Chapter 18 The Biology of Thalassia
429
transient species are frequently juvenile stages of
numerous organisms that seek food and shelter during critical parts of their life cycles, or they may
be daily visitors that use the Thalassia meadows as
feeding grounds. The wealth of literature on fauna
in seagrass systems indicates that the faunal assemblages of seagrass meadows are not specific, but
largely comprise species occurring outside the seagrass meadows as well. The abundance and diversity of fauna associated with Thalassia meadows (as
with other seagrasses) is high compared with that of
unvegetated areas (Hemminga and Duarte, 2000a).
Sanchez (1997), for example, reported densities of
pink shrimp (Penaeus duorarum) in the Gulf of Mexico to be on average 13 times greater in Thalassia testudinum meadows than on nearby unvegetated soft
substrata.
Thalassia meadows play a key role for fauna in
a number of ways. They serve as a feeding habitat
by offering food in the form of fresh seagrass tissue,
epiphytes, detritus, or associated fauna (Greenway,
1995; Schwamborn and Criales, 2000; Sluka and
Miller, 2001; Kirsch et al., 2002). Recent reports
suggest that the importance of leaf herbivory in seagrass meadows is often seriously under-estimated
(Valentine and Heck, 1991; Cebrian and Duarte,
1998; Valentine and Heck, 1999), and highly variable among seagrass species (Cebrian et al., 1998).
Few studies have considered the possibility that herbivores may stimulate rates of primary production,
or the role of belowground nutrient reserves in determining the impacts of grazers on seagrasses (Valentine and Heck, 1999; Stapel and Erftemeijer, 2000).
The meadows also supply a substrate for settlement
for bivalve spat (Bologna and Heck, 2000), juvenile
sea cucumbers (Mercier et al., 2000) or epiphytic
hydroids and foraminifers (Kaehler and Hughes,
1992; Fujita and Hallock, 1999). The importance
of Thalassia meadows as nursery grounds for fishes
and crustaceans is well documented (Aliaume et al.,
1993; Erftemeijer and Allen, 1993; Arrivillaga and
Baltz, 1999; Thayer et al., 1999; Nagelkerken et al.,
2000; Spitzer et al., 2000). One of the factors underlying the attractiveness of Thalassia meadows to
juvenile fishes, shrimps, and other fauna is the provision of shelter by the seagrass canopy, which significantly reduces vulnerability to predation (Ray and
Stoner, 1995; Jordan et al., 1997; Rooker et al., 1998;
Peterson et al., 2001).
The associated fauna, in turn, influence Thalassia growth through herbivory on the seagrass tissue
(Valentine et al., 1997; Sluka and Miller, 2001), leaf
burrowing (van Tussenbroek and Brearley, 1998),
epiphyte grazing (Gacia et al., 1999; Heck et al.,
2000), and fragmentation of detritus (Lawson et al.,
1993; Stoner et al., 1995). Associated fauna may
also affect nutrient cycling, substrate stability, and
water quality in Thalassia meadows through bioturbation (Valentine et al., 1994; Stapel and Erftemeijer,
2000), and suspension feeding (Peterson and Heck,
1999, 2001; Prager and Halley, 1999).
The echinoderm fauna is some of the most conspicuous component of Thalassia seagrass communities. Sea urchins (Echinoids) comprise the main
grazers in the Thalassia ecosystem. Folivory by the
sea urchin Lytechinus variegatus increases the shoot
density and productivity of Thalassia testudinum in
Florida, and controls its seasonal changes in abundance (Valentine et al., 1997, 2000). Sea urchin
grazing in enclosures dramatically affected seagrass
habitat structure for long periods of time (>3.5 year).
Intermittent grazing produced significant reductions
in aboveground plant biomass compared to controls, while continuous grazing produced apparently
permanent loss of seagrasses (Heck and Valentine,
1995). In contrast to findings reported for vertebrate
herbivores, sea urchins were found to feed at higher
rates when offered Thalassia leaves of lower nitrogen
content, compensating for the lower nitrogen levels
by eating more leaves than in treatments where leaf
nitrogen was experimentally elevated using commercial fertilizer (Valentine and Heck, 2001). This
demonstrates that low levels of leaf nitrogen are not
always an effective defense against herbivores. A
demographic study of L. variegatus in the Gulf of
Mexico revealed a catastrophic mortality caused by
a cold front in the spring of 1993, leading to a dramatic decline in sea urchin densities in a Thalassia testudinum meadow. After a pronounced recruitment event in the fall of 1993, the population at this
site recovered within a period of approximately 6
months, which was attributed to the immigration of
new adults (Beddingfield and McClintock, 2000). In
Indonesia, the edible sea urchin Tripneustes gratilla
was observed to prefer grazing on attached fronds of
Thalassia hemprichii, foraging continually throughout the day and night, without evidence of any periodicity (Klumpp et al., 1993). T. gratilla consumed
only 1–5% of Thalassia production between March
and July, while between November and March the
sea urchin consumption increased to over 50% of
seagrass production. They concluded that on an
429
transient species are frequently juvenile stages of
numerous organisms that seek food and shelter during critical parts of their life cycles, or they may
be daily visitors that use the Thalassia meadows as
feeding grounds. The wealth of literature on fauna
in seagrass systems indicates that the faunal assemblages of seagrass meadows are not specific, but
largely comprise species occurring outside the seagrass meadows as well. The abundance and diversity of fauna associated with Thalassia meadows (as
with other seagrasses) is high compared with that of
unvegetated areas (Hemminga and Duarte, 2000a).
