Chapter 18 The Biology of Thalassia
419
Similar rates are reported for T. hemprichii in the
Philippines by Rollon et al. (2001), who found that
this species required ∼2 year to achieve previous values of abundance in 0.25 m
2 plots after experimental
clearance in a mixed species seagrass meadow.
Clonal growth implies that the ramets (sensu
Harper, 1977, which can considered to be the shoots
for seagrasses) have the potential to live independently (van Groenendael and de Kroon, 1990). A
clone is defined here as a group of ramets which
are physically interconnected, which may or may
not be integrated physiologically. For Thalassia, various studies suggest that neighboring shoots of a
clone show physiological integration. Tomasko et al.
(1991) transplanted sections of rhizomes with different numbers of shoots, and found that a minimum
number of two shoots per section was required for
survival, but that sections of four shoots showed
significantly more proliferation (that is formation
of new horizontal rhizome apices through lateral
branching of the vertical rhizomes of the shoots).
Dawes et al. (1997) found that shoots at the edges
of damaged rhizomes showed equal leaf growth as
shoots within the adjacent meadow, indicating that
they were supported by other interconnected shoots
within the meadow. These latter two studies suggest
that resource sharing by interconnected ramets plays
a large role in the functioning of T. testudinum, which
is supported by experimental data. Tomasko and
Dawes (1989) experimentally shaded shoots, which
were either allowed to remain connected via the rhizome (control treatment) or disconnected by cutting
the horizontal rhizome. Isolated shaded shoots had
lower blade growth rates as well as lower protein and
soluble carbohydrate contents than shoots of the control treatment. In a subsequent study, Tomasko and
Dawes (1990) tested whether the degree of shoot
interdependence varied with season along a depth
gradient, and found that there was an effect of shoot
isolation on blade growth (blade growth rates of experimentally isolated shoots were significantly lower
than those of interconnected ones), in the summer
but not in autumn. They also found that shoot integration diminished with depth, which they attributed
to a combination of the more homogeneous light environment at greater depths (all shoots are exposed
to similar conditions), or depth-related differences
in shoot dynamics (all shoots are stressed due to
limitations of the primary resource). These authors
reported that translocation occurred mainly in direction to the active rhizome apices, which was confirmed by Marb` a et al. (2002), who followed incorporation of
13 C and
15 N in the clones of various seagrass
species. They found that the resources from a labeled
shoot of T. hemprichii provided the heavy C and N
isotopes to 7 shoots which corresponded to a translocation distance of ∼28 cm in acropetal direction, and
to 4 shoots (∼12 cm) away from the apex. The shoots
were able to share with their neighbors ∼27 and 17%
of the incorporated
13 C and
15 N, respectively, within
4 day. These few studies of physiological integration in Thalassia spp. have focused on the redistribution of resources from shoots located in sites of resource abundance to shoots located where resources
are scarce (Hartnett and Bazzaz, 1983; Alpert and
Mooney, 1986). However, physiological integration
reaches further than mutual support of stressed ramets (sensu Harper, 1977) by unstressed ones, and it
would be interesting to direct future research efforts
into questions of clonal foraging, i.e. selective placement of resource-acquiring structures in favorable
habitat patches (Hutchings and de Kroon, 1994), or
possible benefits of clonal growth in heterogeneous
vs. homogeneous environments (Hutchings and Wijesinghe, 1997).
B. Sexual Reproduction
The flowers of Thalassia are produced in the axils of
the foliage leaves. As with many other seagrasses,
both species of this genus are dioecious; the male and
female flowers being produced by different clones
(Cox and Tomlinson, 1988). The female flowers of
T. testudinum have 7–8 styles, whereas those of T.
hemprichii have 6: the stigmatic branches are considerably longer in T. testudinum than in T. hemprichii.
The number of stamens of the male flowers is 9 in T.
testudinum and between 3 and 12 (mostly between 6
and 9) in T. hemprichii. Male flowers usually occur in
groups of two or three (but up to five flowers per florescence have been recorded), whereas the female
flowers usually occur singly, and occasionally as a
pair (Durako and Moffler, 1985a). The flowers barely
rise above the substratum, and according to Cox and
Tomlinson (1988), male floral anthesis and dehiscence of anthers is nocturnal, releasing round pollen
grains embedded in ∼1.2 mm long mucilage strands,
whereas anthesis in female flowers is not necessarily
restricted to nocturnal hours. Flowers are not uncommon for either species of Thalassia (Table 2). In
most locations, flowering is clearly seasonal (Table
3), but Durako and Moffler (1987a) on one occasion,
419
Similar rates are reported for T. hemprichii in the
Philippines by Rollon et al. (2001), who found that
this species required ∼2 year to achieve previous values of abundance in 0.25 m
2 plots after experimental
clearance in a mixed species seagrass meadow.
