Abstract
IntroductionGastropoda are guided by several sensory organs in the head region, referred to as cephalic sensory organs (CSOs). These CSOs are innervated by distinct nerves. This study proposes a unified terminology for the cerebral nerves and the categories of CSOs and then investigates the neuroanatomy and cellular innervation patterns of these cerebral nerves, in order to homologise them. The homologisation of the cerebral nerves in conjunction with other data, e.g. ontogenetic development or functional morphology, may then provide insights into the homology of the CSOs themselves.ResultsNickel-lysine axonal tracing (“backfilling”) was used to stain the somata projecting into specific nerves in representatives of opisthobranch Gastropoda. Tracing patterns revealed the occurrence, size and relative position of somata and their axons and enabled these somata to be mapped to specific cell clusters. Assignment of cells to clusters followed a conservative approach based primarily on relative location of the cells. Each of the four investigated cerebral nerves could be uniquely identified due to a characteristic set of soma clusters projecting into the respective nerves via their axonal pathways.ConclusionsAs the described tracing patterns are highly conserved morphological characters, they can be used to homologise nerves within the investigated group of gastropods. The combination of adequate number of replicates and a comparative approach allows us to provide preliminary hypotheses on homologies for the cerebral nerves. Based on the hypotheses regarding cerebral nerve homology together with further data on ultrastructure and immunohistochemistry of CSOs published elsewhere, we can propose preliminary hypotheses regarding homology for the CSOs of the Opisthobranchia themselves.
Highlights
Gastropoda are guided by several sensory organs in the head region, referred to as cephalic sensory organs (CSOs)
Nickel-lysine axonal tracing (“backfilling”) was used to stain the somata projecting into specific nerves in representatives of opisthobranch Gastropoda
The high variability of nervous innervation patterns of homologous structures found in Crustacea [18] and other invertebrates [19,20,21] suggests a further need to refine this criterion for assessment of homology for the cerebral nerves and in consequence the CSOs
Summary
Gastropoda are guided by several sensory organs in the head region, referred to as cephalic sensory organs (CSOs). The high variability of nervous innervation patterns of homologous structures found in Crustacea [18] and other invertebrates [19,20,21] suggests a further need to refine this criterion for assessment of homology for the cerebral nerves and in consequence the CSOs. In the present paper we propose a novel complex character trait for the evaluation of homology of the cerebral nerves by using an axonal filling/tracing technique Our own previous studies led to the proposition of extended criteria for homology of axonal tracing patterns [10,11] These criteria comprise: rent sets of CSOs. These criteria comprise: rent sets of CSOs We compare these cellular innervations patterns with earlier investigations [10,11] of Acteon tornatilis (Acteonoidea) and Haminoea japonica (Cephalaspidea) (in the former studies erroneously identified as Haminoea hydatis) to further test interspecific variability. Due to the extreme diversity of the CSOs within the Opisthobranchia, parallelism cannot be excluded
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