Chapter 5 Seagrass Seeds and Dispersal Strategies
121
one species, Posidonia coriacea, increases the fall
velocity (Orth, 1999). Once settled onto the sediment surface the membrane may serve to retain
the seed near where it settles by influencing hydrodynamics around the seed, minimizing disturbance by waves and currents, and facilitating burial.
Seeds of Phyllospadix have two arms with stiff bristles that facilitate entanglement in a filamentous algae (Turner, 1983; Blanchette et al., 1999). Similarly, seedlings of Amphibolis have a barbed base
that can catch onto algal filaments. While seeds of
other species may appear at first to have no adaptations to either enhance or limit dispersal, we hypothesize that the distinct morphology of seeds influences dispersal distances. For example, the tests of
Halophila seeds are highly sculptured with peg-like
(Halophila spinulosa), or honeycombed projections
(Halophila ovalis) that may aid seed dispersal (by
trapping air bubbles), burial or shedding of the test
during germination (Birch, 1981). During germination, a large mass of fine, long hairs develops on the
seed surface of H. spinulosa that appear to anchor
it in the sediment before the radicle emerges (this
process also appears to occur with Thalassia testudinum seedlings). The actual functional purpose
of seed coat ornamentation in Halophila, and that
displayed on seed coats in other seagrass genera
(e.g. Zostera), is still to be determined. H. uninervis
produces smooth, hard, oval achenes that lack obvious sculpturing or projections on the pericarp. This
morphology appears to be particularly amenable to
tumbling dispersal in mobile sediments, where the
seeds accumulate in large numbers within small pits
and depressions (Fig. 4) (Inglis, 2000b). In terrestrial
plants, accessibility to suitable germination sites has
been shown to be dependent on seed morphology
(smooth vs. rough surfaces and size; Harper, 1977),
but no studies have been conducted to date with seagrasses to test similar hypotheses about the functional importance of seed morphology and size.
Seed dispersal in seagrasses can be influenced by
the position of seed release from the parent plant.
Within the seagrass genera, seeds can be released
either from elevated inflorescences, or at or just below the surface of the sediment (Table 1). For those
species that release seeds below the sediment surface
(Halophila, Cymodocea, and Halodule) dispersal
distances from the parent plant may be on the scale
of centimeters. These genera also have highly persistent seed banks, so this strategy may have evolved
to maximize seed dispersal in time, rather than space
(Venable and Lawlor, 1980). In this case, appropriate
microsites for germination and initial seedling establishment may become available when the parent
plants are disturbed or where storms or intense biotic
activity (rays or manatees) cause plants, sediments,
and seeds to be transported to secondary sites.
Seagrass seed dispersal can be controlled by
both abiotic and biotic elements (Fig. 2). Wind
and currents are important in transporting floating
reproductive fragments or fruits long distances.
Most measurements of seagrass propagule dispersal
are indirect, involving multiplying some metric of
flotation potential to speed of water flow. Harwell
and Orth (2002a) showed that currents alone could
move a Z. marina reproductive fragment up to 23
km in a single 6 h tidal cycle and strong winds
could significantly alter that distance. Small patches
of Z. marina have been found at distances of up to
108 km from the nearest source of donor material and
Harwell and Orth (2002a) hypothesized that these
derived from seeds carried by floating reproductive
fragments. Kaldy and Dunton (1999) calculated
dispersal distances of up to 3 and 15 km for seeds
and fruits, respectively, for T. testudinum, while
Lacap et al. (2002) calculated dispersal distances up
to 3.7 and 63.5 km for seeds and fruits, respectively,
for Enhalus acoroides, and up to 73.5 km for fruit
of Thalassia hemprichii. Lacap et al. (2002) also
postulated that fruits of Thalassia and Enhalus could
be transported 300–400 km, respectively, during typhoons. Kendall et al. (2004) suggest that hurricanes
may be responsible for expansion of a Syringudium
filiforme meadow in St. Croix, US Virgin Islands
by enhancing seed and seagrass fragment dispersal.
Human activities can also influence long distance
dispersal if reproductive fragments are moved in ballast water of ocean going vessels, incorporated as wet
packing material for shipment of live specimens, or
attached to boat trailers. For example, the Japanese
eelgrass, Zostera japonica, is thought to have been
introduced accidentally to the United States west
coast in the early 1900s as seeds transported in live
shipments of Japanese oysters (Harrison and Bigley,
1982). Lipkin (1975) suggested that ship transport
was a source of Halophila stipulacea into the eastern
Mediterranean after Isthmus of Suez was breached.
Nienhuis (1983) suggested the rapid spread of Z. marina into Lake Grevelingen in the Netherlands was a
result of the closure of the estuary, which may have
allowed floating reproductive shoots to be retained
within the closed estuary rather than being exported.
