ULTRASTRUCTURAL STUDY OF SKIN COLORATION IN APLYSIA CALIFORNICA

The intensity of the red-purple skin coloration of Aplysia californica J. G. Cooper, 1863 appears to be due to the abundance of two cell types that have not been described before for sea hares: (1) epidermal cells with variable numbers of pigment vacuoles likely provide maximum color and (2) pigment cells that occur beneath the skin and provide a lighter color in areas where the epidermal cells have few or no pigment vacuoles. The spherical pigment vacuoles in both cell types were statistically similar in both diameter and the size of the electron dense particles filling them. These electron-dense particles were also similar in size to that of ink pigment found in red-purple release vesicles in the ink gland. A third cell type, the rhogocyte, found beneath the skin also contained electron-dense vacuoles but probably had little involvement with skin color. Rhogocytes were both less common and had fewer vacuoles filled with electron dense material than pigment cells and appeared to function in the storage of excess pigment. Distinguishing between pigment cells, rhogocytes and muscle (with several dense mitochondria) required a magnification of at least 30,000-40,000 times. Although not likely involved in skin coloration, a fourth cell type, the packet cell, was found beneath epidermal cells with abundant pigment granules and seldom where such granules were absent. Packet cells had several medium electron-dense vacuoles in a collective membrane, the packet, and an extensive whorl of rough endoplasmic reticulum. These cells fused with (most probable) or inserted themselves between the epidermal cells where the packet was eventually released to the outside as a complete unit.