Chapter 22 Predation in Seagrass Beds
545
reefs) and connectivity of patches and these other
habitats (see Bell et al., Chapter 26 for more discussion of the effects of scale).
III. Effects of Seagrass on Predator–Prey
Interactions in Evolutionary Time
As a preamble to this section, we note that the abundance of many wide-ranging predators is clearly
much lower than what it was in earlier centuries
(Jackson et al., 2001; see also Valetine and Duffy,
Chapter 20), and that ecologists have been studying
the effects of predation in conditions that are different from those in which predator–prey relationships
evolved (Dayton et al., 1998; Jackson et al., 2001;
see also Valentine and Duffy, Chapter 20). With this
caveat in mind, we consider the major hypotheses
concerning the evolution of predator–prey interactions among seagrass-associated animals.
We begin by noting that there are a number of wellcamouflaged invertebrates and small fishes that inhabit seagrass meadows. Familiar examples include
many shrimps and crabs, pipe fishes and seahorses,
and many guidebooks include striking photographs
of how effective the camouflage of certain species
appears to be. The usual interpretation of such cryptic appearances is that they have evolved over many
thousands of generations (i.e. in evolutionary time)
as a means of reducing the effectiveness of predators. However, there have been few attempts to test
the effect of removing or changing the structures or
colors suspected to provide the camouflage on survival rates. A potentially confounding factor is that,
if the protection is provided by living organisms such
as algae or invertebrates, chemical defenses, instead
of camouflage, may be the primary means by which
refuge from predation is obtained (Stachowicz and
Hay, 2000). Thus, carefully designed experiments
capable of distinguishing between the alternatives
of camouflage or chemical defense are required to
evaluate additional suspected cases of camouflaging
behavior involving living organisms.
Several types of behaviors suggest that predation
has affected seagrass-associated animals in important ways. The work of Main (1985, 1987) described
above, showed how stereotyped adult behavior could
evolve in response to the threat of predation. Larval
behavior can also be affected, as the preference for
seagrass shoots by settling blue crab (Callinectes
sapidus) megalopae clearly demonstrates (Forward
et al., 1996). This estuarine crab preferentially settles from the plankton into seagrass meadows [and
can even be induced to settle by the extract of seagrass shoots (Forward et al., 1996)], where predation
rates on juveniles are significantly lower than in adjacent unvegetated substrates (Heck et al., 2001; Orth
and van Montfrans, 2002). Because there are only a
few other investigations of these types, there remains
much to learn about relationships between predation
risk and prey behavior.
There is also evidence that predation rates on
crabs in seagrass meadows vary latitudinally, with
higher predation rates in tropical than temperate
meadows in the western Atlantic Ocean (Heck and
Wilson, 1987; Heck and Coen, 1995). This is consistent with other demonstrations of higher intensity
of predation in tropical intertidal zones (Bertness
et al., 1981; Menge and Lubchenko, 1981), as well as
Vermeij’s (1987) well known thesis that higher rates
of predation by durophages (shell-crushing predators) in tropical regions have led to the evolution
of better armored and predator-resistant prey in low
latitudes. With only a few studies carried out so
far, here too there are many opportunities for more
and better testing of the hypotheses that seagrassassociated animals will be better armored and more
resistant to crushing predators than temperate analogues. Similar comparisons of whether latitudinal
patterns in chemical defenses exist in seagrass inhabitants would also be interesting.
The protection provided by seagrasses has likely
led to many indirect effects on the suite of species
currently associated with seagrass meadows. While
very few well-understood examples exist, we expect
that there are many complex relationships remaining to be discovered. The few cases that have been
investigated all involve suspension-feeding bivalves
that survive at enhanced rates in seagrass meadows
than elsewhere. Reusch et al. (1994) and Reusch
and Williams (1998) have shown that native mussels
may enhance eelgrass growth by fertilizing pore waters, but also that an introduced mussel species can
compete successfully with seagrasses for space belowground. Peterson and Heck (2001) demonstrated
that not only do mussels enhance the growth of turtle
grass by fertilizing pore waters, but they also support
additional numbers of grazers that more effectively
prevent epiphytic algae from growing on seagrass
leaves. Thus, it appears that both mussels and turtle
grass experience elevated survival or growth when
co-occurring than when growing alone.
