Shell Loss in Cephalopods: Trigger for, or By-Product of, the Evolution of Intelligence? A Reply to Mollo et al.

Abstract

In a recent review [1], we discussed the evolution of cephalopods with a focus on the process that may have triggered the emergence of large brains and behavioural flexibility in coleoids (octopuses, cuttlefish, and squid). We suggested that the internalization or loss of the protective shell allowed the ancestors of coleoids to colonize a wide array of new environmental niches, thereby exposing them to increased predatory pressures and fostering complex foraging strategies. These novel ecological challenges may have in turn played a key role in driving intelligence in coleoids.

In their commentary, Mollo et al. question our hypothesis by arguing that coleoids must have evolved intelligence prior to losing their shell, because the opposite sequence of adaptations would have resulted in the extinction of these soft-bodied animals. According to the authors, our hypothesis might only be valid if intelligence is assumed to have evolved as a dramatic mutation across one-generation, which is not a reasonable assumption. Here we highlight that our hypothesis does not hinge on the abrupt evolution of intelligence in cephalopods, and that it concurs with some of Mollo and colleagues’ views. However, we disagree with the authors’ arguments that i) coleoid ancestors developed intelligence prior to losing their shell, and ii) the evolution of alternative defence mechanisms must precede the disappearance of the shell in molluscs.