Skeletons & protection in the water

Vermillion Seastar (Pentagonaster dubeni) Vermillion Seastar (Pentagonaster dubeni), Wild Guide to Moreton Bay Flemfish or Sea Cucumbers (Stichopus horrens) Flemfish or Sea Cucumbers (Stichopus horrens), WildGuide to Moreton Bay

The struggle to survive in the sea has produced many fascinating forms of protection, such as external armour and slimy skins. Invertebrates like crustaceans, shellfish and seastars have a hard external skeleton with muscles attached to the inside of the shell. Armoured fish have thickened scales fused together, and turtles have both an internal and external skeleton where the external shell is formed by specially modified bones and hardened plates of skin attached directly onto the bone. Snails and other shellfish are able to grow simply by adding more to their shells whereas crustaceans cannot grow and must shed (moult) their armour at regular intervals to allow growth.

At the other extreme, many marine animals such as jellyfish lack any form of skeleton, perfectly adapted to floating in the water, full-bodied and rounded, but  small, limp and formless, like a deflated balloon, when washed up on a beach. Animals on land have a rigid skeletal support evolved in response to the forces of gravity, whereas in the sea, body weight is supported by water such that some marine species have no real skeleton at all. These are also often slimy, a characteristic that has evolved purposefully for several good reasons. Sometimes the slime contains highly toxic chemicals as a defense to prevent other creatures eating them, especially useful to soft animals that cannot swim very fast, or are firmly attached to the seabed and cannot swim at all. Slime also act as a protection against drying when exposed to the air, such as in the intertidal zone, preventing desiccation to survive until the next high tide. Slime also acts as a ‘Superman Suit’, with chemicals in the slime containing anti-bacterial agents to prevent infection.

Animals with armour:

Animals with slime:

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