84
H. U. Riisgard and P. Kamermans
4.1.6 Most Examples Among Passive Suspension Feeders
From the above examination of the literature dealing with possible switching
between deposit and suspension feeding, it is striking that most examples are
found among passive suspension feeders that strain food particles from the
near-bottom current without metabolic energetic costs. This attracts attention to possible prerequisites and limitations of adaptation to active suspension feeding.
4.1.7 Adaptation to Suspension Feeding
To assess the adaptation of a suspension feeder to the biotope it is of interest
to know the food energy uptake in relation to the total metabolic energy
requirement of the animal. The latter may conveniently be expressed as the
respiration (R) in amount of oxygen consumed. The former may be expressed
simply as volume of water pumped through the filter device (F) times the food
particle concentration, and will thus depend on the prevailing phytoplankton
concentration. The ratio FIR expresses the lit res of water pumped per ml 02
consumed and may be used as a tool to characterise true suspension feeding
on phytoplankton. A typical FIR-value of 10 1 of water pumped per ml of
oxygen consumed was reported by J0rgensen (1975) as the minimum to
ensure the performance of suspension feeders inhabiting inshore waters.
Using the above FIR-value as a guide, it is possible to discriminate between
suspension feeders and deposit feeders (see Table 4.1). It can be seen that
Nereis diversicolor is able to compete with obligate suspension feeders, like the
blue mussel Mytilus edulis, but the same is obviously not the case for Macoma
balthica or Arenicola marina.
The low concentrations of phytoplankton in the sea is the key to understanding the characteristics of active, obligate suspension feeders which seem
to have evolved according to 'a principle of minimal scaling' according to
which the dimensions of the filter-pumps are sufficient to enable continuous
feeding at low rates, rather than discontinuous feeding at a correspondingly
high rate (J0rgensen 1975). Studies during recent years have shown that active
suspension feeders operate low-energy pumps that continuously process the
surrounding water through filters appropriately dimensioned to cope with the
phytoplankton concentrations of the biotope (Riisgard and Larsen 1995). The
increasing evidence that suspension feeding is based on 'minimal scaling' may
be the key to understanding why switching between deposit and suspension
feeding in Nereis diversicolor is likely to be an exception: Referring to metabolic powers estimated for Mytilus edulis by Riisgard and Larsen (1995) it can
be argued that other measures of efficiency than the overall filter-pump
efficiency (i. e. useful pumping power/total metabolic rate) are relevant. Thus,
H. U. Riisgard and P. Kamermans
4.1.6 Most Examples Among Passive Suspension Feeders
From the above examination of the literature dealing with possible switching
between deposit and suspension feeding, it is striking that most examples are
found among passive suspension feeders that strain food particles from the
near-bottom current without metabolic energetic costs. This attracts attention to possible prerequisites and limitations of adaptation to active suspension feeding.
4.1.7 Adaptation to Suspension Feeding
To assess the adaptation of a suspension feeder to the biotope it is of interest
to know the food energy uptake in relation to the total metabolic energy
requirement of the animal. The latter may conveniently be expressed as the
respiration (R) in amount of oxygen consumed. The former may be expressed
simply as volume of water pumped through the filter device (F) times the food
particle concentration, and will thus depend on the prevailing phytoplankton
concentration. The ratio FIR expresses the lit res of water pumped per ml 02
consumed and may be used as a tool to characterise true suspension feeding
on phytoplankton. A typical FIR-value of 10 1 of water pumped per ml of
oxygen consumed was reported by J0rgensen (1975) as the minimum to
ensure the performance of suspension feeders inhabiting inshore waters.
Using the above FIR-value as a guide, it is possible to discriminate between
suspension feeders and deposit feeders (see Table 4.1). It can be seen that
Nereis diversicolor is able to compete with obligate suspension feeders, like the
blue mussel Mytilus edulis, but the same is obviously not the case for Macoma
balthica or Arenicola marina.
The low concentrations of phytoplankton in the sea is the key to understanding the characteristics of active, obligate suspension feeders which seem
to have evolved according to 'a principle of minimal scaling' according to
which the dimensions of the filter-pumps are sufficient to enable continuous
feeding at low rates, rather than discontinuous feeding at a correspondingly
high rate (J0rgensen 1975). Studies during recent years have shown that active
suspension feeders operate low-energy pumps that continuously process the
surrounding water through filters appropriately dimensioned to cope with the
phytoplankton concentrations of the biotope (Riisgard and Larsen 1995). The
increasing evidence that suspension feeding is based on 'minimal scaling' may
be the key to understanding why switching between deposit and suspension
feeding in Nereis diversicolor is likely to be an exception: Referring to metabolic powers estimated for Mytilus edulis by Riisgard and Larsen (1995) it can
be argued that other measures of efficiency than the overall filter-pump
efficiency (i. e. useful pumping power/total metabolic rate) are relevant. Thus,