| Summary: | Marine plants are known to vary greatly in their growth-supporting value to sea urchins, but the reasons for the differences in growth-supporting value are not well understood.
The major purpose of this study was to determine how well the tropical sea urchin, Tripneustes ventricosus, could use for growth five of the plants available in its habitat, and to determine the causes of any differences in growth by measuring simultaneously the following three phases of the food conversion process: (1) consumption, (2) digestion and absorption,
and (3) conversion of the absorbed food to growth.
The foods varied in growth-supporting value as follows: Sargassum > Padina > Dictyota > Ulva >> Thalassia. Reasons for these differences were found in each of the three phases of the food conversion process:
(1) Consumption rates, expressed in calories/day, varied with diet as follows: Thalassia > Sargassum > Padina > Dictyota > Ulva. This variability
is attributable primarily to differences in the urchin's ability to manipulate and ingest the foods, and to differences in water and ash contents of the foods. There is no evidence that any of the foods are distasteful to the urchin.
(2) Average absorption efficiencies, measured in terms of calories, varied with diet as follows: Ulva (62%), Padina (58%), Dictyota (49%), Sargassum (40%), Thalassia (23%).
The natural foods of sea urchins are usually low in protein, but T. ventricosus improved the calorie : protein ratio by selectively absorbing
protein from most foods. (3) Average net growth efficiencies, measured in terms of calories, varied with diet as follows: for small urchins, Sargassum (23%), Dictyota (19%), Padina (18%), Ulva (16%), Thalassia (7%); for larger urchins, Sargassum (15%), Thaiassia (3%) . When the urchin ate a given food, the net growth efficiency increased with the rate of absorption.
Average net growth efficiencies, measured in terms of protein, were much higher than corresponding efficiencies measured in terms of calories, indicating that J. ventricosus retained for growth a relatively high proportion
of the absorbed protein, and for respiration relied primarily on carbohydrate.
It is concluded that no single phase of the food conversion process was of primary importance in producing the differences in the growth-supporting values of the five plants. Thus, the growth-supporting value of a natural food cannot be inferred simply from the rate at which it is consumed, or from the efficiency with which it is absorbed. Both the protein level and the caloric value of the plants also were poor indicators of growth-supporting value.
The quality of the food also affected the proportion of growth allocated
to the gonad. The gonads were relatively larger in urchins feeding on those foods which supported rapid growth.
The rate of oxygen consumption of T. ventricosus was greater when the urchin fed on a "good" food (Sargassum) than when it fed on a "poor" food (Thalassia). Experiments with the boreo-arctic urchin, Strongylocentrotus droebachiensis, showed that when the urchin ate a given food, the increase in its oxygen consumption above a standard level was linearly related to the rate of absorption of organic matter. This increase in metabolic rate consequent
to feeding (specific dynamic action or SDA) varied with diet from 11% to 18% of the absorbed ration.
Energy budgets calculated for T. ventricosus did not balance. Calories unaccounted for in faeces, growth and respiration represented 15-34% of consumption
and 42-68% of absorption. A loss of dissolved organic matter is hypothesized. Any such loss must be primarily carbohydrate.
The feeding preference of T. ventricosus was positively correlated with the growth-supporting values of the foods. === Science, Faculty of === Zoology, Department of === Graduate
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