Form and function of the mantle edge in Protobranchia (Mollusca: Bivalvia)

Area of Science:

• Marine Biology
• Malacology
• Evolutionary Biology

Background:

• Protobranch bivalves, a diverse group of bivalves, possess unique shell and mantle structures.
• Understanding the morphology and function of the mantle edge, particularly periostracum formation, is crucial for bivalve classification and evolutionary studies.

Purpose of the Study:

• To analyze the morphology and function of the mantle edge and periostracum in ten protobranch species.
• To investigate the cellular basis of periostracum formation and its implications for protobranch monophyly.
• To compare periostracal structures across different protobranch orders (Nuculida, Nuculanida, Solemyida) and identify characteristic features.

Main Methods:

• Optical microscopy
• Transmission electron microscopy (TEM)
• Morphological analysis of mantle edge and periostracum
• Histological examination of cellular structures

Main Results:

• All analyzed protobranchs possess two basal cells in periostracum formation, a potential synapomorphy supporting monophyly.
• Distinct mantle folds and periostracal structures were observed, varying among species and orders.
• Intense glycocalyx secretion and particle uptake were noted in specific mantle epithelia.
• The presence of translucent pouches under periostracal folds in some species may confer flexibility.

Conclusions:

• The two-basal-cell character strongly supports the monophyly of protobranchs.
• Periostracum structure, including the presence of translucent pouches, provides insights into evolutionary relationships within protobranchs.
• The study clarifies the role of the mantle edge in periostracum formation and highlights its significance in bivalve evolution.