Chapter 4
Sexual Reproduction of Seagrasses:
Pollination in the Marine Context
Josef Daniel Ackerman
Faculty of Environmental Sciences and Department of Integrative Biology,
University of Guelph, Guelph, ON, Canada N1G 2W1
I. Introduction
A discussion of sexual reproduction in seagrasses
should begin with an examination of their origin
and include a comparison with their closest freshwater relatives (i.e. the comparative method). There
were no marine plants until angiosperms colonized
marine coastal waters sometime in the Cretaceous,
which is the earliest date of seagrass fossils (i.e.
>100 × 10
6 years ago; den Hartog, 1970; Larkum
and den Hartog, 1989). Prior to that time, bacteria
and protists (i.e. algae) were the only marine photosynthetic organisms, as embryophytic plants had
evolved earlier as a terrestrial group (Niklas, 1997;
Dawes, 1998). The anatomical, morphological, and
molecular evidence indicates that seagrasses evolved
from freshwater ancestors (Arber, 1920; Sculthorpe,
1967; den Hartog, 1970; Tomlinson, 1982; Les et al.,
1997), which are also polyphyletic in origin having evolved multiple times (Cook, 1996a, 1998).
In this context, cetaceans are the zoological equivalent of seagrasses, and the analogy continues in
that both groups are relatively depauperate systematically compared to their terrestrial functional analogues. Specifically, seagrasses represent ∼0.5% of
the total marine flora and 0.02% of all angiosperms
(Dawes, 1998), with 50–60 species in 13 genera (see
below) found in the familiar aquatic plant families
Hydrocharitaceae, Cymodoceaceae, Posidoniaceae,
and Zosteraceae within the Alismatales order of
the monocotoledons (den Hartog, 1970; Tomlinson,
1982; Cook, 1996a; Dawes, 1998; den Hartog and
Kuo, Chapter 1; Waycott et al., Chapter 2). Molecular phylogenies have identified at least three clades
Author for correspondence, email: ackerman@uoguelph.ca
within the seagrasses depending on how one defines
the group (Les et al., 1997), which strengthens the
argument that these plants are polyphyletic and thus
represent a functional group (i.e. a grade of evolution).
Given the polyphyletic origin of the seagrasses,
we would expect significant differences in their reproductive biology. Concomitantly, the possession
of similar apomorphies among the clades would be
evidence of convergence related to the fact that similar evolutionary pressures were faced during the origin and continued evolution of these clades. Arber
(1920) perceived these pressures as: (1) toleration
towards a saline medium; (2) the power of vegetating while wholly submerged; (3) the knack of
developing a sufficiency of anchoring roots to withstand the action of waves and tides; and (4) the capacity for hydrophilous pollination; and den Hartog
(1970) included a fifth: (5) the need to disperse in
the marine context. Indeed, there are a large number of morphological and physiological characters
in seagrasses that support these views (Table 1).
The shared occurrence of many of these characters
in freshwater plants speaks to the freshwater origin of seagrasses, whereas the uncommon and/or
unique characters speak to their evolutionary innovation. Principle among these innovations in reproductive characters are submerged flowers, filamentous pollen, and ssubmarine pollination (Table 1;
Ackerman, 1995, 2000), which are dealt with in detail below. It has been argued that the “capacity for
hydrophilous pollination” in the marine context has
limited seagrass biodiversity, but it is more likely that
the lack of allopatric isolating mechanisms in coastal
environments, especially as compared to freshwater plants, is the cause, given the convergence in
89–109.
A. W. D. Larkum et al. (eds.), Seagrasses: Biology, Ecology and Conservation, pp.
c
2006 Springer. Printed in the Netherlands.
Sexual Reproduction of Seagrasses:
Pollination in the Marine Context
Josef Daniel Ackerman
Faculty of Environmental Sciences and Department of Integrative Biology,
University of Guelph, Guelph, ON, Canada N1G 2W1
I. Introduction
A discussion of sexual reproduction in seagrasses
should begin with an examination of their origin
and include a comparison with their closest freshwater relatives (i.e. the comparative method). There
were no marine plants until angiosperms colonized
marine coastal waters sometime in the Cretaceous,
which is the earliest date of seagrass fossils (i.e.
>100 × 10
6 years ago; den Hartog, 1970; Larkum
and den Hartog, 1989). Prior to that time, bacteria
and protists (i.e. algae) were the only marine photosynthetic organisms, as embryophytic plants had
evolved earlier as a terrestrial group (Niklas, 1997;
Dawes, 1998). The anatomical, morphological, and
molecular evidence indicates that seagrasses evolved
from freshwater ancestors (Arber, 1920; Sculthorpe,
1967; den Hartog, 1970; Tomlinson, 1982; Les et al.,
1997), which are also polyphyletic in origin having evolved multiple times (Cook, 1996a, 1998).
In this context, cetaceans are the zoological equivalent of seagrasses, and the analogy continues in
that both groups are relatively depauperate systematically compared to their terrestrial functional analogues. Specifically, seagrasses represent ∼0.5% of
the total marine flora and 0.02% of all angiosperms
(Dawes, 1998), with 50–60 species in 13 genera (see
below) found in the familiar aquatic plant families
Hydrocharitaceae, Cymodoceaceae, Posidoniaceae,
and Zosteraceae within the Alismatales order of
the monocotoledons (den Hartog, 1970; Tomlinson,
1982; Cook, 1996a; Dawes, 1998; den Hartog and
Kuo, Chapter 1; Waycott et al., Chapter 2). Molecular phylogenies have identified at least three clades
Author for correspondence, email: ackerman@uoguelph.ca
within the seagrasses depending on how one defines
the group (Les et al., 1997), which strengthens the
argument that these plants are polyphyletic and thus
represent a functional group (i.e. a grade of evolution).
Given the polyphyletic origin of the seagrasses,
we would expect significant differences in their reproductive biology. Concomitantly, the possession
of similar apomorphies among the clades would be
evidence of convergence related to the fact that similar evolutionary pressures were faced during the origin and continued evolution of these clades. Arber
(1920) perceived these pressures as: (1) toleration
towards a saline medium; (2) the power of vegetating while wholly submerged; (3) the knack of
developing a sufficiency of anchoring roots to withstand the action of waves and tides; and (4) the capacity for hydrophilous pollination; and den Hartog
(1970) included a fifth: (5) the need to disperse in
the marine context. Indeed, there are a large number of morphological and physiological characters
in seagrasses that support these views (Table 1).
The shared occurrence of many of these characters
in freshwater plants speaks to the freshwater origin of seagrasses, whereas the uncommon and/or
unique characters speak to their evolutionary innovation. Principle among these innovations in reproductive characters are submerged flowers, filamentous pollen, and ssubmarine pollination (Table 1;
Ackerman, 1995, 2000), which are dealt with in detail below. It has been argued that the “capacity for
hydrophilous pollination” in the marine context has
limited seagrass biodiversity, but it is more likely that
the lack of allopatric isolating mechanisms in coastal
environments, especially as compared to freshwater plants, is the cause, given the convergence in
89–109.
A. W. D. Larkum et al. (eds.), Seagrasses: Biology, Ecology and Conservation, pp.
c
2006 Springer. Printed in the Netherlands.