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A combined tidal simulator and actograph for marine animals
137
J. Exp. Mar. Biol. Ecol., 1989, Vol. 125, pp. 137-143 Elsevier
JEM 01198
A combined tidal simulator and actograph for marine animals D. G. Reid, S. R. L. Bolt, D.A. Davies and E. Naylor School of Ocean Sciences, Universiiy College of North Wales, Bangor, Gwynedd, U.K. (Received 26 July 1988; revision received 17 October 1988; accepted 28 October 1988) Abstract: A system is described that controls salinity, temperature, and hydrostatic pressure and monitors locomotor activity in a compartmentalised laboratory animal chamber. The system uses a microprocessor to implement a feedback control technique and is capable of mimicking any commonly experienced variations in the above variables. It incorporates a new microcomputer-controlled valve system to control hydrostatic pressure which eliminates the need for large moving structures required by all previous designs. Key words: Activity recording; Microcomputer; Pressure cycle; Tidal simulation
INTRODUCTION
All animals and plants that live or forage in the intertidal zone and immediate subtidal regions of the shore experience a wide range of variation in many physico-chemical variables. Most important among these are changes in salinity, temperature, and hydrostatic pressure associated with the rise and fall of the tides. Any study of adaptation to this environment will require the ability to manipulate all these variables in the laboratory and to study the responses of organisms to them. Such manipulation requires the construction of some form of tidal simulator which is able to mimic the variable environment of the shore. Many such machines have been designed in the past (see reviews by Underwood, 1972; Morgan, 1984) but few, if any, are capable of reliably controlling all three of the variables described above. Graham et al. (1987) set out six conditions which such a tidal simulator should satisfy. Briefly, it should have a wide and easily adjustable cycle period, have a large cycle amplitude, hold a reasonably large number of animals, have a how-through water supply, log activity automati~~ly, and have the facility to transfer logged data direct into a computer. Graham et al. applied these conditions to a system for controlling pressure change only but they are appropriate also to a system which controls all three of the above variables. To these requirements we would add two more. First, a simulator should operate on a feedback control basis so that the variables under consideration are monitored continuously and Correspondence address: D.G. Reid, School of Ocean Sciences, University College of North Wales, Bangor, Gwynedd LLS7 ZUW, U.K. 0022-0981/89/$03.50 0 1989 Elsevier Science ~b~shers
B.V. (Biomedical Division)
138
D.G. REID ETAL.
then adjusted automatically to comply with the desired regime. Secondly, the system should be able to generate a range of regimes to match conditions on different shores and under different conditions. The design presented here fulfils all these requirements, it is generally trouble free (usually requiring only cleaning), easy to install and requires little space in which to operate. DESIGN AND CONSTRUCTION CONTROLOF TEMPERATUREAND SALINITY The first stage of the system is responsible for monitoring and controlling the temperature and salinity of the water in the header tank and for delivering it to the animal chamber. This section is developed from the system described by Bolt & Naylor (1985) (see Fig. 1). It consists mainly of a single tank in which fresh- and seawater are mixed and brought to the desired temperature. Input of water is controlled by two solenoid valves (Huba 304; Swissenco), one for fresh-, the other for seawater. Temperature is controlled by the cooling coil (Let Refrigeration Ltd.) and by a Tempette water heater. The valves and the heater are switched using power relays (Hamlin Solid-state 2.5A 7653: Mechanical 20A 65.31) connected to the input/output (I/O) port of the microcomputer (BBC, Model “B”). A temperature/salinity probe (designed and built at the Department of Oceanography, University of Southampton) is mounted in the mixing tank and is monitored continuously by the microcomputer. The probe generates two proportional output voltages, one for temperature and one for salinity. These can be connected directly to the analog-to-digital converter (ADC) of the microcomputer. Once calibrated, these readings can be compared to the required levels, pre-set in the computer, by the operator. Salinity readings are accurate to the nearest 0. lx0 and temperature readings to the nearest 0.1 “C. Any deviation from the pre-set levels will cause the computer to change the input to the tank. For instance, if the actual salinity in the tank is lower than the required level, the computer will open the salt water valve and close the freshwater valve, if the salinity is too high the opposite is carried out. The salinity in the tank is monitored every 15 s and the valves opened and closed as required. The cooling coil remains on permanently but, if the temperature is too low, the computer switches on the heater. If the temperature is too high, the heater is switched off and the cooler lowers the temperature. Temperature is monitored once every 2 s. The water is then piped to the animal chamber down z 30 m of flexible reinforced l/2” (12 mm) plastic hose. This provides water of known temperature and salinity with a pressure of 3 atmospheres to the pressure control system which constitutes the second main stage of the construction.
