Department of Biology, Indiana-Purdue University, Fort Wayne, Indiana 46805, USA.
Ecology. 2010 Feb;91(2):540-8. doi: 10.1890/08-2103.1.
Models of feeding and digestion predict that increased body size should result in longer gut passage time and improved assimilation efficiency. We examined the implications of digestion theory for size-structured interactions in a generalist zooplankton herbivore by studying the relationships between body size, ingestion rate, gut passage time (GPT), assimilation efficiency (AE), and growth rate in a clone of Daphnia pulex feeding on seven taxa of green algae that differed in digestibility. We also tested the effect of varying food concentration on GPT and AE while keeping body size constant. Food quality varied markedly among algal taxa, with mean juvenile growth rates at high food concentrations (1-2 mg/L) ranging from 0.10 to 0.61 d(-1). Juvenile growth rate for high food concentrations was highly correlated with juvenile AE (r2 = 0.96), verifying the importance of digestibility for food quality. AE, measured with 14C-labeled algae, increased with increasing age and body size for each of four digestion-resistant taxa but did not vary with age and body size for three readily digested algae. GPT decreased with decreasing body size, supporting the hypothesis that shorter GPT in juveniles leads to lower AE for digestion-resistant resources. Lower food concentrations led to increased GPT and improved AE for juveniles feeding on two digestion-resistant algae, providing further support for a role of longer gut retention in overcoming digestion defenses. The results suggest that increased abundance of digestion-resistant food will lead to growth and recruitment bottlenecks for juvenile herbivores, but that the effectiveness of digestion defenses will be decreased when large-bodied grazers predominate and when low food concentrations result in longer gut passage times. Gut processing constraints may favor either high concentrations of slow-growing, digestion-resistant resources or low concentrations of fast-growing, undefended resources.
摄食和消化模型预测，体型增大应导致肠道通过时间延长和同化效率提高。我们通过研究食性广泛的浮游动物桡足类在摄食 7 种不同可消化性绿藻时的体型、摄食率、肠道通过时间（GPT）、同化效率（AE）和生长率之间的关系，检验了消化理论对体型结构相互作用的意义。我们还在保持体型不变的情况下，测试了食物浓度变化对 GPT 和 AE 的影响。食物质量在各藻类间差异显著，高食物浓度（1-2mg/L）下幼体的平均生长率范围为 0.10-0.61d-1。高食物浓度下幼体的生长率与幼体 AE 高度相关（r2=0.96），验证了可消化性对食物质量的重要性。使用 14C 标记的藻类测量的 AE 随年龄和体型的增加而增加，对于四个抗消化的类群，每个类群均如此，但对于三个易消化的藻类，AE 并不随年龄和体型而变化。GPT 随体型减小而降低，支持了在幼体中较短的 GPT 导致对抗消化资源的 AE 较低的假说。较低的食物浓度导致摄食两种抗消化藻类的幼体 GPT 增加和 AE 提高，进一步支持了延长肠道保留时间以克服消化防御的作用。结果表明，抗消化食物的丰度增加将导致幼年食草动物的生长和补充出现瓶颈，但当大体型捕食者占主导地位和低食物浓度导致肠道通过时间延长时，消化防御的有效性将降低。肠道处理约束可能有利于高浓度生长缓慢、抗消化的资源，或低浓度生长迅速、无防御的资源。
