Using Sea Shells for Defense
There are approximately 150,000 known species of invertebrates in the world's seas and a large number can be found on coral reefs. Of these species, two groups dominate: the mollusks, which include marine snails, clams, and even cephalopods, such as octopuses and cuttlefish; and the crustaceans, represented mainly by prawns, shrimps, and crabs.Whereas soft-bodied inverts may use poisons or, like tubeworms, construct shelters, both mollusks and crustaceans typically use their hard outer shells to protect their vulnerable bodies.
How thick is thick enough for a shell?
Any animal that protects itself from predators with a shell is faced with a conundrum: the more it invests in a heavy shell, the more difficult it becomes to move. Conches have come up with an excellent solution to this problem: the nobbles and hornlike projections that cover their shells make it all but impossible for their predators to get enough of a grip to crush them. Active crustaceans, such as prawns, have comparatively thin shells, which are segmented to permit flexibility, while the better-protected crabs lose some of their flexibility as a result of their armor.
Hermit crabs manage to retain their bodily flexibility by having comparatively thin shells. By sequestering snail shells to protect them, the hermits can afford to be fairly soft bodied. Only their claws, which defend the opening to their shells, are heavily armored.
How can predators get through a shell?
Even the toughest shell faces serious examination from the predators. Mantis shrimps can punch their way through it, while various whelk species can drill through a shell using powerful acids and mechanical drilling. Starfish can force open the shell of a bivalve, such as a clam, by overcoming the muscles used to hold the shell closed. Populations of shelled invertebrates that live among many predators tend to have thicker shells for extra protection than those that exist in low-predation zones.
If an octopus is a mollusk, why doesn't it have a shell?
Mollusks are an extremely diverse group that includes clams and mussels, as well as nudibranchs, octopuses, and cuttlefish. There is clearly a wide range of different body designs within this group. The former have hard external shells, but what about the latter examples? Perhaps surprisingly, nudibranchs, octopuses, and cuttlefish do have shells, but over evolutionary time they have been internalized; their shells are found
within the animal's body. These light shells offer little protection, but do provide support for the body.
What about other shell-less invertebrates?
Plenty of reef invertebrates lack a tough outer shell, so how do they protect themselves? Some, such as the echinoderms (starfish and sea urchins), have tough outer cuticles that allow a fairly free range of movement while providing an element of protection. Many others, such as coral polyps, construct their own shelters, either out of limestone that they produce themselves, or by gluing together small particles of sand and gravel. Many worms do this, so that they can withdraw to protect themselves when danger threatens. Softer-bodied invertebrates, such as nudibranchs, rely instead on chemicals for their protection.