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The spectral sensitivity of larvae in this study was essentially the same as that described for several other species in more detail by Forward (1987), and Forward and Cronin (1979). Larvae were more sensitive to shorter blue wavelengths, which have better penetration, than red wavelengths.

Light adapted larvae were not induced to descend in response to overhead white light of any intensity tested. The experimental design used in this study was intended to

demonstrate the approximate field conditions where sinking may have been induced by light. Consequently, the testing chamber was oriented vertically so that negative phototaxis would be required to counter negative geotaxis in order to produce net downward

movement (light-induced positive geotaxis). Comparisons between other studies where phototactic response was examined with horizontally oriented columns should be made cautiously.

Where phototaxis has been examined independently of geotaxis by the use of horizontal testing columns, larvae tend to exhibit negative geotaxis at low intensities and positive phototaxis at high intensities. This pattern has been observed in: Rhithropanopeus harrisii (Forward et al., 1984); Cancer gracilis, Lophopanopeus bellusbellus, Hemigrapsus oregonensis (Forward, 1987); and Paralithodes camtschatica (Shirley and Shirley, 1988). Forward (1987) attributed this pattern to predator avoidance.

Avoidance behaviour, or negative phototaxis at low light intensity, is not clearly demonstrated in vertically oriented columns where the natural behaviour of negative geotaxis is incorporated. Both Schembri (1982, Ebalia tuberosa) and Jacoby (1982, Cancer magister) tested the response of crab larvae to different light intensities in vertical columns and observed only upward movement. It is tempting to infer that the positive phototactic response of P. gigas (Fig. 7) indicates that the larvae congregate at the surface during the day. However, Forward (1985 & 1988) considers the absence of negative phototaxis in vertically oriented columns to be a laboratory artefact in most studies. For example, Stearns and Forward (1984) found that the copepod Acartia tonsa was positively phototactic to all light intensities although the natural migration pattern is nocturnal. Simulated natural underwater lighting distribution is difficult to achieve, so the observed response of P. gigas may be nothing more than a laboratory artefact despite attempted simulation of natural

Larvae exposed to change in intensity at low light levels responded by downward movement. Conversely, larvae exposed to change in intensity at higher light levels were unaffected. This response was more pronounced at slower rates of change in intensity (in the vertical column only) but was not affected by the sign of intensity change (increasing or decreasing).

Light-induced downward movement in response to change in light intensity, regardless of whether intensity is increasing or decreasing, has not been previously reported. This response may be a variation of the predator avoidance or shadow response proposed by Forward (1986) where negative phototaxis was induced by a rapid decrease in intensity. The shadow response proposed by Forward (1986) was only initiated by rapid decreases in intensity and not increases as was observed in this study. Forward (1986) noted that the change in intensities which resulted in negative phototaxis were too rapid to simulate dusk or dawn. Because of this, he believed that the larval responses did not represent a typical behaviour relevant to diel vertical migration. The simulated rates of intensity changes in this study were also greater than that at dawn or dusk (Table 1), suggesting the response in P. gigas was a predator avoidance, shadow response. The response of stage 1 P. gigas zoeas to change in light intensity, only at low light levels, suggests that the larvae are adapted to respond to low levels; this supports the dismissal of the results of fixed light intensity trials as a laboratory artefact.

There was significantly greater (P<0.05) upward movement of larvae in the vertically oriented column exposed to fixed intensity of light, compared with the column angled directly towards the light source (45°). This suggests that geotaxis is the orienting cue while photokinesis controls locomotory activity. Forward (1985) observed this same interaction in larvae of the xanthid crab Rhithropanopeusharrisii. Geotaxis appeared to be less important in the larval response to changes in light intensity at low light levels. When downward movement occurred in response to rapidly changing intensity, the larval distribution was further from the origin in the angled column. This suggests that larvae

Barokinesis

Larval detection and response to small changes in pressure have been used to explain vertical migration patterns, as it is considered that pressure response provides a negative feedback on vertical movement (Knight-Jones and Morgan, 1966). Stage 1 zoeas of P. gigas did not appear to respond to small pressure changes; this has been reported elsewhere for species where the larvae occupy water of considerable depth: Callinectes sapidus (Sulkin et al., 1980); Geryon quinquedens (Kelly et al., 1982); and Hemigrapsus oregonensis (Arana and Sulkin, 1993). Barokinesis has been shown to change dramatically with ontogeny (Bentley and Sulkin, 1977; Wheeler and Epifanio, 1978) so older larvae of P. gigas may possess greater pressure sensitivity.

Rheotaxis

Stage 1 zoeas of P. gigas were able to detect currents and actively swam against them. Larvae appeared to be unable to detect slight currents below 1.12 cm/s and they were swept along by currents slower than their maximum swimming speed (1.4 and 1.61 cm/s respectively). The combination of the ability of larvae to detect currents, and then to swim against them, resulted in a narrow window within which larvae could maintain position. This suggests that rheotaxis may not be important in larval dispersal. Rheotaxis has also been observed in estuarine species such as the megalopas of Cancer magister (Fernandez et al., 1994) and is thought to assist in movement to and from the estuary. With open ocean species, the function of rheotaxis is less obvious as the environment is more homogeneous. Shirley and Shirley (1988) also observed rheotaxis in an oceanic species, Paralithodes

camtschatica, and suggested that the function of rheotaxis in the oceanic environment may be important for zoeal feeding and predator avoidance.