Basic Biology of Invertebrate Exotic Pets

Invertebrates – animals without backbones – account for around 98 per cent of all animals on the planet; unsurprisingly with such a diverse collection of species, their biology and natural history is often fascinating, varied and full of surprises.

When it comes to keeping invertebrate pets, most of the animals commonly offered for sale fall into one of two groups – the arthropods and the molluscs. In terms of their biology, appearance and lifestyles, they could hardly be more different.

Arthropods

If it’s an invertebrate and it’s got legs, it’s going to be an arthropod – a group that includes insects, arachnids (spiders and scorpions), myriapods (centipedes and millipedes) and crustaceans (crabs, shrimps and woodlice), the name being derived from the Greek, meaning “jointed foot”.

All arthropods have a segmented body and legs attached to at least one of those segments – and typically more. They are all covered with an exoskeleton – the thickest examples being found amongst the crustaceans – and need to shed this hard outer layer periodically to allow themselves to grow. The largest group in the Animal Kingdom by far, they have a history which according to the fossil record stretches back to the late proterozoic era – around 600 million years ago.

Their characteristic legs obviously allow them to move about – often amazingly swiftly – and rely on a well developed muscle system, with hydraulic assistance provided by the animal’s blood pressure. This quirk of their biology is particularly apparent in spiders, which extend or contract their limbs hydraulically, depending on how much blood they pump into them, and only use their actual muscles for running.

Arthropods are common throughout the world, in marine, freshwater, terrestrial and even aerial habitats. They range in size from microscopic creatures to the largest living arthropod – the Japanese Spider Crab – which has a leg span of up to 12ft (3.5m) across; some of the prehistoric arthropods grew even bigger.

Molluscs

Although the mollusc clan numbers far fewer, it can claim the largest invertebrate known to biology – the Colossal Squid (Mesonychoteuthis hamiltoni), which is said to be between 12–14 metres (39–46 feet) long and weighing an estimated 2,000 kg (4,400 lb) or more.

Most molluscs are marine, though some live in freshwater and they also have land-living representatives – including some of those most commonly kept as exotic pets – snails and slugs. Molluscs are well represented in the fossil record, largely because the hard shell that many kinds of them possess preserves exceptionally well; as a result it’s possible to trace their contribution to the world’s biology back over roughly as long as the arthropods. However, unlike the arthropods, molluscs show no body segmentation.

Feeding amongst the mollusc group shows a great deal of variety – with many marine forms being filter feeders, while the octopuses and squids are highly competent predators, equipped with a strong, horny beak. However, slugs and snails have a uniquely mollusc feature – the radula – a horny, rasp-like tongue used to scrape away at plant material, or even the shells and bodies of other molluscs, in the case of some of the carnivorous kinds.

On the whole, most people are inevitably much more familiar with our own group – the vertebrates – than the numerically superior invertebrates which make up the vast majority of animal life on the planet. For the exotic pet keeper, however, the invertebrates represent a real treasure-trove of biology’s oddest and most fascinating creatures.