Chapter 18 The Biology of Thalassia
421
reported early stage inflorescences in October in
Tampa Bay, and Moffler et al. (1981) observed earlystage fruits in midwinter, which is outside the normal flowering season for this region. Male and female clones usually grow intermixed, but Durako
and Moffler (1985a) reported spatial segregation of
T. testudinum sexes at one site in Tampa Bay, with female clones growing in shallower waters than males.
They suggested that this segregation may reflect the
age structure of the seagrass meadow, with the male
shoots being older than females, but it is also possible
that females occur in shallower waters because they
have higher light demands, and more studies are necessary. In another study, Durako and Moffler (1985b)
reported that in Tampa Bay, shoots with male flowers
had broader leaves than those having female inflorescences, suggesting that sexual dimorphism may
occur in this species, but in this case also, further
studies are required to establish this. The diameter of mature fruits is between 20 and 25 mm (Kuo
et al., 1991; van Tussenbroek, unpublished data). The
number of seeds per fruit is usually two or three in
both species, but can vary from one to six (Orpurt
and Boral, 1964; Kuo et al., 1991; Kaldy and Dunton, 2000; Rollon et al., 2001).
The seeds are rich in proteins and starch; germination is precocious and they form a shoot apex with
leaf primordia (or intraseminal leaves) while still
within the fruit (Orpurt and Boral, 1964; Moffler and
Durako, 1984; Kuo et al., 1991). Thus, the seeds are
cryptoviviparous (zygotes develop significantly but
do nor penetrate the fruit pericarp before dispersal:
sensu Tomlinson, 1986), with no dormancy period.
Seeds are negatively buoyant, but may float for a
short period immediately after fruit dehiscence, as
was reported for T. testudinum by Kaldy and Dunton
(1999; buoyant <1 day) and for T. hemprichii by
Lacap et al. (2002; median time of buoyancy 0.3
h). The seeds are released from the disintegrating
fruits (which are positively buoyant) either before
or after these becomes detached from their peduncles (Orpurt and Boral, 1964; Kuo et al., 1991).
Thus they can be deposited within or outside the
mother meadow, which increases the potential for
seed dispersal. Lacap et al. (2002) found that seeds
moved ∼100 cm day
−1 over the sediment within a
T. hemprichii bed in a Philippine reef flat system.
These seeds disappeared from their experimental
area within 2–3 day, which they thought to be due to
herbivory or burial by invertebrates. Floating fruits,
on the other hand, have a considerably higher dispersal capacity and Kaldy and Dunton (1999) estimated
a dispersal distance of <1.5–15 km for detached
fruits of T. testudinum in the Laguna Madre, Texas.
And Lacap et al. (2002) recorded a mean in situ dispersal speed of 0.43 km h
−1 for floating fruits of
T. hemprichii, which combined with a flotation time
between 2 and 7 day, corresponded with a dispersal
distance of 23–74 km.
It is not uncommon to report young seedlings (<1year old) unattached or anchored in the field (Lewis
and Phillips, 1980; Williams and Adey, 1983; Kaldy
and Dunton, 1999), but very few survive in the long
term (Kaldy and Dunton, 1999), although seedlings
seem to be able to survive quite well under laboratory conditions (Williams and Adey, 1983; Moffler and Durako, 1984; Durako and Moffler, 1981,
1984, 1987b; Kaldy and Dunton, 1999). Field studies
have reported that population expansion and maintenance of Thalassia spp. occurs almost exclusively
through vegetative propagation (see above), and
rarely through successful establishment of seedlings.
Although recently, Kaldy and Dunton (1999) found
that seedling establishment was important for the
expansion of T. testudinum meadows in Laguna
Madre, Texas, and Whitfield et al. (2004) reported
that seedlings of T. testudinum were common source
of new recruits in physically disturbed areas (by a
grounded vessel and scarring of beds by propellers
of small boats) of seagrass beds in the Florida Keys.
Currently, the principal role of sexual reproduction in the evolution of these two species is not clear.
Sexual reproduction might play a role in maintenance of genetic variability within populations, or
on the other hand, its major role may be the recolonization of areas from which the seagrass has
been eradicated by major disturbances (rescue effect, Ouborg et al., 1999), but data to support either hypothesis are still insufficient. In this context
it is important to note that long-distance dispersal
by vegetative fragments has been reported for some
seagrass species (Cambridge et al., 1983; Kuo et al.,
1987; Campbell, 2003), but colonization of virgin
areas by fragments has never been reported for either Thalassia species. An exceptional study is that
of Rollon et al. (2001) who discovered a disjunctive and young population (average shoot age ∼1
year) of T. hemprichii on an island of the Kalayaan
Island group in the Indo-Pacific ocean, >500 km
away from major populations of this species. Interestingly, seven other nearby islands within 10s of km
distance, did not show signs of seagrass colonization.
