8 things you didn’t know about jellyfish

Lisa-Ann Gershwin reveals fascinating facts about these ancient, marine coelenterates.  

Jellyfish_Max_Xie_iStock_623-928fe72

Jellyfish © Max_Xie/iStock

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1 The typical jellyfish life-cycle has two alternating generations. Medusae, the floating form, reproduce sexually to spawn seabed polyps that resemble sea anemones, which clone lots of replicates. When conditions are right, these bud off to form ephyrae – baby jellyfish.

2 A sting from a box jellyfish, the most venomous animal on Earth, can kill a human in two minutes.

3 Box jellyfish have well-developed eyes with lens, retina and cornea, and exhibit sophisticated behaviours including light attraction, coloured-light response, terrestrial navigation and courtship – yet have no brain.

4 Of 2,500 or so jellyfish species, some 90 per cent are smaller than a human thumbnail and rarely researched. The sole study of the bloom dynamics of these species found a five-fold increase in 20 years.

5 Jellyfish fossils, though rare, do exist, some claimed to be over 600 million years old. Many closely resemble living species. 

6 One jellyfish species, Turritopsis dohrnii, is biologically immortal. After an individual medusa dies, its cells re-aggregate into the polyp form and the life-cycle begins again.

7 Jellyfish grow very rapidly – some can expand from a 2mm ephyra to the size of a dinner plate in a matter of weeks.

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8 The largest ‘new’ invertebrate discovered in the 20th century, found off Los Angeles in 1997, was the jellyfish Chrysaora achlyos. Its body is 1m across and its oral arms 8m long.