D .T. Dy, H.T. Yap J. Exp. Mar. Biol. Ecol. 251 2000 227 –238
231
2.5. Statistical analysis Tests for significant differences due to the recent feeding history of the organisms
which was nested under time of incubation which in turn was nested under species were carried out using nested hierarchical analysis of covariance ANCOVA at P 5 0.05
with temperature and salinity acting as covariates. Dependent variables oxygen consumption, ammonium and phosphate excretion were initially tested for normality
and homoscedasticity. Data were transformed i.e. oxygen consumption using log ,
10
ammonium excretion using square root and phosphate excretion using the sine function because the dependent variables were heteroscedastic. In cases where significant
differences were detected among the dependent variables after ANCOVA, Tukey’s H.S.D. test was used for post-hoc comparisons of the main effects. Planned comparisons
were carried out to compare effects of recent feeding history and time of incubation for each echinoderm species. To check whether there was a correlation between oxygen
consumption and nutrient excretion in each species, we used Pearson product-moment correlation Zar, 1984.
3. Results
Oxygen consumption differed significantly among species and their recent feeding history, but less so with time of day Fig. 2; Table 2. T
. gratilla had significantly higher rates of oxygen consumption than the other two echinoderm species. Significantly higher
oxygen consumption rates were also measured for starved T . gratilla during the day
compared to night. Both O . incrassata and P. nodosus consumed more oxygen when
recently collected than when the organisms were subjected to short-term starvation. Interaction between species and recent feeding history was significant during the day.
Ammonium excretion differed significantly among species, time of day and recent feeding history Fig. 2; Table 2. Interaction between species and recent feeding history
was also significant. T . gratilla had the highest rates of ammonium excretion followed
by P . nodosus and then by O. incrassata. Post-hoc analysis indicated significant
differences among the three species of echinoderms. Ammonium excretion by T . gratilla
varied significantly in terms of time of day and recent feeding history. Starved organisms of this species excreted significantly more ammonium than recently collected in-
dividuals. Ammonium excretion was significantly higher during the day compared to night.
Recently collected specimens of the brittle star, O . incrassata excreted significantly
more ammonium during the day, while starved specimens excreted significantly more ammonium at night. Similarly, higher values were measured during the day compared to
night for the starfish, P . nodosus but only recently collected specimens had significantly
higher ammonium excretion during the day. Rates of ammonium excretion by P .
nodosus did not vary significantly with recent feeding history. There was a significant interaction between time of day and recent feeding history for ammonium excretion by
O . incrassata but not so for the starfish, P. nodosus.
232 D
.T. Dy, H.T. Yap J. Exp. Mar. Biol. Ecol. 251 2000 227 –238
Fig. 2. Rates mean6S.E. of oxygen consumption, and ammonium and phosphate excretion. D 5 day, N 5 night, F 5 recently collected, S 5 starved. Top values are mean6S.E. of four treatments.
There were no significant differences in phosphate excretion among the three species of echinoderms, their recent feeding history and time of day Fig. 2; Table 2.
For T . gratilla, there was a significantly positive, but weak, correlation between
oxygen consumption and ammonium excretion r 5 0.48, P 5 0.018, and between oxygen consumption and phosphate excretion r 5 0.41, P 5 0.047. For O
. incrassata, there was no significant correlation between oxygen consumption and either ammonium
r 5 0.34, P 5 0.100 or phosphate r 5 2 0.31, P 5 0.146 excretion. The same was
D .T
. Dy
, H
.T .
Yap J
. Exp
. Mar
. Biol
. Ecol
. 251
2000 227
– 238
233 Table 2
Summary of nested ANOVA results testing for the effect of species, time of day and recent feeding history on oxygen consumption, and ammonium and phosphate excretion
Summary of all effects df Effect
MS Effect df Error
MS Error F
P level Oxygen consumption
Species 2
1.27 58
0.01 85.37
0.0000 Time of day nested under species
3 0.03
58 0.01
2.18 0.0999
Recent feeding history nested under time of day 2
0.05 58
0.01 3.39
0.0405 Species 3 Recent feeding history
4 0.06
58 0.01
4.24 0.0044
Ammonium excretion Species
2 543.01
58 10.15
53.50 0.0000
Time of day nested under species 3
407.53 58
10.15 40.15
0.0000 Recent feeding history nested under time of day
2 948.87
58 10.15
93.49 0.0000
Species 3 Recent feeding history 4
1137.14 58
10.15 112.04
0.0000 Phosphate excretion
Species 2
0.75 58
0.46 1.62
0.2061 Time of day nested under species
3 0.46
58 0.46
0.98 0.4065
Recent feeding history nested under time of day 2
0.25 58
0.46 0.54
0.5854 Species 3 Recent feeding history
4 0.62
58 0.46
1.35 0.2631
Significant at P , 0.05.
234 D
.T. Dy, H.T. Yap J. Exp. Mar. Biol. Ecol. 251 2000 227 –238
true for P . nodosus oxygen consumption vs. ammonium excretion r 5 0.24, P 5 0.250;
oxygen consumption vs. phosphate excretion r 5 2 0.27, P 5 0.198. 3.1. O:N ratio
An estimate of the atomic ratio of oxygen-consumed to ammonia-nitrogen excreted O:N ratio was computed to provide an indication of the nutritional requirement of the
three species of echinoderms. T . gratilla had an O:N ratio of 68.7619.5 while O.
incrassata had 13.261.3 and P . nodosus had 7.660.7.
3.2. Dry wt volume ratio Of the three echinoderm species used in this study, the brittlestar, O
. incrassata had the highest dry wt volume ratio 0.6060.04 followed by the starfish, P
. nodosus 0.3760.01 and then by the sea urchin, T
. gratilla 0.1560.00. The P. nodosus individuals used in this study were heavier than the other two echinoderms.
4. Discussion
