English: A lion's mane jellyfish (Cyanea capillata) using its tentacles catch a moon jellyfish (Aurelia aurita) in Gullmarn fjord at Sämstad, Lysekil Municipality, Sweden. The jelly medusas are still quite small, about 5–6 cm (2.0–2.4 in) in diameter.
The jellies are disturbing the top water layer of the fjord. The photo was taken on a calm, sunny morning after a rain, so it's likely that the layer is warmer water with some freshwater added. In this case that would make a thermocline coincide with a halocline and resulting in a very thin but visible pycnocline. This difference in density and the altered refractive index is what makes the water look "oily".