Barnacles often settle higher on the shore than most other organisms. They are adapted to live part of their life, sometimes most of it, actually out of water – being able to get by on splashes of water that extend beyond the high-tide line.
The way that animals and plants are distributed across the shore is known as zonation. Zonation is generally accepted as meaning a vertical separation of different groups of organism, often into distinct bands of different colour when living on hard substrates, resulting from the tolerances of individual species to dessication, temperature, and wave action – otherwise termed ‘exposure’. The barnacles and mussels occupy the mid-shore level. Around the world, although the species differ, the same phenomenon is found, with zonation more clearly visible to the casual observer on steep exposed rocky shores.
An extreme example of this zonation can often be seen on the artificial structures of a waterfront harbour where wooden wharf-sides, timber pier pilings, and metal revetments substitute for rock surfaces on which organisms can settle. Many of these artificial substrates are vertical and therefore the zoning of the organisms may be exaggerated and clearer to see.
The pictures in this post show a pale band or stripe, made up almost entirely of cream-coloured sessile or acorn barnacles, naturally cemented onto harbour-side structures, sometimes wholly encircling them. A few common periwinkle gastropod molluscs move around the barnacles, feeding on the bio-film that accumulates on their shells. Fronds of spiral wrack and sea lettuce type of seaweed, both also fairly tolerant of exposure out of water, are sometimes scattered over the barnacle zone. The barnacles have special adaptations that allow them to survive dehydration at low water but they are none-the-less vulnerable to predating dog whelks at all stages of the changing tides.
Below the barnacle zone, a darker, almost black band, is composed of edible mussels attached by byssus threads. Mussels are less tolerant to air exposure than the barnacles so they survive best lower down where they are not out of the water for so long. They are a sitting target, though, for starfish which use their tube feet to sucker onto these bivalves, forcing them to open, and then everting and inserting their starfish stomach into the mollusc so that they can feed upon the living contents.
All these photographs were taken on the waterfront in Halifax, Nova Scotia. Most of the piers and jetties are still traditionally made of timber because it is such an abundant commodity in Canada. There are generations of timber structures: new; old and decaying; and derelict examples. All of these show the barnacle banding. So do the more recently built rusting metal revetments to the edges of the renovated wharves in the more developed areas.
Interestingly, many modern high-rise buildings in that location have been constructed right on the water’s edge where they are supported by foundations of steel piles driven deep down into the very hard metamorphosed bed-rock. The pilings can be seen projecting below the buildings on the waterside elevations, disappearing into the harbour water. Each white-painted column displays at its base a lower ring of black mussels and a higher ring of paler barnacles – the structures themselves being reflected in the seawater with an odd abstract effect.
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