Monday, 24 September 2012

Hydras fight for food

   Hydras and daphnias became a "classical" pair of predator and prey model of pond microscopic animals. However, what is often forgotten, hydras tend to aggregate together in high numbers. Usually, the encounter is not like one hydra vs one daphnia. It's a real mess! During warm days it's possible to spot hundreds of these tiny hungry monsters on a single leaf laying underwater. Inevitably, they start competing for food. In this post I'll show how two hydras fight for a single water flea. Unlike in my other posts, the photographs are not made through the microscope. I tried Canon MP-E mounted on 7D for videography purposes, but made lots of stills during my project. May be these pictures will not impress you after previous hydra posts (1,2) but they show natural behavior.

The cnidarians normally extend their tentacles searching for prey, their tentacles get entangled, and large hydras have young polyps growing on their sides. Overall, it's a mess and distinguishing where's a single organisms might become a pain. Even not involving such philosophical questions as when an organism becomes an individual...

   Very often a water flea (in this example it's a ceriodaphnia) gets caught by multiple predators at the same time. That's when the fun begins.Many scenarios can take place. Usually the hydra pulls the prey and shoves it in the mouth quickly and other hydras don't have time and the only thing they can do is to apply some force to extract the food straight from the mouth. But I recorded something slightly different: two hydras caught a flea at the same time and started pulling it without an obvious winner in the beginning.

It resulted in a lazy fight that lasted several minutes.

   Finally one of the hydras managed to get the flea close enough to its mouth and could start devouring. But the less successful hydra didn't want to give up and starting trying to suck the flea out of the mouth of its competitor.

Technically, both hydras were swallowing the same flea at the same time.

   In a few seconds the winner was obvious. "Realizing" that these efforts are fruitless, the smaller hydra had to abandon that idea. In the picture below I captured the last second of their contact - something that looks like a kiss.

   If you looked at the pictures carefully, you could have spotted some vorticella ciliates sitting on algae.

   And that's ingested prey inside the hydra on top.

   Apparently, watching microscopic animals through ultra-macro lenses is actually more interesting than doing so with a microscope. I could see more of natural behavior and didn't have to stress them with very bright light. Although a microscope reveals more details and pictures look better and sharper than what I display today, I can't ignore the obvious advantages.

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