Plankton Portal Talk

Direction of motion assumptions - push but not pull?

  • ecovrar by ecovrar

    I'm curious as to the basis for the assumption that "tails" on these organisms are pushing rather than pulling. While it's an intuitively obvious assumption for us "macros" that appendages like these will push, given the spatial scale of these organisms viscous forces will predominate. This is life at low Reynolds and it's a very different world where inertia is dwarfed by viscosity. It's analogous to trying to push a string.

    So then why are we assuming that its a "head" being pushed by a "tail" rather than a "body" being pulled by an pulled by a corkscrew or oars ( for lack of a better term)


  • jessicaluo by jessicaluo scientist, admin

    Hi @ecovrar,

    Yes, it is certainly valid to question whether the "tails" on organisms are pushing or pulling. In many organisms we do know what direction they are moving -- i.e. video of rocketship siphonophores show that they indeed are propelled forward by their swimming bells (those are the "heads" that we are calling) - check out this video that Casey Dunn put together here:, and go to 0:35 to see these siphonophores swimming. Likewise, we also have video of ctenophores and many medusae swimming.

    However, in some other cases, such as the narcomedusa Solmaris, they indeed swim with their tentacles facing forward. This was something we thought about when we asked people to mark their direction. The reality is -- we cannot ask people to mark from bell to tentacles on one species and then from tentacles to bell on another species. we would end up with completely un-usable data because people would get too mixed up. It is much easier for us to flip the orientation later, in data analysis, which is what we intend on doing. What we are aiming to do with this project is "the simplest thing that works." That's the Zooniverse model, which has worked very well for them thus far.

    It is also a very interesting train of thought to explore -- the issue of low Reynolds numbers for small gelatinous zooplankton. For an organism that is over 95% water, how do you deal with life in an viscous environment?




  • scopedriver by scopedriver

    @jessicaluo, that video is mind-boggling! The #sipho #corncob looks huge! Perhaps you could link to it again in a more prominent place on the site, so that folks that don't have time to search can find it, and be amazed by it. The quality of images is really stunning. Thanks for your consideration and appreciation of us "citizen scientists"!