The influence of substrate type on the ecology of the sponges Coscinoderma matthewsi and Hyrtios erecta was examined on the coral reefs at Masig Island and Kodall Island, in central Torres Strait.Twelve 4 m² permanent quadrats, located 2-40 m apart, were established at each reef on the reef slope (30-50°) at depths between 10 and 12 m. Six quadrats at each island were dominated by rock (>=66% of the seafloor). The other 6 quadrats at each island were dominated by rubble (>=66% of the seafloor). The remaining substrate in each quadrat consisted mostly of sand or was occupied by living organisms such as hard and soft coral species.Quadrats were surveyed every 3-5 months for a period of 27 months (December 2004 to March 2007). At the start of the study, each quadrat had between 0-3 individuals of Coscinoderma matthewsi and Hyrtios erecta. During each survey, each quadrat was thoroughly checked for the presence of Coscinoderma matthewsi and Hyrtios erecta and the position of each sponge marked on gridded data sheets. Sponges that moved out of the quadrat, were not counted even if they were found alive a short distance away.The effect of substrate type on sponge growth was examined by measuring the volume of 27 Coscinoderma matthewsi and 30 Hyrtios erecta individuals attached to either rock or rubble at Kodall and Masig Islands, using a photographic method. For Coscinoderma matthewsi, a close-up photo was taken looking directly down, thus sponge length and width could be accurately determined. For Hyrtios erecta, photos of individuals were taken along their full length including any branches; if needed several photos from different angles were taken so that length of all branches could be calculated.The tagged sponges were measured each time quadrats were surveyed. Sponges were also measured if they moved out of the quadrat area. Water temperature was recorded using HOBO Water Temp Pro recorders, deployed at a depth of 10 m at Kodall and Masig Islands. Data on wind direction and speed was obtained from the Australian Government's Bureau of Meteorology (www.bom.gov.au) weather station at neighbouring Poruma (Coconut) Island, approximately 50 km away. For each month, the number of days where wind direction was NE, N and NW and wind strength was >50 km/h were counted.
Rock and rubble are common substrates on coral reefs in Torres Strait, constituting upwards of 70% of the benthic cover. Rubble is typically unbound or unstable in Torres Strait, being moved around by storms and daily water action driven by currents or wind. This study was initiated to determine if there were differences in the population dynamics and growth of Coscinoderma matthewsi and Hyrtios erecta on rock or rubble substrates in central Torres Strait within and between years.
