Underwater fairy lights?

Found on Brighton Beach: a golden spray of tinsel? gossamer seaweed? washed up underwater fairy lights? botanical jewellery? 

None of the above. This is actually the fine, branched stems of a species of marine hydroid called Amphisbetia operculata. Hydroids are part of a larger group of animals called Hydrozoa (within the phylum Cnidaria which also includes jellyfish and corals).

Hydroids exist in a colonial polyp form. Colonies can be a few centimeters in length, but individual polyps are much smaller. Unlike many hydrozoans that alternate between polyp and medusa stages, Amphisbetia operculata does not have a medusa stage. Instead, it reproduces by releasing planula larvae, which swim for a short time before settling and growing into new colonies.

Like other hydrozoans, it is a filter feeder, relying on cnidocytes (stinging cells) for prey capture. Though they can be found in freshwater, they mostly live in shallow sea waters, attached to rocks, shells, seaweed, or other underwater surfaces. See more about Amphisbetia operculata at Aphotomarine.

Cool facts. While Amphisbetia operculata might be considered a rather lovely but otherwise benign and uninteresting sea creature, some species of hydroids have a little more pizazz about them. For example, Turritopsis dohrnii is sometimes called the ‘immortal jellyfish’. It has the unique ability to reverse its aging process - instead of dying, it can revert to an earlier life stage and start over, meaning it could, in theory, live forever under the right conditions. 

Certain hydroids, like those in the genus Aglaophenia, can grow on discarded fishing nets, ropes, or other debris, creating ghostly underwater structures that float in the currents.

And, finally, there are some hydroids that form colonies in which different polyps specialise in different tasks (some feed, some reproduce, and some defend the colony with stinging tentacles) - making for a superorganism!