Sanchez (1997), for example, reported densities of
pink shrimp (Penaeus duorarum) in the Gulf of Mexico to be on average 13 times greater in Thalassia testudinum meadows than on nearby unvegetated soft
substrata.
Thalassia meadows play a key role for fauna in
a number of ways. They serve as a feeding habitat
by offering food in the form of fresh seagrass tissue,
epiphytes, detritus, or associated fauna (Greenway,
1995; Schwamborn and Criales, 2000; Sluka and
Miller, 2001; Kirsch et al., 2002). Recent reports
suggest that the importance of leaf herbivory in seagrass meadows is often seriously under-estimated
(Valentine and Heck, 1991; Cebrian and Duarte,
1998; Valentine and Heck, 1999), and highly variable among seagrass species (Cebrian et al., 1998).
Few studies have considered the possibility that herbivores may stimulate rates of primary production,
or the role of belowground nutrient reserves in determining the impacts of grazers on seagrasses (Valentine and Heck, 1999; Stapel and Erftemeijer, 2000).
The meadows also supply a substrate for settlement
for bivalve spat (Bologna and Heck, 2000), juvenile
sea cucumbers (Mercier et al., 2000) or epiphytic
hydroids and foraminifers (Kaehler and Hughes,
1992; Fujita and Hallock, 1999). The importance
of Thalassia meadows as nursery grounds for fishes
and crustaceans is well documented (Aliaume et al.,
1993; Erftemeijer and Allen, 1993; Arrivillaga and
Baltz, 1999; Thayer et al., 1999; Nagelkerken et al.,
2000; Spitzer et al., 2000). One of the factors underlying the attractiveness of Thalassia meadows to
juvenile fishes, shrimps, and other fauna is the provision of shelter by the seagrass canopy, which significantly reduces vulnerability to predation (Ray and
Stoner, 1995; Jordan et al., 1997; Rooker et al., 1998;
Peterson et al., 2001).
The associated fauna, in turn, influence Thalassia growth through herbivory on the seagrass tissue
(Valentine et al., 1997; Sluka and Miller, 2001), leaf
burrowing (van Tussenbroek and Brearley, 1998),
epiphyte grazing (Gacia et al., 1999; Heck et al.,
2000), and fragmentation of detritus (Lawson et al.,
1993; Stoner et al., 1995). Associated fauna may
also affect nutrient cycling, substrate stability, and
water quality in Thalassia meadows through bioturbation (Valentine et al., 1994; Stapel and Erftemeijer,
2000), and suspension feeding (Peterson and Heck,
1999, 2001; Prager and Halley, 1999).
The echinoderm fauna is some of the most conspicuous component of Thalassia seagrass communities. Sea urchins (Echinoids) comprise the main
grazers in the Thalassia ecosystem. Folivory by the
sea urchin Lytechinus variegatus increases the shoot
density and productivity of Thalassia testudinum in
Florida, and controls its seasonal changes in abundance (Valentine et al., 1997, 2000). Sea urchin
grazing in enclosures dramatically affected seagrass
habitat structure for long periods of time (>3.5 year).
Intermittent grazing produced significant reductions
in aboveground plant biomass compared to controls, while continuous grazing produced apparently
permanent loss of seagrasses (Heck and Valentine,
1995). In contrast to findings reported for vertebrate
herbivores, sea urchins were found to feed at higher
rates when offered Thalassia leaves of lower nitrogen
content, compensating for the lower nitrogen levels
by eating more leaves than in treatments where leaf
nitrogen was experimentally elevated using commercial fertilizer (Valentine and Heck, 2001). This
demonstrates that low levels of leaf nitrogen are not
always an effective defense against herbivores. A
demographic study of L. variegatus in the Gulf of
Mexico revealed a catastrophic mortality caused by
a cold front in the spring of 1993, leading to a dramatic decline in sea urchin densities in a Thalassia testudinum meadow. After a pronounced recruitment event in the fall of 1993, the population at this
site recovered within a period of approximately 6
months, which was attributed to the immigration of
new adults (Beddingfield and McClintock, 2000). In
Indonesia, the edible sea urchin Tripneustes gratilla
was observed to prefer grazing on attached fronds of
Thalassia hemprichii, foraging continually throughout the day and night, without evidence of any periodicity (Klumpp et al., 1993). T. gratilla consumed
only 1–5% of Thalassia production between March
and July, while between November and March the
sea urchin consumption increased to over 50% of
seagrass production. They concluded that on an