by
Jenny Edwards
If you visit rock platforms in different parts of NSW you will notice how those near many of our cities are just about devoid of shellfish; even periwinkles are missing. Our nature coast is still fortunate to have many intertidal areas relatively intact and hopefully they will stay that way.
Human gatherers often target periwinkles although it must take hundreds to make a meal of their flesh, and surely no one in Australia is that hungry. “Periwinkle” is a common name which clumps together many different unrelated species. In our shire two of the most numerous are the Black Nerites (Nerita atramentosa) and the Striped Conniwink (Bembicium nanum).
Black Nerites have black shells with a white lining. Sometimes the older shells are so worn by erosion that they are grey or mostly white. They grow to about 28mm diameter but most are smaller. They are often found clustered under ledges or at the edges of pools.
Striped Conniwinks are about the same size as the Black Nerites. The shell is a low conical shape with brown wavy lines running obliquely across the lower parts and orange coloration near the apex. Young shells are striped all over.
Both are found near high tide level, with the Striped Conniwink often in large numbers out in more exposed parts. Like most of the other shellfish you will see above the mean high water mark, the animals graze the algal film on the rocks with their rasp-like tongues (radulas).
They both lay eggs in wet places on the rock platform. Black Nerites attach white egg capsules less than 2mm in diameter to submerged rock surfaces. Each capsule contains hundreds of eggs. The Striped Conniwinks lay their eggs in tiny yellowish jelly blobs in similar locations, usually under rocks. Larvae hatch from the eggs and are washed away to spend time drifting as plankton before landing ashore as tiny shelled molluscs.
From the millions of eggs only a few will make it to a suitable shore and fewer still will survive to adulthood. Animals in the intertidal zone have to survive the extremes of weather and all the marine predators that feed on them. Stormwater drains that flow across rocky shores can make life very difficult for all intertidal life. Even clean freshwater is stressful for them; let alone polluted stuff.
Periwinkles and other shellfish can certainly do without human gatherers adding to their woes.