-------- Original Message -------- Subject: Re: arching effect and allometry in fish geometric morphometrics Date: Tue, 23 Nov 2010 14:31:02 -0500 From: Carmelo Fruciano <[email protected]> To: [email protected] morphmet <[email protected]> ha scritto:
-------- Original Message -------- Subject: arching effect and allometry in fish geometric morphometrics Date: Tue, 16 Nov 2010 07:22:42 -0500 From: Rebeca <[email protected]> To: <[email protected]>
[CUT]
Thus, I started studying the possibility of having an arching effect in my dataset and after doing some tests, I realized that the strenght of the arching effect could be also related to the size of the fish specimens (small specimens are more rigid than medium and large sized specimens, which tend to bend more easily). Therefore, to understand better what is happening, I would like to test if the arching effect is related to size and if the deformtion model (1^st mean eigenvector) and the allometric vector (regression vector of the regression of shap on size) are related, to determine what is the best approach to correct the shape variables before the comparison of fish from different areas, I mean, if I need to correct for allometry and for the arching effect separately or if both explain similar shape changes and when I correct for allometry I also eliminate some of the variability due to the arching effect. So, to see if the deformation model is related to size, I was planning to do a regression of the scores of the first PC of the deformation model on size (centroid size) and to check if the deformation model (1^st mean eigenvector) and the allometric vector are related I was thinking of simply calculating the angle between these two vectors. Since I am not an expert in these subjects, I would really appreciate your comments or ideas on this.
Dear Rebecca, I'm not a big expert either but the problem of measurement error in fish geometric morphometrics has interested me since a few years ago. For my PhD thesis (not as the main topic but as a sort of "addendum") I did some tests which I hope will surface in a paper some time in the future. One of the things that I've found is, indeed, that there can be an association between measurement error and fish size. In your particular case, if I understand correctly, the problem is that you want to avoid confounding effects in your analysis (that is, a practical problem with your particular analysis). If this is the case, if you think the same arching model is applicabile to both small and large fish and the difference between large and small fish is only in the magnitude (that is, the position along the first vector) of the arching problem, my feeling is that using FIRST the procedure described by Valentin and colleagues and then performing the other analyses is the way to go. I think of it this way: - first I remove a source of error which I know and I modelled - then I assume residual morphological variation is due to other causes (not arching) such as allometric change during growth, variation in geographic space and so on and I analyse it accordingly (if allometric change during growth is not of interest I remove this component from my data and obtain "allometry-free data", which can be, in turn, another can of worms) I hope I expressed myself clearly All the best Carmelo -- Carmelo Fruciano Dipartimento di Biologia Animale University of Catania Tel. +39 095 7306023 Cell. +39 349 5822831 e-mail [email protected] ---------------------------------------------------------------- Universita' di Catania - A.P.Se.Ma. Servizio di Posta Elettronica
