Most young fishes lack the ability to function as herbivores, which has been attributed to two aspects of the digestive system: elevated nitrogen demand and a critical gut capacity. We compared the digestive morphology and biochemistry of two size classes of the marine herbivore Hyporhamphus regularis ardelio, preontogenetic trophic shift (pre-OTS, b100 mm) and post-ontogenetic trophic shift (post-OTS, N100 mm), to determine what limits the onset of herbivory and how their digestive processes fit with current models of digestion. Two gut-somatic indices comparing gut length to body length (relative gut length) and body mass (Zihler's Index) demonstrated a significant decrease (RGL 0.59→0.49, Pb0.01; ZI 3.24→2.44, Pb0.01) in gut length relative to body size. There was little difference in enzyme activity between the two classes, with juveniles showing similar levels of carbohydrase and lipase and less protease compared with adults, indicating that juveniles did not preferentially target nitrogen and were as capable of digesting an herbivorous diet. These findings suggest that herbivory in this fish is not limited by the function of the post-oesophageal digestive tract, but rather the ability of the pharyngeal mill to mechanically process plants. Our findings offer partial support for the current model of stomachless digestion, indicating that further refinement may be necessary.

Item Type:	Refereed Article
Keywords:	α-amylase, trypsin, lipase, Hemiramphidae, halfbeak, RGL, Zihler Index, plug-flow reactor, adaptive modulation hypothesis, pharyngeal mill
Research Division:	Biological Sciences
Research Group:	Zoology
Research Field:	Animal structure and function
Objective Division:	Environmental Management
Objective Group:	Marine systems and management
Objective Field:	Marine biodiversity
UTAS Author:	Day, R (Dr Ryan Day)
ID Code:	95697
Year Published:	2011
Web of Science® Times Cited:	10
Deposited By:	Sustainable Marine Research Collaboration
Deposited On:	2014-10-07
Last Modified:	2017-10-31
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