保護色指生物體表的顏色、斑紋、質感等與環境相似。鈍額曲毛蟹（Camposcia retusa; 圖一）是運用保護色的一個好例子，他們生活在岩質的海床和海岸，會就地取材，將周圍的生物—如海葵、海綿、海藻等—黏貼在自己身上，以達至隱身的效果。
另外，無毒的福清白環蛇（Lycodon futsingensis；圖五甲）身體黑白相間，驟眼看跟有劇毒的銀環蛇（Bungarus multicinctus multicinctus；圖五乙）難以分別。
To survive, organisms must develop ways to avoid or counter dangers around them. We call the methods they use to cope with enemy organisms (mostly predators) “defence”. Defence can be classified into different levels:
1. Avoid being discovered
As the saying goes, “prevention is better than cure”. The best way to cope with enemies are often to conceal oneself and avoid being detected. To achieve this, there are mainly two ways – camouflage and mimicry.
Camouflage refers to the method in which an organism develops a body surface with colours, patterns and texture similar to the environment it inhabits. A good example of camouflage is the spider decorator crab (Camposcia retusa; Fig. 1), which inhabits rocky seabeds and seashores. They make themselves invisible by using local resources – sticking anemones, sponge and seaweed onto themselves.
Some organisms that perform mimicry (mimics) imitate non-living things. For example, stick insects (Order Phasmida; Fig. 2) and some caterpillars will mimic dead branches and leaves (Fig. 3).
On the other hand, some mimics make themselves look similar to other organisms. The caterpillars of some swallowtail butterflies (Family Papilionidae; FIg. 4) have two large eyespots on their frontal segment, one on each side. In addition to this, when threatened, they will also protrude a forked “snake tongue”, which makes them look like a snake curled on the branch.
Another example is the non-venomous Futsing Wolf Snake (Lycodon futsingensis; Fig. 5a), which is covered in black and white bands. A first glance it looks exactly the same as the venomous Many-banded Krait (Bungarus multicinctus multicinctus; Fig. 5b), so it is very difficult to distinguish between the two.
When discovered by an enemy, organisms will try their best to avoid a fight, and now intimidation becomes the best strategy. Intimidation is classified into two types – by appearance or behaviour. The former is often achieved by the use of warning colouration. For example, the toxin-secreting Hong Kong Newt (Paramesotriton hongkongensis; Fig. 6) has a bright orange-and-black pattern on their abdomen.
As for intimidation by behaviours, some organisms perform a threat display towards enemies, such as by making themselves look bigger than their actual size. The Copperhead Racer (Coelognathus radiata; Fig. 7), for instance, turns its front body into loops and inflates it while facing an enemy.
3. Fighting off enemies
If the strategy of scaring away the enemy fails, organisms will resolve to a fight until one side falls. A lot of frogs secrete large volumes of mucus when attacked, while some inflate their bodies. All these make them difficult to be devoured by predators. Slug moth (Family Limacodidae) larvae (Fig. 8) are covered with sharp stinging hairs, and are ready to punish any predator who dares to eat it. Some organisms have even developed chemical defence mechanisms. To fight off enemies, ground beetles (Family Carabidae) produce irritating or even corrosive secretions from the glands in the lower back of their abdomen.
Often times, the battle does not end, even when death is certain. There are many poisonous organisms that do not use their poisons outwardly, such as millipedes, milkweed butterflies, and some amphibians. It is only at the moment of death when the toxins in their bodies come in to use. After the organism is devoured, the toxins enter the predator’s blood and poison them, and may even lead to death. Even when the predator escapes its doom, it will learn not to prey on that poisonous species. Hence, from the species’ point of view, having toxins is a great defence mechanism.
Defensive strategies of organisms come in different fascinating ways and are, of course, not limited to those mentioned above. Not only do they make ecological observations more interesting, they also show us the amazingly strong will of organisms to survive.