GLMM-based modelling of growth in juvenile Octopus maya siblings: does growth depend on initial size?

Abstract

In most studies on cephalopod growth, variability in initial size is masked by the assumption of a similar growth " starting point" for all hatchlings and by calculating the growth rate (GR) through modelling the average size of individuals through time. Statistical interpretations based on such models are limited because regression assumptions (e. g. homoscedasticity and independence between subjects) are frequently violated. To avoid these limitations, generalized linear mixed modelling was used to model the early growth of two sets of siblings of the holobenthic octopus Octopus maya under controlled conditions. The aim was to (i) determine the effect of initial weight (IW) on the GR of individuals grouped in three size categories (small, medium, and large), (ii) obtain statistically reliable estimates of parameters in an exponential growth model for juveniles up to 105 d old, and (iii) evaluate the influence of hatching date on weight at hatching. Using restricted maximum likelihood, linear models were fitted between (i) IW and final weight (FW) for octopuses in each size category, and (ii) the natural logarithm of IW as a function of time. The models were validated by visually inspecting the residuals. Individual FW depended on IW, but GR did not differ between juveniles of different sizes. The exponential growth model for individuals of all size categories was, with 1i N(0, s2 [ agei] 2d). Hatching date had no effect on hatching size (F 1.93

p 0.11). The GR value is similar to those reported for other holobenthic species, and one of the first estimates of the magnitude (d 0.20) and structure of the increase in variance of individual weight through time is provided.