【明報專訊】Are horseshoe crabs really crabs?

Despite their name, horseshoe crabs are not crabs but are related to spiders and scorpions (蠍子). They have a unique body shape like a helmet (頭盔) or a horseshoe. Under their ''helmet'', there are jointed (有關節的) legs, mouth and gills. They also have a long, straight and rigid tail that balances their body and allows them to flip their body over when they are upside down.

◆Where do horseshoe crabs live?

Horseshoe crabs live in coastal waters. Although a horseshoe crab mother can lay several thousands of eggs, the survival rate (存活率) of the babies is extremely low in their first year of life — only one in ten thousand!

There are four species of horseshoe crabs. Two of them, the Chinese horseshoe crab and the mangrove horseshoe crab, can be found in Hong Kong in areas such as Ha Pak Nai (下白泥).

◆What is special about horseshoe crab blood?

Similar to some other invertebrates (無脊椎動物), horseshoe crabs have blue but not red blood. The blood extract can be made into a special indicator for detecting toxins (毒素) of bacteria. Therefore, it is valuable for the medical industry and is widely used for detecting contamination in related products. Companies with responsible practices would collect just a portion of blood from each horseshoe crab and release them alive, so as to ensure the sustainability of the animals.

◆What threats are horseshoe crabs facing?

Horseshoe crabs have declined dramatically worldwide due to the loss of habitats. Many beaches and mudflats are either lost to urban development or pollution, or disturbed by visitors so that they are no longer suitable for horseshoe crabs to breed and grow. On the other hand, some horseshoe crabs are entangled in fishing nets, and are consumed as food or bait (魚餌) .

◆Conservation of horseshoe crabs

In collaboration with City U, Ocean Park Conservation Foundation, Hong Kong (OPCFHK) has been holding the ''Juvenile Horseshoe Crab Rearing Programme'' since 2009. It provides secondary school students with an opportunity to take care of baby horseshoe crabs and release them into the wild. In 2014, OPCFHK established the Mainland, Taiwan and Hong Kong Horseshoe Crab Conservation Alliance (兩岸三地鱟保育聯盟) with other institutions, which enabled regional conservation organisations to join hands in protecting horseshoe crabs and the coastal ecosystem.

Text & photos: Ocean Park Hong Kong

■Glossary

rigid (adj) 堅硬的

extract (n) 提取物

detect (v) 偵測

contamination (n) 污染

[Smarties' Power English 第205期]