reef organisms. The appearance of these herbivores
paralleled profound changes in reef ecology, including
the rise of well-defended, highly tolerant coralline algae
(Steneck, 1985), a notable increase in branching corals
since the late Cretaceous (Jackson and McKinney,
1991), and the loss of many functional organisms that
prove to be intolerant to excavatory attack (Table 1). This
suggests a cause–effect system, where adaptation to predatory attack has been intimately bound to the origin and
assembly of modern reefs.
Secure attachment to a hard substrate
Organisms without secure attachment to a stable substrate
are susceptible to the effects of all disturbances, including
bioturbation, as inversion and burial will cause death.
Most modern suspension feeders require a hard substrate,
even if these are only isolated patches within areas of
unstable, soft substrate.
Possession of an edge zone in all but the most primitive
scleractinian corals allows them to gain permanent attachment to a stable substrate. As a result, scleractinian corals
dominate modern reef framework environments, especially those in high-energy settings, where there is also
an abundance of wave-swept, extensive hard substrata
for colonization. Permanent attachment also allows the
development of very large branching morphologies.
Cambrian archaeocyath sponges usually bore small
holdfasts that enabled limited attachment to hard substrates (Figure 7a). But many mid-to- late Palaeozoic reefs
were dominated by large, sheet-like invertebrates
(stromatoporoid sponges, tabulate and rugose corals, and
trepostome and cystoporate bryozoans) that were initially
attached to small, ephemeral skeletal debris and then grew
over the surrounding sediment (Figure 5). Small,
branching forms (some stromatoporoids and bryozoans)
lacking extensive attachment sites were also common,
and they were presumably partially rooted in soft
sediment.
The late Paleozoic decline of immobile epifauna coincides with the rise of major bulldozing taxa, which passed
through the end-Permian extinction unscathed. This
coincidence must remain conjectural until tested
experimentally.
Resistance to partial mortality
Predation that actively excavates underlying skeleton
often results only in partial mortality, that is, sublethal
damage. In such cases, the capacity of the prey to heal or
replace damaged areas of soft tissue becomes critical to
survival. Strategies that rely upon herbivores/predators
to remove competing algae therefore often entail the loss
of the prey’s own tissues.
In coralline algae, a protective outer epithallus and conceptacles that contain reproductive structures have been
demonstrated to protect the delicate reproductive anatomy
from intensive grazing (Steneck, 1982). Many coralline
algae can also tolerate intense herbivory due to their ability to rapidly regenerate removed material. Thickened
crusts are more tolerant to attack than thin encrusting or
branching forms (Steneck, 1985), but in modern reefs,
the dominance of a particular growth form appears to be
a trade-off between the cost of investment in increased
defence, and the reduction in growth rate or competitive
ability. As a result, thickened crusts dominate only in areas
of high wave energy and biological disturbance.
After the Eocene, herbivore-susceptible, delicately
branched coralline algae reduced in abundance in the tropics, the proportion of thickened encrusting forms
increased, and the first algal ridges appeared – all coincident with the rise of excavatory herbivorous fish.
Many sessile reef organisms possess a modular or colonial habit where partial predation and boring may remove
either individual or a few modules, or large areas may be
cleared of living tissue, sometimes together with the excavation of underlying skeleton. But the modular organization also reduces soft tissue to a relatively thin veneer
over a larger basal skeleton. This not only decreases accessibility and the ease of prey manipulation by predators but
also minimizes the tissue biomass while maximizing the
cost of collection. For example, in a typical domal colony
of Porites, only about 0.5% of the colony’s radius is occupied by soft tissue (Rosen, 1986). In branching and platy
colony forms, the relative proportion of skeleton is even
higher.
Cambrian archaeocyath sponges show a steady and
marked increase in the proportion of complex modular
forms during their history (Wood et al., 1992), as do
scleractinian corals since the mid-Triassic, which appears
to be uninterrupted by the end-Cretaceous extinction event
(Coates and Jackson, 1985).
Regeneration after breakage
Some morphologies are more resistant to breakage than
others. For example, colonies with closely spaced
branches can make predator access difficult by forming
hidden, protected areas. The flattening of branch terminations can also offer greater resistance to all forms of breakage and shearing, and this character is found in erect
species of bryozoans, gorgonian corals and stylasterine
corals.
A multiserial modular organization, however, in addition to promoting architectural diversity and flexibility,
also allows compartmentalization of damage and enables
some colonies to regenerate from fragments. Most significantly, branching corals also show tremendous powers of
regeneration: Acropora palmata has one of the highest
rates recorded. Indeed, unlike massive, platy or encrusting
forms, damage to branching corals often leads to an immediate increase in growth rate so causing an increase in size
rather than simply repairing damaged tissue. The staghorn
coral (Acropora cervicornis) is able to re-anchor fragments and rapidly regenerate and grow, often fusing with
other colonies. Such branching corals have turned adversity into considerable advantage, and appear to flourish
because, and not in spite, of breakage.
