Chapter 19 Epiphytes of Seagrasses
451
the communities of epiphytic invertebrates on seagrass leaves deserves attention.
There appears to be no clear successional pattern in the colonization by macrophytes or invertebrates, rather additional new species recruit to the
seagrass over time mainly as a function of propagule availability and local hydrodynamics, leading to
increased species richness as the seagrass leaf or
stem ages (Bulthuis and Woelkerling, 1983a; Heijs,
1985a,b; Borowitzka et al., 1990). The importance
of the availability of propagules is illustrated by the
study of Lethbridge et al. (1988) using artificial Amphibolis-like seagrasses. They also observed that
some algal epiphytes colonized directly by the attachment of larger detached thallus fragments rather
than from spores. For example, the hook-like branch
tips of the rhodophyte, Hypnea episcopalis, entangle with the stems of Amphibolis and then form
new attachments within a few hours. Similar attachment structures have been observed in Polysiphonia forfex, Dicranema revolutum, and other
algae.
The apico-basal distribution of epiphytic algae observed on the strap-like leaves of seagrasses such as
Posidonia, Zostera, and Cymodocea are unlikely to
be the result of changes in surface chemistry of the
leaves as suggested by Harrison and Durance (1985),
but rather are a function of localized differences in
turbulence, since exactly the same pattern of settlement is observed on artificial leaves made of plastic tape (Horner, 1987; Trautman and Borowitzka,
1999). Some invertebrates, however, show preferential settlement on to particular parts of the seagrass.
For example, the density of newly settled larvae of
the spirorbid polychaete Neodexiospira brasiliensis
were more abundant on the basal part of the leaves
of both Zostera and Phyllospadix (Hamamoto and
Mukai, 1999) and this may be the result of a distinct
preference for shaded substrata (Saunders and Connell, 2001). On the other hand, the hydroids Clytia
edwardsi and Tubularia mesembryantheum initially
settle near the leaf apex of Z. marina. Tubularia
then spread to the lower leaf surface via actinula
larvae (Nishihara, 1968). Similarly, the hydroid Sertularia perpusilla, an obligate epiphyte of P. oceanica, grows downwards on the leaves maximizing the
residence time on the leaf and facilitating transfer of
hydroids to the short young leaves by stolonization,
a method of asexual reproduction (Hughes et al.,
1991a,b).
The few long-term studies of seagrass epiphytes
show distinct seasonality in both species composition and abundance (e.g. Heijs, 1984; Alcoverro
et al., 1997; Reyes et al., 1998; Lepoint et al., 1999;
Wear et al., 1999); however, most of these studies are
only for a 12-month-period and they therefore provide no information on interannual variation. This
seasonality in abundance and diversity appears to be
a function of substratum availability (i.e. new seagrass surfaces), substratum stability (i.e. life-time
of the seagrass leaf or stem which changes over
the year), availability of larvae and propagules of
the potential epiphyte, and environmental conditions
favoring the growth of the epiphytes. Not surprisingly seagrass epiphytes can be classified into groups
based on their seasonal distribution: (a) epiphytes
occurring throughout the year, (b) epiphytes with
a distinct seasonal pattern in their occurrence, and
(c) transient colonizers (Heijs, 1985a). The percent
cover and biomass of the epiphytes also shows seasonal variation; however, a distinct pattern is more
difficult to determine because the temporal scale of
sampling (monthly or quarterly) in many studies is
often not well matched to the scale of epiphyte and
seagrass leaf turnover.
It has been hypothesized that differences in growth
rate of individual seagrass species can account
for differences in the spatial patterns of epiphyte
diversity. A comparison of A. griffithii and P. coriacea revealed different relationships between epiphyte diversity and the distance apart of the seagrass meadows (Lavery and Vanderklift, 2002). In
A. griffithii meadows, there was an almost exponential increase in differences between assemblages as
distance between sites increased, whereas in P. coriacea meadows differences among samples separated by tens or hundreds meters apart were relatively small. This can be explained in terms of the
interaction between the lifespan of the host and the
reproductive lifespan of the epiphytes (cf. Reyes and
Sans´ on, 1997). Where the host is long-lived, as for A.
griffithii, local recruitment from existing epiphytes
with fast reproductive strategies can continually reinforce the local composition. While this occurs in
P. coriacea, its faster rate of leaf turnover results
in a reduction in the influence of the local recruitment but an increase in the relative importance of
regional recruitment. The result is less dissimilarity
among distant P. coriacea assemblages compared to
A. griffithii.
451
the communities of epiphytic invertebrates on seagrass leaves deserves attention.
