larval development of the european lobster and how small heterochronic shifts lead to a more pronounced metamorphosis

We redescribe the larval stages of the European lobster, Homarus gammarus, based on autofluorescence composite imaging. We focus on larval stages (II) to (IV). Compared to the American lobster, Homarus americanus, differences are most apparent in stage (III). This stage appears more mature in H. gammarus; for example, the rostrum is already curved and bears spines, and the appendages are better developed and longer and more differentiated. In H. americanus stage (III) shows a stronger resemblance to stage (II). As a result of the morphology of stage (III), the “metamorphic” moult between stage (III) and stage (IV) in H. gammarus is less drastic than in H. americanus. Metamorphosis is characterised by two criteria. It involves (1) a drastic change in morphology in (2) a short amount of time. It has hence been suggested that a more pronounced metamorphosis evolves by two factors affecting these criteria, namely, (1) the evolution of specialised larval features, which increase the morphological disparity between larva and adult that makes the change of morphology more drastic, and (2) the skipping of entire stages. This means larval forms ancestrally moult over several intermediate forms into the definite adult morphology. Yet, in more derived forms the stages with intermediate morphologies are no longer expressed; highly specialized larvae moult into the adult within a single moult (in the most extreme case) hence bridging the morphologies of larvae and adult in a shorter amount of time. The example of the two Homarus species demonstrates that this explanation is not the only possible one. Additionally, differences of a single larval stage (in this case larval stage (III)) can lead to a more or less metamorphic-appearing ontogenetic sequence.