Chapter 18 The Biology of Thalassia
423
annual shoot mortality and recruitment rates and
age-independent mortality, required by this model
are tenable (Jensen et al., 1996; Kaldy et al., 1999;
van Tussenbroek, 2000; Peterson and Fourqurean,
2001). Another application of shoot demography
has been that of van Tussenbroek (2002), who applied a classic static life-table analysis to various
stable shoot-populations of T. testudinum, and found
that mortality of the shoots was age-dependent, with
lower mortality for the younger shoots. This is not
uncommon for clonal plants, and is an indication
of support of the younger ramets by the older ones
(Sarukh´ an and Harper, 1973; Hartnett and Bazzaz, 1983). Also, elasticity analysis of a derived
Leslie matrix showed that ramification of 1-yearold shoots was by far more important to net population growth than survival of the foliar shoots of
all other age classes. The latter indicates that proliferation through branching is not only important
for expansion of populations to nearby virgin areas
(see above), but also for maintenance of established
Thalassia populations.
B. Population Genetics
The first study on population genetics on both
species of Thalassia, was that by McMillan (1980),
who found that allozymes separated the two species
T. testudinum and T. hemprichii, but did not detect any intra-specific variability for either species
throughout their distribution area. Since this pioneering work, attempts have been made to separate
populations of T. testudinum using allozyme, RAPD,
ISSR-DNA Fingerprinting, or AFLP techniques.
Schlueter and Guttman (1998) employed fourteen
allozymes of which five proved to be polymorphic,
and found that the population in the Florida Keys
consisted of at least four genets. However, across all
sampling sites, they reported low genetic diversity
(>0.975 similarity) and high gene flow over large
distances (>120 km). This tendency was confirmed
by Kirsten et al. (1998), who employed RAPDs
for comparison of populations from N-Florida, the
Florida Keys, and Jamaica. These authors found high
genetic variability on a local scale (81%), whereas
only 19% of the variation was explained by divergence on a macro-geographical scale. Davis et al.
(1999) used the more unusual technique ISSR (Inter
Sample Sequence Repeat) DNA Fingerprinting to
determine the genetic variability of T. testudinum in
an area of ∼25 km
2 in Florida Bay; they reported 22
genetically distinct individuals out of the 72 sampled shoots, and different individuals were found
on small spatial scales (half of the genetic variability was within ∼25 cm distance), suggesting that
the clones grow intertwined. More recently, Waycott and Barnes (2001) applied the AFLP (Amplified
Fragment Length Polymorphism) technique to differentiate populations from San Blas, Panama, and
Bermuda. AFLP is a relatively new fingerprinting
technique, and has a higher resolution than RAPDs.
Even with this technique they found a very low degree of differentiation of genotypes between Panama
and Bermuda in spite of the fact that the sites are
separated by 2700 km, and only 5.4% of the genetic variability was attributed to the sites between
the regions. In summary, all studied populations of
T. testudinum were multiclonal, but inter-population
genetic diversity was low, suggesting a high gene
flow between populations. Suggested reasons for the
high genetic uniformity of populations which are
up to hundreds or thousands km apart, have been
long-distance movement of vegetative, or transport
of sexual offspring over large distances on a geographical scale (Kirsten et al., 1998; Waycott and
Barnes, 2001; Waycott et al., Chapter 2).
VIII. Community Ecology
A. Vegetation Structure
The structure of seagrass communities in general
is regarded as quite simple, as it consists of one
or a few seagrass species of a rather similar morphology (den Hartog and Phillips, 2001). Thalassiadominated seagrass meadows, however, are generally multi-taxon conglomerates composed of various
species of seagrasses, rooted calcareous and fleshy
algae (rhizophytic algae), drifting fleshy or filamentous algae, and epiphytes (Tables 4 and 5). The rhizophytic calcareous algae often form an important
component of these communities, which together
with the calcareous epiphytes (see section 2), are
important producers of carbonate sediments in many
tropical localities (Land, 1970; Bosence, 1989). For
example, psammophytic species Halimeda incrassata produces generally in the order of tens or hundreds gram calcium carbonate per year depending on
the locality (Bach, 1979; Wefer, 1980; Multer, 1988;
Payri, 1988; Freile and Hillis, 1997). These carbonate sediments are subsequently trapped and bound
by the seagrasses (Scoffin, 1970). In the Western
423
annual shoot mortality and recruitment rates and
age-independent mortality, required by this model
are tenable (Jensen et al., 1996; Kaldy et al., 1999;
van Tussenbroek, 2000; Peterson and Fourqurean,
2001). Another application of shoot demography
has been that of van Tussenbroek (2002), who applied a classic static life-table analysis to various
stable shoot-populations of T. testudinum, and found
that mortality of the shoots was age-dependent, with
lower mortality for the younger shoots. This is not
uncommon for clonal plants, and is an indication
of support of the younger ramets by the older ones
(Sarukh´ an and Harper, 1973; Hartnett and Bazzaz, 1983). Also, elasticity analysis of a derived
Leslie matrix showed that ramification of 1-yearold shoots was by far more important to net population growth than survival of the foliar shoots of
all other age classes. The latter indicates that proliferation through branching is not only important
for expansion of populations to nearby virgin areas
(see above), but also for maintenance of established
Thalassia populations.
