Mussels

by Georgina Jones and Geoff Spiby in February 2012 - Marine Scene

Lifecycles are always fascinating. In the great winter storm of 2008, huge waves overwhelmed the Kalk Bay seafront. Underwater, scenes of devastation were everywhere. Smitswinkel Bay’s reefs were stripped bare.

That December, a plankton soup swept into the bay. Mussel larvae must have been among those millions of minuscule floating animals because by January of 2009, tiny mussels had begun to cover the denuded reefs.

By May, almost the entire reef was covered with mussels, but their rule wasn’t long-lasting. By the end of 2009, they had been evicted over large areas, and the rocks had been re-colonised by other bottom-dwelling organisms such as sponges, redbait and anemones.

Along with competition for space, predation also took its toll. Specifically, the spiny starfish, Marthasterias glacialis, which appeared on the reefs in large numbers. These starfish are voracious predators and one of their favourite foods is fresh mussel meat. The starfish hunch themselves over the prey mussel and use their hydraulically powered tubefeet, operating on either side of the mussel’s shell to open it. The tubefeet alternate until the mussel tires. Even a small gap allows the starfish to extrude its stomach into the mussel, and digest its flesh. It takes at least 2.5 hours to completely digest a large mussel.

Today, the remaining mussels are so enormous that humans can’t eat them in one bite, and even the starfish may be satisfied after eating just one.

Until the next big storm, when the process will start all over again.