Presence of Egg Wrack indicates a sheltered shore
WHEN the tide has ebbed, the common brown seaweeds that are exposed in the middle of a rocky seashore are called ‘wracks’. There are five common species and since they tend to occupy different levels on the shore they are used to divide rocky seashores into three broad zones: upper, middle and lower.
Channelled Wrack defines the uppermost shore and it is followed down-shore by Flat Wrack. Neither of these two species has air bladders. The lower shore is defined by Saw Wrack. It is also a bladderless species and is unique among the wracks in having a saw edge to its leaves or fronds.
The remaining two species have air bladders and occupy the middle shore. Bladder Wrack has bladders that are small, roundish and occur in pairs side by side. Egg Wrack has bladders that are larger, more oval or egg-shaped and they occur singly.
Seaweeds cannot have wood to support their bodies as the force of the waves would snap them off. Instead they have floppy bodies that go with the flow. Species with air bladders use these gas-filled chambers to support themselves to get more light to make food.
In the image above of Egg Wrack, the air bladders are smooth, large, more olive-green and are in the middle of fronds. A good example may be seen left of centre at the very top of the image.
The other structures visible are not air bladders; they are the seaweed’s reproductive organs or receptacles.
The receptacles have a rough and grainy texture, are usually smaller than air bladders, are more yellowish in colour and are located at the ends or tips of fronds rather than in the middle.
Egg Wrack cannot tolerate exposure to rough sea conditions so its absence on a rocky shore is an indication that the shore is moderately or severely exposed. On the other hand, the presence of Egg Wrack indicates a sheltered shore. Sometimes, an exposed shore may not support any Egg Wrack except where a headland or pier juts out and creates a sheltered corner where the plant can thrive.
Egg Wrack is also known as Asco a contraction of its Latin botanical name Ascophyllum. It is commonly harvested by individuals as a soil conditioner and commercially by the seaweed industry for use in alginates, fertilisers, manufacture of seaweed meal, packaging, etc. Commercial exploitation is controversial in many coastal communities.