421
reported early stage inflorescences in October in
Tampa Bay, and Moffler et al. (1981) observed earlystage fruits in midwinter, which is outside the normal flowering season for this region. Male and female clones usually grow intermixed, but Durako
and Moffler (1985a) reported spatial segregation of
T. testudinum sexes at one site in Tampa Bay, with female clones growing in shallower waters than males.
They suggested that this segregation may reflect the
age structure of the seagrass meadow, with the male
shoots being older than females, but it is also possible
that females occur in shallower waters because they
have higher light demands, and more studies are necessary. In another study, Durako and Moffler (1985b)
reported that in Tampa Bay, shoots with male flowers
had broader leaves than those having female inflorescences, suggesting that sexual dimorphism may
occur in this species, but in this case also, further
studies are required to establish this. The diameter of mature fruits is between 20 and 25 mm (Kuo
et al., 1991; van Tussenbroek, unpublished data). The
number of seeds per fruit is usually two or three in
both species, but can vary from one to six (Orpurt
and Boral, 1964; Kuo et al., 1991; Kaldy and Dunton, 2000; Rollon et al., 2001).
The seeds are rich in proteins and starch; germination is precocious and they form a shoot apex with
leaf primordia (or intraseminal leaves) while still
within the fruit (Orpurt and Boral, 1964; Moffler and
Durako, 1984; Kuo et al., 1991). Thus, the seeds are
cryptoviviparous (zygotes develop significantly but
do nor penetrate the fruit pericarp before dispersal:
sensu Tomlinson, 1986), with no dormancy period.
Seeds are negatively buoyant, but may float for a
short period immediately after fruit dehiscence, as
was reported for T. testudinum by Kaldy and Dunton
(1999; buoyant <1 day) and for T. hemprichii by
Lacap et al. (2002; median time of buoyancy 0.3
h). The seeds are released from the disintegrating
fruits (which are positively buoyant) either before
or after these becomes detached from their peduncles (Orpurt and Boral, 1964; Kuo et al., 1991).
Thus they can be deposited within or outside the
mother meadow, which increases the potential for
seed dispersal. Lacap et al. (2002) found that seeds
moved ∼100 cm day
−1 over the sediment within a
T. hemprichii bed in a Philippine reef flat system.
These seeds disappeared from their experimental
area within 2–3 day, which they thought to be due to
herbivory or burial by invertebrates. Floating fruits,
on the other hand, have a considerably higher dispersal capacity and Kaldy and Dunton (1999) estimated
a dispersal distance of <1.5–15 km for detached
fruits of T. testudinum in the Laguna Madre, Texas.
And Lacap et al. (2002) recorded a mean in situ dispersal speed of 0.43 km h
−1 for floating fruits of
T. hemprichii, which combined with a flotation time
between 2 and 7 day, corresponded with a dispersal
distance of 23–74 km.
It is not uncommon to report young seedlings (<1year old) unattached or anchored in the field (Lewis
and Phillips, 1980; Williams and Adey, 1983; Kaldy
and Dunton, 1999), but very few survive in the long
term (Kaldy and Dunton, 1999), although seedlings
seem to be able to survive quite well under laboratory conditions (Williams and Adey, 1983; Moffler and Durako, 1984; Durako and Moffler, 1981,
1984, 1987b; Kaldy and Dunton, 1999). Field studies
have reported that population expansion and maintenance of Thalassia spp. occurs almost exclusively
through vegetative propagation (see above), and
rarely through successful establishment of seedlings.
Although recently, Kaldy and Dunton (1999) found
that seedling establishment was important for the
expansion of T. testudinum meadows in Laguna
Madre, Texas, and Whitfield et al. (2004) reported
that seedlings of T. testudinum were common source
of new recruits in physically disturbed areas (by a
grounded vessel and scarring of beds by propellers
of small boats) of seagrass beds in the Florida Keys.
Currently, the principal role of sexual reproduction in the evolution of these two species is not clear.
Sexual reproduction might play a role in maintenance of genetic variability within populations, or
on the other hand, its major role may be the recolonization of areas from which the seagrass has
been eradicated by major disturbances (rescue effect, Ouborg et al., 1999), but data to support either hypothesis are still insufficient. In this context
it is important to note that long-distance dispersal
by vegetative fragments has been reported for some
seagrass species (Cambridge et al., 1983; Kuo et al.,
1987; Campbell, 2003), but colonization of virgin
areas by fragments has never been reported for either Thalassia species. An exceptional study is that
of Rollon et al. (2001) who discovered a disjunctive and young population (average shoot age ∼1
year) of T. hemprichii on an island of the Kalayaan
Island group in the Indo-Pacific ocean, >500 km
away from major populations of this species. Interestingly, seven other nearby islands within 10s of km
distance, did not show signs of seagrass colonization.