98
Josef Daniel Ackerman
of access to sites, and societal factors such as interest, and number of scientific personnel has also
influenced the understanding of local and regional
conditions (Walker et al., 2001). Consequently, there
are geographic regions, such as the North Atlantic,
Mediterranean, and parts of the North Pacific coasts,
where phenological patterns are well described, but
the majority of coasts, including the regions of
high diversity, are less well known (Walker et al.,
2001).
Coincident with large-scale phenomena is the spatial and temporal variation in flowering phenology
that exists on local scales. This is most evident in taxa
in which flowering is periodic, rare and/or in which
reproductive material has not been described (e.g.
Cymodocea angustata Osterfeld, Halodule beaudettei (den Hartog) den Hartog, Halodule bermudensis den Hartog, Halodule emarginata den Hartog,
Halophila johnsonii Eiseman; Kuo and den Hartog,
2001).
Notwithstanding these gaps in understanding,
there are taxa in which the phenological patterns are
well known. For example, the phenology of Zostera
marina L has been described (de Cock, 1980), and
reproductive shoots can represent from 0 to 100%
of a given population with variation existing due to
photoperiod and water temperature (Phillips et al.,
1983; Olesen, 1999). Walker et al. (2001) provide a
detailed review of this topic from a geographic perspective, which reveals that most genera are reproductive in the spring and summer (i.e. increase in day
length and warmer water temperatures) (e.g. Thalassia, Halophila, Syringodium, Halodule, Cymodocea, Thalassodendron, Phyllospadix, Nanozostera,
Zostera), a number flower in the fall and winter (i.e.
decrease in day length and cooler water temperatures) (e.g. Amphibolis, Posidonia, Heterozostera),
and one genus is reproductive throughout the year
(Enhalus), although there are exceptions especially
for tropical species.
VII. Pollination
Whereas aspects of pollination in seagrasses have
been addressed by a number of authors (Sculthorpe,
1967; den Hartog, 1970; Tomlinson, 1982; Pettitt,
1984; Kuo and McComb, 1989; McConchie and
Knox, 1989a; Walker et al., 2001; Okubo et al.,
2002), this treatment will follow a functional approach and focus on pollination mechanisms, following from Ackerman (1995, 2000). As indicated
above, seagrasses are among a limited number of
taxa that pollinate underwater using water currents,
the exception being Enhalus, which as discussed previously is ephydrophilous (see below for detailed
description). Submarine pollination is restricted to
the coastal marine environment, where there is more
energetic and regular water motion (e.g. tidal and
wind-generated currents) than in freshwater systems, where movements vary greatly and are a function of basin morphometry (Nixon, 1988; Schindler,
1991; Kalff, 2002). There can be little doubt that
some shallow intertidal seagrass populations may
be exposed to air during low tides, however, subtidal populations are, by definition, never exposed
(see den Hartog, 1970; Ackerman, 1986). This may
be less evident during spring tides when tidal exchanges are the largest, however, as indicated above,
latitudinal and depth gradients in flowering phenology (Phillips et al., 1983) limit surface pollination
to local situations. It is also important to note that
pollen released at the surface will be maintained on
the surface unless sufficient external force is expended to overcome the surface tension (see Ackerman, 1997b). Cox and co-workers (reviewed in
Cox, 1988) have used a mathematical model (random search theory) to assert that selection pressures
for surface pollination have led to the evolution of
filiform pollen in seagrasses and arrays of pollen
in freshwater plants (the so-called elongate search
vehicle). Whereas the mathematical models are correct, they are not applicable to seagrass pollination
in a mathematical or biological sense because wind
and water currents have directional components (i.e.
particle trajectories are not recurrent), pollen rotate
in the boundary layers around flowers (i.e. not perpendicular to the pathline; see Fig. 4 and 5, below),
and filiform pollen is found only in seagrasses and
not in related freshwater plants (see treatments in
McConchie and Knox, 1989a; Ackerman, 1995).
Notwithstanding the fact that surface pollination
does occur in some situations, it is clear that filiform pollen has evolved for pollination underwater under dynamic flow conditions due to mechanical advantages of increased pollen length. This advantage is revealed when filiform shapes are compared to spherical shapes under flowing conditions
where a particle’s motion is due to fluid translation, fluid rotation, and fluid deformation (Fig. 5;
Visser, 2001). Velocity gradients (boundary layers) exist around flowers and inflorescences under
Josef Daniel Ackerman
of access to sites, and societal factors such as interest, and number of scientific personnel has also
influenced the understanding of local and regional
conditions (Walker et al., 2001). Consequently, there
are geographic regions, such as the North Atlantic,
Mediterranean, and parts of the North Pacific coasts,
where phenological patterns are well described, but
the majority of coasts, including the regions of
high diversity, are less well known (Walker et al.,
2001).
