Ultrastructural evidence for nutritional exchange between brooding unionid mussels and their glochidia larvae

The life history of unionid bivalve molluscs includes retention of developing embryos within the gills of parental mussels. This brooding behavior may facilitate nutrient transfer to the glochidia larvae, i.e., matrotrophy. To address this possibility, morphological relationships between brood chambers and developing larvae of Pyganodon cataracta and Utterbackia imbecillis were examined with TEM, and larval shells were observed with SEM, for features that could be associated with the uptake of dissolved materials. Early in brooding, glochidia are enclosed in a vitelline membrane that physically contacts numerous cilia and microvilli of the epithelia] cells lining the brood chamber (marsupium). The vitelline membrane subsequently disappears. Lamellar tissues of parental mussels initially have large deposits of glycogen that diminish during the course of brooding. Septa separating brood chambers from adjacent secondary water tubes have numerous mitochondria and microvilli, suggesting the potential for active transport of materials into or out of the marsupia. Since punctae (pores) in the larval shells become filled with an organic matrix early in brooding, they are unlikely to be involved in nutrient exchange. Ultrastructure of the brood chamber and physical contact between the parental mussel and larvae are consistent with a nutritive role for retention of glochidia in the marsupia.