PRIMARY RESEARCH PAPER Tied hands: synchronism between beak development and feeding-related morphological changes in ommastrephid squid paralarvae Rita Melo Franco-Santos . Erica Alves Gonzalez Vidal Received: 5 July 2019 / Revised: 1 March 2020 / Accepted: 5 March 2020 Ó Springer Nature Switzerland AG 2020 Abstract Ommastrephid squid produce some of the smallest cephalopod hatchlings, whose feeding behav- ior has not been observed. The present study aimed at indirectly filling this knowledge gap by describing ontogenetic changes in beak morphology and mor- phometry and integrating these results with published datasets on Illex argentinus arm crown morphology and gut contents. Individuals [0.7–15 mm mantle length (ML)] were measured, weighed, and had their buccal mass extracted. Jaw measurements were cor- related with ML to determine whether jaw develop- ment occurred linearly with ML. For a 10 mm increment in ML, weight increased 430-fold. The jaws of hatchlings were rudimentary, but in larger paralarvae the rostrum protrudes and the jaw features (teeth, slit, groove) disappear. Increases in ML were predicted by beak robustness indices and rostrum protrusion, with growth discontinuities pointing to faster growth in individuals B 2 mm ML. Morpho- logical changes in the beak and arm crown are in synchrony with a transitional event in the feeding ecology of paralarvae: the onset of active predation on crustaceans and masticating their exoskeletons for ingestion. Integration of the results with published data has led to the proposal of a hypothesis of four size-differentiated developmental stages in the feed- ing ecology of I. argentinus rhynchoteuthions. Keywords Argentine shortfin squid Á Beak ontogeny Á Cephalopod Á Feeding ecology Á Proboscis Á Rhynchoteuthion Introduction Ommastrephid flying squids are widely distributed throughout the world’s oceans, from sub-Arctic to sub-Antarctic seas (Roper et al., 2010). These are robust, fast swimming and growing squid that play a central role in the trophic structure of marine pelagic food webs. Ommastrephids are extensively fished, and represent the bulk of the cephalopod fishing industry (i.e., 48% of the total catch in 2010; FAO, 2012; Arkhipkin et al., 2015). The ommastrephid Argentine Handling editor: Iacopo Bertocci Electronic supplementary material The online version of this article (https://doi.org/10.1007/s10750-020-04223-z) con- tains supplementary material, which is available to authorized users. R. M. Franco-Santos Á E. A. G. Vidal (&) Cephalopod-early-life stages Laboratory, Center for Marine Studies, Universidade Federal do Parana ´ – UFPR, Caixa Postal 61, Pontal do Parana ´, PR CEP 83.255-976, Brazil e-mail: ericavidal2000@yahoo.com.br Present Address: R. M. Franco-Santos Institute for Marine and Antarctic Studies (IMAS), University of Tasmania, 20 Castray Esplanade, Battery Point, Hobart, TAS 7004, Australia 123 Hydrobiologia https://doi.org/10.1007/s10750-020-04223-z