466
GENERAL EVOLUTION OF CARBONATE REEFS
paralleled profound changes in reef ecology, including
the rise of well-defended, highly tolerant coralline algae
(Steneck, 1985), a notable increase in branching corals
since the late Cretaceous (Jackson and McKinney,
1991), and the loss of many functional organisms that
prove to be intolerant to excavatory attack (Table 1). This
suggests a cause–effect system, where adaptation to predatory attack has been intimately bound to the origin and
assembly of modern reefs.
Secure attachment to a hard substrate
Organisms without secure attachment to a stable substrate
are susceptible to the effects of all disturbances, including
bioturbation, as inversion and burial will cause death.
Most modern suspension feeders require a hard substrate,
even if these are only isolated patches within areas of
unstable, soft substrate.
Possession of an edge zone in all but the most primitive
scleractinian corals allows them to gain permanent attachment to a stable substrate. As a result, scleractinian corals
dominate modern reef framework environments, especially those in high-energy settings, where there is also
an abundance of wave-swept, extensive hard substrata
for colonization. Permanent attachment also allows the
development of very large branching morphologies.
Cambrian archaeocyath sponges usually bore small
holdfasts that enabled limited attachment to hard substrates (Figure 7a). But many mid-to- late Palaeozoic reefs
were dominated by large, sheet-like invertebrates
(stromatoporoid sponges, tabulate and rugose corals, and
trepostome and cystoporate bryozoans) that were initially
attached to small, ephemeral skeletal debris and then grew
over the surrounding sediment (Figure 5). Small,
branching forms (some stromatoporoids and bryozoans)
lacking extensive attachment sites were also common,
and they were presumably partially rooted in soft
sediment.
The late Paleozoic decline of immobile epifauna coincides with the rise of major bulldozing taxa, which passed
through the end-Permian extinction unscathed. This
coincidence must remain conjectural until tested
experimentally.
Resistance to partial mortality
Predation that actively excavates underlying skeleton
often results only in partial mortality, that is, sublethal
damage. In such cases, the capacity of the prey to heal or
replace damaged areas of soft tissue becomes critical to
survival. Strategies that rely upon herbivores/predators
to remove competing algae therefore often entail the loss
of the prey’s own tissues.
In coralline algae, a protective outer epithallus and conceptacles that contain reproductive structures have been
demonstrated to protect the delicate reproductive anatomy
from intensive grazing (Steneck, 1982). Many coralline
algae can also tolerate intense herbivory due to their ability to rapidly regenerate removed material. Thickened
crusts are more tolerant to attack than thin encrusting or
branching forms (Steneck, 1985), but in modern reefs,
the dominance of a particular growth form appears to be
a trade-off between the cost of investment in increased
defence, and the reduction in growth rate or competitive
ability. As a result, thickened crusts dominate only in areas
of high wave energy and biological disturbance.
After the Eocene, herbivore-susceptible, delicately
branched coralline algae reduced in abundance in the tropics, the proportion of thickened encrusting forms
increased, and the first algal ridges appeared – all coincident with the rise of excavatory herbivorous fish.
Many sessile reef organisms possess a modular or colonial habit where partial predation and boring may remove
either individual or a few modules, or large areas may be
cleared of living tissue, sometimes together with the excavation of underlying skeleton. But the modular organization also reduces soft tissue to a relatively thin veneer
over a larger basal skeleton. This not only decreases accessibility and the ease of prey manipulation by predators but
also minimizes the tissue biomass while maximizing the
cost of collection. For example, in a typical domal colony
of Porites, only about 0.5% of the colony’s radius is occupied by soft tissue (Rosen, 1986). In branching and platy
colony forms, the relative proportion of skeleton is even
higher.
Cambrian archaeocyath sponges show a steady and
marked increase in the proportion of complex modular
forms during their history (Wood et al., 1992), as do
scleractinian corals since the mid-Triassic, which appears
to be uninterrupted by the end-Cretaceous extinction event
(Coates and Jackson, 1985).
Regeneration after breakage
Some morphologies are more resistant to breakage than
others. For example, colonies with closely spaced
branches can make predator access difficult by forming
hidden, protected areas. The flattening of branch terminations can also offer greater resistance to all forms of breakage and shearing, and this character is found in erect
species of bryozoans, gorgonian corals and stylasterine
corals.
A multiserial modular organization, however, in addition to promoting architectural diversity and flexibility,
also allows compartmentalization of damage and enables
some colonies to regenerate from fragments. Most significantly, branching corals also show tremendous powers of
regeneration: Acropora palmata has one of the highest
rates recorded. Indeed, unlike massive, platy or encrusting
forms, damage to branching corals often leads to an immediate increase in growth rate so causing an increase in size
rather than simply repairing damaged tissue. The staghorn
coral (Acropora cervicornis) is able to re-anchor fragments and rapidly regenerate and grow, often fusing with
other colonies. Such branching corals have turned adversity into considerable advantage, and appear to flourish
because, and not in spite, of breakage.
466
GENERAL EVOLUTION OF CARBONATE REEFS