Miraculous mussels heal in days

Abstract

For many of us, breaking a bone is rare and hopefully no more than a once in a lifetime experience. It's excruciating and recovery can take weeks. But the shells of mussels (Mytilus californianus) clinging to the shoreline are continually at risk of harm, battered by waves and debris hurled by the surf. Even when the tide is out, mussel shells are vulnerable to hungry crustaceans taking a nip with their powerful claws. ‘The intertidal zone can be a particularly dangerous place. It is a harsh environment’, says Rachel Crane from Stanford University, USA. So how much damage do these resilient structures sustain after a substantial assault and how quickly can they recover? Crane, along with José Diaz Reyes and Mark Denny, both from the same university, headed down to the seashore below the Hopkins Marine Station, USA, to find out.Back in the lab with a bucket full of the molluscs, Crane and Diaz Reyes repeatedly compressed the mussels between two plates to stress them, adjusting the level of force depending on their size. Then, the duo checked the shells for damage by testing how much total force bearing down they could withstand. Sure enough, the shells of the squashed mussels were significantly weaker than shells that had not been compressed; they could only withstand forces that were 54 N less (equivalent to the weight of 5.4 kg) than unstressed mussels of the same size. And when Crane and Diaz Reyes opened up the mussels and looked at the surface of the shells within, they sometimes saw networks of cracks radiating across the dome of the shell or larger individual cracks. Repeatedly pounding the mussels had taken a toll on their shells, but how well and how quickly could the molluscs recover?Allowing the mussels 1, 2 and 4 weeks to recuperate, the team was astonished to see that the squashed shells were every bit as tough as the unharmed shells within a week. ‘We were amazed when we saw the data. We were expecting the mussels to take weeks or maybe even months to repair their shells; I never thought the response would be so fast’, says Crane. And, when she and Diaz Reyes rechecked the mussels 4 weeks later, they were impressed that the shells dealt the hardest blows were the most resilient, coming back even stronger. However, the effort of recovery had taken a toll on the molluscs, with the soft internal part of the mussels weighing 7% less after 4 weeks of recovery than the soft tissue of the mussels that had remained unassailed.In addition, when the team scrutinised the shells for signs of recovery, although some of the shells showed evidence of regrowth in the area near and surrounding cracks, many of the fissures remained unhealed, even though the shells were restored to full strength. ‘Mussels may have a self-healing or compensatory mechanism that does not involve shell deposition’ says Crane. And when the team compared the mollusc's healing rates with those of other armoured surf residents, the mussels were streaks ahead, recuperating in a matter of days, whereas snails take weeks and even months to repair damage. Crane also suggests that the mussels’ ability to recover rapidly could account for their undisputed success in one of the world's most turbulent environments. ‘Molluscs protect themselves with this amazing dynamic armour – these animals not only can resist repeated insults, but they can respond, repair and strengthen so quickly’, says Crane.