Sea Oak seaweed at Studland

Sea Oak, Halidrys siliquosa (L.) Lyngbye, is another of the brown seaweeds or Phaeophyceae that can be found washed ashore on the sands of Studland Bay, Dorset. It is a perennial seaweed growing from 30 – 90 centimeteres long. It lives in sheltered water below low tide level or in deep rock pools; and is fairly common and widely distributed throughout the British Isles. A cone-shaped holdfast attaches the seaweed to rock.

The Sea Oak is flattened rather than rounded in cross-section. ‘Branches’ arise in a regular manner on alternate sides of the ‘stem’ which has a very characteristic wavy zig-zag shape. Unlike many other types of seaweed, the air bladders are long and narrow instead of spherical, elliptical or ovoid. There are from ten to twelve internal compartments in each air bladder; the divisions between the compartments give rise to a series of external constrictions along the length of the bladder. The resemblance to a seed pod or siliqua is the feature from which this seaweed gets its specific name. The word Halidrys comes from two Greek words meaning sea and oak but no-one can remember why this was considered an appropriate generic name for the group.

When Sea Oak washes up on the strandline, it is often in a jumbled heap and frequently occurs mixed in with other types of seaweed. You need to pick up the weed and spread it out if you are interested in identifying the different species. That is the only way to clearly see what you have. It lets you examine the whole structure and you may find other interesting seaweeds or small creatures attached to it. Spreading out the alga in water is even more helpful because you can see the natural overall shape and the individual diagnostic features, at the same time as rinsing away the sand sticking to it. Smaller seaweeds destined for herbarium preservation in museums are floated first and then captured in their natural configuration onto paper that is slipped underneath them in the water.