418
B. I. van Tussenbroek, J. A. Vonk, J. Stapel et al.
shoots in patches where T. testudinum had suffered
die-off in Florida Bay showed ‘twinning’, which he
attributed to abundant light and nutrients conditions.
It is possible that under these conditions, apical dominance is low and the branched apex forms foliar
tissue, instead of becoming a functional horizontal
rhizome apex. On the horizontal rhizomes, branching occurs at regular intervals, and for both Thalassia
species, the number of scale leaves between each pair
of branches is always odd (usually varying between
7 and 19), as the branches are born alternatively left
and right (Fig. 1B). The vertical rhizomes, in contrast, branch more sporadically and at irregular intervals (Fig. 1B). Tomlinson (1970) marveled at the
highly specialized capacity of the apical meristem
of the horizontal rhizome of Thalassia, stating that
‘it can count, (and) it can distinguish between left
and right,’ and mentioned that the precise organization of the shoot system sets Thalassia apart from the
other members of its family and from all other monocotyledonous plants. But this highly organized and
rigid growth pattern implies that Thalassia depends
strictly on the apex of the horizontal rhizome for
vegetative expansion, differing in this respect with
other seagrass genera, which to a greater or lesser
degree have more flexible proliferation patterns, because the distinction between horizontal and vertical
axes is less absolute, and, thus, the apical meristems
have the potential to produce either horizontal or
vertical rhizomes (Tomlinson, 1974).
The strict dependence of the horizontal rhizome
apex for clonal expansion, together with the low plasticity in internode length and absence of dormant
axillary buds (Tomlinson, 1974), led van Tussenbroek et al. (2000) to raise the question of how T.
testudinum could respond to changes in resource
availability, as this rigid growth pattern apparently
does not allow for regulation of the spacing of the
resource-acquiring shoots. Although under conditions of high resource availability, this seagrass does
show some flexibility in its growth form as was
demonstrated by the presence of ‘twin shoots’, and
exceptionally high leaf formation rates in die-off
patches in Florida Bay by Durako (1995, see above).
Van Tussenbroek et al. (2000) found that the foliar development of the vertical rhizomes of T. testudinum in a reef lagoon with a limited nutrient
supply, was inhibited at increasing rhizome density,
but that the meristems of these vertical rhizomes
maintained functionality by forming scale-like foliar structures instead of foliage leaves. Van Tussenbroek et al. (2000) named these structures ‘dormant
meristems’ (Figs. 1G–I). The existence of such a
‘dormant-meristem bank’ allowed the seagrass to
exploit a sudden increase in resources (provided as
nutrient application to the sediments) through reactivation of the foliar development of these dormant
meristems, before other plant species could invade
the area. Apparently, the dormant apical meristems
have the same function as the dormant axillary buds
in most other clonal species, with the advantage that
an apical meristem can pass through the stages of
dormancy and foliar reactivation at various times
during its life. It is important to note, however, that
dormant meristems have also been registered under adverse climatic conditions; for example P´ erez
and Galindo (2000) reported a mass defoliation of
T. testudinum in Venezuela (total area 320 km
2 ) after a short episode of extreme low salinity caused
by torrential rain. The underground rhizomes were
left intact, and had many dead but also defoliated
dormant shoots. Reactivation of the foliar development of these dormant shoots allowed for a complete
recovery of these meadows within 5 months.
Extension of the horizontal rhizome of Thalassia
spp. is slow in comparison with that of many other
seagrass species (Marb` a and Duarte, 1998); rates of
horizontal rhizome extension in T. testudinum vary
between 19 and 35 cm year
−1 apex
−1 depending on
the sampling site (Gallegos et al., 1993; van Tussenbroek, 1998), and those of T. hemprichii vary between 21 and 56 cm year
−1 apex
−1 (Vermaat et al.,
1995; Rollon et al., 2001). The restricted growth pattern of Thalassia spp. (see above), combined with
these slow expansion rates, poses restrictions for recolonization of cleared areas due to disturbances.
Dawes et al. (1997) found that the regrowth of T. testudinum in clearances caused by propellors in Tampa
Bay was slow due to limited production of horizontal rhizome meristems, and could not be attributed to
changes in sediment conditions due to the clearing of
the seagrasses. These authors estimated a recovery
period of 2.3–4.1 year, which is in accordance with
the observation of Zieman (1976), that tracks by propellors persisted for 2–5 year in T. testudinum beds
in Biscayne Bay, Florida. Durako et al. (1992) estimated that T. testudinum required between 3.6 and
6.0 year to obtain pre-disturbance densities experimental propellor scars in Tampa Bay, and Williams
(1990) found that T. testudinum required more than
4.3 year to obtain pre-cleared density in 1 m
2 denuded experimental plots in the US Virgin Islands.
