FISHER, G.R.*; DIMOCK, R.V. Jr.: The Role of the Larval Mantle Cells During Metamorphosis of Utterbackia imbecillis (Bivalvia: Unionidae)

Larvae of Utterbackia imbecillis undergo a period of metamorphosis to the juvenile while parasitic on a host fish, but can also be induced to metamorphose in a modified cell culture medium. This metamorphosis involves some dramatic changes in the structure of the larval mantle cells which are thought to be involved in digesting the degenerating larval adductor muscle and the host fish tissue. Transmission electron microscopy and histochemistry for digestive enzymes were used to investigate the role of these cells in both fish-reared and in vitro-reared animals. TEM analysis revealed endocytosis in the larval mantle cells but not in any other larval tissues. This was true even for in vitro-reared animals which presumably ingest the larval adductor muscle since there is no fish tissue present. Fish-reared larvae showed no acid phosphatase activity; however, the larval mantle cells of in vitro-reared larvae possessed lysosomes containing acid phosphatase and were also releasing acid phosphatase into the extracellular environment at cell boundaries. The differences in acid phosphatase activity between the two rearing conditions may be due to the presence of glycogen in the fish-reared animals. Prior to the onset of metamorphosis, larvae possess large amount of glycogen stores in the tissues. In the in vitro-reared animals, these stores are depleted very early in metamorphosis, while in fish-reared animals they remain until much later. Perhaps the acid phosphatase is not activated until the cells have used up their glycogen reserves and require an external source of nutrition. It appears that the larval mantle cells play a role in the digestion of both the larval adductor muscle and host fish tissue during metamorphosis.