121
one species, Posidonia coriacea, increases the fall
velocity (Orth, 1999). Once settled onto the sediment surface the membrane may serve to retain
the seed near where it settles by influencing hydrodynamics around the seed, minimizing disturbance by waves and currents, and facilitating burial.
Seeds of Phyllospadix have two arms with stiff bristles that facilitate entanglement in a filamentous algae (Turner, 1983; Blanchette et al., 1999). Similarly, seedlings of Amphibolis have a barbed base
that can catch onto algal filaments. While seeds of
other species may appear at first to have no adaptations to either enhance or limit dispersal, we hypothesize that the distinct morphology of seeds influences dispersal distances. For example, the tests of
Halophila seeds are highly sculptured with peg-like
(Halophila spinulosa), or honeycombed projections
(Halophila ovalis) that may aid seed dispersal (by
trapping air bubbles), burial or shedding of the test
during germination (Birch, 1981). During germination, a large mass of fine, long hairs develops on the
seed surface of H. spinulosa that appear to anchor
it in the sediment before the radicle emerges (this
process also appears to occur with Thalassia testudinum seedlings). The actual functional purpose
of seed coat ornamentation in Halophila, and that
displayed on seed coats in other seagrass genera
(e.g. Zostera), is still to be determined. H. uninervis
produces smooth, hard, oval achenes that lack obvious sculpturing or projections on the pericarp. This
morphology appears to be particularly amenable to
tumbling dispersal in mobile sediments, where the
seeds accumulate in large numbers within small pits
and depressions (Fig. 4) (Inglis, 2000b). In terrestrial
plants, accessibility to suitable germination sites has
been shown to be dependent on seed morphology
(smooth vs. rough surfaces and size; Harper, 1977),
but no studies have been conducted to date with seagrasses to test similar hypotheses about the functional importance of seed morphology and size.
Seed dispersal in seagrasses can be influenced by
the position of seed release from the parent plant.
Within the seagrass genera, seeds can be released
either from elevated inflorescences, or at or just below the surface of the sediment (Table 1). For those
species that release seeds below the sediment surface
(Halophila, Cymodocea, and Halodule) dispersal
distances from the parent plant may be on the scale
of centimeters. These genera also have highly persistent seed banks, so this strategy may have evolved
to maximize seed dispersal in time, rather than space
(Venable and Lawlor, 1980). In this case, appropriate
microsites for germination and initial seedling establishment may become available when the parent
plants are disturbed or where storms or intense biotic
activity (rays or manatees) cause plants, sediments,
and seeds to be transported to secondary sites.
Seagrass seed dispersal can be controlled by
both abiotic and biotic elements (Fig. 2). Wind
and currents are important in transporting floating
reproductive fragments or fruits long distances.
Most measurements of seagrass propagule dispersal
are indirect, involving multiplying some metric of
flotation potential to speed of water flow. Harwell
and Orth (2002a) showed that currents alone could
move a Z. marina reproductive fragment up to 23
km in a single 6 h tidal cycle and strong winds
could significantly alter that distance. Small patches
of Z. marina have been found at distances of up to
108 km from the nearest source of donor material and
Harwell and Orth (2002a) hypothesized that these
derived from seeds carried by floating reproductive
fragments. Kaldy and Dunton (1999) calculated
dispersal distances of up to 3 and 15 km for seeds
and fruits, respectively, for T. testudinum, while
Lacap et al. (2002) calculated dispersal distances up
to 3.7 and 63.5 km for seeds and fruits, respectively,
for Enhalus acoroides, and up to 73.5 km for fruit
of Thalassia hemprichii. Lacap et al. (2002) also
postulated that fruits of Thalassia and Enhalus could
be transported 300–400 km, respectively, during typhoons. Kendall et al. (2004) suggest that hurricanes
may be responsible for expansion of a Syringudium
filiforme meadow in St. Croix, US Virgin Islands
by enhancing seed and seagrass fragment dispersal.
Human activities can also influence long distance
dispersal if reproductive fragments are moved in ballast water of ocean going vessels, incorporated as wet
packing material for shipment of live specimens, or
attached to boat trailers. For example, the Japanese
eelgrass, Zostera japonica, is thought to have been
introduced accidentally to the United States west
coast in the early 1900s as seeds transported in live
shipments of Japanese oysters (Harrison and Bigley,
1982). Lipkin (1975) suggested that ship transport
was a source of Halophila stipulacea into the eastern
Mediterranean after Isthmus of Suez was breached.
Nienhuis (1983) suggested the rapid spread of Z. marina into Lake Grevelingen in the Netherlands was a
result of the closure of the estuary, which may have
allowed floating reproductive shoots to be retained
within the closed estuary rather than being exported.