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The Oddities on the Rock (Ⅰ)

Besides starfish, the intertidal zones of marine parks are inhabited with various kinds of marine life. Chiton is one of the odd ones which may draw your attention.

Chiton is a primitive marine mollusk. It bears eight overlapping plates on its oval flattened body which serve like an armour. It is usually found creeping slowly on its muscular feet on the lower intertidal rocky shores. To withstand the strong wave actions in the intertidal rocky shores, chiton is able to create powerful suction and attach itself tenaciously on rock surfaces; however, if dislodged from the underlying surface, it would slowly curls up into a ball. There are over 800 species of chitons worldwide. The ones commonly found in Hong Kong are about 3 to 5 cm in length but a giant one—the largest one ever found in the Northwest Pacific—was 43cm in length!

Acanthopleura japonica is the chiton species commonly found in Hong Kong. It mainly feeds on algae by scraping the rocky substrate with its radula (a tongue like structure with rows of teeth). Like other species of chitons, it relies on the special sensory cells just below its shell and on its feet to detect the changes in light/ vibration in the surrounding environment.

Have you ever wondered how the chiton may generate such a strong adhesion force to the rock surface? In fact, when it is about to attach onto a surface, its feet clamp down tightly against the substratum while the inner margin is raised. The vacuum area created enables the chiton to grip the substratum with great tenacity. It takes a great deal of effort to dislodge the chiton and it may cause injury to the chiton.

Hence, it is always better to observe from a distance!

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Profile
Name in Chinese: 日 本 花 棘 石 鱉
Scientific name: Acanthopleura japonica
Site of discovery: The intertidal rocky shore in Tung Ping Chau Marine Park
Status: Uncommon (more abundant in the exposed/ semi-exposed rocky shore of Kang Lau Shek and Lung Lok Shui)

 

Acanthopleura japonica 

Acanthopleura japonica

 

Chiton and juvenile common rockfish in the rock pool
 

 

 
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Last Review Date : 10 July 2017