There appears to be no clear successional pattern in the colonization by macrophytes or invertebrates, rather additional new species recruit to the
seagrass over time mainly as a function of propagule availability and local hydrodynamics, leading to
increased species richness as the seagrass leaf or
stem ages (Bulthuis and Woelkerling, 1983a; Heijs,
1985a,b; Borowitzka et al., 1990). The importance
of the availability of propagules is illustrated by the
study of Lethbridge et al. (1988) using artificial Amphibolis-like seagrasses. They also observed that
some algal epiphytes colonized directly by the attachment of larger detached thallus fragments rather
than from spores. For example, the hook-like branch
tips of the rhodophyte, Hypnea episcopalis, entangle with the stems of Amphibolis and then form
new attachments within a few hours. Similar attachment structures have been observed in Polysiphonia forfex, Dicranema revolutum, and other
algae.
The apico-basal distribution of epiphytic algae observed on the strap-like leaves of seagrasses such as
Posidonia, Zostera, and Cymodocea are unlikely to
be the result of changes in surface chemistry of the
leaves as suggested by Harrison and Durance (1985),
but rather are a function of localized differences in
turbulence, since exactly the same pattern of settlement is observed on artificial leaves made of plastic tape (Horner, 1987; Trautman and Borowitzka,
1999). Some invertebrates, however, show preferential settlement on to particular parts of the seagrass.
For example, the density of newly settled larvae of
the spirorbid polychaete Neodexiospira brasiliensis
were more abundant on the basal part of the leaves
of both Zostera and Phyllospadix (Hamamoto and
Mukai, 1999) and this may be the result of a distinct
preference for shaded substrata (Saunders and Connell, 2001). On the other hand, the hydroids Clytia
edwardsi and Tubularia mesembryantheum initially
settle near the leaf apex of Z. marina. Tubularia
then spread to the lower leaf surface via actinula
larvae (Nishihara, 1968). Similarly, the hydroid Sertularia perpusilla, an obligate epiphyte of P. oceanica, grows downwards on the leaves maximizing the
residence time on the leaf and facilitating transfer of
hydroids to the short young leaves by stolonization,
a method of asexual reproduction (Hughes et al.,
1991a,b).
The few long-term studies of seagrass epiphytes
show distinct seasonality in both species composition and abundance (e.g. Heijs, 1984; Alcoverro
et al., 1997; Reyes et al., 1998; Lepoint et al., 1999;
Wear et al., 1999); however, most of these studies are
only for a 12-month-period and they therefore provide no information on interannual variation. This
seasonality in abundance and diversity appears to be
a function of substratum availability (i.e. new seagrass surfaces), substratum stability (i.e. life-time
of the seagrass leaf or stem which changes over
the year), availability of larvae and propagules of
the potential epiphyte, and environmental conditions
favoring the growth of the epiphytes. Not surprisingly seagrass epiphytes can be classified into groups
based on their seasonal distribution: (a) epiphytes
occurring throughout the year, (b) epiphytes with
a distinct seasonal pattern in their occurrence, and
(c) transient colonizers (Heijs, 1985a). The percent
cover and biomass of the epiphytes also shows seasonal variation; however, a distinct pattern is more
difficult to determine because the temporal scale of
sampling (monthly or quarterly) in many studies is
often not well matched to the scale of epiphyte and
seagrass leaf turnover.
It has been hypothesized that differences in growth
rate of individual seagrass species can account
for differences in the spatial patterns of epiphyte
diversity. A comparison of A. griffithii and P. coriacea revealed different relationships between epiphyte diversity and the distance apart of the seagrass meadows (Lavery and Vanderklift, 2002). In
A. griffithii meadows, there was an almost exponential increase in differences between assemblages as
distance between sites increased, whereas in P. coriacea meadows differences among samples separated by tens or hundreds meters apart were relatively small. This can be explained in terms of the
interaction between the lifespan of the host and the
reproductive lifespan of the epiphytes (cf. Reyes and
Sans´ on, 1997). Where the host is long-lived, as for A.
griffithii, local recruitment from existing epiphytes
with fast reproductive strategies can continually reinforce the local composition. While this occurs in
P. coriacea, its faster rate of leaf turnover results
in a reduction in the influence of the local recruitment but an increase in the relative importance of
regional recruitment. The result is less dissimilarity
among distant P. coriacea assemblages compared to
A. griffithii.