TIDAL SIMULATOR
CONTROL
OF HYDROSTATIC
AND ACTOGRAPH
FOR MARINE MAMMALS
139
PRESSURE
The animal chamber is a box, 40 x 80 x 5 cm deep, which is constructed of 1 cm thick Perspex. The chamber is provided with inflow and outflow pipes and two hatches for the introduction and removal of the animals. The hatches are sealed by rubber “0” rings and wing-nut closures. Other designs of animal chamber could easily be fitted to the system because all the control and monitoring equipment is external to the animal chamber proper. Inflow into the chamber is through a constant flow valve (Platon Flowbits FC/+” B) which gives a constant flow rate of 11. min - i provided the pressure drop across the valve is > 1 atmosphere (other flow rates are available). Outflow from the animal chamber is through a fine control needle valve (Platon Flowbits FCV/l/4” S/M). The valve is controlled by a stepper motor (RS Type 23) controlled via a stepper motor control board (RS Unipolar 2A) by the microcomputer. Pressure in the animal chamber is monitored constantly with a pressure transducer (RS piezo-resistive) which is connected to the ADC of the microcomputer. The precise hydrostatic pressure within the animal chamber is then controlled by varying the setting of the motor driven valve. Because the volume of the animal chamber is constant and water is incompressibIe, any volume of water entering the chamber must be matched by an equal volume leaving. Thus, if the aperture of the outlet valve is reduced, the pressure in the animal chamber must increase to force 11 water * min _ ’ out through the now smaller outlet. Therefore, closing the valve will increase the pressure and opening it will decrease the pressure. For feedback control, the microcomputer reads the input from the pressure transducer once every 10 s and compares this with the pre-set value. If the two values are sufficiently different, the computer will then open or close the valve a fixed number of steps (usually IO) and then re-check the pressure reading 10 s later. It will continue to adjust the valve until the two readings are reasonably close. We found that, provided the two values are within 0.01 atmospheres of each other, no adjustment is necessary; this prevents continuous oscillation around the pre-set value. It is also possible to simulate very low amplitude cycles with this equipment (e.g., ~0.5 m) by reducing the number of steps adjusted after each pressure reading and by increasing the time between readings. With the present design it is possible to control the pressure in the animal chamber to the nearest 0.001 atmosphere (1 cm of water). It should be noted that the accuracy of the system is limited mainly by the performance of the pressure transducer. In addition to the motorised valve outlet, the system is also fitted with two other outlets. The first is a simple shunt which can be set at a particular aperture to control the amplitude and range of the pressure cycles. The second is a safety overllow. The animal chamber used in this design is able to withstand up to 1.5 atmospheres above ambient atmospheric pressure. The water supply from the header tank, however, has a pressure of = 3 atmospheres above ambient and, to prevent accidental subjection of the chamber to this full pressure, a safety outlet is fitted. This outlet consists of a pipe extending to a height of 15 m above the animal chamber (15 m of water = 1.5 atmospheres) and then running to waste. If the pressure in the animal chamber should
D.G. REID ETAL.
140
ever rise to 1.5 atmospheres above ambient, the safety valve will overflow and prevent any further rise (Fig. 1).
TW:=SO I I
MICRO
CR
Fig. 1. Diagrammatic representation of combined system for cycling salinity, temperature, and hydrostatic pressure and recording locomotor activity. Continuous lines represent electrical connections; dotted lines represent piped water connections. AC, animal chamber; ADC, ~~og-to-~~tal converters; C, cooler; CR, cassette recorder; CV, constant flow valve; FS, freshwater supply; H, Tempette water heater; IRD, infrared detector; IRG, infrared beam generator; I/O, input/output port; MP, multiplexer; MV, motorised needle valve; PT, pressure transducer; RB, relay box; Sl, solenoid valve 1; S2, solenoid valve 2; SB, stepper motor control board; SM, stepper motor; SO, safety overflow; ST, salinity/temperature measuring probe; SS, seawater supply; SV, shunt valve; TW, to waste.
