Extracellular and intracellular digestion in bivalves, studied by magnetic resonance imaging with a contrast reagent

We investigated extracellular and intracellular digestion in bivalves, employing magnetic resonance imaging (MRI). Ruditapes philippinarum clams and Mytilus galloprovincialis mussels were incubated in seawater containing a contrast reagent [gadolinium-diethylenetriamine pentaacetic acid (GdDTPA)] at 20°C. The digestive systems, from the esophagus to the rectum, were visualized at a high signal intensity by the T1-weighted MRI. The crystalline style of the clam was also identified, which turned counterclockwise when viewed from a ventral-posterior position at a rate of 16 revolutions min-1. Determined using the T1 relaxation rate, the uptake and excretion rates of the GdDTPA in the mussel's digestive glands were 2.9 and 0.25 day-1, respectively, indicating that intracellular digestion in the gland acinar cells is slower than extracellular digestion. These results demonstrate that MRI with contrast reagents is useful to study the activity of the digestive system in bivalves, and that this technique could be applied to study other invertebrates.