Animals without backbones (known as invertebrates) include insects, snails and worms. They play an important role in freshwater ecosystems. They feed on living and dead plant matter, and on each other, and are an important food for fish and some birds.
A diverse group
Most invertebrate families have members that live in fresh water. There are sponges, flatworms, annelids (worms, leeches), molluscs (snails, mussels), crustaceans (crayfish, shrimps, copepods), insects (caddisflies, dragonflies, mayflies) and mites. Lesser-known groups include rotifers, hydras, hairworms, ribbon worms and bryozoans.
Freshwater invertebrates range from tiny animals that can barely be seen with the naked eye (such as planktonic rotifers) to quite large crayfish (kōura) and mussels (kākahi), a traditional food source for Māori.
Testing the water
Because they are so diverse and plentiful, invertebrates are ideal indicators of water quality and habitat health. If pollution-tolerant species are present, this shows the water is in poor health. Species that are sensitive to pollution signal water in good condition. For example, certain mayflies and stoneflies are only found in streams with high-quality water.
Many are highly mobile, while other types, such as sponges, spend much of their life attached to a single spot. Some have shells (for example, molluscs) or cases (for example, caddisflies) to protect them from predators. Others move to refuges such as the darkness of deep water, where it is more difficult for predators to see them.
Freshwater invertebrates have a wide range of reproductive methods:
- Hydra and some worms reproduce asexually.
- Leeches and flatworms are often hermaphrodites (with both male and female reproductive organs).
- Some snails and crustaceans lay eggs that develop without fertilisation.
- Young freshwater shrimps are initially male and then become female as they grow.
- Sexual reproduction is common for most aquatic insects, many of which breed only once and then die.
Freshwater crayfish put great effort into parental care and carry their eggs and offspring under their tail for many months. Some other invertebrates rely on a numbers game, releasing thousands of eggs that each have only a limited chance of survival.
Crayfish are one of the longest-lived invertebrates, sometimes surviving for more than 16 years. Several of the larger aquatic insects live for several years, although most have a shorter span, with one or two generations per year. The tiny rotifers live for only 8–10 days.
Like most other animals, invertebrates have specialised eating habits:
- Some species are grazers and eat phytoplankton or algae.
- Some prey on other invertebrates.
- Some feed on dead and decaying organic matter.
- A group common in flowing waters have net-like appendages to catch passing food particles.
A connection with Gondwana
New Zealand’s freshwater invertebrates are quite distinctive, and many species are not found elsewhere. However, many groups are also related to similar species found in South America, New Caledonia, Australia and South Africa. This suggests that they have common ancestors that once lived on the ancient supercontinent of Gondwana, before it began to split apart some 85 million years ago.