TIDAL MICROCOMPUTER
SIMULATOR
AND ACTOGRAPH
FOR MARINE
MAMMALS
141
CONTROL
The control for all three variables described above (temperature, salinity, and hydrostatic pressure) is effected using a feedback loop mechanism. The microcomputer is pre-programmed, usually with a constant or sinusoidal pattern for each variable. It then receives constantly updated readings from the three transducers and by controlling the solenoid valves, Tempette and motorised valve, respectivefy, can alter the salinity, temperature, or pressure to conform with the required value from the pre-set program. The software to control this is very simple and for each variable is similar to the flow diagram for the control of pressure outlined in Fig. 2. Start
t
IS
PR>SV+O-Ol?
Valve
no 5
yeslO:teps
Is PRc SV -0.01? no I
yes
Close Valve =
1
l--Yeps
Fig. 2. Flow diagram describing type of feedback loop used to control hydrostatic pressure. PR, pressure reading from pressure transducer; SV, set value, stored in microcomputer memory.
ACTIVITY
MONITORING
The inside of the animal chamber is divided into eight indi~du~ sections by plastic mesh (Netlon) partitions. Each of the access hatches opens into four of these cages. The activity of the animals in each chamber is monitored using a vertically mounted infra-red beam system (Fig. 1). Such systems are easily built or can be obtained ~o~erci~y. In this case, the output from the infra-red detector is interfaced via a custom-built multiplexer to one of the ADC channels on the microcomputer. Any interruptions of the beam are detected as voltage changes by the computer and recorded as events which are summed every 15 m or other required time period. These values are then downloaded onto magnetic tape with the temperature, salinity, and pressure readings for subsequent analysis. It is atso necessary to consider the possibility that an animal might remain static under the beam. To deal with this possibility, the software is written so that an event is recorded only if the interruption of the beam is a new occurrence. If
142
D. G. REID ETAL.
the beam is interrupted, an event is recorded, but another event will not be recorded until the animal moves out of the beam and then back in again. DISCUSSION
The system described here has three main advantages over the tidal simulation equipment designed by previous workers. First, it is capable of cycling the three main environmental variables on the shore: salinity, temperature, and hydrostatic pressure. Secondly, by using feedback control and transducers which constantly monitor the hydrostatic pressure in the animal chamber and the salinity and temperature in the mixing tank, it is possible to control the conditions precisely to conform with any pre-set pattern. One can impose any required pattern of cyclic variation of these three important tidal variables which are known to affect the behaviour and physiology of animals (Naylor & Williams, 1984). Unlike many previous systems used to control pressure cycles, the one described here has no manometer or tank on a large rotating arm (Graham et al., 1987). Manometer systems involve altering the height of a column of mercury that is in contact with the water in the animal chamber under pressure and such systems preclude the incorporation of control of variations in salinity and temperature. There would also be the danger exposing both animals and researchers to large volumes of mercury. Similar closed designs, using a syringe (Enright, 1962) or compressed air (Digby, 1972), also preclude any control of other parameters. There are also a number of drawbacks with the rotating arm design. First, the amplitude of the pressure cycles is restricted to the size of room in which the machine can be constructed. Secondly, the rotating arm method can normally only produce a sinusoidal form of cycle. Any other pattern would require some form of pressure transducer and feedback mechanism as described here. This would require the rotation of the arm to be controlled by a computer. The system described here is much more flexible because the amplitude of the pressure cycle is restricted only by the pressure of the water supplied and by the strength of the animal chamber. This supply can be either from a header tank, as in this design, or from a pump which would then permit the generation of pressure cycles of as great an amplitude as the animal chamber is capable of withstanding. Using a pump to provide the pressure head is, therefore, a possibility but it would normally result in changes in flow-rate with imposed pressure (see Blaxter & Tytler, 1972). The present design, based partly on a manually operated valve system (Naylor & Atkinson, 1972), avoids the changes in flow rate associated with a pump and functions simply by continuous adjustment of a single tap or valve. Thus, at its simplest, this system requires only two small working areas, one for the mixing header tank (preferably as high in the building as possible) and one for the animal chamber and motorised valve, the two areas connected by a single flexible pipe.
