Environmentally influenced variability in the morphology of Cinachyrella australiensis (Carter 1886)

Full description

This study examines populations of Cinachyrella australiensis across a range of habitats from three sites within Darwin Harbour, Northern Territory, Australia to assess the potential influence environmental factors may have on sponge morphological variability. Seven environmental variables were measured at each site on a monthly basis: conductivity, pH, turbidity, dissolved oxygen, water temperature, salinity and water flow. Ten sediment samples were collected haphazardly from each reef and specific attributes of the sponges were recorded to test their correlation with particular environmental variables along with spicule dimensions (length, shaft width and head width) and morphological characteristics.

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Statement: - Study sites - Darwin Harbour is a relatively shallow tropical estuarine system with a diurnal macrotidal environment, with an 8 m maximum tidal range. It differs from other tropical systems in that the tidal range is significantly higher, and exposure of intertidal organisms occurs during the hottest part of the day (1300-1400 hours). Tidal currents are generally very strong throughout the harbour (ranging from 0.25 to 2 m s-1), whereas nearby bays can be relatively calm (Semeniuk 1985; Byrne 1987). The harbour has a high concentration of rivers, streams and creeks that flow into it, each with accompanying discharge and strong tidal movement. These inputs promote excessive sediment mixing, high turbidity and high nutrient input from upstream mangroves; consequently, much of the harbour is heavily scoured, particularly the main channel (Michie 1987). Three study sites were used in this study. They differ in environmental and geomorphological characteristics. The Channel Island (CI) site is on a small reef between Channel Island and the mainland, between which is a short stretch (4 m) of mud leading into mangrove forests. Sediments at this site have a relatively large grain size (>0.9 mm) and there is a considerable amount of coral rubble in the sub-tidal zone. The area is subject to extreme water flow (2.2 m s?1), which has been observed to move portions of coral, up to 1 m in diameter, large distances (McDonald unpublished). The East Point (EP) site also experiences considerable water flow (1.4 m s-1), but has finer sediments (<=0.53 mm). The EP reef is more extensive than the CI reef, extending for ~500 m off shore, and it is subjected to greater wave action than the other sites. The Fannie Bay (FB) site is in a relatively sheltered part of the harbour, and has very fine, silty sediments (<0.53 mm) covering much of the reef flat. This site experiences very little disturbance from water flow (0.5 m s-1) and has relatively uniform and calm conditions throughout the year. The reefs studied are all fringing lateritic rock with large numbers of hard and soft coral. They provide a habitat for many species of sponge, and have been described as the most diverse and species-rich sponge region in Australia (Hooper, unpublished.).

Statement: - Environmental variables - Seven environmental variables were measured at each site on a monthly basis: conductivity, pH, turbidity, dissolved oxygen, water temperature, salinity and water flow. The first six variables were measured with a Horiba water checker U-10, and water flow with a Schiltknecht MiniAir 2 flow meter. To determine the relationship between the environmental variables and tissue variability a correlation analysis was conducted with Statview 4.5. Ten sediment samples were collected haphazardly from each reef. Samples were dried for 48 h at 100°C to determine dry weight (DW) and then passed through a series of sieves to determine the relative proportions in each size class. - Morphological characteristics - Specific attributes of the sponges were recorded to test their correlation with particular environmental variables. Individuals of different size, and consequently different age classes, were collected at random points along the length of each reef. Random sampling was adopted to prevent collection of similar morphotypes and/or age groupings that might have biased the data. Four morphological attributes were measured in situ from twenty individuals taken haphazardly from each site: height and diameter, oscule and porocalyx diameter. Height was measured from the highest point of the sponge perpendicular to the substratum, and diameter measured across the widest point of the sponge. Measurements were made prior to removal of specimens. Means were compared by a one factor ANOVA at a 0.05 significance level.

Statement: - Tissue variability - Of the twenty sponges measured for morphological characteristics, ten of haphazard size and distribution were removed from each site at low tide, ensuring that all the anchoring tufts were collected. In the laboratory, wet weight (WW) was determined by allowing each sponge to drip dry for 1 h then recording its weight. Sponges were then dried to a constant weight in a drying oven for 24 h at 100°C to determine DW. Ash-free dry weight (AFDW) was determined after the material had been burnt in a furnace for 24 h at 500°C. Proportions of sponge tissue devoted to organic matter (DW-AFDW) and inorganic matter (AFDW) were determined. Proportions of structural silica, oxea width and oxea length were plotted relative to CI, the site with the largest structural component and oxea size. - Spicule morphology - Spicules were extracted from additional sponge tissue by boiling small pieces of sponge, including ectosome and choanosome, in concentrated nitric acid. The solution was centrifuged through distilled water then absolute alcohol. This process was performed twice. Spicule extracts were sputter-coated with gold and examined in a JEOL JSM-T330 scanning electron microscope. Spicule dimensions (length, shaft width and head width) were measured with a digital measurement device attached to the microscope.

Notes

Credit
Grey T. Coupland for her help in data collection and editing of the manuscript