545
reefs) and connectivity of patches and these other
habitats (see Bell et al., Chapter 26 for more discussion of the effects of scale).
III. Effects of Seagrass on Predator–Prey
Interactions in Evolutionary Time
As a preamble to this section, we note that the abundance of many wide-ranging predators is clearly
much lower than what it was in earlier centuries
(Jackson et al., 2001; see also Valetine and Duffy,
Chapter 20), and that ecologists have been studying
the effects of predation in conditions that are different from those in which predator–prey relationships
evolved (Dayton et al., 1998; Jackson et al., 2001;
see also Valentine and Duffy, Chapter 20). With this
caveat in mind, we consider the major hypotheses
concerning the evolution of predator–prey interactions among seagrass-associated animals.
We begin by noting that there are a number of wellcamouflaged invertebrates and small fishes that inhabit seagrass meadows. Familiar examples include
many shrimps and crabs, pipe fishes and seahorses,
and many guidebooks include striking photographs
of how effective the camouflage of certain species
appears to be. The usual interpretation of such cryptic appearances is that they have evolved over many
thousands of generations (i.e. in evolutionary time)
as a means of reducing the effectiveness of predators. However, there have been few attempts to test
the effect of removing or changing the structures or
colors suspected to provide the camouflage on survival rates. A potentially confounding factor is that,
if the protection is provided by living organisms such
as algae or invertebrates, chemical defenses, instead
of camouflage, may be the primary means by which
refuge from predation is obtained (Stachowicz and
Hay, 2000). Thus, carefully designed experiments
capable of distinguishing between the alternatives
of camouflage or chemical defense are required to
evaluate additional suspected cases of camouflaging
behavior involving living organisms.
Several types of behaviors suggest that predation
has affected seagrass-associated animals in important ways. The work of Main (1985, 1987) described
above, showed how stereotyped adult behavior could
evolve in response to the threat of predation. Larval
behavior can also be affected, as the preference for
seagrass shoots by settling blue crab (Callinectes
sapidus) megalopae clearly demonstrates (Forward
et al., 1996). This estuarine crab preferentially settles from the plankton into seagrass meadows [and
can even be induced to settle by the extract of seagrass shoots (Forward et al., 1996)], where predation
rates on juveniles are significantly lower than in adjacent unvegetated substrates (Heck et al., 2001; Orth
and van Montfrans, 2002). Because there are only a
few other investigations of these types, there remains
much to learn about relationships between predation
risk and prey behavior.
There is also evidence that predation rates on
crabs in seagrass meadows vary latitudinally, with
higher predation rates in tropical than temperate
meadows in the western Atlantic Ocean (Heck and
Wilson, 1987; Heck and Coen, 1995). This is consistent with other demonstrations of higher intensity
of predation in tropical intertidal zones (Bertness
et al., 1981; Menge and Lubchenko, 1981), as well as
Vermeij’s (1987) well known thesis that higher rates
of predation by durophages (shell-crushing predators) in tropical regions have led to the evolution
of better armored and predator-resistant prey in low
latitudes. With only a few studies carried out so
far, here too there are many opportunities for more
and better testing of the hypotheses that seagrassassociated animals will be better armored and more
resistant to crushing predators than temperate analogues. Similar comparisons of whether latitudinal
patterns in chemical defenses exist in seagrass inhabitants would also be interesting.
The protection provided by seagrasses has likely
led to many indirect effects on the suite of species
currently associated with seagrass meadows. While
very few well-understood examples exist, we expect
that there are many complex relationships remaining to be discovered. The few cases that have been
investigated all involve suspension-feeding bivalves
that survive at enhanced rates in seagrass meadows
than elsewhere. Reusch et al. (1994) and Reusch
and Williams (1998) have shown that native mussels
may enhance eelgrass growth by fertilizing pore waters, but also that an introduced mussel species can
compete successfully with seagrasses for space belowground. Peterson and Heck (2001) demonstrated
that not only do mussels enhance the growth of turtle
grass by fertilizing pore waters, but they also support
additional numbers of grazers that more effectively
prevent epiphytic algae from growing on seagrass
leaves. Thus, it appears that both mussels and turtle
grass experience elevated survival or growth when
co-occurring than when growing alone.