TIDAL
SIMULATOR
AND ACTOGRAPH
FOR MARINE
MAMMALS
143
REFERENCES Blaxter, J.H. S. & P. Tytler, 1972. Pressure discrimination in teleost fish. Symp. Sot. Exp. Biol., Vol. 26, pp. 417-444. Bolt, S.R.L. & E. Naylor, 1985. Interaction of endogenous factors controlling locomotor activity rhythms in Carcirrus exposed to tidal salinity cycles. J. Exp. Mar. Biol. Ecol., Vol. 85, pp. 47-56. Digby, P.S.B., 1972. Detection of small changes in hydrostatic pressure by Crustacea and its relation to the electrode action in the cuticle. Symp. Sot. Exp. Biol., Vol. 26, pp. 445-472. Enright, J.T., 1962. Responses of an amphipod to pressure changes. Camp. Biochem. Physiol., Vol. 7, pp. 131-145. Graham, J. M., R. Bowers & R.N. Gibson, 1987. A versatile tide machine and associated activity recorder. J. Mar. Biol. Assoc. U.K., Vol. 67, pp. 709-716. Morgan, E., 1984. The pressure responses of marine invertebrates: a psychophysical perspective. 2001. J. Linn. Sot., Vol. 80, pp. 209-230. Naylor, E. & R. J. A. Atkinson, 1972. Pressure and the rhythmic behaviour of inshore animals, Symp. Sot. Exp. Biol., Vol. 26, pp. 345-415. Naylor, E. & B.G. Williams, 1984. Environmental entrainment of tidally rhythmic behaviour in marine animals. Zool. J. Linn. Sot., Vol. 80, pp. 201-208. Underwood, A. J., 1972. Sinusoidal tide models: design, construction and laboratory performance, J. E.xp. Mar. Biol. Ecol., Vol. 8, pp. 101-l 11.
J. Exp. Mar. Biol. Ecol., 1989, Vol. 125, pp. 137-143 Elsevier
JEM 01198
A combined tidal simulator and actograph for marine animals D. G. Reid, S. R. L. Bolt, D.A. Davies and E. Naylor School of Ocean Sciences, Universiiy College of North Wales, Bangor, Gwynedd, U.K. (Received 26 July 1988; revision received 17 October 1988; accepted 28 October 1988) Abstract: A system is described that controls salinity, temperature, and hydrostatic pressure and monitors locomotor activity in a compartmentalised laboratory animal chamber. The system uses a microprocessor to implement a feedback control technique and is capable of mimicking any commonly experienced variations in the above variables. It incorporates a new microcomputer-controlled valve system to control hydrostatic pressure which eliminates the need for large moving structures required by all previous designs. Key words: Activity recording; Microcomputer; Pressure cycle; Tidal simulation
INTRODUCTION
All animals and plants that live or forage in the intertidal zone and immediate subtidal regions of the shore experience a wide range of variation in many physico-chemical variables. Most important among these are changes in salinity, temperature, and hydrostatic pressure associated with the rise and fall of the tides. Any study of adaptation to this environment will require the ability to manipulate all these variables in the laboratory and to study the responses of organisms to them. Such manipulation requires the construction of some form of tidal simulator which is able to mimic the variable environment of the shore. Many such machines have been designed in the past (see reviews by Underwood, 1972; Morgan, 1984) but few, if any, are capable of reliably controlling all three of the variables described above. Graham et al. (1987) set out six conditions which such a tidal simulator should satisfy. Briefly, it should have a wide and easily adjustable cycle period, have a large cycle amplitude, hold a reasonably large number of animals, have a how-through water supply, log activity automati~~ly, and have the facility to transfer logged data direct into a computer. Graham et al. applied these conditions to a system for controlling pressure change only but they are appropriate also to a system which controls all three of the above variables. To these requirements we would add two more. First, a simulator should operate on a feedback control basis so that the variables under consideration are monitored continuously and Correspondence address: D.G. Reid, School of Ocean Sciences, University College of North Wales, Bangor, Gwynedd LLS7 ZUW, U.K. 0022-0981/89/$03.50 0 1989 Elsevier Science ~b~shers
B.V. (Biomedical Division)
138
D.G. REID ETAL.
