering), which probably ensure
outcrossing and limit self pollination (Smith and