Shea, E. K., and Vecchione, M. 2010. Ontogenic changes in diel vertical migration patterns compared with known allometric changes in three mesopelagic squid species suggest an expanded definition of a paralarva. - ICES Journal of Marine Science, 67: 000-000. Planktonic and newly hatched cephalopods are routinely called paralarvae. Currently, the onset of diel vertical migration (DVM) marks the end of the paralarval phase, although changes in ontogenic growth trajectories may also be used. Patterns of DVM are reported for the first time for three poorly understood mesopelagic squid species. Discrete-depth samples taken during the Amsterdam Mid North Atlantic Plankton Expeditions (AMNAPE) of 1980-1983 are used to examine the timing of ecological and morphological changes in Chtenopteryx sicula, Mastigoteuthis magna, and Brachioteuthis sp. 3. DVM patterns are species-specific, and ontogenic changes in DVM coincide with allometric changes in the arm, fin, and funnel characters of C. sicula at 7 mm mantle length. Mastigoteuthis magna is not concentrated in the upper 250 m of the water column during the day, and no clear DVM pattern is found in Brachioteuthis sp. 3, meaning that the endpoint of the paralarval phase cannot be defined ecologically in these species. Other ecological transformations, e.g. changes in prey-capture ability, are therefore explored as alternatives to DVM. The pad-shaped club and long neck are proposed as visual markers of the end of the paralarval phase of C. sicula and Brachioteuthis sp. 3, respectively.