10
1. INTRODUCTION
simulation of the physical environment, including hydrodynamics and local
light intensities, has become feasible. The advances in developmental biology
may even make it possible that diffusive patterning mechanisms and gradi -
ents of morphogens to be included and represented by similar computational
models.
There exists a fairly large amount of information on experiments done
in vivo and in vitro with marine sessile organisms and the impact of the
physical environment. Water flow has a strong influence on the distribution of food particles in the immediate environment of suspension feeders.
In several studies (for example Frechette et al. 1989; Buss and Jackson 1981;
Pile et al. 1997) it was demonstrated that locally around a sessile suspension
feeder areas may occur which are depleted in food particles. There is also
a (species-specific) relation between the ability of species to capture food particles and the flow rate. A review of the effects of flow on suspension feeders
is given by Shimeta and [umars (1991). Water movement also has an important mechanical impact on marine sessile organisms (see Koehl 1998, Vogel
1994), which might have very different effects on flexible organisms, for example many seaweeds, and rigid organisms, for example the stony corals.
Mechanical forces may lead to breakage and abrasion in the organisms, but
also strongly influence the capture of food particles. In the photosynthetic
marine sessile organisms, for example the seaweeds and many of the stony
corals, the available local light intensity is a dominant environmental parameter (Goreau et al. 1971). In many of these organisms the physiology is
adapted to a certain range of light intensities and wave lengths, allowing the
species to inhabit a certain range of depths.
A third reason why simulation models of growth and form are required
is that this type of model might be useful to study the regeneration capabilities and the impact of changes in the global climate on the growth of,
for instance, reef organisms. Recently coral reefs have been affected severely,
on a worldwide scale, by so-called "coral bleaching". Coral bleaching occurs
when the coral colony expels its photosynthetic symbionts, the zooxantellae,
leading to a white-colored "bleached" colony. The process is reversible, but if
this bleaching event continues for a longer period, the colony will ultimately
die. There is currently much evidence that elevated sea temperatures, in
combination with increased ultraviolet radiation, is the main cause of mass
bleaching events (see Lesser 1996). In 1998 the most severe coral bleaching
event until now was observed worldwide. The bleaching event coincided
with elevated sea temperatures during a strong El Nino period in 1998. The
consequences of this event were summarized by the International Tropical
Marine Ecosystems Management Symposium:
"A summit meeting on coral bleaching by world experts on coral bleaching
held in Townsville on 24 November 1998 released the following statement
on the status of reefs following the 1998 global coral bleaching event."
"Tropical sea surface temperatures in 1997/98 have been higher than
at any other time in the modern record. Record sea surface temperature
increases over the tropics in the past 15 years are not explained by existing climate models. The coral bleaching associated with the high sea
surface temperatures has affected almost all species of corals. Loss of some
corals more than 1000 years old indicates the severity of this event. Associated reef invertebrates have been severely affected by unusually high sea
temperatures."
1. INTRODUCTION
simulation of the physical environment, including hydrodynamics and local
light intensities, has become feasible. The advances in developmental biology
may even make it possible that diffusive patterning mechanisms and gradi -
ents of morphogens to be included and represented by similar computational
models.
There exists a fairly large amount of information on experiments done
in vivo and in vitro with marine sessile organisms and the impact of the
physical environment. Water flow has a strong influence on the distribution of food particles in the immediate environment of suspension feeders.
In several studies (for example Frechette et al. 1989; Buss and Jackson 1981;
Pile et al. 1997) it was demonstrated that locally around a sessile suspension
feeder areas may occur which are depleted in food particles. There is also
a (species-specific) relation between the ability of species to capture food particles and the flow rate. A review of the effects of flow on suspension feeders
is given by Shimeta and [umars (1991). Water movement also has an important mechanical impact on marine sessile organisms (see Koehl 1998, Vogel
1994), which might have very different effects on flexible organisms, for example many seaweeds, and rigid organisms, for example the stony corals.
Mechanical forces may lead to breakage and abrasion in the organisms, but
also strongly influence the capture of food particles. In the photosynthetic
marine sessile organisms, for example the seaweeds and many of the stony
corals, the available local light intensity is a dominant environmental parameter (Goreau et al. 1971). In many of these organisms the physiology is
adapted to a certain range of light intensities and wave lengths, allowing the
species to inhabit a certain range of depths.
A third reason why simulation models of growth and form are required
is that this type of model might be useful to study the regeneration capabilities and the impact of changes in the global climate on the growth of,
for instance, reef organisms. Recently coral reefs have been affected severely,
on a worldwide scale, by so-called "coral bleaching". Coral bleaching occurs
when the coral colony expels its photosynthetic symbionts, the zooxantellae,
leading to a white-colored "bleached" colony. The process is reversible, but if
this bleaching event continues for a longer period, the colony will ultimately
die. There is currently much evidence that elevated sea temperatures, in
combination with increased ultraviolet radiation, is the main cause of mass
bleaching events (see Lesser 1996). In 1998 the most severe coral bleaching
event until now was observed worldwide. The bleaching event coincided
with elevated sea temperatures during a strong El Nino period in 1998. The
consequences of this event were summarized by the International Tropical
Marine Ecosystems Management Symposium:
"A summit meeting on coral bleaching by world experts on coral bleaching
held in Townsville on 24 November 1998 released the following statement
on the status of reefs following the 1998 global coral bleaching event."
"Tropical sea surface temperatures in 1997/98 have been higher than
at any other time in the modern record. Record sea surface temperature
increases over the tropics in the past 15 years are not explained by existing climate models. The coral bleaching associated with the high sea
surface temperatures has affected almost all species of corals. Loss of some
corals more than 1000 years old indicates the severity of this event. Associated reef invertebrates have been severely affected by unusually high sea
temperatures."