Ascophyllum nodosum is a brown algae (phaeophycea) is one of the dominant seaweed species on the rocks which litter the west side of Runswick Bay beach. A. nodosum (common name knotted wrack) is a sexually reproducing algae, and these granular yellow blebs are the receptacles which contain the gamete-producing conceptacles. A survey of the rocky beach today showed that Ascophyllum was absent only at the upper limit of the splash zone. What was obvious was that Polysiphonia lanosa, the red seaweed that grows on Ascophyllum, was far less common on the upper beach and much more widely found the closer I sampled to the low tide mark. Possibly P. lanosa is less resistant to dehydration than its host Ascophyllum, and less able to survive the more prolonged periods of being exposed to the air that would result from living further up the beach. The beach fell 3 metres over a 67 metre transect from the splash zone to the water 1 hour before low water (which was 1.6m above chart datum today).
A. nodusum can live for up to 15 years and its fronds can reach 2 metres long.
Above: Ascophyllum nodosum with its epiphytic red seaweed Polysiphonia lanosa. The small yellowish ovals are the reproductive receptacles, the larger ovals are air bladders that support the algae in the water column.