Feeding, assimilation, and scope for growth in the scavenging sandy-beach neogastropod Bullia digitalis (Dillwyn)

Abstract

Ingestion rates, absorption efficiency, and excretion rates in the scavenging sandy-beach gastropod Bullia digitalis (Dillwyn) were measured in the laboratory using bivalve tissue as food. A clear distinction was made between time spent feeding and the duration of the postfeeding phase. Intake of a meal was completed within l h and the period before the next meal was 7–10 days. All data were fitted to allometric equations to describe the relationship between physiological rates and body sizes ranging from 16 to 1000 mg dry tissue weight. The daily ingestion rate was described by IR (mg dry wt · day −1) = 0.34 W 0.64 and excretion rates as U ( μg NH 4- N · h −1) = 1.03 W 0.58. Absorption efficiency was independent of size and meaned at 88 %. Published respiration rates were recalculated as IR (μg O 2· h −1) = 12.31 W 0.60 . All energy data were converted to Joules and used to construct a daily energy budget for a size range of Bullia in which the absorbed ration was balanced against the cost of excretion and respiration. All sizes maintained a highly positive scope for growth. Most of the energetic expenditure is due to aerobic respiration, followed by growth and reproduction and then excretion. Adult Bullia show the highest efficiency in converting ingested energy into tissue and this may be associated with the prevailing reproductive condition of adults. The need to be specific in expressing ingestion rates for marine carnivorous gastropods in publications is emphasized.