B. I. van Tussenbroek, J. A. Vonk, J. Stapel et al.
shoots in patches where T. testudinum had suffered
die-off in Florida Bay showed ‘twinning’, which he
attributed to abundant light and nutrients conditions.
It is possible that under these conditions, apical dominance is low and the branched apex forms foliar
tissue, instead of becoming a functional horizontal
rhizome apex. On the horizontal rhizomes, branching occurs at regular intervals, and for both Thalassia
species, the number of scale leaves between each pair
of branches is always odd (usually varying between
7 and 19), as the branches are born alternatively left
and right (Fig. 1B). The vertical rhizomes, in contrast, branch more sporadically and at irregular intervals (Fig. 1B). Tomlinson (1970) marveled at the
highly specialized capacity of the apical meristem
of the horizontal rhizome of Thalassia, stating that
‘it can count, (and) it can distinguish between left
and right,’ and mentioned that the precise organization of the shoot system sets Thalassia apart from the
other members of its family and from all other monocotyledonous plants. But this highly organized and
rigid growth pattern implies that Thalassia depends
strictly on the apex of the horizontal rhizome for
vegetative expansion, differing in this respect with
other seagrass genera, which to a greater or lesser
degree have more flexible proliferation patterns, because the distinction between horizontal and vertical
axes is less absolute, and, thus, the apical meristems
have the potential to produce either horizontal or
vertical rhizomes (Tomlinson, 1974).
The strict dependence of the horizontal rhizome
apex for clonal expansion, together with the low plasticity in internode length and absence of dormant
axillary buds (Tomlinson, 1974), led van Tussenbroek et al. (2000) to raise the question of how T.
testudinum could respond to changes in resource
availability, as this rigid growth pattern apparently
does not allow for regulation of the spacing of the
resource-acquiring shoots. Although under conditions of high resource availability, this seagrass does
show some flexibility in its growth form as was
demonstrated by the presence of ‘twin shoots’, and
exceptionally high leaf formation rates in die-off
patches in Florida Bay by Durako (1995, see above).
Van Tussenbroek et al. (2000) found that the foliar development of the vertical rhizomes of T. testudinum in a reef lagoon with a limited nutrient
supply, was inhibited at increasing rhizome density,
but that the meristems of these vertical rhizomes
maintained functionality by forming scale-like foliar structures instead of foliage leaves. Van Tussenbroek et al. (2000) named these structures ‘dormant
meristems’ (Figs. 1G–I). The existence of such a
‘dormant-meristem bank’ allowed the seagrass to
exploit a sudden increase in resources (provided as
nutrient application to the sediments) through reactivation of the foliar development of these dormant
meristems, before other plant species could invade
the area. Apparently, the dormant apical meristems
have the same function as the dormant axillary buds
in most other clonal species, with the advantage that
an apical meristem can pass through the stages of
dormancy and foliar reactivation at various times
during its life. It is important to note, however, that
dormant meristems have also been registered under adverse climatic conditions; for example P´ erez
and Galindo (2000) reported a mass defoliation of
T. testudinum in Venezuela (total area 320 km
2 ) after a short episode of extreme low salinity caused
by torrential rain. The underground rhizomes were
left intact, and had many dead but also defoliated
dormant shoots. Reactivation of the foliar development of these dormant shoots allowed for a complete
recovery of these meadows within 5 months.
Extension of the horizontal rhizome of Thalassia
spp. is slow in comparison with that of many other
seagrass species (Marb` a and Duarte, 1998); rates of
horizontal rhizome extension in T. testudinum vary
between 19 and 35 cm year
−1 apex
−1 depending on
the sampling site (Gallegos et al., 1993; van Tussenbroek, 1998), and those of T. hemprichii vary between 21 and 56 cm year
−1 apex
−1 (Vermaat et al.,
1995; Rollon et al., 2001). The restricted growth pattern of Thalassia spp. (see above), combined with
these slow expansion rates, poses restrictions for recolonization of cleared areas due to disturbances.
Dawes et al. (1997) found that the regrowth of T. testudinum in clearances caused by propellors in Tampa
Bay was slow due to limited production of horizontal rhizome meristems, and could not be attributed to
changes in sediment conditions due to the clearing of
the seagrasses. These authors estimated a recovery
period of 2.3–4.1 year, which is in accordance with
the observation of Zieman (1976), that tracks by propellors persisted for 2–5 year in T. testudinum beds
in Biscayne Bay, Florida. Durako et al. (1992) estimated that T. testudinum required between 3.6 and
6.0 year to obtain pre-disturbance densities experimental propellor scars in Tampa Bay, and Williams
(1990) found that T. testudinum required more than
4.3 year to obtain pre-cleared density in 1 m
2 denuded experimental plots in the US Virgin Islands.