Larval Development of the European Lobster and How Small Heterochronic Shifts Lead to a More Pronounced Metamorphosis
Evolutionary scenario for the changes in ontogeny in Homarus. Presumed ancestral developmental pattern as seen in H. gammarus (and hybrids of H. gammarus and H. americanus), with rather gradual developmental steps. Derived developmental pattern as seen in H. americanus; the stage (III) is less far developed than that in the ancestral pattern. Hence the developmental step from (II) to (III) is “shorter” and leads to a more juvenile appearing larva (III) paedomorphosis. The larva was simply “stopped,” as it is also slightly smaller progenesis. The developmental step from (III) to (IV) on the other hand is now more pronounced, it appears more metamorphic, and this moult is often interpreted as a “metamorphosis.” This type of heterochronic shift is a peramorphosis, more precise, and it represents a case of acceleration. All drawings modified by Charmantier et al.  and Charmantier and Aiken .