Sunday, 25 February 2007

Shell company

The tide was on its way out at Williamstown but I couldn't stay around to see what it might uncover. So I strolled along the water's edge while the usual suspects watched me from their grandstand in Port Phillip Bay.

Shelley Beach at Point Gellibrand is flanked by two natural breakwaters. The basalt that forms them is part of a lava flow that was once quarried for bluestone. Its resistance to erosion—the property that makes it such a good building material—and the dark coloration tend to discourage molluscs from settling. There is little shelter in the smooth, even surface and the black rock heats up rapidly in the sun. Only a few hardy limpets cling to exposed boulders.

Although molluscs don't favour the basalt, the lichen love it. Orange Caloplaca spreads over the rock above the high tide level. Its success here might be a consequence of the grazers' inability to get a foothold. In other places, the combinations of fungi and algae that make up lichens support large populations of upper shore snails. Littonids, especially, feed on lichens around the splash zone.

Even though the rocks are nearly free of molluscs, thousands of shells cover the beach between the boulders. Most of them belong to one species of bivalve—Anadara trapezia. Anadara occurs in large numbers on mud in shallow water from Port Phillip Bay north to southern Queensland. The species was once much more widely distributed along the temperate coasts of Australia and New Zealand but declined since the Pleistocene. A relict population occurs near Albany in Western Australia.

Just before I returned to my car, I spotted a stranded jellyfish. They're difficult enough to identify while they're swimming around but they're almost impossible when left high and dry on the shore. I think this might be the jelly blubber Catostylus, a large species that is very common in the Bay. Whichever one it is, I know just how it feels.

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