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Gas exchange within Asteriodae sea stars is different to other groups within the Echinodermata family. Asteriodae stars have evolved to have extra gas exchange organs on the aboral body surface, known as papulae. Each papula is a small membranous outgrowth of the internal pervisceral coelom. Papulae are not a part of the water vascular system, despite being similar in function and composition to ampullae and tube feet (Ruppert et al. 2004).
Papulae are specialised gills that supply oxygen to the aboral, internal organs of the star, unlike the tube feet which supples oxygen to the water vascular system, to maintain internal pressure. Papulea directly supplies oxygen to the pervisceral coelom, which is then dispersed throughout the gut, gonads and muscles within the arms and central disc (Ruppert et al. 2004).
Nardoa novaecaledonia prefers to inhabit areas with oxygen levels between 4.379 – 4.685 ml/L (EoL 2011). Despite this they are able to thrive in the back reef and on the reef crest. There is normally significant difference between the oxygen levels within these areas of the reef. The reef crest usually has much higher levels of dissolved oxygen as there is a greater exchange of tidal water. For this reason it was hypothesised that sea stars that inhabited the reef crest would have less papulae than stars that occurred in the shallow back reef, as there is more oxygen in the reef crest environment. N. novaecaledonia that occurred in the back reef should have more papulae to be able to extract the required amount of oxygen from the water.
This experiment was performed on Heron Island, Great Barrier Reef, Australia. Individuals were collected from various parts of the back reef and reef crest and brought back to a laboratory for analysis. Each star was placed under a dissection microscope, and the number of individual papula in 50 clusters of papulae were counted and an average was taken. The average number of papula in a cluster of papulae was 10.67. The number of clusters of papulae on 1/5 of the star was counted, and then extrapolated to estimate the number of papula on the entire star. This was done for each of the stars collected from both of the areas.
The average number of papula within the two areas was calculated and plotted. A student t-test was performed to investigate if there was a significant difference between the numbers of papula within the two areas.
Figure 1: The average number of papula on N. novaecaledoniae from different areas of the reef. P=0.131 (n=10).
The results suggest there is no significant difference between the amounts of papula on the stars on the reef crest, compared to others that inhabit the shallow back reef. This would suggest that there are no physiological constraints, limiting the dispersal of N. novaecaledoniae throughout the back reef and reef crest.
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