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

We investigated the extracellular and intracellular digestion of bivalves employing magnetic resonance imaging (MRI). Ruditapes philippinarum clams and Mytilus galloprovincialis mussels were incubated in seawater containing a contrast reagent (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 rpm. 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 d⁻¹, respectively, indicating that the intracellular digestion in the gland acinar cells is slower than the extracellular digestion. These results demonstrate that MRI with contrast reagents is useful to study the activity of digestive system in the bivalves, and this technique can likely be applied to the study of other invertebrates.