Species by community type

In contrast to being sessile or fixed to the reef, motile organisms can move about on the reef or in the water column. Motile animals that stay on or close to the reef or seabed live in the benthic zone. The density and diversity of benthic reef organisms is largely influenced by the physical structure of the reef, which can vary from flat reef, to a complex system of boulders and crevices. More cracks and crevices mean an increase in the availability of shelter for animals, as well as increased surface area available to search for food. A diverse range of molluscs live on Derwent reefs, either grazing on algae from the surface of rocks (e.g, abalone, chiton), or preying on other reef animals (e.g., sea slugs, octopus). Echinoderms are spiny skinned benthic invertebrates that move around using hydraulic pressure from their tube feet (e.g., sea urchins, starfish, brittle stars, sea cucumbers and sea slugs). They can feed on dead plants and animals (detritivores), but many are also predators. Another highly diverse group of benthic reef animals are crustaceans. Common examples on Derwent reefs are the southern rock lobster, soldier crab, shrimp, and barnacle. A range of fish are also termed ‘benthic’ as they spend most of their lives resting and feeding on the bottom. Most live on soft sandy sediment (e.g., flathead, stinkfish, and flounder), but rays, skate and sharks often inhabit reefs. The draughtboard shark is a commonly sighted benthic fish on Derwent reefs.

The rocky shoreline supports a community of intertidal reef organisms that are visible in rock pools, and on boulders and rock slabs at low tide. Algae and lichen plant communities often occur in prominent bands or zones of the foreshore. For example, rust coloured lichens are often found along the high tide mark, bright green sea lettuce occurs along the mid tide mark and macroalgae or seaweed such as Neptune’s Necklace often occurs in rock pools at the low tide mark. Characteristic bands occur as a result of the different physical conditions between the ‘splash zone’, ‘tidal zone’ and ‘low shore fringe’. The splash zone is above the high tide mark, where sea spray from waves wets the rocks and vegetation. The tidal zone is the area covered and uncovered by the tide twice a day, being further separated into the high tide, mid tide and low tide zones. The low shore fringe is where the waves break when the tide is at its lowest. In which zone an animal lives depends upon their ability to tolerate desiccation, immersion, and wave action. Mobile inter-tidal animals such as crabs, sea stars, turbo shells, and limpets, can move into rock pools or crevices during low tide to avoid drying out. However, many inter-tidal animals are ‘sessile’, meaning they fix themselves permanently to the rocky shoreline. This is a handy strategy to overcome wave action, but means these animals have had to develop ways of retaining water to keep their gills wet in order to breath during low tide. One of these special inter-tidal animals is the Critically Endangered Derwent River Seastar (Patiriella littoralis, previously known as Marginaster littoralis - see page 113 of the State of the Derwent estuary 2020 for more details of this rare species).

Saline shrublands include a number of woody shrubs with salt tolerance capabilities, enabling them to cope with saline saltmarsh conditions. Bonsai-like in appearance the shrubby glasswort Tecticornia arbuscula, and Grey Saltbush Atriplex cinerea, growing to only 50–100 cm height, but be a few meters wide, creating a patchy distribution across saltmarshes. They are slow growing, with large bushes thought to be hundred years old. Being highly fragile they are easily damaged and slow to re-colonise after land clearing or disturbance. The salt tolerant shrub Austral Seablite Suaeda australis is also common in some areas, such as the Lauderdale saltmarsh.

Succulent herblands are dominated by a number of small to medium sized ground hugging herbs that grow in a thick carpet less than 20cm height. They are the most salt tolerant so generally occur in low lying areas of saltmarshes more frequently inundated by the tide. Succulence involves the storage of water in leaves to dilute the salt taken up from the soil, and has arisen independently in a range of different saltmarsh plant types. Within the Derwent estuary the most common flowering succulent herbs are pigface, wilsonias and samphire (also known as glasswort). Succulent herbs form a patchy mosaic with succulent shrubs, sedges, grasses and rushes. Multiple species of succulent herbs commonly grow together and form a dense multicoloured carpet.