Complex sentence: The mangrove whipray is dark brown to gray above with many white dots and flecks, that become more dense with size; this dark coloration is due to a layer of mucus, without which the body is light orange-gray.
Split into simpler sentences: The mangrove whipray is dark brown to gray above with many white dots and flecks, that become denser with increasing size. The dark coloration is due to a layer of mucus, without which the body is light orange-gray.