then adjusted automatically to comply with the desired regime. Secondly, the system should be able to generate a range of regimes to match conditions on different shores and under different conditions. The design presented here fulfils all these requirements, it is generally trouble free (usually requiring only cleaning), easy to install and requires little space in which to operate. DESIGN AND CONSTRUCTION CONTROLOF TEMPERATUREAND SALINITY The first stage of the system is responsible for monitoring and controlling the temperature and salinity of the water in the header tank and for delivering it to the animal chamber. This section is developed from the system described by Bolt & Naylor (1985) (see Fig. 1). It consists mainly of a single tank in which fresh- and seawater are mixed and brought to the desired temperature. Input of water is controlled by two solenoid valves (Huba 304; Swissenco), one for fresh-, the other for seawater. Temperature is controlled by the cooling coil (Let Refrigeration Ltd.) and by a Tempette water heater. The valves and the heater are switched using power relays (Hamlin Solid-state 2.5A 7653: Mechanical 20A 65.31) connected to the input/output (I/O) port of the microcomputer (BBC, Model “B”). A temperature/salinity probe (designed and built at the Department of Oceanography, University of Southampton) is mounted in the mixing tank and is monitored continuously by the microcomputer. The probe generates two proportional output voltages, one for temperature and one for salinity. These can be connected directly to the analog-to-digital converter (ADC) of the microcomputer. Once calibrated, these readings can be compared to the required levels, pre-set in the computer, by the operator. Salinity readings are accurate to the nearest 0. lx0 and temperature readings to the nearest 0.1 “C. Any deviation from the pre-set levels will cause the computer to change the input to the tank. For instance, if the actual salinity in the tank is lower than the required level, the computer will open the salt water valve and close the freshwater valve, if the salinity is too high the opposite is carried out. The salinity in the tank is monitored every 15 s and the valves opened and closed as required. The cooling coil remains on permanently but, if the temperature is too low, the computer switches on the heater. If the temperature is too high, the heater is switched off and the cooler lowers the temperature. Temperature is monitored once every 2 s. The water is then piped to the animal chamber down z 30 m of flexible reinforced l/2” (12 mm) plastic hose. This provides water of known temperature and salinity with a pressure of 3 atmospheres to the pressure control system which constitutes the second main stage of the construction.
TIDAL SIMULATOR
CONTROL
OF HYDROSTATIC
AND ACTOGRAPH
FOR MARINE MAMMALS
139
PRESSURE
The animal chamber is a box, 40 x 80 x 5 cm deep, which is constructed of 1 cm thick Perspex. The chamber is provided with inflow and outflow pipes and two hatches for the introduction and removal of the animals. The hatches are sealed by rubber “0” rings and wing-nut closures. Other designs of animal chamber could easily be fitted to the system because all the control and monitoring equipment is external to the animal chamber proper. Inflow into the chamber is through a constant flow valve (Platon Flowbits FC/+” B) which gives a constant flow rate of 11. min - i provided the pressure drop across the valve is > 1 atmosphere (other flow rates are available). Outflow from the animal chamber is through a fine control needle valve (Platon Flowbits FCV/l/4” S/M). The valve is controlled by a stepper motor (RS Type 23) controlled via a stepper motor control board (RS Unipolar 2A) by the microcomputer. Pressure in the animal chamber is monitored constantly with a pressure transducer (RS piezo-resistive) which is connected to the ADC of the microcomputer. The precise hydrostatic pressure within the animal chamber is then controlled by varying the setting of the motor driven valve. Because the volume of the animal chamber is constant and water is incompressibIe, any volume of water entering the chamber must be matched by an equal volume leaving. Thus, if the aperture of the outlet valve is reduced, the pressure in the animal chamber must increase to force 11 water * min _ ’ out through the now smaller outlet. Therefore, closing the valve will increase the pressure and opening it will decrease the pressure. For feedback control, the microcomputer reads the input from the pressure transducer once every 10 s and compares this with the pre-set value. If the