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Does trophic position of the omnivorous signal crayfish (Pacifastacus leniusculus) in a stream food web vary with life history stage or density?

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Food mixing behavior of omnivorous consumers can be difficult to predict. We undertook an enclosure experiment to explore the effects of ontogenetic stage and density on food choices of the omnivorous signal crayfish, Pacifastacus leniusculus. Juvenile or adult crayfish were placed in in-stream enclosures for 6 weeks at three different densities. Gut and stable isotope analyses were used to determine the food sources utilized by these organisms. In addition, we analyzed the guts of both adult and juvenile crayfish from outside the enclosures to account for experimental effects. We found few differences in the gut contents between juvenile and adult crayfish from either inside or outside the enclosures, as the majority of food consumed by both ontogenetic stages and all densities was allochthonous detritus. Stable isotope results indicate that crayfish of both ontogenetic stages were relying primarily on detrital biofilms for nutrition, despite an additional laboratory experiment showing that growth would be far more rapid on invertebrates. The dietary choices made by P. leniusculus in its native environment seem to be primarily affected by factors other than the nutritional value of food sources, contrary to expectations that food mixing behavior of omnivores should be based on this factor alone.

Le comportement de mélange alimentaire chez les consommateurs omnivores peut être difficile à prédire. Une expérience en enclos nous a permis d'explorer les effets du stade ontogénique et de la densité sur les choix alimentaires de l'écrevisse omnivore Pacifastacus leniusculus. Nous avons placé des écrevisses juvéniles ou adultes dans des enclos dans un cours d'eau pendant six semaines à différentes densités. Des analyses du tube digestif et des iso topes stables ont servi à déterminer les sources de nourriture utilisées par ces organismes. De plus, nous avons analysé les tubes digestifs d'adultes et de juvéniles provenant de l'extérieur des enclos afin de tenir compte des effets du protocole expérimental. Il y a peu de différences entre les contenus des tubes digestifs des jeunes écrevisses et des adultes, tant de l'extérieur que de l'intérieur des enclos, puisque la plus grande partie de la nourriture consommée par les deux stades ontologiques, à toutes les densités, est constituée de détritus allochtone. Les résultats de l'analyse des isotopes stables indique que les écrevisses des deux stades ontogéniques dépendent principalement des biofilms sur le détritus pour leur alimentation, bien que des expériences supplémentaires en laboratoire indiquent que la croissance serait beaucoup plus rapide s'ils se nourrissaient d'invertébrés. Les choix alimentaires faits par P. leniusculus dans son habitat indigène semblent affectés surtout par des facteurs autres que la valeur nutritionnelle des sources de nourriture, ce qui est contraire à nos attentes que le comportement de mélange alimentaire devrait se baser sur ce seul facteur.[Traduit par la Rédaction]

Document Type: Research Article

Publication date: November 1, 2005

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  • Published continuously since 1901 (under various titles), this monthly journal is the primary publishing vehicle for the multidisciplinary field of aquatic sciences. It publishes perspectives (syntheses, critiques, and re-evaluations), discussions (comments and replies), articles, and rapid communications, relating to current research on cells, organisms, populations, ecosystems, or processes that affect aquatic systems. The journal seeks to amplify, modify, question, or redirect accumulated knowledge in the field of fisheries and aquatic science. Occasional supplements are dedicated to single topics or to proceedings of international symposia.
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