Invertebrates
Invertebrates are animals that don't have a segmented spinal column. They are a diverse group of animals ranging from single-celled animals (like protozoans) to more complex animals (such as crabs and prawns).
There are many types of aquatic invertebrates, many of which are invisible to the naked eye. Some of the more common ones found in estuaries are the polychaetes, crustacea and molluscs.
Polychaetes are segmented marine worms. These worms can be found buried in the sediments in burrows or tubes. They appear at night and feed on detritus (decaying plant and animal matter). A glow is given off by these worms during breeding.
Jellyfish belong to this group of invertebrates. Two species of jellyfish are commonly found in the waterways: the brown jellyfish and the transparent jellyfish. It is the brown jellyfish which stings swimmers.
Gastropods include snails, periwinkles, abalone and sea hares. Snails are often found in the waterways attached to seagrasses.
These molluscs are characterised by a shell with two parts which are hinged together. Bivalve molluscs may be found attached to the ground or solid objects such as jetties and pylons, or buried slightly in the sediments. Molluscs are filter feeders. They filter plankton from the water. The bivalve mollusc Mytilus edulis planulatus is common in estuaries and rivers. It is a small black mussel about two centimetres long.
Besides crabs and prawns, other small crustaceans live in the shallows and amongst weed, feeding on detritus and smaller plants and animals. They are mostly aquatic and have two pairs of antennae which may be used for sensing, swimming or attachment. They have one pair of mouth parts, and limbs which are in pairs that may be similar or dissimilar. For example, some crabs have one large and one small claw.
Amphipods and copepods can be found at the water's edge. They hop away like fleas. Amphipods have several different kinds of legs, some for feeding, some for walking, some for swimming and some for hopping. They are usually straw-coloured or reddish brown.
Isopods are found in similar parts of the water to amphipods but sometimes crawling along, often in large groups. They are different to amphipods in that they are flat, and only have legs for walking (the wood-lice or slaters found on land are isopods). They are usually dark brown or reddish in colour.
The shrimp Palaemonetes australis is common in the shallows amongst weed and seagrass and can be spotted by moving dry algae on the foreshore. It has a wide tolerance to salinity differences and is found in both estuarine and freshwater environments.
Some invertebrates float or drift along in the water. These animals are collectively known as 'zooplankton'. The term 'plankton' (which means 'wandering') may also be used. This can be used to describe both plants and animals which float in the water (plant plankton is called 'phytoplankton'). Other invertebrates may be found attached to the algae and seagrass of estuaries. The majority of invertebrates are bottom-dwelling organisms and can be found in the top 5 cm of the sediments.
Most invertebrate species are marine and so the range of invertebrates decreases upstream, particularly in winter when there is more fresh water in the river. Most invertebrates prefer sandy, shallow, well-aerated conditions like on shallow banks. Few species inhabit the fine, oxygen-depleted mud of the deeper water.
Invertebrates are a very important part of a waterway's ecosystem. They are the food of most estuarine fish and wading birds. Most invertebrates feed on other zooplankton, phytoplankton and bacteria. Zooplankton generally feed on other smaller zooplankton or phytoplankton. Invertebrates which live on the bottom of a waterway often feed on protozoa and bacteria which help with the decay of seagrasses and algae. Because of this a large number of invertebrates are found in seagrass beds. Their presence attracts fish looking for food to the seagrass beds.
The salinity and temperature of the water, the type of bottom sediments and other factors determines what species of invertebrates live in a waterway. Some species are able to survive a range of conditions while others can't. They either have to move to a more favourable environment or die. An example of a species able to survive a range of conditions is the shrimp Palaemonetes australis which has a wide tolerance to salinity differences. It is found in both estuarine and freshwater environments. It is common in shallows amongst seagrass.