104
Josef Daniel Ackerman
As in other aquatic plants the dispersal of seagrass
diaspores (fruit, seed) is not well described (Van der
Pijl, 1972; Hay et al., 2000), even though the Greek
Botanist Theophrastus noted the occurrence of the
“sea oak”, which was the free-floating fruit of Posidonia oceanica (L.) Delile (Cavolini, 1792 translated by K¨ onig, 1805). Fortunately, the recognition
of the importance and relevance of seagrass diaspore
dispersal phenomena has received more attention of
late (Orth et al., Chapter 5). There is a great deal of
diversity in the morphology and dispersal ecology
of seagrass diaspores although a number of patterns
are evident (Table 2). For example, seven seagrass
genera have dormant seeds, and Halodule, Cymodocea, and Halophila release their seeds at or under
the sediment (geocarpy), which may facilitate recovery from disturbances in Halodule (Table 2; Inglis,
2000). At least two genera (Amphibolis and Thalassodendron) have viviparous seedlings that germinate on the female plant and disperse following
their detachment at a larger size, much like mangroves (den Hartog, 1970; Fig. 15, Chapter 3). Interestingly, Amphibolis has a comb-like orientation
and grappling device on the seedling that apparently
aids in seedling establishment (den Hartog, 1970).
Vivipary and vegetative propagation of reproductive
shoots has also been noted in other genera (Zostera
and Heterozostera), but this appears to be an occasional/rare event that may serve in the dispersal
phase of the detached reproductive material (e.g.
Addy, 1947; Cambridge et al., 1983). A number of
genera (Phyllospadix and Zostera) release relatively
small, negatively buoyant diaspores that would likely
disperse ∼1–10 (<100) m horizontally under normal
conditions (Okubo et al., 2002). It is interesting to
note that the bifid, barbed fruits of Phyllospadix require regions with branched turf forming algae (with
cylindrical thalli) onto which they recruit (Turner,
1983). However, as indicated above for Z. marina,
there are opportunities for detached fruiting material
to disperse over great distances (see below) and for
seeds to float on bubbles, which can extend dispersal distances to ∼10–100 m (Churchill et al., 1985).
Floatation of buoyant diaspores (Enhalus, Thalassia, and Posidonia) and detached infructescences
and reproductive material extend dispersal distance
to ∼100 m–10 km (e.g. Harwell and Orth, 2002;
Lacap et al., 2002; Orth et al., Chapter 5). It has
long been speculated that animal mediated dispersal
in the guts of birds, sea turtles, and fish could lead
to very large dispersal distances to ∼100 m–1000
km, especially in migrating birds (e.g. Baldwin and
Lovvorn, 1994).
IX. Evidence for Pollination Success
There are a number of ways in which to ascertain the level of pollination success in seagrasses including morphological analysis, phenological observation, field observation of fruiting
and seedling establishment, and population genetics inferred from allozyme variation and molecular
analysis. The determination of the pollen to ovule
ratios has been used to estimate the relative contribution of pollen to ovules and thus establish the
efficiency of pollination syndromes (e.g. Faegri and
van der Pijl, 1979). Unfortunately, this approach led
to the incorrect conclusion that wind pollination is
a wasteful process because of high pollen–ovule ratios (i.e. >10
6 :1). These high rates are likely a function of metabolic costs and/or intermale competition and consequently can be equally high in insect
pollinated plants (see Ackerman, 2000). Regardless,
this approach has been applied to a limited number
of seagrasses and freshwater plants. Pollen–ovule
ratios in seagrasses range from 10:1 in Enhalus,
to 10
4 :1 in Zostera, Phyllospadix, and Amphibolis,
and 10
5 :1 in Posidonia (Kausik, 1941; Ackerman,
1993; Williams, 1995; Verduin et al., 2000; Smith
and Walker, 2002). Whereas the variability appears
rather large, it should be recalled that Enhalus is
an obligate ephydrophilous plant, and wide variation has also been noted in hydrophilous freshwater
plants (e.g. 10
2 :1 to 10
4 :1; Philbrick and Anderson,
1987; Huang et al., 2001). Given this variation and
the limitations of this approach, it is difficult to draw
any clear conclusions about the pollen–ovule ratios
in seagrasses.
