controlled room. All water used was 0.2 µm filtered seawater of consistently 35 ppt salinity.
The movement of larvae was measured by placing approximately 30 larvae into a clear plexiglas column divided into eleven 5 cm segments (30 mm internal diameter x 550 mm length) and recording their position after subjecting to a stimulus. Larvae were introduced into the middle of the column with a transparent 60 ml syringe. The light intensity that larvae are adapted to has been shown to influence their behaviour in response to light stimulus (Forward, 1974). To prevent confounding of experiments from previous light exposure, larvae were acclimatised to the lighting for 10 min within the transparent syringe. Preliminary trials established suitable duration of trials to be 2 min. Longer periods than this resulted in all the larvae gathering at either end of the testing chamber. Also, no change in the nature of the larval response occurred in trials of 15 min compared with 2 min trials. Barokinesis trials were reduced to 1.5 min due to constraints on the apparatus used to alter pressure.
Injection of larvae into the testing column caused currents which tended to move the larvae vertically upwards. To compensate for this effect, the initial position of the larvae was determined by repeatedly introducing larvae into the testing column and recording their position immediately (433 zoeas in the angled column, 394 in the vertical column). The mean column position of these larvae was used as the point of origin in all trials.
Significant difference between treatments was determined using the method outlined by Sulkin et al. (1980). "Mean position value" was calculated by assigning weights from 1-11 for each of the sections along the testing chamber, multiplying the weights by the number of larvae in each section and dividing the product by the total number of larvae. The mean position value was calculated for each replicate and these values were then used to compare treatments with the non-parametric Mann-Whitney U test (Zar, 1974). Differences in means were considered significant at P<0.05.
Geotaxis
Significant upwards movement of the larvae in the absence of light or pressure changes was attributed to negative geotaxis. The apparatus used to study geotactic response is illustrated in Fig. 1. Geotactic response of larvae was tested at: immediately post-hatch, 15 h, 20 h, 2 d, 6 d, 9 d, and 13 d. Larvae were maintained in a 1000 l tank on a recirculating
water system with UV sterilisation and biofiltration. Larvae were fed 2nd instar artemia
nauplii enriched with Protein Selco™ and they moulted to second stage zoeas after 7 days. At the culmination of the geotaxis trials (13 d), larvae were still at second stage zoea.
Figure 1. Detail of testing chamber and experimental apparatus used to measure larval response in darkness and also to light of 617 nm and 478 nm. Wavelength and light intensity was altered with optical filters held in a rack between the light source (quartz halogen flood light) and the testing chamber. Larvae were introduced through the entry port and water and air bubbles displaced through the exit port.
DRAIN ENTRY PORT EXIT PORT OPTICAL FILTERS LIGHT SOURCE BALL VALVE TESTING CHAMBER
Phototaxis
Several aspects of phototactic response were investigated: spectral sensitivity, response to constant light intensity, response to change in light intensity, and the effect of incident
The apparatus used to study spectral sensitivity is illustrated in Fig. 1. Lighting was from beneath and wavelength was altered with red (Kodak #25, dominant wavelength = 617 nm) and blue (Kodak #47A, dominant wavelength = 478 nm) gelatine filters.
Response to constant light intensity, response to change in light intensity, and the effect of incident angle of light source were determined with the apparatus illustrated in Fig 2. Natural underwater distribution of light was approximated by submerging the testing chamber in a 400 l tank, using angled light and diffusing the light source with neutral density filters. The walls of the outer tank were blackened walls and the tank was filled with 0.2 µm filtered seawater. All trials with constant light intensity were conducted with the light source in the lower position. Test intensities ranged from 3 lux to 40000 lux, recorded from the top of the testing chamber.
To test the effect of change in light intensity, the light source was moved towards or away from the testing chamber by a variable speed 12 V electric motor (Fig. 2). The change in intensity commenced as the larvae were introduced to the column, and continued for the duration of each trial. The effect of change in light intensity was examined for both increasing and decreasing intensities for intensities between 6 and 2000 lux. The range of intensities experienced by larvae for each treatment is given in Fig. 8. Rates of change in light intensity, under natural conditions at sunset, were determined by measuring decline in light intensity on two days in September. Readings were taken every two minutes and rates of change averaged for the two days. Simulated declines in intensity were considerably faster than that which occurs at sunset (Table 1).
Figure 2. Experimental apparatus used to measure larval response to fixed intensity white light and change in light intensity. The light source was moved up or down the track to adjust intensity with a variable speed, 12 V, electric motor connected to a gearbox so as to reduce speed of revolution and increase torque. Initial light intensity was adjusted with neutral density filters or by changing the wattage of the quartz halogen globe. GEARBOX AND SPOOL SPEED ADJUSTMENT TESTING CHAMBERS EXIT PORT ENTRY PORTS MOVEABLE LIGHT SOURCE
The effect of incident angle of light source was tested by comparing larval distributions in a testing chamber angled directly towards the angled light source with a testing chamber oriented vertically (Fig. 2). The angle of light incident on the testing chambers was 45° to vertical after refraction through the water surface. Trials conducted to compare the effect of incident angle of light source were conducted simultaneously for the vertical and angled
Table 1. Change of light intensity during natural sunset compared with experimental rates of intensity decline.
Simulated change in intensity Initial Intensity
(lux)
Naturala (lux/min) Slow
(lux/min) Rapid (lux/min) 15 2.8 3 4.5 230 20.9 60 95 900 57.5 175 395 2000 103.9 450 875
aValues for natural change in intensity at sunset are derived from a regression fitting recorded intensity
changes.