The medusa of Tripedalia cystophora is about in diameter. Its boxy dome-shaped bell has a flattened top and is slightly wider than it is high. From a single point on each of the four corners of the bell arise three tentacles, each on a pedalium or stalk. The outer surface of the bell is granulated with small wart-like structures armed with nematocysts. About a quarter way up the bell are four rhopalia, cylindrical structures containing light-sensitive cells. About half way up the bell, the four flattened gonads can be seen on the inside of the bell. The underside of the bell is partially constricted by a velarium, a horizontal ring of tissue, and in the centre is a manubrium, a tube-like structure which hangs down with the mouth at its tip. This box jellyfish is a translucent yellowish-brown colour.
Box jellyfish swim by expanding and contracting their bells vigorously. During the dayTripedalia cystophora is mostly to be found within of the surface, in sunlit positions among the prop roots of mangroves. These warm sunlit areas are where its main food item, the copepodDioithona oculata, are to be found during the day. Dense swarms of these copepods form in the illuminated patches of water where shafts of sunlight shine through the mangrove canopy. Tripedalia cystophora forages by allowing itself to sink slowly towards the bottom with its tentacles spread out around it to snare its prey. Box jellyfish have complex visual systems. Each of the four rhopalia on the bell of Tripedalia cystophora consists of a group of six eyes of four morphological types. The uppermost and the lowest eye each contains a lens, and there are also a pair of slit eyes and a pair of pit eyes. It has been found that Tripedalia cystophora displays complex behaviours such as the avoidance of obstacles and fast swimming in a particular direction and is able to maintain its position in the shaft of sunlit water presumably by using visual cues. The presence or absence of the copepod prey or the quantity present does not seem to affect its turning behaviour or swimming speed but by remaining in the sunlit water it maximises the number of copepods on which it can feed. It can detect a cylindrical object such as a root when it gets close enough to it and can then swerve sharply to one side to avoid it. At night Tripedalia cystophora moves away from the mangroves a few metres further into the shallow lagoon and sinks to the bottom. It is not clear whether it settles on the bed of the lagoon or whether it swims slowly about among the seagrasses and green algae. This is because when an investigator shines a light to observe it, it reacts by rising towards the surface. Each individual medusa of Tripedalia cystophora is gonochoristic and produces gametes. After fertilisation, the zygote develops into a planula larva which is brooded by the female inside the bell. It is later expelled and settles to the bottom where it undergoes metamorphosis into a polyp about long with four, knob-tipped tentacles surrounding a mouth. This may produce further polyps by budding and these creep across the substrate before attaching themselves. The oral end of each polyp later differentiates into a proto-medusa which detaches itself from the base of the polyp to become a juvenile medusa and complete the life cycle.
