The Coastal Hermit - Sea Cucumber
The coastal environment is complex and ever-changing, with its landscape constantly shaped by the rise and fall of tides. To thrive in these harsh conditions, inhabiting creatures have to employ a variety of strategies to survive. Sea cucumbers are among such creatures. However, sea cucumbers lack high mobility or the hard protective shells that other coastal creatures have. With only their thick and soft bodies, they lie quietly in crevices or sand and mud, slowly extending their tentacles to search for food. Sea cucumbers have few predators, so they do not need to constantly hide from hunters. They are like reclusive hermits living peacefully in the ever-changing coastal environment, surviving from one generation to another.
Sea cucumbers are a type of invertebrate belonging to the echinoderm family. The impression that most people have of sea cucumbers is that they are just grey or black, however, some species are actually quite colourful. Their bodies are typically tubular, with a texture that is soft and elastic. They have numerous small bumps on their bodies called "tube feet", which they use for movement and respiration.
The sea cucumber’s mouth is located at its front end, surrounded by tentacles that can be extended outside its body to forage. Depending on their diet, different species have different types of tentacles to help them seek different foods. Species that feed on plankton, organic particles, and food debris in the water tend to have dendritic tentacles, while those that gather organic matter and small organisms from the seabed have feather-like or finger-like tentacles; and those that feed on microorganisms and organic matter in sand or algae on rocks have peltate tentacles. Regardless of their diet, sea cucumbers contribute to the circulation and purification of seabed sediments during their feeding process, playing a vital role in maintaining the balance of marine ecosystems.
Most people are familiar with sea cucumbers because they are a common food ingredient. In fact, the Red Sea Cucumber meat and Sea Cucumber meat that are frequently consumed also come from sea cucumbers. Although their Chinese names contain the word "mussel", they are not bivalves, but the internal muscles of sea cucumbers. Red Sea Cucumber meat and Sea Cucumber meat come from different species of sea cucumbers, and they are named differently due to their difference in colour.
Unique Anti-Predator Mechanisms
Although sea cucumbers have few natural predators, especially in adulthood, they still possess a variety of defence mechanisms. Some species shoot sticky tubules from their anus at predators, which rapidly expand and entangle them. The sticky tubules also emit a special odour that further deters predators. Some species secrete anaesthetic substances on their skin to ward off predators.
Furthermore, when under serious attack, sea cucumbers may exhibit "autotomy", expelling their intestines or respiratory tree from the anus, or shedding or dissolving their back muscles to distract predators and buy time to escape. Besides escaping from predators, sea cucumbers may also exhibit autotomy under adverse environmental conditions, such as low oxygen levels. Fortunately, their regenerative ability is strong—once the environment returns to normal, they can usually regenerate within a month.
However, no matter how strong their defence mechanisms and regenerative abilities are, sea cucumbers cannot withstand the impact of human overfishing. Sea cucumbers are rich in protein and low in fat. They are regarded as a premium delicacy in East Asia, leading to them being fished in huge quantities. As the sea cucumbers inhabiting Hong Kong are species that are not economically valuable, they are mainly threatened by water pollution and habitat destruction.