This photo - I lifted from another site - better represents what the swarms would have looked like from underwater. We witnessed them floating in groups that were literally 100s strong. Big brown ones, some almost white and some small baby ones. We've seen them many times before around this time in Pittwater and in the Broken Bay National Park area...but never in these numbers. So I did some research....this is what Wikipedia says...
The Jelly Blubber (Catostylus mosaicus), also known as the Blue Blubber Jellyfish, is a species of jellyfish from coastal regions in the Indo-Pacific. It is the most commonly encountered jellyfish along the Australian eastern coast and large swarms sometimes appear in estuarine waters.
In Sydney waters, the Jelly Blubber's large bell is a creamy white or brown colour, but farther north in Australia it is usually blue. This is because the jellyfish has developed a symbiotic relationship with algal plant cells that are kept inside its body. These plants vary in colour from region to region. It grows to a diameter of 35 centimetres (14 in). The sting can be painful but generally poses no serious risk to humans.
So now we know. Its important to learn something new every day! Unfortunate name though don't you think - Jelly Blubber? Mr. G and I chased them all around Stingray Bay trying to get a good enough picture for this blog. We didn't do so well...but here is a sample.