Field observations of phenology, proportion of
reproductive activity, seedling recruitment and outcrossing (Waycott et al., Chapter 2) provide necessary information to support the importance of successful sexual reproduction. This is especially true
in the aforementioned annual populations that must
recruit each year from seed banks due to excessive heat or ice scouring (e.g. Keddy and Patriquin,
1978; Meling-Lopez and Ibarra-Obando, 1999). The
number and diversity of local studies of phenology of individual species are too numerous to include here, but Roll´ on et al. (2003) provide an excellent example in the case of Enhalus acoroides. The
Josef Daniel Ackerman
As in other aquatic plants the dispersal of seagrass
diaspores (fruit, seed) is not well described (Van der
Pijl, 1972; Hay et al., 2000), even though the Greek
Botanist Theophrastus noted the occurrence of the
“sea oak”, which was the free-floating fruit of Posidonia oceanica (L.) Delile (Cavolini, 1792 translated by K¨ onig, 1805). Fortunately, the recognition
of the importance and relevance of seagrass diaspore
dispersal phenomena has received more attention of
late (Orth et al., Chapter 5). There is a great deal of
diversity in the morphology and dispersal ecology
of seagrass diaspores although a number of patterns
are evident (Table 2). For example, seven seagrass
genera have dormant seeds, and Halodule, Cymodocea, and Halophila release their seeds at or under
the sediment (geocarpy), which may facilitate recovery from disturbances in Halodule (Table 2; Inglis,
2000). At least two genera (Amphibolis and Thalassodendron) have viviparous seedlings that germinate on the female plant and disperse following
their detachment at a larger size, much like mangroves (den Hartog, 1970; Fig. 15, Chapter 3). Interestingly, Amphibolis has a comb-like orientation
and grappling device on the seedling that apparently
aids in seedling establishment (den Hartog, 1970).
Vivipary and vegetative propagation of reproductive
shoots has also been noted in other genera (Zostera
and Heterozostera), but this appears to be an occasional/rare event that may serve in the dispersal
phase of the detached reproductive material (e.g.
Addy, 1947; Cambridge et al., 1983). A number of
genera (Phyllospadix and Zostera) release relatively
small, negatively buoyant diaspores that would likely
disperse ∼1–10 (<100) m horizontally under normal
conditions (Okubo et al., 2002). It is interesting to
note that the bifid, barbed fruits of Phyllospadix require regions with branched turf forming algae (with
cylindrical thalli) onto which they recruit (Turner,
1983). However, as indicated above for Z. marina,
there are opportunities for detached fruiting material
to disperse over great distances (see below) and for
seeds to float on bubbles, which can extend dispersal distances to ∼10–100 m (Churchill et al., 1985).
Floatation of buoyant diaspores (Enhalus, Thalassia, and Posidonia) and detached infructescences
and reproductive material extend dispersal distance
to ∼100 m–10 km (e.g. Harwell and Orth, 2002;
Lacap et al., 2002; Orth et al., Chapter 5). It has
long been speculated that animal mediated dispersal
in the guts of birds, sea turtles, and fish could lead
to very large dispersal distances to ∼100 m–1000
km, especially in migrating birds (e.g. Baldwin and
Lovvorn, 1994).
IX. Evidence for Pollination Success
There are a number of ways in which to ascertain the level of pollination success in seagrasses including morphological analysis, phenological observation, field observation of fruiting
and seedling establishment, and population genetics inferred from allozyme variation and molecular
analysis. The determination of the pollen to ovule
ratios has been used to estimate the relative contribution of pollen to ovules and thus establish the
efficiency of pollination syndromes (e.g. Faegri and
van der Pijl, 1979). Unfortunately, this approach led
to the incorrect conclusion that wind pollination is
a wasteful process because of high pollen–ovule ratios (i.e. >10
6 :1). These high rates are likely a function of metabolic costs and/or intermale competition and consequently can be equally high in insect
pollinated plants (see Ackerman, 2000). Regardless,
this approach has been applied to a limited number
of seagrasses and freshwater plants. Pollen–ovule
ratios in seagrasses range from 10:1 in Enhalus,
to 10
4 :1 in Zostera, Phyllospadix, and Amphibolis,
and 10
5 :1 in Posidonia (Kausik, 1941; Ackerman,
1993; Williams, 1995; Verduin et al., 2000; Smith
and Walker, 2002). Whereas the variability appears
rather large, it should be recalled that Enhalus is
an obligate ephydrophilous plant, and wide variation has also been noted in hydrophilous freshwater
plants (e.g. 10
2 :1 to 10
4 :1; Philbrick and Anderson,
1987; Huang et al., 2001). Given this variation and
the limitations of this approach, it is difficult to draw
any clear conclusions about the pollen–ovule ratios
in seagrasses.
Field observations of phenology, proportion of
reproductive activity, seedling recruitment and outcrossing (Waycott et al., Chapter 2) provide necessary information to support the importance of successful sexual reproduction. This is especially true
in the aforementioned annual populations that must
recruit each year from seed banks due to excessive heat or ice scouring (e.g. Keddy and Patriquin,
1978; Meling-Lopez and Ibarra-Obando, 1999). The
number and diversity of local studies of phenology of individual species are too numerous to include here, but Roll´ on et al. (2003) provide an excellent example in the case of Enhalus acoroides. The