Even though chemical defense plays the major role in the marine and aquatic environment, some organisms are still mechanically protected. For example, some crustaceans have a solid chitin exoskeleton that provides armor against aggressors. The hermit crab (Eupagurus bernhardus) protects itself by the use of an empty snail shell.
The crown-of-thorn starfish (Acanthasterplanci) has no need to produce defensive compounds because it is very well protected by solid and sharp thorns. Furthermore, starfish easily recover from loss of some of their arms. Indeed, the well-protected crown-of-thorn starfish has become a serious pest feeding on corals.
Also, behavior can provide some protection against attack. Many small fish swim synchronously in swarms that may appear as a huge organism to predatory fish and it also increases the chances for escape in case of attack because the fishes in the swarm will swim apart in all directions making it hard for the attacker to select its prey.
Squids (Euprymna scolopes) make use of their light organs to blur recognition by optical illusion pretending to be in another position in the water than where they really are, which supports their escape. In fact, squids benefit from the symbiotic bacteria that they host in their light organ because it is the bacteria that produce the light.
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