Stomolophus meleagris - “cannonball jelly”, “cabbagehead jelly”
Rounded, rigid bell, milky in color with reddish-brown pattern becoming darker towards bell margin. Tentacles and oral arms are presented as a short, rigid stalk extending from underneath bell. Although typically reddish-brown, the pattern may be white instead.
Size: the rounded bell gives these jellies ball-like appearances, from tennis ball size to soccer balls.
Mild to no sting.
Source: Dock Watch