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Translucent gelatinous creature about three inches long and two inches wide. Bell-shaped in the ocean, flat when on shore, with thin, delicate, venomous tentacles extending from the open end of the bell.
Nearshore tropical waters.
Spotted washed up on the sand at Haleiwa Beach Park on Oahu's north shore. Box jellies are not normally found here, though they are periodically plentiful on the south and west shores of Oahu. As it was still alive when spotted, I returned it to the sea after taking its picture. About a hour later it, or a close relative, stung my 3-year old granddaughter. No good deed goes unpunished.
Thanks for the ID suggestion, chickenparmesan24. Unfortunately, I don't believe it is correct. Although Copula sivickisi is found in Hawaii, its diameter, per Wikipedia, is much smaller (<10 mm) than that of this spotting (>50 mm). Additionally, there is a noticeable difference in the appearance of the tentacles of Copula sivickisi and the appearance of the micro-filament tentacles of this spotting.