Clonal growth implies that the ramets (sensu
Harper, 1977, which can considered to be the shoots
for seagrasses) have the potential to live independently (van Groenendael and de Kroon, 1990). A
clone is defined here as a group of ramets which
are physically interconnected, which may or may
not be integrated physiologically. For Thalassia, various studies suggest that neighboring shoots of a
clone show physiological integration. Tomasko et al.
(1991) transplanted sections of rhizomes with different numbers of shoots, and found that a minimum
number of two shoots per section was required for
survival, but that sections of four shoots showed
significantly more proliferation (that is formation
of new horizontal rhizome apices through lateral
branching of the vertical rhizomes of the shoots).
Dawes et al. (1997) found that shoots at the edges
of damaged rhizomes showed equal leaf growth as
shoots within the adjacent meadow, indicating that
they were supported by other interconnected shoots
within the meadow. These latter two studies suggest
that resource sharing by interconnected ramets plays
a large role in the functioning of T. testudinum, which
is supported by experimental data. Tomasko and
Dawes (1989) experimentally shaded shoots, which
were either allowed to remain connected via the rhizome (control treatment) or disconnected by cutting
the horizontal rhizome. Isolated shaded shoots had
lower blade growth rates as well as lower protein and
soluble carbohydrate contents than shoots of the control treatment. In a subsequent study, Tomasko and
Dawes (1990) tested whether the degree of shoot
interdependence varied with season along a depth
gradient, and found that there was an effect of shoot
isolation on blade growth (blade growth rates of experimentally isolated shoots were significantly lower
than those of interconnected ones), in the summer
but not in autumn. They also found that shoot integration diminished with depth, which they attributed
to a combination of the more homogeneous light environment at greater depths (all shoots are exposed
to similar conditions), or depth-related differences
in shoot dynamics (all shoots are stressed due to
limitations of the primary resource). These authors
reported that translocation occurred mainly in direction to the active rhizome apices, which was confirmed by Marb` a et al. (2002), who followed incorporation of
13 C and
15 N in the clones of various seagrass
species. They found that the resources from a labeled
shoot of T. hemprichii provided the heavy C and N
isotopes to 7 shoots which corresponded to a translocation distance of ∼28 cm in acropetal direction, and
to 4 shoots (∼12 cm) away from the apex. The shoots
were able to share with their neighbors ∼27 and 17%
of the incorporated
13 C and
15 N, respectively, within
4 day. These few studies of physiological integration in Thalassia spp. have focused on the redistribution of resources from shoots located in sites of resource abundance to shoots located where resources
are scarce (Hartnett and Bazzaz, 1983; Alpert and
Mooney, 1986). However, physiological integration
reaches further than mutual support of stressed ramets (sensu Harper, 1977) by unstressed ones, and it
would be interesting to direct future research efforts
into questions of clonal foraging, i.e. selective placement of resource-acquiring structures in favorable
habitat patches (Hutchings and de Kroon, 1994), or
possible benefits of clonal growth in heterogeneous
vs. homogeneous environments (Hutchings and Wijesinghe, 1997).
B. Sexual Reproduction
The flowers of Thalassia are produced in the axils of
the foliage leaves. As with many other seagrasses,
both species of this genus are dioecious; the male and
female flowers being produced by different clones
(Cox and Tomlinson, 1988). The female flowers of
T. testudinum have 7–8 styles, whereas those of T.
hemprichii have 6: the stigmatic branches are considerably longer in T. testudinum than in T. hemprichii.
The number of stamens of the male flowers is 9 in T.
testudinum and between 3 and 12 (mostly between 6
and 9) in T. hemprichii. Male flowers usually occur in
groups of two or three (but up to five flowers per florescence have been recorded), whereas the female
flowers usually occur singly, and occasionally as a
pair (Durako and Moffler, 1985a). The flowers barely
rise above the substratum, and according to Cox and
Tomlinson (1988), male floral anthesis and dehiscence of anthers is nocturnal, releasing round pollen
grains embedded in ∼1.2 mm long mucilage strands,
whereas anthesis in female flowers is not necessarily
restricted to nocturnal hours. Flowers are not uncommon for either species of Thalassia (Table 2). In
most locations, flowering is clearly seasonal (Table
3), but Durako and Moffler (1987a) on one occasion,