Coincident with large-scale phenomena is the spatial and temporal variation in flowering phenology
that exists on local scales. This is most evident in taxa
in which flowering is periodic, rare and/or in which
reproductive material has not been described (e.g.
Cymodocea angustata Osterfeld, Halodule beaudettei (den Hartog) den Hartog, Halodule bermudensis den Hartog, Halodule emarginata den Hartog,
Halophila johnsonii Eiseman; Kuo and den Hartog,
2001).
Notwithstanding these gaps in understanding,
there are taxa in which the phenological patterns are
well known. For example, the phenology of Zostera
marina L has been described (de Cock, 1980), and
reproductive shoots can represent from 0 to 100%
of a given population with variation existing due to
photoperiod and water temperature (Phillips et al.,
1983; Olesen, 1999). Walker et al. (2001) provide a
detailed review of this topic from a geographic perspective, which reveals that most genera are reproductive in the spring and summer (i.e. increase in day
length and warmer water temperatures) (e.g. Thalassia, Halophila, Syringodium, Halodule, Cymodocea, Thalassodendron, Phyllospadix, Nanozostera,
Zostera), a number flower in the fall and winter (i.e.
decrease in day length and cooler water temperatures) (e.g. Amphibolis, Posidonia, Heterozostera),
and one genus is reproductive throughout the year
(Enhalus), although there are exceptions especially
for tropical species.
VII. Pollination
Whereas aspects of pollination in seagrasses have
been addressed by a number of authors (Sculthorpe,
1967; den Hartog, 1970; Tomlinson, 1982; Pettitt,
1984; Kuo and McComb, 1989; McConchie and
Knox, 1989a; Walker et al., 2001; Okubo et al.,
2002), this treatment will follow a functional approach and focus on pollination mechanisms, following from Ackerman (1995, 2000). As indicated
above, seagrasses are among a limited number of
taxa that pollinate underwater using water currents,
the exception being Enhalus, which as discussed previously is ephydrophilous (see below for detailed
description). Submarine pollination is restricted to
the coastal marine environment, where there is more
energetic and regular water motion (e.g. tidal and
wind-generated currents) than in freshwater systems, where movements vary greatly and are a function of basin morphometry (Nixon, 1988; Schindler,
1991; Kalff, 2002). There can be little doubt that
some shallow intertidal seagrass populations may
be exposed to air during low tides, however, subtidal populations are, by definition, never exposed
(see den Hartog, 1970; Ackerman, 1986). This may
be less evident during spring tides when tidal exchanges are the largest, however, as indicated above,
latitudinal and depth gradients in flowering phenology (Phillips et al., 1983) limit surface pollination
to local situations. It is also important to note that
pollen released at the surface will be maintained on
the surface unless sufficient external force is expended to overcome the surface tension (see Ackerman, 1997b). Cox and co-workers (reviewed in
Cox, 1988) have used a mathematical model (random search theory) to assert that selection pressures
for surface pollination have led to the evolution of
filiform pollen in seagrasses and arrays of pollen
in freshwater plants (the so-called elongate search
vehicle). Whereas the mathematical models are correct, they are not applicable to seagrass pollination
in a mathematical or biological sense because wind
and water currents have directional components (i.e.
particle trajectories are not recurrent), pollen rotate
in the boundary layers around flowers (i.e. not perpendicular to the pathline; see Fig. 4 and 5, below),
and filiform pollen is found only in seagrasses and
not in related freshwater plants (see treatments in
McConchie and Knox, 1989a; Ackerman, 1995).
Notwithstanding the fact that surface pollination
does occur in some situations, it is clear that filiform pollen has evolved for pollination underwater under dynamic flow conditions due to mechanical advantages of increased pollen length. This advantage is revealed when filiform shapes are compared to spherical shapes under flowing conditions
where a particle’s motion is due to fluid translation, fluid rotation, and fluid deformation (Fig. 5;
Visser, 2001). Velocity gradients (boundary layers) exist around flowers and inflorescences under