At first sight, we might not appear to have much in common with mussels. However, when Yoshiteru Seo from Dokkyo Medical University School of Medicine, Japan, was looking for an animal system that might provide insight into how the minute hairs (cilia) that line our spinal cords and brain ventricles waft cerebrospinal fluid through the tissues, he realised that we could learn a lot from the humble mollusc. Mussels fan water through their body cavity with microscopic (15 μm long) cilia located on the gills to feed and breathe, but when Seo began investigating the parallels in more detail, he realised that as far as the fluid motion through the mollusc's body was concerned, the mussel was little more than a black box. ‘There were large gaps in our knowledge of the beating of the cilia’, says Seo, who also realised that MRI was the perfect technique to visualise fluid motion inside the mollusc's body. Teaming up with mussel experts Eriko Seo, Kazue Ohishi, Tadashi Maruyama, Yoshie Imaizumi-Ohashi and Masataka Murakami, Seo set about teasing apart the tiny details of how fluid flows through the bodies of Mediterranean mussels (Mytilus galloprovincialis) (p. 2277).
The team immersed a muscle in sea water inside a tube, placed this inside an MRI scanner and then waited for the mussel to begin breathing. Seo admits that working with the molluscs required patience as the animals do not breathe on demand: ‘We did not know when it would start or stop’, he says. However, when the shell opened and the mussel began to inhale, Seo recalls that the MRI image changed dramatically. The team could see water gushing into the inhalant aperture at speeds ranging from 20 to 40 mm s−1 and squirting out of the exhalant siphon at 50 mm s−1. However, when water entered the mussel's body, the speed fell to 5–10 mm s−1 as it flowed from the lower mantle cavity to the upper cavity over the gills.
The team was also noticed that water began moving in the mussel's body before the shell opened and they were impressed by the dramatic way that flow ceased almost instantaneously when the mussel closed its shell. In addition, the flow continued increasing rapidly during the first minute after the shell opened. The mussel also seemed to be capable of controlling flow through the left and right body cavities independently and when the team calculated the flow rate of water through the body it was an impressive 400 mm3 s−1, allowing water to pass through the mollusc in less than 3 s as it processed 1.4 l h−1 .
Seo says, ‘This is direct evidence of the lateral cilia as the primary drive of water flow in the mussel’. He also admits that he is impressed that the mussel has such a sophisticated flow control system that allows them to produce flow changes within seconds and mobilise high flow rates over brief periods. ‘Mytilus is not just a boring black shell’, he concludes with a chuckle.
Having shown how impressive the Mediterranean mussel is at dextrously controlling water flow through its body mantel, Seo and his colleagues are eager to investigate how more exotic Bathymodiolus mussels manipulate flow across their gills. Explaining that Bathymodiolus live deep under the ocean and depend on symbiotic chemosynthetic bacteria residing in their gills for survival, the team is keen to discover how it regulates water flow across the gills.