two values are sufficiently different, the computer will then open or close the valve a fixed number of steps (usually IO) and then re-check the pressure reading 10 s later. It will continue to adjust the valve until the two readings are reasonably close. We found that, provided the two values are within 0.01 atmospheres of each other, no adjustment is necessary; this prevents continuous oscillation around the pre-set value. It is also possible to simulate very low amplitude cycles with this equipment (e.g., ~0.5 m) by reducing the number of steps adjusted after each pressure reading and by increasing the time between readings. With the present design it is possible to control the pressure in the animal chamber to the nearest 0.001 atmosphere (1 cm of water). It should be noted that the accuracy of the system is limited mainly by the performance of the pressure transducer. In addition to the motorised valve outlet, the system is also fitted with two other outlets. The first is a simple shunt which can be set at a particular aperture to control the amplitude and range of the pressure cycles. The second is a safety overllow. The animal chamber used in this design is able to withstand up to 1.5 atmospheres above ambient atmospheric pressure. The water supply from the header tank, however, has a pressure of = 3 atmospheres above ambient and, to prevent accidental subjection of the chamber to this full pressure, a safety outlet is fitted. This outlet consists of a pipe extending to a height of 15 m above the animal chamber (15 m of water = 1.5 atmospheres) and then running to waste. If the pressure in the animal chamber should
D.G. REID ETAL.
140
ever rise to 1.5 atmospheres above ambient, the safety valve will overflow and prevent any further rise (Fig. 1).
TW:=SO I I
MICRO
CR
Fig. 1. Diagrammatic representation of combined system for cycling salinity, temperature, and hydrostatic pressure and recording locomotor activity. Continuous lines represent electrical connections; dotted lines represent piped water connections. AC, animal chamber; ADC, ~~og-to-~~tal converters; C, cooler; CR, cassette recorder; CV, constant flow valve; FS, freshwater supply; H, Tempette water heater; IRD, infrared detector; IRG, infrared beam generator; I/O, input/output port; MP, multiplexer; MV, motorised needle valve; PT, pressure transducer; RB, relay box; Sl, solenoid valve 1; S2, solenoid valve 2; SB, stepper motor control board; SM, stepper motor; SO, safety overflow; ST, salinity/temperature measuring probe; SS, seawater supply; SV, shunt valve; TW, to waste.
TIDAL MICROCOMPUTER
SIMULATOR
AND ACTOGRAPH
FOR MARINE
MAMMALS
141
CONTROL
The control for all three variables described above (temperature, salinity, and hydrostatic pressure) is effected using a feedback loop mechanism. The microcomputer is pre-programmed, usually with a constant or sinusoidal pattern for each variable. It then receives constantly updated readings from the three transducers and by controlling the solenoid valves, Tempette and motorised valve, respectivefy, can alter the salinity, temperature, or pressure to conform with the required value from the pre-set program. The software to control this is very simple and for each variable is similar to the flow diagram for the control of pressure outlined in Fig. 2. Start
t
IS
PR>SV+O-Ol?
Valve
no 5
yeslO:teps
Is PRc SV -0.01? no I
yes
Close Valve =
1
l--Yeps
Fig. 2. Flow diagram describing type of feedback loop used to control hydrostatic pressure. PR, pressure reading from pressure transducer; SV, set value, stored in microcomputer memory.
ACTIVITY
MONITORING
The inside of the animal chamber is divided into eight indi~du~ sections by plastic mesh (Netlon) partitions. Each of the access hatches opens into four of these cages. The activity of the animals in each chamber is monitored using a vertically mounted infra-red beam system (Fig. 1). Such systems are easily built or can be obtained ~o~erci~y. In this case, the output from the infra-red detector is interfaced via a custom-built multiplexer to one of the ADC channels on the microcomputer. Any interruptions of the beam are detected as voltage changes by the computer and recorded as events which are summed every 15 m or other required time period. These values are then downloaded onto magnetic tape with the temperature, salinity, and pressure readings for subsequent analysis. It is atso necessary to consider the possibility that an animal might remain static under the beam. To deal with this possibility, the software is written so that an event is recorded only if the interruption of the beam is a new occurrence. If
