Morphogenesis of the carapace from phyllosoma to puerulus in the Japanese spiny lobster

Molting in exoskeleton-bearing organisms can result in dramatic body shape transformations. While this phenomenon has been well-studied in insects, limited knowledge exists for other animals, including crustaceans. Among crustaceans, achelate lobsters, such as Palinuridae and Scyllaridae, exhibit a unique metamorphic transition from a flattened phyllosoma larva to a three-dimensional puerulus or nisto stage in a single molt. This study investigates the mechanisms underlying this transformation in the Japanese spiny lobster (Panulirus japonicus), focusing on carapace deformation through histological analysis, live imaging, and computational modeling. Three-dimensional micro-computed tomography revealed a significant reduction in body length and a shift in mouth position during metamorphosis. Live imaging captured dynamic morphological changes, including epithelial contraction and subsequent reshaping. Furthermore, observation of the transient metamorphosis phase identified a distinctive furrow structure on the carapace surface in a mid-metamorphosis specimen that died during the metamorphosis process, absent in post-metamorphosis puerulus larvae, suggesting a possible role in the transformation process. To explore the formation of this furrow pattern, a computational model was developed based on differential shrinkage rates in vertical and horizontal directions. The model successfully reproduced similar patterns observed in natural specimens, implying that controlled anisotropic contraction could contribute to morphogenesis. Furthermore, three-dimensional shrinkage simulations demonstrated that local contraction with constrained out-of-plane deformation can generate folds, which later expand to form curved structures. This study provides novel insights into the biomechanics of arthropod molting, highlighting a previously unrecognized mechanism of two-dimensional-to-three-dimensional transformation. The findings enhance our understanding of larval development in Achelata and offer broader implications for exoskeletal morphogenesis across arthropods.