Comparative 3D microanatomy and histology of the eyes and central nervous systems in coleoid cephalopod hatchlings

Abstract

Adaptive radiation of an animal group is the evolutionary variation of morphology, physiology, and behavior opening up new habitats and resources. An impressive example of the reciprocal interdependency of form and function is found in the anatomy of cephalopod visual and central nervous systems. Interspecific differences of sensory organs and signal processing structures reflect the eco-functional context, e.g., the species-specific demands emanating from habitat and foraging behavior. To substantiate this, we investigated the eyes and brain neuropils of early post-hatching stages of six coleoid cephalopod species (Sepia officinalis, Rossia macrosoma, Sepietta obscura, Idiosepius notoides, Loligo vulgaris, and Octopus vulgaris), showing different size and inhabiting different ethoecological niches. Comprehensive 3D structure data sets were produced in light microscopic resolution, i.e., semithin section series of the head region (histology presented for I. notoides, R. macrosoma, and S. obscura for the first time) and 3D surface renderings of the neuropils, enabling the display of all components in arbitrary perspectives and combinations, and comparative volumetic anaylsis of homologous lobe neuropils. Differing in absolute size considerably, the visual and central nervous systems of the six species follow the same bauplan in adult-like configuration. The visual sense obviously is of paramount importance already after hatching, but also, equilibrioception and olfaction are well developed. The species-specific shapes of various components show that some plasticity and distinct differences in volumetric ratios are found, subject to their functional relevance and to different demands of the lifestyle on the brachial and swimming motor function, on camouflage, as well as on sensoric and cognitive abilities.