Research Article

Functional Dorsoventral Symmetry in Relation to Lift-Based Swimming in the Ocean Sunfish Mola mola

  • Yuuki Watanabe mail,

    Affiliations: International Coastal Research Center, Ocean Research Institute, The University of Tokyo, Otsuchi, Iwate, Japan, National Institute of Polar Research, Itabashi, Tokyo, Japan

  • Katsufumi Sato

    Affiliation: International Coastal Research Center, Ocean Research Institute, The University of Tokyo, Otsuchi, Iwate, Japan

  • Published: October 22, 2008
  • DOI: 10.1371/journal.pone.0003446
  • Featured in PLOS Collections

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Academic Editor Comments

Posted by SHUM on 23 Oct 2008 at 08:59 GMT

These comments refer to a version of the MS before acceptance. The Authors have dealt with most comments.

Notes on “Functional Dorsoventral Symmetry in Relation to Lift-based Swimming in the Ocean Sunfish Mola mola “

Minor points:

P4L21. I would recommend the use of a word other than “lazy” here. I’m not sure that it’s anthropomorphic meaning is helpful. In addition, although I’m not familiar with the literature, I had always assumed that the behaviour of lying on their sides near the surface was related to regaining heat after dives to depth.

P4L32. I’m not hugely convinced by the Reynolds number and separation argument. Perhaps some more discussion on the potential impact of the lateral flattening of the sunfish might be appropriate.

Other comments to consider:

1. Strouhal numbers
From a rough estimate, it appears that your sunfish may well be operating in the expected (i.e. most efficient) Strouhal number range (see Taylor et al. 2003). I think this is worth mentioning given the assumptions about their poor swimming ability. If we take your smallest tagged animal, then St = f A / U, with f as frequency (0.56 1/s), A as fin amplitude (I estimate perhaps 0.5 m??), and U as mean velocity (0.7 m/s), the resultant Strouhal number is of the order 0.4. This is within the expected range of 0.2 to 0.4, although my guesstimate at amplitude needs to be checked.

2. Fin aspect ratios
I think the implications from your aspect ratio estimates might actually be very interesting. In the bird and bat aerodynamics literature, the assumption is that high aspect ratios are an adaptation to high speed, but at the cost of higher energetic expenditure (as you imply). On the other hand, the argument could be turned on it's head - perhaps the large size of adult Mola frees them from a lot of the predation risk of smaller individuals, which may require high escape speeds? Perhaps speed just isn’t that important to larger individuals, who may benefit from the postulated higher maneuverability of low aspect ratio fins?

3. Allometry of fin area
You seem to have the data here to say something about the scaling of both aspect ratio and actual fin area, rather than simply saying that it changes with size. Estimation of the allometric exponent for fin area/aspect ratio may be helpful in terms of estimating the forces experienced or generated by the fins.
4. WiG effects
Wing-in-ground effects are expected for any animals that move near the air-water interface, including those below the water. Whales are thought to benefit from this effect (see Vogel 1994 for a start, Rayner 1991 for more detail). I find it intriguing that Mola appear to swim ‘normally’ most of the time, even when near the surface - if they were taking advantage of WiG effects then they might be expected to spend more time on their sides near the surface, or to have asymmetrical fins.

An additional point to think about might be the assumptions regarding Mola being a conventionally streamlined shape (see above) - what happens if you consider the body to be the equivalent of a wing section? In this instance we might postulate that it should produce lift and drag in the same way as a wing, and in addition to allowing the whole body to be used to produce turns etc, if the animal were to swim on it’s side near the surface this might allow it to use drag/lift to offset any residual buoyancy issues near the surface, where density/bouyancy changes are most extreme.

J. M. V. Rayner, Phil. Trans. Roy. Soc. Lon. B 334, 119 (1991).

Taylor et al. Flying and swimming animals cruise at a Strouhal number tuned for high power efficiency. Nature (2003) vol. 425 (6959) pp. 707-11