Limpets, barnacles, tortoises and armadillos
Chitons and woodlice
Lobsters, crabs and scorpions
Sea urchins
Turtles
Snails
Scales
Slimy shells
Synopsis
The seashore is a very hostile environment and animals that live there have to survive rising and falling tides, pounding waves, desiccation and predation by other animals. One solution to these problems is the use of armour plating. Combined with an ability to clamp down tightly to rock surfaces, this provides an effective way of weathering harsh conditions.
Here we compare some seashore and marine animals with land animals that have developed similar methods of protection.
Limpets and barnacles
At first glance a small limpet and a barnacle look very similar, but they are very different animals. Barnacles are crustaceans, related to crabs, prawns and lobsters. Limpets are molluscs and related to snails, clams and squid. If you look closely at the shells of these two animals there is one major difference. The limpet shell is a single structure, but the barnacle shell has several interlocking plates. Both shells serve the same purpose: when they clamp down, these animals can survive hours, even days, out of the sea on rocky surfaces. The seal is so firm, and the shell so hard that few animals can remove them.
Tortoises and armadillos
On the land, animals have evolved similar strategies for protection. The tortoises are armour plated reptiles and if threatened can retreat into their shells, just as the limpet does. Like the barnacle, armadillos have a series of plates for protection. If intimidated they roll into a tight ball forming an impenetrable shell.
Chitons and woodlice
Another shore dweller which could be compared to the armadillo is the chiton. These molluscs have a series of eight shell plates surrounded by a flexible girdle. They eat algae from the rocks and move sluggishly. They roll up slowly if detached from the rock. Some larger chitons are carnivorous and can trap small animals under their shell or even rasp whole barnacles off rocks.
On land an animal that acts and looks similar to the chiton is the woodlouse. Woodlice are crustaceans, related to crabs, prawns and barnacles. Their hard exoskeleton offers protection against many predators. They have a row of plates over their back, allowing flexibility. Ancestors of woodlice were marine and the shoreline lifestyle is still favoured by sea-slaters - the UK's largest species of woodlouse. Another group of land animals with bands of armour (called tergites) are the pill millipedes. Some woodlice and the pill millipedes can roll into a tight ball for defence just like the armadillo and chiton.
Lobsters, crabs and scorpions
Lobsters also have a tough covering - the exoskeleton. As they grow they moult regularly to allow the body to grow. Their claws are a good defence against larger predators. They can also rapidly flick their tails to kick attackers. Crabs have exoskeletons too and most have a large powerful pair of pincers. Hermit crabs borrow armour by living in old discarded shells.
On the land there are similar creatures. Scorpions have a much the same basic body shape a lobsters with two large pincers. They have a hard exoskeleton which is shed to allow the scorpion to grow. They also have a special tail for defence, tipped with a venomous sting.
Scorpions are excellent at surviving dry environments. Their exoskeleton is coated with a layer of wax, which locks in moisture. Even if they dehydrate, they can lose as much as 40 per cent of their body fluids without harm.
Sea urchins
Not only do sea urchins have a hard spherical exoskeleton to protect the soft body parts within, but they have a battery of spines - a formidable barrier against predators. Many land organisms have also developed spines for defence. These include hedgehogs, spiny anteaters, echidnas and porcupines. Many fish have spines for protection and some inject poisons if touched. The stonefish, which lies camouflaged amongst weed and rocks in shallow water, contains one of the most powerful venoms in the animal kingdom.
Turtles
Unlike land turtles, sea turtles can't withdraw their body parts completely into their thick carapaces. Ocean-going turtles have a problem that their land cousins do not. In the sea there are animals and plants which can attach on to the carapace and grow there. This can slow the turtle down by increasing drag and reducing streamlining. Luckily turtles can get a 'shell service' around coral reefs where fish feed on the growth, cleaning it off.
Snails
Snails are found on land and in the sea. Their spiral shells are an ideal protective housing into which they can retreat when threatened by predators or adverse conditions. Different species of snail are found on different levels or zones of the rocky shore. The nearer the sea you look, the more sea living adaptations you find. For example, some shoreline marine snails have an operculum. This acts like a stopper for the shell - effectively sealing the animal inside - and protects against predators and drying out.
Most land snails do not have operculums, because they are adapted for life out of water. They are still prone to drying out, so many produce a slime which can set to form a skin over the opening of the shell. This skin (or epiphragm) is tough and contains crystals of calcite, the mineral that forms mollusc shells and crab exoskeletons.
Scales
Bony fish
Most fish have scales for armour. Early fish were covered with hard bony plates, but the evolution of lightweight scales and swim bladders have allowed bony fish to become more efficient and faster swimmers. Scales form a coating over the skin, protecting fish from skin parasites. Large scales can make fish harder to eat. Some primitive fish such as the sturgeon still have bony plates or scutes on their skin.
Sharks and rays
Cartilaginous fish such as sharks and rays have an incredibly tough sandpaper-like skin, abrasive against the skin of many other species. The skin is covered with thousands of tiny denticles which have a similar structure to teeth. In the fish's mouth these become enlarged and form rows of cutting or crushing teeth.
Scaly reptiles
On land, scaled creatures include the reptiles. Their scales not only protect the skin beneath by acting like natural chainmail, but help reduce water loss and conduct heat to the body. Birds also have scales on their legs and heads.
Pangolin
A mammal which has developed scales is the pangolin. Its armour is made of horny overlapping scales. Other mammals have thickened skin or plates of bone, but pangolin scales, which grow from the tough underlying skin, make it look more like a reptile than a mammal. This amazing protective coat covers every part of the body except the underside and the inner surfaces of the limbs. The scales are shed and replaced individually.
Slimy shells
Armour doesn't have to be hard and bony to be an effective protection. Parrotfish retreat to crevices on coral reefs to sleep at night. They are vulnerable to predators that follow their scent trail. The parrotfish therefore secretes a cocoon of mucus around its body which stops many predators from sniffing them out. Fish also use slime as a protective device, mainly against small parasites that grow on the skin. It's all very well having armour, but you need to keep it polished!
Animals on land use slime as protection. Some frogs and toads are capable of secreting cocoons to survive dry periods in desert areas. Other amphibians such as salamanders are experts in producing slime laced with toxic chemicals.
Synopsis
Armoury appears in many shapes and forms in the animal kingdom. It is just as important to creatures living in the sea as those on land. As a result animals in both realms have produced similar solutions and designs - convergent evolution. Animals that are totally unrelated can resemble one another because they have similar lifestyles imposing similar constraints or selective pressures - and predation is a very strong selective pressure.
You may have noticed that most of the animals described here have relatively slow moving or even sedentary lifestyles. This is because they have invested energy into producing armoury and no longer need to be as fast or vigilant. Since there is a limited number of practical armour designs, many land and sea animals look very similar.
