Structures Catch Prey

photograph of blue and purple physalia physalis on green sea kelp at beach

Biological Strategy

Portuguese man-of-war

AskNature Team

Image: Enrique Ortega Miranda / Unsplash / Free non-commercial use

Move in/on Liquids

Water is not only the most abundant liquid on earth, but it’s vital to life–so it’s no surprise that the majority of life has evolved to thrive on and under its surface. Moving efficiently in and on this dense and dynamic substance presents unique challenges and opportunities for living systems. As a result, they have evolved countless solutions to optimize drag, utilize surface tension, fine tune buoyancy, and take advantage of various types of currents and fluid dynamics. For example, sharks can slide through water by reducing drag due to their streamlined shape and specially shaped features on their skin.

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Capture, Absorb, or Filter Organisms

Many living systems must secure organisms for food. But just as one living system must capture its prey to survive, its prey must escape to survive. This results in capture and avoidance strategies that include trickery, speed, poisons, constructed traps, and more. For example, a carnivorous plant called the pitcher plant has leaves formed into a tube that collect water. Long, slippery hairs within the tube face downward. When insects enter the tube seeking nectar, they lose their footing and slide inside, unable to climb out and escape being eaten and digested by the plant.

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Jellyfish

Subphylum Medusozoa (“serpent-haired monster animals”): Jellyfish, box jellyfish, hydrozoans

Medusozoans spend at least part of their life cycle in the medusa stage, where they form a bell-like shape with tentacles, allowing them to move freely. Within this subphylum, Class Scyphozoa (“cup animals”) include true jellyfish that have a thick, gelatinous body and live their adult lives as medusa. There are a few hundred species of scyphozoans. Class Cubozoa, often called “box jellyfish” get their name from their cube-shaped bodies, may be incredibly venomous, and have complex eyes, which are unusual among cnidarians.

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The float and tentacles of the Portuguese man-of-war capture prey using synergy for efficient sail-driven trawling.

Image: U.S. Department of Commerce, National Oceanic and Atmospheric Administration / Wikipedia /

Portuguese man-of-war

Image: Sonnymt / Wikipedia /

Portuguese man-of-war at Miami beach

“The most conspicuous results of this study can be summarized in three points as follows: (i) The tilt angle (from vertical) of a trailing tentacle increases with the sailing velocity and decreases with the tentacle diameter. Since the tentacles are of different diameters, trailing causes them to spread fan-like in the vertical plane. The spread angle vanishes both at small and at large trailing speeds. Using plausible figures for the diameters, densities and drag coefficients of the tentacles, the spread angle is estimated as 10–15º at the same speeds the Portuguese man-of-war were observed sailing. (ii) The force needed to trail a tentacle at a given speed decreases as the tilt angle of the tentacle increases. The tentacles bud from the float in such a way that this change in drag sheets out the sail as the tentacles lift up, which serves to keep the sailing speed in that particular range where the tentacles spread best. The Portuguese man-of-war were observed sailing with their sails aligned with the wind in strong winds. (iii) The trim of the sail is very delicate and a minute increase in camber may luff the sail. We believe that these results can be rationalized by adopting a hypothesis that the Portuguese man-of-war has evolved for efficient sail-driven trawling: the tentacles’ spreading obviously increases the likelihood of catching the prey, whereas the Portuguese man-of-war ‘does’ all it possibly can to keep them spread.” (Iosilevskii and Weihs. 2009: 4016)