What does "claming up" mean here? Based on their findings, the researchers built a metal mimic. Its two halves contract and expand, and a rod digs through the sand. They believe that an energy-efficient Roboclam could anchor an autonomous underwater vehicle, search for mines or lay underwater cables. The scientists discuss their efforts in the journal Bioinspiration and Biomimetics. Rather than clamming up. What does "claming up" mean here?