Spotted jelly

The spotted jelly is so named because of the little dots that garnish its jelly. It usually measures between 3 and 10 cm (1.2–3.9 in) in length and between 2 and 7 cm (0.8–2.8 in) in diameter but some individuals can reach 30 cm (12 in) long. Contrary to most medusozoans, Mastigias papua does not have stinging tentacles. However, some individuals may contain some rare cnidocytes spread on the arms of the animal but they are inoffensive because they have lost their stinging power.[3]

Like all medusozoans, Mastigias papua is 95% composed of water. This water similar density enables it to easily float.

Jellyfishes with stinging tentacles are usually hunters. Cnidocyte cells enable them to catch their preys before eating them. The spotted jelly has developed another way to feed itself; it lives in symbiosis with a unicellular photosynthetic organism called zooxanthellae. This unicellular organism settles in the tissue of jellyfishes. It provides products of photosynthesis to the jellyfish, and in return, the jellyfish provides it minerals and nutrients from the soil and the sea water.[4]

In addition to this symbiosis, the spotted jelly has several small mouths used to grab animal plankton. These mouths are disposed all along its oral arms.

Spotted jellies have been recorded many times in the Indo-Pacific Ocean, between Japan and Australia. They live in shallow waters, and for this reason they are usually found in coastal and lagoonal waters, but also in marine lakes.

This species of jellyfish is well known for living in huge groups, forming aggregates. This atypical behaviour becomes a tourist attraction.[5] The most famous spot to admire these organisms is the Ongeim’l Tketau Lake in Palau. This lake has been formed 15,000 years ago. Like many other lakes of this region, it was initially joined to the Pacific Ocean, and because of geological movements, the lake has progressively become separated from the rest of the ocean. Mastigias papua has therefore been isolated in this closed lake, with other species of medusa. Out of reach of predators, it has progressively lost its cnidocyte cells, and is therefore now totally harmless to scuba divers. The lake of Palau now counts around 10 million individuals of this species.

The spotted jelly lives in trophic mutualism with a unicellular organism capable of photosynthesis: zooxanthella. This mutualism is based on a life cycle which permits an exchange of energy between the two species.

Mastigias papua has two different ways of life through 24 hours. During the day, it stays at the surface of the water, in the photic zone. The photic zone is located between the surface of the sea and approximately 100 meters deep. It corresponds to the zone where photosynthetic organisms can use sunlight as an energy source. The jelly swims almost 2 kilometres a day, following the sun, therefore allowing zooxanthellae living in its tissue to optimize their photosynthetic activity. Organic matter produced from this biochemical process is shared between the algae and its host. When the sun goes down, Mastigias papua gains deeper areas and zooxanthellae stop their photosynthetic activity. The jelly takes over the role of energy provider. It absorbs nutrients in the soil and stores them in its tissues. When the sun rises again, the jelly returns to the photic zone of and makes the absorbed nutrients available to the zooxanthellae.[6]

• Media related to Mastigias papua at Wikimedia Commons
• Mastigias papua at Animal Diversity Web
• Mastigias papua at Monterey Bay Aquarium
• Photos of Spotted jelly on Sealife Collection