Innervation patterns of the cerebral nerves in Haminoea hydatis (Gastropoda: Opisthobranchia): a test for intraspecific variability

Abstract

This study describes the innervation patterns for the cerebral nerves which project to the cephalic sensory organs (CSOs) in the opisthobranch Haminoea hydatis (Linnaeus 1758) and uses axonal tracing techniques (backWlling) to reveal the central cellular origins for these cerebral nerves. Cell clusters projecting into the cerebral nerves can be deWned by their positions in the ganglion relative to other clusters, nerve roots and lobes. The number of cell clusters and the relative sizes of somata are constant in a given cluster, whereas the absolute number of somata and absolute sizes of single somata in a given cluster increase with the size of the animal. Additionally, the invariable morphological characteristics of the cell clusters are used to deWne criteria for the assessment of possible homology for the clusters innervating the CSOs in Opisthobranchia. The data suggest two diVerent strategies to accommodate the increasing body size; Wrst, the additions of nerve cells and second, the growth of nerve cells.

Use of axonal projection patterns for the homologisation of cerebral nerves in Opisthobranchia, Mollusca and Gastropoda

“…As in our previous study [11], we defined clusters of nerve cells here on the basis of their close proximity within the ganglia and the tight fasciculation of their axons projecting into the filled nerve. Assignment of cells to clusters followed a conservative approach based primarily on relative location of the cells.…”

“…We focus on the tracing patterns of Aplysia punctata and Aplysia californica (Aplysiomorpha), Archidoris pseudoargus (Nudibranchia) and Pleurobranchaea meckeli (Pleurobranchomorpha) possessing different 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. Our study aims at examining whether variability in morphology of CSOs is reflected by differences in the patterns of somata projecting into the nerves innervating the organs.…”

“…Homology at the cellular level has already been discussed in Gastropoda [23-27] and in Crustacea [21] and several criteria for cellular homology have been established. Additionally, our own previous studies led to the proposition of extended criteria for homology of axonal tracing patterns [10,11]. These criteria comprise:…”

“…3) the distribution of axonal pathways; whereas the final arborization of the axons can be highly variable [21,24], the general pathways of major bundles of axons seem to be relatively invariant [10,11]. …”

“…The earlier investigations in H. japonica and A. tornatilis showed little variation between the axonal tracing patterns. Both species share similar but not equal sets of CSOs, which have been assumed to be an adaptation to their life history [10,11]. Therefore, similarity of the axonal tracing patterns could be explained by sharing a similar gross morphology or a similar functional adaptation of the CSOs.…”