142
D. G. REID ETAL.
the beam is interrupted, an event is recorded, but another event will not be recorded until the animal moves out of the beam and then back in again. DISCUSSION
The system described here has three main advantages over the tidal simulation equipment designed by previous workers. First, it is capable of cycling the three main environmental variables on the shore: salinity, temperature, and hydrostatic pressure. Secondly, by using feedback control and transducers which constantly monitor the hydrostatic pressure in the animal chamber and the salinity and temperature in the mixing tank, it is possible to control the conditions precisely to conform with any pre-set pattern. One can impose any required pattern of cyclic variation of these three important tidal variables which are known to affect the behaviour and physiology of animals (Naylor & Williams, 1984). Unlike many previous systems used to control pressure cycles, the one described here has no manometer or tank on a large rotating arm (Graham et al., 1987). Manometer systems involve altering the height of a column of mercury that is in contact with the water in the animal chamber under pressure and such systems preclude the incorporation of control of variations in salinity and temperature. There would also be the danger exposing both animals and researchers to large volumes of mercury. Similar closed designs, using a syringe (Enright, 1962) or compressed air (Digby, 1972), also preclude any control of other parameters. There are also a number of drawbacks with the rotating arm design. First, the amplitude of the pressure cycles is restricted to the size of room in which the machine can be constructed. Secondly, the rotating arm method can normally only produce a sinusoidal form of cycle. Any other pattern would require some form of pressure transducer and feedback mechanism as described here. This would require the rotation of the arm to be controlled by a computer. The system described here is much more flexible because the amplitude of the pressure cycle is restricted only by the pressure of the water supplied and by the strength of the animal chamber. This supply can be either from a header tank, as in this design, or from a pump which would then permit the generation of pressure cycles of as great an amplitude as the animal chamber is capable of withstanding. Using a pump to provide the pressure head is, therefore, a possibility but it would normally result in changes in flow-rate with imposed pressure (see Blaxter & Tytler, 1972). The present design, based partly on a manually operated valve system (Naylor & Atkinson, 1972), avoids the changes in flow rate associated with a pump and functions simply by continuous adjustment of a single tap or valve. Thus, at its simplest, this system requires only two small working areas, one for the mixing header tank (preferably as high in the building as possible) and one for the animal chamber and motorised valve, the two areas connected by a single flexible pipe.
TIDAL
SIMULATOR
AND ACTOGRAPH
FOR MARINE
MAMMALS
143
REFERENCES Blaxter, J.H. S. & P. Tytler, 1972. Pressure discrimination in teleost fish. Symp. Sot. Exp. Biol., Vol. 26, pp. 417-444. Bolt, S.R.L. & E. Naylor, 1985. Interaction of endogenous factors controlling locomotor activity rhythms in Carcirrus exposed to tidal salinity cycles. J. Exp. Mar. Biol. Ecol., Vol. 85, pp. 47-56. Digby, P.S.B., 1972. Detection of small changes in hydrostatic pressure by Crustacea and its relation to the electrode action in the cuticle. Symp. Sot. Exp. Biol., Vol. 26, pp. 445-472. Enright, J.T., 1962. Responses of an amphipod to pressure changes. Camp. Biochem. Physiol., Vol. 7, pp. 131-145. Graham, J. M., R. Bowers & R.N. Gibson, 1987. A versatile tide machine and associated activity recorder. J. Mar. Biol. Assoc. U.K., Vol. 67, pp. 709-716. Morgan, E., 1984. The pressure responses of marine invertebrates: a psychophysical perspective. 2001. J. Linn. Sot., Vol. 80, pp. 209-230. Naylor, E. & R. J. A. Atkinson, 1972. Pressure and the rhythmic behaviour of inshore animals, Symp. Sot. Exp. Biol., Vol. 26, pp. 345-415. Naylor, E. & B.G. Williams, 1984. Environmental entrainment of tidally rhythmic behaviour in marine animals. Zool. J. Linn. Sot., Vol. 80, pp. 201-208. Underwood, A. J., 1972. Sinusoidal tide models: design, construction and laboratory performance, J. E.xp. Mar. Biol. Ecol., Vol. 8, pp. 101-l 11.