Invertebrates

As their name indicates, an invertebrate is any animal that does not have a backbone, and incredible as it may sound, more than 90 percent of all animal species on our planet fall into this scientific category.

Invertebrates include sea stars, sea urchins, earthworms, sponges, jellyfish, lobsters, crabs, insects, spiders, snails, clams and squid. These creatures not only provide food for humans, but also play a pivotal role in food chains which sustain birds, fish, and many other vertebrate species.

Apart from the absence of a vertebral column, different invertebrates have very little in common and compose 35 distinct sub-categories. While all vertebrates (animals with a backbone) are contained within a single phylum: Chordata.

In general, invertebrates are soft-bodied animals without an internal skeleton but often with a hard outer skeleton – think of crayfish, crabs, mussels and oysters. These tough outer shells provide protection for these amazing creatures.

  • Phylum: CNIDARIA (anemones, corals and jellyfish)


    Look out for the sea anemones, corals and jellies (jellyfish). They all have hollow, sac-like bodies with one opening, the mouth. A ring of tentacles that have stinging cells surrounds the mouth. These stinging cells are used to sting the prey. The helpless prey is then pulled into the mouth. Corals are also cnidarians except that they live inside the hard coral skeleton which they produce. Many live together in large groups, and are famous for the formation of huge coral reefs such as the Great Barrier Reef off the Australian Coast.

    Moon jellyfish are found in coastal regions

    • Moon jellyfish (Aurelia aurita)

      This creature is also known as the moon jelly, common jellyfish or saucer jellyfish. Moon jellyfish, unlike other species of jellyfish, have both oral arms and tentacles to facilitate feeding. The medusa, bell and moon jellyfish can range from 5cm to 40cm in size. Moon jellyfish are commonly found in coastal regions in warm and temperate waters. The sting of the moon jellyfish is not dangerous to humans.

 

  • Phylum: ANNELIDA (segmented worms)


    If you look very carefully in the Coral Reef you might see a fan worm with its feathery tentacles sticking out of its tube. The fan worm is similar to the earthworm, however it does not move around and builds itself a tube from which it sends out tentacles to collect food and for breathing. Bristle worms have hundreds of tiny feet for crawling around. They are scavengers and can bite.

     

  • Phylum: ARTHROPODA (jointed limbs)


    The rock lobster and crab are related to insects such as locusts, beetles and flies. Crabs and rock lobsters, however, belong to a sub-group called crustaceans. They have a hard outer skeleton for protection, and jointed legs for locomotion. The outer skeleton cannot stretch as the animal grows, so it moults, replacing its small shell with a new one. After moulting its shell is very soft and the animal must grow quickly before the outer shell hardens again. Since these animals are eaten by humans, they are protected by regulations to ensure that people do not overexploit them (collect too many).

    A rock lobster sometimes sacrifices a leg to avoid capture

    • Rock lobster (Panulirus homarus)

      Common on inshore reefs along the KwaZulu-Natal coast, rock lobsters are a popular delicacy in many restaurants. These crustaceans reach 25cm in length. They feed primarily on brown mussels, emerging from their holes at night to feed. When threatened, they may shed a leg to distract the predator. The missing leg regrows. Rock lobsters are protected by bag limits in KwaZulu-Natal, and may not be sold.

 

  • Phylum: MOLLUSCA (mussels, octopus, cuttlefish)


    Many types of shells, the octopus and the cuttlefish are all members of the mollusc family. These animals all have a soft body and are often protected by a hard shell, with the exception of the octopus and the cuttlefish. They have excellent eyesight and are able to quickly change colour or swim away, to evade predators.

    An octopus can fit its body into a tiny crevice

    • Common octopus (Octopus vulgaris)

      These amazing eight-legged creatures are widely regarded as the most intelligent of all invertebrates. Their lack of a skeleton means that they are able to squeeze their muscular bodies into tiny crevices, which is very useful as this helps them to escape predators and to capture prey. They are often found living on reefs and in mussel beds and are able to camouflage themselves by changing colour and texture. In addition to their many amazing adaptations they are also able to expel ink and shoot rapidly away from predators.

 

  • Phylum: ECHINODERMATA (sea stars, urchins, cucumbers)


    The brighter the sea apple, the more toxic it is to predators

    Echinoderms are very interesting animals, examples of which include sea stars, sea urchins, sea cucumbers and the brittle stars. All these animals have rough skins and bodies divided into five segments. Rows of little tube feet stick out of their skin to enable them to move around. Look carefully at the sea stars attached to glass panels in the aquarium and you will be able to clearly see their tube feet. Sea urchins have many sharp spines covering their round bodies. When the animal dies the spines fall off, leaving the shell behind.