Tied hands: synchronism between beak development and feeding-related morphological changes in ommastrephid squid paralarvae

Ommastrephid squid produce some of the smallest cephalopod hatchlings, whose feeding behavior has not been observed. The present study aimed at indirectly filling this knowledge gap by describing ontogenetic changes in beak morphology and morphometry 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 correlated with ML to determine whether jaw development 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 ≤ 2 mm ML. Morphological 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 feeding ecology of I. argentinus rhynchoteuthions.