Comparative Study of the Gastrointestinal System of Bathymodiolus Bivalves Harbouring Chemosymbiotic Bacteria and Mytilus Using a Liquid-Phase MRI Tracer

In order to assess the gastrointestinal transport function in bathyal bivalves harbouring chemosymbiotic bacteria, the Bathymodiolus japonicus, Bathymodiolus platifrons and Bathymodiolus septemdierum, the uptake of a liquid-phase MRI tracer, gadolinium-diethylenetriamine pentaacetic acid (GdDTPA), was observed for 24 h. The excretion of GdDTPA was then observed for more than 8 days at 5°C. GdDTPA concentration was estimated from the MRI T1 relaxation rate (R1=1/T1). GdDTPA was not taken up into the haemolymph from the seawater, largely reflecting the fluid transport in the gastrointestinal canal and partly the endocytosis/exocytocis of acinar cells. Compared with Mytilus galloprovincialis at 20°C, the uptake rate to the stomach (kis) varied from 60 to 150%, and that of the digestive gland (kidg) was around 70%. The excretion rate from the stomach (kes) was the same as for the Mytilus at 20°C, and almost 3 times that of the Mytilus at 5°C. Therefore, the kis, kes, kidg in the Bathymodiolus species at 5°C were similar to that for the Mytilus at 20°C. The excretion rate from the digestive gland (kedg) was 30-70% of that for the Mytilus at 20°C, and similar to the Mytilus at 5°C. However, the lower kedg value, which indicated a longer retention time, suggested a lower activity of the digestive gland in the Bathymodiolus species, but the gastrointestinal transport was still functioning more than the Mytilus at 5°C. These results suggested that the Bathymodiolus species has two nutrition routes that enable the mussel to adapt and survive in a wide range of environments.