The effect of damage on the growth, reproduction and storage of lipids in the scleractinian coral Pocillopora damicornis (Linnaeus)

Brief description

A population of the scleractinian coral Pocillopora damicornis (Linnaeus) was examined at Rottnest Island, off the Western Australian coast. This is a temperate zone and an area under considerable tourist pressure. Experiments were conducted at Little Salmon Bay and Mary Cove to investigate the effect of mechanical damage on the growth, reproduction and storage of lipid to see whether patterns of energy allocation were affected. These parameters were measured simultaneously 14 times over 13 months from July 1988 to September 1989.

Lineage

Maintenance and Update Frequency: notPlanned

Statement: - Experimental design - Manipulative experiments were set up at Mary Cove and Little Salmon Bay (see thumbnail). All P. damicornis colonies chosen for this study appeared healthy, were between 30 and 50 cm in diameter, and were not shaded. Two treatments were used: fragmentation and dislodgement. Fragmentation involved breaking some branches from the colony with a hammer but leaving the colony in place. Approximately 50% of branches were broken off throughout the colony rather than just one section of the colony. The remaining colony was sampled. Dislodged corals were knocked from the reef onto the substratum below which was either sand, rock or seagrass bed. Corals were placed face up at the time of damage and at each sampling time. An orthogonal design was used for this experiment so all possible combinations of these two treatments were used. At both sites there were fragmented only, dislodged only, fragmented and dislodged and not fragmented, not dislodged corals. - Sampling procedure - Sampling of colonies involved breaking three branches, ~5 cm long from each coral at each date. Branches from the lowest parts of the colonies were avoided. Samples were used to monitor growth, reproduction and lipid storage. The branch tips were used for growth measurement and the rest of each branch for lipids and evaluation of the gametogenic cycle. Sampling began in July 1988 and continued until September 1989. During the winter, samples were taken each month and during the summer months they were taken every 2 wk.

Statement: - Growth - In order to measure rates of linear extension of branches, colonies of P. damicornis at both sites were stained in situ using the vital stain sodium alizarin sulfonate. The dye was incorporated into the skeleton of the coral colouring it purple where carbonate deposition occurred. Between July and November, plastic bags were placed over the corals just after dawn and tied in place with long strips of Dayglo tape in order to isolate them from the surrounding sea water. Alizarin was injected into the plastic bags at a concentration of 15 mg per litre. The bags were left in place until just before sunset nine hours later. Three branches were taken from the colony and the tips were placed into fresh water. After 2-3 days the branch tips were washed with a jet of tap water until all the tissue had been removed. They were then cut on a trim saw with a diamond blade so that there was a flat face on which to measure the linear extension from the stain mark. Once the coral had dried, the growth was measured using calipers (to +- 0.1 mm) under a magnifying lamp. - Lipids - The remaining branch sections from the above growth samples were placed in 10% formaldehyde in seawater for 24 h. The samples were then immersed in Bouin's fixative for 2 wk to decalcify the skeleton. The tissue samples were placed in embedding cassettes and dried in an oven for 24 h at 55 degrees C. The samples were then weighed (to +- 0.0001 g) and immersed on a 2:1 chloroform-methanol solution for 24 h to dissolve the lipids. The tissue samples were then redried for 3 h and reweighed. The total lipid content was calculated by subtraction and expressed as a percentage of the total tissue biomass. - Larval capture - To estimate the number of larvae produced, portions of colonies were taken from each coral 4 days before the new moon in March and transported to the laboratory. Colonies were placed in fresh sea water and larvae were collected, counted and stored in a solution of 10% formaldehyde in sea water. The number of planulae shed per piece of coral was standardised by calculating the relative surface area of each branch. Branches were weighed, placed in a Miles Tissue Tek consul of paraffin wax at 60 degrees C for 5 s and then reweighed when cooled. The weight of wax was then calculated and the number of planulae shed was divided by the weight of wax. Captured larvae were analysed for lipid content following the methods used by Barnes & Blackstock (1973) and Stimson (1987). Larvae from individual colonies were placed in small vials and dried in an oven at 60 degrees C for 24 h. They were then covered with a 2:1 solution of chloroform methanol for 24 h to extract the lipid before being dried again for 3 h and reweighed. - Gametogenesis - The number and position of samples taken for gametogenesis was the same as that of the lipid samples except that extra samples were taken 4 days before and 1 day after the new moon in March. The samples were prepared for histology by placing them in a solution of 10% formaldehyde in sea water for 24 h before being decalcified in Bouin's solution for ~2 wk. Tissue was then peeled off and placed into embedding cassettes. Samples were processed through a graded series of alcohols and then cleared in 100% chloroform. They were then impregnated in paraplast wax and embedded in paraffin wax. Sections of 6 um were cut and these were stained with Gills Haematoxylin and Eosin. On each slide all polyps (50-100) were scored for their stage of gametogenesis and their planulae were counted. Stages were identified basically following those of Stoddart (1984a,b). There were four stages of spermatogenesis and two stages of oogenesis. Reproduction includes both production of planulae and gametes, so a measure of reproductive output which included both components was devised. An index of reproductive potential (IRP) was calculated from the percentage of polyps with ova and planulae for each colony at each date. The largest sum of the percentage of polyps containing the two oogenic stages and/or planulae at each time point of each coral was termed the IRP. **References** Barnes, D.J. & J. Blackstock, 1973. Estimation of lipids in marine animals and tissues: detailed investigation of the sulphophosphovanillin method for total lipids. J. Exp. Mar. Biol. Ecol., Vol. 12, pp. 103-118. Stimson, J.S., 1987. Location, quantity and rate of change in quantity of lipids in tissue of Hawaiian hermatypic corals. Bull. Mar. Sci., Vol. 41, pp. 889-904. Stoddart, J.A., 1984a. The genetical structure of populations of the coral Pocillopora damicornis. Ph.D., University of Western Australia. Stoddart, J.A., 1984b. Genetical structure within populations of the coral Pocillopora damicornis. Mar. Biol., Vol. 81, pp. 19-30.