RED-COLORED DIGESTIVE GLANDS IN CULTURED MUSSELS AND SCALLOPS - THE IMPLICATION OF MESODINIUM-RUBRUM

In April 1992, cultured mussels from Ship Harbour, a major estuary on the shore of Nova Scotia, Canada, were found to have red-coloured dige stive glands. Examination of digestive gland tissue using spectrofluor ometry and epifluorescence microscopy revealed the presence of the pho tosynthetic pigment phycoerythrin. A survey of the local phytoplankton suggested that the most likely source of the pigment was the ciliate Mesodinium rubrum. In general, the intensity of the red colouration in the digestive glands of the mussels varied consistently with the abun dance of M. rubrum. Control (phycoerythrin-free) mussels transferred i nto Ship Harbour accumulated sufficient phycoerythrin to closely resem ble the indigenous mussels within a week. Epifluorescent examination o f the control mussels indicated the presence of phycoerythrin in the d igestive tubules within 4 h of deployment. In depuration experiments, macroscopic evidence of red colouration disappeared within 48 h, but e pifluorescence microscopy indicated that phycoerythrin persisted in th e digestive gland for as long as 3 wk. Control scallops transferred in to Ship Harbour accumulated phycoerythrin, but not to the extent of de veloping red-coloured digestive glands. The abundance of M. rubrum dec lined from 30,000 cells . l(-1) in nearly May to <10,000 cells . l(-1) in early June, coincident with an increase in wafer temperature and t he disappearance of the red colouration in the mussels. During July an d August, numbers remained low and the population was generally confin ed to the coldest, deepest (8-13 m) section of the estuary.