The Jawbone area consists of sandy beaches flanked by basalt rock outcrops that are the seaward end of flows of basalt lava that characterise Melbourne’s western plains.
Subtidal and intertidal soft sediments:
The subtidal and intertidal soft sediments of the sanctuary comprise approximately 7 ha of the sanctuary. The intertidal soft sediments are at the western end of the sanctuary, and are inhabited by micro-algae, polychaete worms, bivalves and amphipods. The mudflats are feeding areas for fish including stingarees, globefish, flounder, flathead and whiting. Subtidal soft sediments are diverse and nutrient-rich habitats. Species composition often depends on the grain size and physical structure of the sediment. The spaces between the sand grains provide a habitat for numerous species of small organisms such as nematodes and copepods. Invertebrates such as bivalve molluscs, amphipods, echinoderms and polychaetes are associated with the upper layers of the sediment, while large animals such as benthic fish, crabs and seastars are associated with the sediment surface.
The subtidal and intertidal soft sediments of the sanctuary also support two hectares of seagrass beds mainly in Jawbone Bay and Wader Beach. The seagrass beds are composed of Zostera and Heterozostera spp. Seagrass beds are complex and productive environments that provide habitat, nursery and feeding grounds for a wide range of organisms including some commercially important species. They provide habitat for epiphytic algae and diatoms, grazing invertebrates such as gastropod molluscs and amphipod crustaceans, and other animals such as juvenile fish, pipefish and crabs.
Intertidal and subtidal rocky reefs:
Extensive intertidal and subtidal rocky reefs in the sanctuary occupy half of the sanctuary (15 ha) and are formed by a wide band of fractured basalt and boulders up to 30 m wide, extending for several hundred metres along the shoreline. Resident species are adapted to environmental extremes and high wave energy. The composition of species depends on the dynamic processes of competition, predation, recruitment, disturbance and frequency of tidal submergence. Macro-algae are generally absent from the upper intertidal area; the algal assemblage on the lower intertidal reef is composed principally of turfing algae, coralline algae and Ulva spp. The intertidal reef is characterised by molluscs, in particular herbivorous gastropods including the Variegated Limpet, Blue Mussel, Scaled Nerite, Striped-mouth Conniwink, Ribbed and Zebra top shells (Hart et al. 2003). Carnivorous molluscs, including Wine-mouthed Lepsiella, and the encrusting tubeworm Galeolaria , occur in the lower intertidal area.
The subtidal reef consists of large basalt boulders providing a complex habitat. The algal assemblage is characterised by low abundance of large algal species and is instead dominated by low turfs of the filamentous brown algae Ectocarpales spp. The presence of Ectocarpales spp. in high abundance is often an indicator of high nutrient concentrations and may reflect a large estuarine influence of nearby Kororoit. A variety of other algal species cover the reef including the iridescent Dictyota dichotoma , Ulva spp. and red algal species. The subtidal reef is also covered by sponges, encrusting coralline algae and large areas of the temperate hard coral Plesiastrea versipora . These provide habitat for many sessile invertebrates, including the very abundant Common Sea Urchin and Black-lip Abalone as well as many crustaceans, seastars, molluscs and nudibranchs. Common fish species include the Southern Hulafish, Zebra Fish, Dusky Morwong and Six-spined Leatherjacket.
Common free-swimming animals include squid, sharks, rays and bony fish such as whiting, garfish and mullet.
The Marine Sanctuary includes one of the few mangrove stands within Port Phillip Bay. The Avicennia marina mangrove community at Jawbone Marine Sanctuary is very unusual because it occurs amongst massive basalt boulders rather than on intertidal mudflats which is unique for Victoria. The mangroves are behind a wide band of intertidal boulders and provide important habitat for a number of bird and fish species as well as crustaceans and molluscs. They are highly productive environments that provide a good food source for many species while the roots or pneumatophores provide shelter. Mangroves are sensitive to disturbance including trampling and take a long time to recover. Mangroves at the mouth of Kororoit Creek were almost completely destroyed in 1950 by oil discharged by a vessel in the bay. (They were replanted there around the 1980s).
Saltmarsh plant communities are generally found landward of mangroves above and around the mean high water mark, preferring to be occasionally submerged. The saltmarsh community within the sanctuary is found among the basalt boulders and soft sediment and occasionally seaward of the mangroves. The upper saltmarsh area is characterised by glasswort and the lower or intertidal saltmarsh area is characterised by austral seablite, beaded and shrubby glasswort. The fauna of the saltmarsh community is characterised by airbreathing gastropods and the littorinid Bembicium melanostomum . The saltmarsh is also potential habitat for the endangered Orange-bellied Parrot.