Invertebrates make up 97% of animal species, including worms, insects, and molluscs. They split from vertebrates 525-520 million years ago and have been the most successful animals in terms of biomass. Some, like nematodes and fruit flies, are used as model organisms in science. Macroinvertebrates are important indicators of environmental status and are a crucial link in the food chain.
Invertebrates are spineless animals. They constitute 97% of all animal species, including all worms, insects, arachnids, crustaceans, molluscs, jellyfish, starfish, nematodes, sponges and all microfauna. The smallest vertebrate, the female Paedocypris fish, is 7.9 mm long, so anything smaller belongs to this group. Genetically, one of the main ways they differ from vertebrates is that invertebrates have only one set of Hox genes, while vertebrates have several. Humans have 4 Hox clusters. All but a subset of one, the Chordata, of the 38 animal phyla are invertebrates.
Vertebrates and invertebrates split evolutionarily between 525 and 520 million years ago, when eight early vertebrate animals appear in the fossil record, most notably the jawless fish Myllokunmingia. Prior to this, animals had existed for at least 80 million years in spineless forms. Since then, they have been the world’s most successful animals in terms of biomass, particularly crustaceans in the sea, arthropods on land, and nematodes in both.
In terms of size, vertebrates tend to have the advantage, although there have been some rather large invertebrates throughout history, including Mesonychoteuthis hamiltoni, the colossal squid, with a maximum size of 40-46m; the extinct eurypterids (sea scorpions), which were up to 12 feet (14 m) in size; Arthropleura (giant centipede), at 8 feet (2.5 m); and the griffon vulture, Meganeuropsis permiana, with a wingspan of 8.5 cm. On average, however, invertebrates tend to be microscopic or the size of an insect, while the typical vertebrate is the size of a fish or rodent.
Because of their rapid reproductive rates and comparative structural simplicity, some invertebrates, such as nematodes and fruit flies, are commonly used in science as model organisms. Much of what we know about genetics and biomechanics comes from detailed studies of these creatures.
A subset of spineless animals called aquatic invertebrates visible to the naked eye have been given the name “macroinvertebrates.” The abundance and diversity of these animals is considered an important indicator of the environmental status of an area. They are one of the most important links in the food chain above the level of microfauna and below the level of top predators. Some macroinvertebrates obviously precede other macroinvertebrates.
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