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- There are plenty of tentacular things close to the surface, but they're not (generally) big. Hydra, for example. Most of the larger creatures that live (or can live) closer to the surface are vertebrates.
Maybe that's because tentacles (and feelers, such as antennae and multiple legs) work better in low-light conditions, and arthropods and molluscs are better at those. Whereas eyes and ears (which are specializations with greater refinement in vertebrates) work better in shallower layers.Jul 11, 2012
- Sorry, guys, I gotta step in. The most abundant multicellular animal ANYWHERE in the ocean are copepods, which are (mostly) swimming arthropods.Jul 11, 2012
- Yes, but they don't get BIG. And by that I mean something you would notice if it bit you.Jul 11, 2012
- How do copepods move? I watched some crustaceans on youtube, the small ones swam like ciliates; larger ones rowed by propelling water backwards with a single large pair of legs (or antennae or whatever - but on arthropods, almost everything is a leg anyway), possibly reaching Re>1 and making use of the water's inertia.
In my first comment I had been thinking more about walking and grabbing things than about swimming (for swimming, one wants drag, at least sometimes), I should have said so.
My general idea about swimming thins goes like this: Tiny beings (Re<<1) must rely on drag - there is nothing else. This begins with bacterial flagella, gets replaced with cilia (or similar things) for bigger ones. Another way to do this is being a flagellum - I believe eels swim like this. This gets better when the body is vertically flattened, as this improves the ratio of transversal to longitudinal drag. Big things can take a bunch of water and throw it backwards, with tuna being the most visible example.
Cephs are exempt from the rules since they know rocket science (i.e. they can accelerate water backwards to reach useful Reynolds numbers even when they are small themselves (Bartol gives Re=1..100 for paralarvae)).Jul 11, 2012
- Copepods are about Re=10 when "jumping" for their escape response, but their normal swimming is just about Re=1. Most copepods swim up (using "scooping" motions of their appendanges) and then float gently down. Interestingly, their mouthparts are Re=0.1 or so and exist in a very viscous environment.Jul 11, 2012
- Oh, right, so what I meant to say was that their mouthparts are paddles, not sieves. They are too tiny to be able to "grab" things and must push them instead. Here's a classic paper on this: http://www.aslo.org/lo/toc/vol_26/issue_6/1062.pdfJul 11, 2012
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