Rotational Maneuvers of Copepod Nauplii at Low Reynolds Number

Round 1

Reviewer 1 Report

In the manuscript “Rotational Maneuvers of Copepod Nauplii at Low Reynolds Number”, the authors report high-speed video observations of copepod nauplii swimming behaviors; in particular, they illustrated how the movement of the three pairs of appendages (A1, A2 and M) lead to the three rotational movements: yaw, roll and pitch. The reported observations will be an excellent addition in the research area of micro-organisms swimming and low Reynolds number robotic design.

I do have a few suggestions, which hopefully would better benefit the research community out of this research:

1. It is interesting that N1/N2 stage nauplii cannot perform the pitch rotation. Does this mean that N1/N2 stage nauplii can only swim in a 2D plane since the third rotation direction is disabled? In relate to this, as the authors mentioned in line 131, N1/N2 nauplii has not developed the tail, thus considering the two hinge Purcell model in Figure 8, N1/N2 nauplii has only 1 hinge providing only 1 degree of freedom, thus necessarily from the Scallop theorem it could not perform the pitch rotation. Maybe the authors can add some comments to the paragraph related to Figure 8, since it’s highly related to the Scallop theorem and LRN robotic design. 

2. In lines 169-186 where the authors explained the asymmetry in the oscillation of the A2 appendages, is there a quantitative way to present this asymmetry? If some figure like Figure 3 can show the asymmetry / phase delay in the A2 oscillation, that’ll be very helpful for future studies.
3. While yaw and pitch clearly turn the swimming direction of the nauplii, a perfect roll rotation should not change much of the swimming direction, as is shown in Figure 5. So does this roll rotation enhance the swimming speed? Otherwise what is the purpose of this mode of rotation? Could the authors provide a comparison between a roll rotation swimming and a rotation-free straight swimming, so that it might help people see the role of roll rotation in directed swimming?

Author Response

We thank the reviewer for the helpful suggestions. We have considered them very carefully and revised the manuscript as described below.

1. Concerning N1/N2 stage nauplii, even if they cannot rotate directly around the pitch axis, they can still perform yaw turns to reach any point in 2D, and they can rotate this 2D plane with roll rotations to reach any point in 3D. Thus, they can still maneuver in 3D, albeit with a time-consuming combination of yaw and roll rotations. We agree that the lack of tail in N1/N2 nauplii corresponds to the Purcell model with only one hinge, which cannot rotate in theory. As suggested, we have added comments about the one-hinge model at the end of section 3.4 and clarified the 3D maneuverability of N1/N2 nauplii at the end of the Introduction and Discussion sections.
2. We have thought about quantifying the main asymmetry between the pair of A2 appendages. We noted a larger rotation of the appendage around its long axis on one side than the other, as is evident from the appearance/overlap of the two branches. But the rotation angle is difficult to quantify because we do not have an accurate estimate of the distance between the branches. 7 lines from the end of section 3.2, we have added a sentence acknowledging the difficulty of quantifying the asymmetry in the left and right A2 appendages.
3. No, the roll rotation does not enhance the swimming speed. We found no significant difference in the displacement of the swimmer with and without roll rotation. The main purpose of the roll rotation appears to be to change the orientation of the yaw axis. This prevents the body from remaining confined to 2D and enables 3D maneuverability with yaw and roll rotations only, as discussed earlier. 5 lines into the section 3.3, we have added a sentence comparing the displacement of nauplii swimming with and without roll rotation. In the Discussion, we have expanded the text about maneuvering in 3D with only yaw and roll rotations.

Reviewer 2 Report

This manuscript reports on high-speed observations of the motion of early to late nauplii of B. similis (?) and P. crassirostris (Genera?). The authors report in detail on rotational maneuvers in 3-D. Did these copepods receive food while they were being observed? What was their feeding history, excluding NI and N II ? Temperature and Salinity?

The authors produced a lot of data. What do those data imply ecologically?  Or: Why do those nauplii rotate? Those nauplii want to survive. Therefore: Are those motions reported here significant for their existence and survival? Are those motions continuous or intermittent?

The data which the authors have produced ought to have some implications for those nauplii’s existence. There are publications available (not cited by the authors) which could support the authors towards advancing their evaluations/discussions of their findings.

Also: We need to recognize that “A nauplius is not a nauplius”; there are major differences in behavior  among genera, and also within genera. Those facts need to be considered in a manuscript like this one.

This reviewer would like to suggest to the authors to position their findings as part of a nauplius’ activities towards existence and survival while it is in the water column of the ocean, exposed to predators continuously while having to find food organisms almost continuously.

Author Response

Text in the introduction and discussion was changed to address reviewer #2 comments:

1. Genera and information on culturing conditions were added to the Materials and Methods.
2. References were added to provide more ecological context. The introduction and discussion have been edited to include more information on the ecology of calanoid nauplii. The motions reported in this study are significant for their existence and survival in terms of evasion of predation and possible feeding mechanisms. The motions are intermittent, as is explicitly stated in the Materials and Methods.
3. The reviewer is right, “a nauplius is not a nauplius.” This point has been strengthened to include the degree of variation that has been reported within the Calanoida.
4. The discussion includes a section describing nauplius activities, in particular those centered around escaping from predators and those associated with feeding (searching, finding and capturing food).

Round 2

Reviewer 1 Report

In the revision, the authors have addressed my earlier questions and concerns, I sincerely appreciate it. I think the manuscript in the current form is in good status to be published.

Author Response

Thank you.