Influence of re-orientation on alignment to flow and tissue production in a Spongia sp.

Full description

Spongia sp. individuals on intertidal reefs in Darwin Harbour were rotated by 90 degrees so tissue was re-orientated to face across the prevailing currents. Differences in growth between re-orientated sponges, and both moved control and
undisturbed control treatments were made over a 3 month period.

Lineage

Maintenance and Update Frequency: notPlanned

Statement: - Study sites - Darwin Harbour is a relatively shallow tropical estuarine system with a semi-diurnal macro-tidal environment and an 8-m maximum tidal range (Collins, 1994). Tidal currents are generally very strong throughout the harbour (ranging from 0.25 to 2 m s-1) (Semeniuk, 1985; Byrne, 1987). East Point reef (latitude 12 degrees 24 minutes 3 seconds S, longitude 130 degrees 49 minutes 2 seconds E), situated at the mouth of Darwin Harbour, is a fringing lateritic reef extending approximately 500 m off-shore. The reef provides habitat for many species of sponge, and has been described as one of the most diverse and species rich sponge sites in Australia. It has an unusually high population density and species diversity in a relatively small area (Hooper, 1992, unpublished). This site experiences considerable water flow (mean 1.4 m s -1) and has fine sediments (<=0.53 mm). - Orientation - Preliminary observations on individuals of Spongia sp. revealed that sponges were orientated with their longest axis perpendicular to the prevailing water current. Compass bearings of twenty haphazardly selected individuals were taken to determine if this orientation was prevalent throughout the population (90 degrees perpendicular to flow, 180 degrees parallel to flow). Rayleigh's uniformity test was used to calculate the distribution of the observed directional data relative to that of a random orientation (at 0.05 significance level).

Statement: - Re-orientation experiment - An experiment was conducted at East Point (see thumbnail) over a three month period to evaluate the effects of re-orientation of Spongia sp. to water flow and its effect upon the morphology and growth of Spongia individuals. The following treatments were established using individuals located in the mid-intertidal zone (exposed at sub 2 m tides). (a) Undisturbed controls - Five sponges were haphazardly selected. These sponges were neither moved nor rotated. This treatment allowed comparisons to be made between 'un-manipulated' and 'experimentally manipulated' animals. (b) Moved controls - Three sponges were detached from the reef with a portion of substratum and reattached to the substratum in their original orientation. These sponges acted as controls for the movement process (moved controls). (c) Rotated animals - Five sponges were detached from the reef with a portion of substratum, rotated 90 degrees relative to their original position and prevailing water flow and reattached to the reef. Sponges used in this experiment were of comparable size and each treatment (reoriented, moved control and undisturbed control) was randomly allocated. A small number of replicate animals were used due to the relatively low number of Spongia sp. present on this reef. Each sponge individual had its maximum height, length and width measured. These linear measures were then used to calculate sponge volume at each census. Each sponge was measured prior to disturbance then at each monthly census. Both re-orientated and moved control sponges were fixed to the substratum using plastic mesh. Holes were cut in the mesh, large enough to leave a margin for growth, and the mesh then placed over each sponge. Sponges were maintained in position as the substratum still attached to the sponge was confined beneath the mesh. The mesh structure was fixed in place using concrete nails hammered into the surrounding substratum.

Notes

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