Seasonal Variation in Morphology of the Freshwater Sponge, Clypeatula cooperensis
Unlike other freshwater sponges in the northern Rocky Mountains, Clypeatula cooperensis does not die back during the winter months but instead maintains its structure through the winter, although in an altered form. In addition, since this sponge does not produce gemmules, it apparently relies exclusively on sexual reproduction for propagation. In the present study, the morphology of the sponge was followed by light microscopy over the course of one year. Samples were collected at two-month intervals from August to May and at two-week intervals during June and July (when sexual reproduction occurs). During the winter, the sponge underwent a modified form of regression in which the choanocyte chambers disappeared, the mesohyl increased in density, and the volume of the canal system increased. Regression reached its peak in February. Afterwards the changes that gave rise to the regressed state were reversed: choanocyte chambers reformed and attached to the canal system; the mesohyl became less dense; and the volume of the canal system decreased. These changes were followed in June and July by gamete production, embryogenesis, and larval development. A previously undescribed type of cell, characterized by large eosinophilic granules, became concentrated in the dermal membrane during October. The role of these eosinophilic cells remains to be determined.