2.1. THE PHYSICAL ENVIRONMENT
momentum) of the particle in the fluid, the difference in charge between the
particle and the filter, and the tendency of the particle to sink due to gravity. The size and even species of prey item captured from the water column
by a sessile suspension feeder is thus to a large extent determined both by
the characteristics of the particle and by the interaction of the filter feeder
with the characteristics of the ambient flow environment. Both theoretical
and empirical approaches have been undertaken to address the role of organism morphology in driving prey capture by sessile invertebrates. While
theoretical approaches can offer considerable insight into the factors most
likely to affect particle capture, extrapolation to organisms living in the field
can be difficult. In contrast, empirical measurements account for more of
this natural variability, but in doing so can reduce our ability to generalize .
Both,levels of approach are therefore necessary and complementary in order to gain a better understanding of the interactions of coral and sponge
architecture with the hydrodynamic environment.
SPONGES. As active filter feeders, sponges are able to generate currents
through the action of flagella, which line the walls of the interior of the
sponge. Bacteria and other microscopic food particles are absorbed from the
moving water and incorporated into food vacuoles, and then transported into
the main body of the cell. As a result of this active filtration, sponges are often
able to thrive in areas oflow to moderate flow, where more passive feeders are
excluded (Reiswig 1974, Leichter and Witman 1997). Nonetheless, the overall
morphology and architecture of sponges can also have a significant impact
on particle transport. The intake of water, and thus particulate matter, into
the sponge occurs through myriad pores called ostia . The fluid then travels
through chambers of varying length and complexity to a central chamber,
where the water is expelled through a large opening called an osculum. Generally, the total combined surface area of the incurrent pores exceeds that of the
exhalant osculum (Bidder 1923, Vogel1974). The effect of the reduced surface
area is thus akin to a jet, and water exiting the sponge is accelerated, reducing
the chances that the sponge will refilter the water that it just processed.
Sponges and other sponge-like organisms also benefit from water movement not induced by flagellar action. As water flows around and over the top
of a sponge , the fluid is accelerated. This faster-moving water induces a region oflower pressure, which in concert with viscous entrainment within the
fluid serves to induce flows out of the top of the sponge, further enhancing
the transport of water throughout the organism (Vogel1974). The movement
of water outside the sponge also serves to replenish nutrient- and particledepleted water, as sponges at high densities can compete for food resources
with one another (Buss and Jackson 1981).
CORALS. While corals display some ciliary activity within the coelenteron,
and in some cases have been shown to generate weak currents (Helmuth and
Sebens 1993), particle capture is to a large degree dependent on the delivery of
zooplankton and particulate matter directly to the coral's tentacles, where the
particle is ensnared by a series of harpoon-like nematocysts. The interaction
of ambient flow with a coral's morphology thus can have a significant impact
on rates of particle capture, as can the presence of neighboring organisms.
Quantifying the interaction of coral morphology with flow is, however, a very complex undertaking, especially given the wide array of flow
27
momentum) of the particle in the fluid, the difference in charge between the
particle and the filter, and the tendency of the particle to sink due to gravity. The size and even species of prey item captured from the water column
by a sessile suspension feeder is thus to a large extent determined both by
the characteristics of the particle and by the interaction of the filter feeder
with the characteristics of the ambient flow environment. Both theoretical
and empirical approaches have been undertaken to address the role of organism morphology in driving prey capture by sessile invertebrates. While
theoretical approaches can offer considerable insight into the factors most
likely to affect particle capture, extrapolation to organisms living in the field
can be difficult. In contrast, empirical measurements account for more of
this natural variability, but in doing so can reduce our ability to generalize .
Both,levels of approach are therefore necessary and complementary in order to gain a better understanding of the interactions of coral and sponge
architecture with the hydrodynamic environment.
SPONGES. As active filter feeders, sponges are able to generate currents
through the action of flagella, which line the walls of the interior of the
sponge. Bacteria and other microscopic food particles are absorbed from the
moving water and incorporated into food vacuoles, and then transported into
the main body of the cell. As a result of this active filtration, sponges are often
able to thrive in areas oflow to moderate flow, where more passive feeders are
excluded (Reiswig 1974, Leichter and Witman 1997). Nonetheless, the overall
morphology and architecture of sponges can also have a significant impact
on particle transport. The intake of water, and thus particulate matter, into
the sponge occurs through myriad pores called ostia . The fluid then travels
through chambers of varying length and complexity to a central chamber,
where the water is expelled through a large opening called an osculum. Generally, the total combined surface area of the incurrent pores exceeds that of the
exhalant osculum (Bidder 1923, Vogel1974). The effect of the reduced surface
area is thus akin to a jet, and water exiting the sponge is accelerated, reducing
the chances that the sponge will refilter the water that it just processed.
Sponges and other sponge-like organisms also benefit from water movement not induced by flagellar action. As water flows around and over the top
of a sponge , the fluid is accelerated. This faster-moving water induces a region oflower pressure, which in concert with viscous entrainment within the
fluid serves to induce flows out of the top of the sponge, further enhancing
the transport of water throughout the organism (Vogel1974). The movement
of water outside the sponge also serves to replenish nutrient- and particledepleted water, as sponges at high densities can compete for food resources
with one another (Buss and Jackson 1981).
CORALS. While corals display some ciliary activity within the coelenteron,
and in some cases have been shown to generate weak currents (Helmuth and
Sebens 1993), particle capture is to a large degree dependent on the delivery of
zooplankton and particulate matter directly to the coral's tentacles, where the
particle is ensnared by a series of harpoon-like nematocysts. The interaction
of ambient flow with a coral's morphology thus can have a significant impact
on rates of particle capture, as can the presence of neighboring organisms.
Quantifying the interaction of coral morphology with flow is, however, a very complex undertaking, especially given the wide array of flow
27