by Jenny Edwards
If Blue Australwinks could breathe air they may give an audible sigh of relief that the worst trampling time of the year is nearly over, at least until Christmas holidays come around again.
These tiny smooth blue shells (Nodilittorina unifasciata if you want to be precise) live high on rock platforms scattered over the surface and clustered in cracks and crevices where possible. It is hard not to tread on hundreds of them if you walk around the rocks. Fortunately we humans often have bare feet, soft soled shoes or thongs so the shells are not always fatally squashed.
Trampling is just one hazard that Blue Australwinks have to contend with as they graze the film of lichen and microscopic algae on the surface of the rocks. They live mainly in the splash zone so are rarely under water when the tide comes in but they breathe by gills which must be kept moist. On hot days they close their doors tight and often cluster together to conserve moisture while they suffer the heat. That’s one reason you’ll often see them in crevices where there might be a bit of shade.
Hot weather evaporates the shallow water left in the splash zone making it extra salty. This water could dehydrate any shellfish foolish enough to crawl in it. Things are not much better in rainy weather when water around the Australwinks can be so fresh it would kill them if they ventured out of their shell. Then there are those cold winter nights to contend with.
Despite all these hazards it is pretty obvious from their numbers that Blue Australwinks are good survivors. This is pretty amazing when you know what their babies have to go through. Firstly the mother’s egg capsules have to be timed to be released on the highest of spring tides so the hatching planktonic larvae will be able to float and feed in the sea for a while. Then the baby Australwinks that survive all the hungry mouths in the sea have to be washed up onto a rocky shore – a beach simply will not do. You will see that the smallest of these shellfish are often closest to the high water mark and seem to need more moisture. Adults grow to 15mm.
A close relative, the Noddiwink (Nodilittorina pyramidalis), is lumpier and slightly larger and lives a similar lifestyle in the highest part of the splash zone.