【明報專訊】Though sometimes called jellyfish, sea jellies are not fish at all! They belong to the animal group "cnidarians" (刺胞動物), which means "stinging creatures". All the animals in this group have tentacles (觸手) with stinging cells (刺絲細胞) that are used for defence and feeding. There are more than 200 species of sea jellies, and most surprisingly, they came into existence on Earth millions of years ago, even before dinosaurs!

Heartless yet graceful

Sea jellies' bell-shaped bodies, with the tentacles, can range from 3 millimetres to 2 metres in diameter. They consist of 95% water, and have no brain, heart, eye or backbone (脊椎). They usually drift along with the ocean current (洋流), but they are able to contract their bell in order to propel themselves up and down in the water like graceful dancers.

The multi-purpose gut

Sea jellies feed on zooplankton (浮游生物), fish eggs, baby fish and even other sea jellies. They hunt passively using their tentacles as drift nets and food items are brought into the mouth by the oral arms. The waste after digestion is removed also through the same mouth. This two-way, tube-like gut is not only where food and poo passes through, but also where eggs and sperm are released!

The interesting life cycle

The life cycle (生命周期) of a sea jelly passes through two different forms: the smaller, immobile polyp (水螅體), and the larger, drifting medusa (成體). Take the moon jelly as an example. Each fertilised egg (受精卵) stays on the mother's oral arms and is released into open water when it grows into a tiny pancake-shaped larva (幼體). The larva rides the currents until it reaches a hard surface, to which it is attached and transforms into a polyp. The polyp grows and forms a stack of young medusas. Eventually, they break off and continue to grow into adult jellies.

Plastics, the true threat out there...

Most sea jellies are harmless to humans. These stingers have low toxicity and only give us a feeling of needle pricks but a few of them are deadly!

Not every creature avoids sea jellies. Some fishes treat them as moving shelters; while leatherback turtles (棱皮龜) and sunfish (翻車魚) feed mainly on sea jellies. However, they often mistake plastic bags in the sea for sea jellies and swallow the litter. This may kill them!

Text & photos: Ocean Park Hong Kong

■Glossary

defence (n) 防衛

graceful (adj) 優雅的

contract (v) 收縮

passively (adv) 被動地

[Smarties' Power English 第213期]