Ontogenetic and evolutionary trends on cephalopod digestive systems

Abstract Meropelagic octopuses hatch as planktonic paralarvae, being the subject of progressive morphological and behavioural changes ending with settlement on the seafloor as juveniles. The comparative morphological study of digestive systems can help to understand the adaptation to particular niches during this challenging plankton-benthos transition. Here, the morphometric development of the digestive system of the common octopus ( Octopus vulgaris ) is described through 3D microscopy techniques over the first two months of life. This morphological development is compared with those of adults and the holobenthic cuttlefish Sepia officinalis , as well as with the holopelagic squids Loligo vulgaris and Illex coindetii . Based on present and published results, we suggest four stages for the early development of O. vulgaris : (1) post embryonic, (2) early paralarval, (3) advanced paralarval and (4) early juvenile. The digestive system development was variable over time, with faster growth during the first days of planktonic life and after settlement. All the paralarvae-bearing species showed very proportionally enlarged posterior salivary glands at hatching compared both to more developed conspecifics and to the cuttlefish hatchling. This could reflect a potential common feeding mechanism through external digestion and probably a compensatory mechanism for an early digestive gland underperformance. The proportionally smallest digestive system has been found in L. vulgaris , mainly because of their remarkably smaller digestive glands. All species showed major changes in digestive organs between hatchlings and developed phases, indicating how different ecological contexts over ontogeny are reflected in their digestive structures. Graphical abstract