WHILE WE HUMANS are vertebrates—creatures with backbones—and the animals with which we are most familiar (birds, cats, fish, dogs, cows, sheep, etc.) are also vertebrates, when it comes to the number of animal species in the world, the vertebrates are far outnumbered by invertebrates. All animals with backbones belong to just a single phylum—Chordata—but there are some 33 other phyla of animals whose members all lack backbones. To confuse things a little, not even all chordates have backbones, however they all possess a structure that is thought to be a forerunner of a backbone—a notocord. The native lancelet Epigonichthys hectori—a 100 mm long bland fish-like creature—is an invertebrate chordate, as are the three New Zealand hagfish species, which also lack backbones.

Of the 35,112 known New Zealand species of Animalia, 33,582 species (95.6 per cent) are invertebrates. Of these, about 60 per cent are terrestrial, 33 per cent are marine, and 7 per cent live in freshwater. These proportions could change when the sea is better explored. The most species-rich phylum is Arthropoda, to which insects and spiders belong on land and crabs, barnacles and shrimps in the sea.

However, of the 34 phyla currently recognized, only nine have representatives on land. Yet examples from all but four phyla are found in the seas around New Zealand, and three of the absent phyla are also marine, though each is known from only one or two species. Hence the sea holds a much greater structural and genetic diversity of creatures than the land, and almost all this diversity is among the invertebrates.

Invertebrates perform critically important ecological roles and many are significant to humans as pests, food or other resources. The following “Cook’s tour” of marine phyla considers them in terms of their broad zoological relationships, and many of these relationships are controversial. Until a decade ago, relationships between groups of organisms were determined from anatomy and also from embryological similarities—the stages through which animals pass as they develop from egg to newborn. Now, analyses of RNA and DNA have been added to the mix and the nucleic acid results are not always in accord with traditional views.