Cephalopod sucker design and the physical limits to negative pressure

Abstract

Two factors determine the strength of pressure-based adhesive mechanisms such as suction: the magnitude of the pressure differential that their musculature and mechanics can produce and the pressure differential that water can sustain. This paper compares the adhesive strength of the primary cephalopod sucker types: the stalked suckers of decapods (cuttlefish and squid) and the unstalked suckers of octopods. These results are compared with the physical limits imposed by cavitation, the failure of water under negative pressure. The maximum pressure differentials that suckers can produce were measured using a wettable pressure transducer or by measuring their force of attachment on a wettable surface and dividing by the area exposed to reduced pressure. The maximum pressure differentials that water can sustain on a typical marine surface were measured in a Z-tube. Fifteen cephalopod species representing three orders were studied. At sea level, cavitation limits all suckers to the same range of pressure differentials (100-200 kPa), regardless of their morphology. As ambient pressure increases with depth, cavitation ceases to be limiting. In this case, stalked decapod suckers produce greater pressure differentials than unstalked octopod suckers. In addition, small suckers produce greater pressure differentials than large suckers. Suckers larger than 7.5 mm2, both decapod and octopod, typically achieve pressure differentials of 100 kPa. As their size decreases below 7.5 mm2, octopod suckers get slightly stronger, sometimes producing pressure differentials of 250-300 kPa, while decapod suckers get exponentially stronger, sometimes producing pressure differentials near 800 kPa. There were no differences in sucker strength among the four octopod species, but seven of the ten decapod species differed from the overall decapod regression curve. The strongest suckers belonged to the fast-swimming, open-water species in the decapod sub-order Oegopsida.