The veliger larva of Trichotropis cancellata and comparative aspects of the caenogastropod apical ganglion
Date
2000
Authors
Parries, Shawn Constantine
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Abstract
The early life history of the caenogastropod, Trichotropis cancellata, was observed and described. The planktotrophic veliger larvae were reared from hatching through to metamorphosis. Specimens were prepared for scanning electron microscopy and histological examination to complement observations of live larvae, and progressive changes in development are described at a number of stages: late embryo, hatching veliger, post-hatching veliger and post-metamorphic juvenile.
The apical ganglion (also called the apical or cephalic sensory organ) is a key structural landmark in a wide range of invertebrate phyla. One of the hallmarks of the apical ganglion (AG) is the presence of serotonin (5-hydroxytryptamine, 5-HT) in the neurons associated with it. The gastropod apical ganglion has been most thoroughly studied in planktotrophic opisthobranch larvae (subgroup of the Heterobranchia). Using light microscopy, immunohistochemical localization of 5-HT antigenicity, and scanning and transmission electron microscopy, the present study provides comparative data on the apical ganglion in five caenogastropod species (Trichotropis cancellata, Euspira [Polinices} lewisii, Marsenina stearnsii, Lacuna vincta, and Amphissa columbiana). Ultrastructure of the AG is similar amongst these species and the overall layout corresponds to the tri-lobed arrangement described for opisthobranch larvae. The serotonergic neurons of the AG show interspecific variation in these caenogastropods with the number of somata ranging from 3 to 6; quite unlike the characteristic 5 neurons associated with opisthobranch larvae. The AG is a larval specific structure and completely degenerates within four days of metamorphosis in T. cancellata. These and other comparative data suggest that common ancestry is a major determinant of overall AG design within the Gastropoda.
The larval nervous system of T. cancellata was fmiher probed with antibodies against FMRFamide. The pattern of immunoreactivity for FMRFamide revealed two apical cells that transiently express the peptide in late embryo and early veliger stages, as well as a series of cells and fibres along the metatroch (post-oral ciliary band) of the velum and in the cerebral and pedal ganglia. An immuncytochemical assay for dopamine 13-hydroxylase and a histochemical assay for nitric oxide synthase yielded negative results.