An Investigation into Biochemical Relationships Associated with Shell Reduction and Loss in Opisthobranch Molluscs

Takeshi Furuhashi

doi:10.25907/00398

Shell loss in molluscs is considered to be a relatively recent evolutionary development, with the Opisthobranchia (Mollusca: Gastropoda) representing the full range of morphogenesis from obvious external shell to complete shell absence. The evolutionary progression from shell reduction to shell loss has been correlated with inhibition of the mineralisation process in shell formation. While it has been recognised that sugars play a principle role in biomineralisation through the provision for nucleation sites for crystallisation, the type and proportion of sugars involved in this process in molluscs has not been investigated. This study utilised a range of histological staining methods, along with the techniques of gas chromatography and colorimetric assay, to investigate the relationships between mucus sugars and shell biomineralisation status in 23 gastropod species. These species represent the full range of shell morphology from fully calcified shell to reduced shell and shell absence. Histological staining indicated that there are no clear differences in the types of sugars present in the mantle tissue across the spectrum of gastropods investigated. However, there were consistent differences in stain intensity that suggest a correlation between a decrease in the concentration of neutral sugars and shell reduction and loss. Analysis of the ratio of N-acetylated amino sugars to mannose sugars, and the ratio fo sulphated sugars to neutral sugars, revealed a positive relationship between low relative ratios of mannose sugars and progressive shell loss. It is hypothesised that this change need not be complicated. Simple changes in the ratio of sugars may render the mineralisation process ineffective, through a reduction in the acid macromolecules required for crystal nucleation. Alternatively, a decrease in mannose-rich glycoproteins could reduce the binding capacity of sulphated sugars or mineralisation proteins with the organic matrix, effectively decreasing the number of nucleation sites. Ultimately, changes in the ratios of sugars involved in the mineralisation process could result in shell thinning or complete shell loss. The elimination of the energy expensive process of shell biomineralisation is consistent with the requirement for optimisation of energy in organisms that exhibit rapid growth patterns, as occurs in the shell-loss molluscs. Hence, the evolution of mucus sugars in hypothesised to be related to rapid growth in mollusc species, and parallels the evolution of shell loss.

An Investigation into Biochemical Relationships Associated with Shell Reduction and Loss in Opisthobranch Molluscs

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