The High Tide Level is also called the Upper Barnacle Zone by some researchers, most notably Isobel Bennett and the late William John Dakin in their various well-known books titled "Australian Seashores".
At first sight a barnacle may appear to be another type of mollusc, something like a limpet. Instead of holding fast onto the rock with a strong foot, their shells are cemented onto the rock.
Barnacles are crustaceans, related to crabs and prawns. There are about seven different types of barnacle found along the south-eastern shores of Australia. They are all found at various tide levels. In some zonation classification systems the barnacles are considered to be key zonation indicator species.
The highest occurring barnacle on the shore is the Six-plated Barnacle, Chthamalus antennatus. It is distinguished by the distinctive breaks (sutures) between the shell plates, and some surfaces weathering to a glistening white "Tooth enamel" appearance.
It often occurs with another barnacle, the smaller four-plated Honeycomb Barnacle, Chamaesipho tasmanica. Sometimes they are difficult to tell apart if they are crowded together or are weathered. The small Honeycomb Barnacle may be very common to abundant over the mid to upper intertidal areas of many rocky shores. On some shores there may be so many of these barnacles, that the rock surface is white over very large areas. The pattern formed by these tight-fitted barnacles gives the common name, "Honeycomb" barnacle.
There are two other very common swift-footed crustaceans that live on the high shore. They sometimes roam above the highest tide levels. They are members of the Decapod crab group. Decapod means ten limbs, where there are eight walking or swimming legs and two feeding claws called chelae.
.The most common crab is the Variegated Shore Crab, Leptograpsus variegatus. Its other common name is "steelback" which well describes its colour and pattern. This abundant, conspicuous, swift crab is found on most non-tropical rocky shores.
Another, not quite so common crab is Smooth Shore Crab, Cyclograpsus audouinii. It occurs on the open coast at all tide levels and below, usually sheltering under rocks on rocky or silt bottoms. It also occurs commonly in estuaries. There is great colour variation, but its lack of shell markings (smooth) and very rounded claws (cyclo = round; grapsus = claws) make it distinctive.
An unusual mollusc also lives at these highest tide levels. Unlike the Noddiwinks, it doesn't have an operculum door to keep in the scarce moisture. It must clamp itself down onto the rock to conserve moisture.
The limpet which lives at these high shore levels is Petterd's Limpet, Notoacmea petterdii. It is often overlooked and missed by an inexperienced observer. Although I knew this limpet was common. The books told me so. It took me months to find one with very careful searching. When I finally found one, I soon realised that this small brown-coloured limpet is quite widespread and common. It is an excellently camouflaged mollusc. Petterd's Limpet may be found commonly on exposed vertical rock faces.
As we walk further down the Eastern Warm Temperate Zone shore we now reach the Mid Tide Level.
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