Australia’s southern waters contain spectacular and diverse organisms which rival those of tropical reefs in colour and form, though most are not corals. Marine animal communities flourish under low light conditions on deep rocky reefs, caves, crevices, and the undersides of overhangs. Because not much sunlight penetrates to deepwater habitats, they lack the rich plant growth found on shallower reefs. Instead, these communities are dominated by a breathtaking array of free-swimming and permanently attached invertebrate animals.  Vertical and overhanging rock walls are covered with delicate lacework bryozoans, soft branching corals, and long thin sea whips. Sponges large and small, in different shapes and colours are abundant in these sponge gardens.

Caption: Sponge gardens are highly colourful and diverse 📷 Kade Mills

Sponge gardens are most are commonly found in low light environments on reefs more than 20 metres deep, and on rubble in areas where currents aren’t very strong. These habitats are of great ecological significance and scientific value, and are favoured sites for divers and underwater photographers. Deep reefs and sponge gardens contain a great diversity of species, many of which are endemic (found nowhere else in the world). In many groups, for example ascidians and bryozoans, 80 to 90% of the species are endemic. Victoria’s sponge gardens are richer in species and have a greater proportion of endemic species than comparable habitats around the world.

Sponges, hydroids and anemones, sea pens, sea whips, tube-building worms, bryozoans, barnacles, ascidians and soft corals are the major groups of animals in sponge garden communities. These habitats flourish in shaded or deeper water as they are not dependent on sunlight as algae and seagrasses are. Sponges can have fundamental roles in ecosystem processes, including altering the texture of reef structures, stabilising boulder and rubble fields, substantially influencing nutrient cycling and energy flow, structuring benthic (sediment-dwelling) communities, and providing food and secondary production.   The significance of these roles is not widely appreciated.

Caption: A Southern Blue Devil (Paraplesiops meleagris) swims over soft corals and sponges on the Lonsdale Wall, Port Phillip Heads 📷 Peter Beaumont

Like all life in Victoria’s temperate (cool) marine waters, the habitat-forming invertebrates in sponge gardens rely on clean water and sufficient nutrients and food particles to survive. Because many of the animals are permanently attached to the rock and unable to move, they cannot go in search of food or flee from harm.  They can also be smothered by silt, harmed by abrasive material suspended in the water, and suffer the effects of pollution. Their delicate structures and slow growing life cycle makes sponge gardens susceptible to physically damaging processes such as storms, and human activities like trawling and dredging.

There is a unique sponge garden known as the Entrance Deep Canyon Community at the entrance of Port Phillip Bay. This is a biodiversity hotspot for sea sponges and one of the most ecologically significant parts of the bay. More than 271 species of sponges have been identified in the Port Phillip Heads area, more than half of those known only from Victoria –  and 115 of these species can only be found at Port Phillip Heads. This uniqueness also means that temperate Victorian species are more vulnerable to extinction. The dredging of shipping channels in Port Phillip Bay, undertaken to allow access to larger ships, has had serious impacts on the unique sponge garden community in the entrance canyon. Dredging activities have scoured the delicate life forms from reef surfaces, leaving nothing but bare rock in 55% of the area previously covered by the gardens.