B. Population Genetics
The first study on population genetics on both
species of Thalassia, was that by McMillan (1980),
who found that allozymes separated the two species
T. testudinum and T. hemprichii, but did not detect any intra-specific variability for either species
throughout their distribution area. Since this pioneering work, attempts have been made to separate
populations of T. testudinum using allozyme, RAPD,
ISSR-DNA Fingerprinting, or AFLP techniques.
Schlueter and Guttman (1998) employed fourteen
allozymes of which five proved to be polymorphic,
and found that the population in the Florida Keys
consisted of at least four genets. However, across all
sampling sites, they reported low genetic diversity
(>0.975 similarity) and high gene flow over large
distances (>120 km). This tendency was confirmed
by Kirsten et al. (1998), who employed RAPDs
for comparison of populations from N-Florida, the
Florida Keys, and Jamaica. These authors found high
genetic variability on a local scale (81%), whereas
only 19% of the variation was explained by divergence on a macro-geographical scale. Davis et al.
(1999) used the more unusual technique ISSR (Inter
Sample Sequence Repeat) DNA Fingerprinting to
determine the genetic variability of T. testudinum in
an area of ∼25 km
2 in Florida Bay; they reported 22
genetically distinct individuals out of the 72 sampled shoots, and different individuals were found
on small spatial scales (half of the genetic variability was within ∼25 cm distance), suggesting that
the clones grow intertwined. More recently, Waycott and Barnes (2001) applied the AFLP (Amplified
Fragment Length Polymorphism) technique to differentiate populations from San Blas, Panama, and
Bermuda. AFLP is a relatively new fingerprinting
technique, and has a higher resolution than RAPDs.
Even with this technique they found a very low degree of differentiation of genotypes between Panama
and Bermuda in spite of the fact that the sites are
separated by 2700 km, and only 5.4% of the genetic variability was attributed to the sites between
the regions. In summary, all studied populations of
T. testudinum were multiclonal, but inter-population
genetic diversity was low, suggesting a high gene
flow between populations. Suggested reasons for the
high genetic uniformity of populations which are
up to hundreds or thousands km apart, have been
long-distance movement of vegetative, or transport
of sexual offspring over large distances on a geographical scale (Kirsten et al., 1998; Waycott and
Barnes, 2001; Waycott et al., Chapter 2).
VIII. Community Ecology
A. Vegetation Structure
The structure of seagrass communities in general
is regarded as quite simple, as it consists of one
or a few seagrass species of a rather similar morphology (den Hartog and Phillips, 2001). Thalassiadominated seagrass meadows, however, are generally multi-taxon conglomerates composed of various
species of seagrasses, rooted calcareous and fleshy
algae (rhizophytic algae), drifting fleshy or filamentous algae, and epiphytes (Tables 4 and 5). The rhizophytic calcareous algae often form an important
component of these communities, which together
with the calcareous epiphytes (see section 2), are
important producers of carbonate sediments in many
tropical localities (Land, 1970; Bosence, 1989). For
example, psammophytic species Halimeda incrassata produces generally in the order of tens or hundreds gram calcium carbonate per year depending on
the locality (Bach, 1979; Wefer, 1980; Multer, 1988;
Payri, 1988; Freile and Hillis, 1997). These carbonate sediments are subsequently trapped and bound
by the seagrasses (Scoffin, 1970). In the Western