Sponges are immobile, aquatic animals that live on rocks and sediment at the bottom of salt water environments. There are many different species of sponge. Most species of sponge are sensitive to environmental stress. Human activity causes some environmental stress. Scientists sampled the marine habitats along a length of coastline in Algeciras Bay in southern Spain to study the species diversity of sponges． The scientists selected 12 sampling stations， $$\(\mathbf{A}\)$$ to $$\(\mathbf{L}\)$$ ，as shown in Fig．2．2．The scientists noted the land use or human activity along the coast next to each sampling station． The sampling stations were chosen to compare the effects of land use or human activity on the species diversity of sponges in the bay． At each marine sampling station，the scientists： －placed permanent line transects， 50 m in length，on the seabed －photographed all sponges sighted at a distance of 1 m either side of the transect －sampled each transect for the same length of time －sampled each transect four times a year． The scientists compared the species diversity of sponges between different sampling stations using an index of diversity known as beta diversity. The higher the beta diversity index, the higher the species diversity at a sampling station. A simplified formula for beta diversity is shown: $$\[ \text { beta diversity }=\frac{n}{\bar{x}-1} \]$$ Key to symbols: $$\(n=\)$$ the number of sponge species recorded at a sampling station $$\(\bar{x}=\)$$ the mean number of sponge species across all sampling stations At sampling station A, 37 sponge species were recorded ( $$\(n=37\)$$ ). Use Fig. 2.3 and the formula for beta diversity to calculate the mean number of sponge species across all sampling stations $$\((\bar{x})\)$$. Show your working. Write your answer to the nearest whole number. mean number of sponge species across all sampling stations $$\((\bar{x})=\)$$

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