R .B. Forward et al. J. Exp. Mar. Biol. Ecol. 248 2000 225 –238
229
2.4. Light dark preference of larvae in the laboratory The foregoing experiment was repeated in the laboratory using the blue-green
stimulus light system described for the larval release experiments. Light intensity ranged
12 14
22 21
from 10 to10
photons cm s
to determine the lowest intensity to evoke a significant light dark preference. The data were analyzed as described for experiments in
sunlight and presented as the mean percentage of larvae 6standard error found in the light section.
3. Results
3.1. Larval release Sunlight induced larval release but the time course varied with light intensity. The
mean time until larval release increased as the sunlight intensity decreased Fig. 1, and
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21
release was not observed at light intensities below 0.036 3 10 photons cm
s . The
actual time course for larval release Fig. 2 became wider as the light intensity decreased. For example, the width of the larval release time interval in unscreened
15 22
21
sunlight 110.5 3 10 photons cm
s was 30 min but increased to 105 min at the
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21
lowest light intensity 0.0364 3 10 photons cm
s to induce larval release Table
1.
Fig. 1. The mean time of larval release for Polyandrocarpus zorritensis colonies exposed to different intensities of sunlight. Means and standard errors are plotted. The average sample size at each light intensity
was 16.
230 R
.B. Forward et al. J. Exp. Mar. Biol. Ecol. 248 2000 225 –238
Fig. 2. The percentage of the total number of larvae released over time at each sunlight intensity. The number
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21
adjacent to each plot is the sunlight intensity 3 10 photons cm
s , which corresponds to the intensities
in Fig. 1. Plots were selected to show the time course at log unit differences in sunlight intensity.
The time until the first larva was released latency increased as the light intensity decreased Table 1. At the highest sunlight intensities, larval release began in the
15-min interval after placement in light, but very few larvae were released in this interval Fig. 2. The most pronounced delay in larval release was observed upon
15
exposure to the lowest light intensity to induce larval release 0.0364 3 10 photons
21 21
cm s
, where the latency was 60 min. It was hypothesized that larval release would be affected by current flow. When
R .B. Forward et al. J. Exp. Mar. Biol. Ecol. 248 2000 225 –238
231 Table 1
a
The time of larval release of colonies exposed to sunlight and blue-green light intensities Intensity
Latency Width
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21
3 10 photons cm
s min
min Sunlight
110.5 15
30 65
15 45
25.5 15
60 10
15 60
1.56 30
75 0.29
30 105
0.036 30
105 Blue-green light
6.0 15
90 0.175
15 135
0.0175 30
150 0.00875
90 –
a
Latency is the time after exposure to light that the first larva was released. Width is the time interval during which the central 90 of the larvae were released. A dash indicates that too few larvae were released to
reliably calculate the width.
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21
exposed to unscreened sunlight 110.5 3 10 photons cm
s , the mean time of
21
larval release remained relatively constant at current speeds between 0 and 15 cm s Table 2. There was no significant difference between the mean time of larval release at
21
current speeds of 0 and 15 cm s Student’s t-test. Thus, larval release is not
21
influenced by current speeds up to 15 cm s .
Larval release in the laboratory was measured upon exposure to light filtered to the spectral region blue-green: 480–540 nm, to which larvae are predicted to be maximally
sensitive. The laboratory light stimulus system could not produce light that was equivalent in intensity to unscreened sunlight. Nevertheless at lower intensities, the same
types of relationships were evident. The mean time of larval release became longer as the light intensity decreased Fig. 3. Larval release was not observed at light intensities
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21
below 0.00875 3 10 photons cm
s . Mean times in sunlight and blue-green light
were very similar at equivalent intensities. For example, the mean time in sunlight at
15 22
21 15
10 3 10 photons cm
s was 49.5 min while in blue-green light at 6 3 10
photons
22 21
cm s
the time was 52.4 min. The time course for larval release became very wide at the lower intensities of
blue-green light Fig. 4, as the width of the larval release time interval increased Table
Table 2 Larval release in current flow
Current speed Mean time of
S.D. n
21
cm s larval release min
36.4 5.8
19 5
36.9 3.7
5 15
31.1 3.1
5
232 R
.B. Forward et al. J. Exp. Mar. Biol. Ecol. 248 2000 225 –238
Fig. 3. The mean time of larval release of colonies exposed to different intensities of blue-green light in the laboratory. Means and standard errors are plotted. The average sample size for each light intensity was nine.
1. Also the latency was longer as the light intensity decreased Table 1. For example,
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21
at 6 3 10 photons cm
s the first larva was released within 15 min of placement in
15
light, where as at the lowest intensity to induce larval release 0.00875 3 10 photons
22 21
cm s
, larvae were first evident after 90 min in light. 3.2. Larval light–dark preference
When given a choice between light and dark, larvae were attracted to light areas. At the end of the 1-h exposure to light, most larvae were either attached to the bottom of
the dish or resting on the bottom. The attraction of light did not change with age Fig. 5. The mean percentage of larvae attracted to the light by larvae , 2 h after release
80 was not significantly different from the mean percentage 24 h later 73; Student’s t-test using arcsine transformed data. When exposed to sunlight, the
15
preference of larvae for light was evident at intensities down to 0.29 3 10 photons
22 21
15 22
21
cm s
Fig. 6. At 0.045 3 10 photons cm
s , larvae no longer preferred light,
as the mean percentage in the light section was 53. Similar results were observed in
15
the laboratory under blue-green light Fig. 7. At light intensities of 0.005 3 10
22 21
15 22
photons cm s
and higher, larvae preferred the light. At 0.001 3 10 photons cm
21
s , there was no preference, as the mean percentage in the light section was 52.
R .B. Forward et al. J. Exp. Mar. Biol. Ecol. 248 2000 225 –238
233
Fig. 4. The percentage of the total number of larvae released over time at each blue-green light intensity. The
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21
number adjacent to each plot is the blue-green light intensity 3 10 photons cm
s , which corresponds
to the intensities in Fig. 3.
4. Discussion
