Foods, feeding, & digestion
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Carnivory

  This part of carnivory deals with specific dynamic action, while topics of MECHANICS OF CRUSHING, and carnivory in Metacarcinus magister and CANCER PRODUCTUS are dealt with in other sections.
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Specific Dynamic Action

Feeding in animals, including crabs, obviously requires that energy be expended.  This can include direct costs of handling/dismembering a prey, costs of “excitement”, swallowing, and digestion, and also “indirect” costs of Specific Dynamic Action.  SDA is a postprandial and post-digestion metabolic cost that can account for 10-30% of the energy value of a meal.  It is highest on protein diets, suggesting a strong component of amino-acid deamination and transamination costs, and is generally considered to represent the broad range of metabolic-conversion costs involved with growth.  Long thought to be wasted energy, SDA is now accepted as an integral cost of processing a meal into tissue growth.  SDA is defined by some authors (e.g., RS1 below) as having both mechanical (e.g., feeding, swallowing, and movement of food through the gut) and chemical components, but the traditional definition of SDA excludes mechanical costs.  These mechanical costs, also known as “apparent” SDA, must be teased out to obtain "true" SDA.

NOTE the original work was done on unconscious dogs using intubations and/or intravenous injections of amino acids and other nutrients

NOTE more information on separation of mechanical and chemical components on SDA in crustaceans can be found at LEARNABOUT ISOPODS/FOOD, FEEDING, & NUTRITION/LIGIA

 

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Research study 1
 

graph showing effects of different food rations on oxygen consumption in crabs Cancer gracilisA large investigation done at Bamfield Marine Sciences Centre, British Columbia, University of Nevada, and Memorial University of Newfoundland concerns the effect of body size and meal size on specific dynamic action (SDA) in several west- and east-coast crustaceans.  Only detailed results for Cancer gracilis, a large omnivorous, subtidal crab are presented here.  SDA in this study is measured only as those costs (ultimately expressed in kJ per unit body mass) reflected by increased oxygen consumption after a meal (postprandial), and thus include both mechanical (mastication in the gastric mill, peristalses to move food along the gut) and chemical histograms showing transit times of food through the gut of crabs Cancer gracilis(production of enzymes and “true” SDA) costs related to SDA.  As expected, meal size contributes substantially to increased oxygen consumption, but only when meal size reaches the upper feeding limit of 3% live body mass (see graph above Right).  This lack of difference may be at least partly explained by a lack of effect of meal size on transit time of the food through the gut (see histograms on Right). Body size has even a lesser relative effect on oxygen consumption.  The study represents a strong contribution to our understanding of digestive physiology of higher crustaceans and, like any good study, raises more questions than it answers.  McGaw & Curtis 2013 Comp Biochem Physiol A 166: 414. Photograph courtesy Dave Cowles, Walla Walla University, Washington.

NOTE  these include crabs (Callinectes sapidus, Hemigrapsus nudus, Pugettia producta, Carcinus maenas, Cancer gracilis, M. magister, and C. irroratus), lobsters (Homarus americanus), and crayfishes (Procambarus clarkii)

NOTE  measured using a fluoroscope device.  Meals generally consist of fish or shrimp, depending upon species, but the one used for monitoring gut-transit times is a specially formulated radio-opaque one (fish/shrimp muscle and electrolytic iron powder bound in gelatin)

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