Rhodophyta

The Rhodophyta (or red algae) occur in many different growth forms and have colours ranging from pink to almost black. In strong sunlight plants can appear a greeny yellow when pigments become bleached. The red colour comes from pigments called phycoerythrin and phycocyanin which absorb blue light and reflect red light. These pigments allow red algae to live deeper in the sea than green or brown seaweeds because the short wavelengths of blue light penetrate deepest through the water. Red algae also contain a characteristic storage product known as floridean starch.

There are around 350 species of red algae in the British Isles belonging to 2 main groups or genera:

The Bangiophyceae – which include the edible laver bread ‘Porphyra purpurea’. This group have a relatively simple structure.

The Floridophyceae – which includes many diverse forms from fine filamentous and elaborately branched plants to large plants with a simple flat thallus (eg dulse or Palmaria palmata)

Identification of red seaweeds takes time, patience and practice. A single species may look different at different times of its life cycle, in reproductive phases or when growing under different environmental conditions (extreme wave-exposure or shelter). Even an expert can get caught out by the variations in growth form. Sometimes several individuals need to be examined in order to come to a successful identification but despite the difficulties, time and persistence will pay off and success is very satisfying.

The best way to learn is from an expert during field work and a good beginning is to attend a field course run by an organisation such as the Field Studies Council. For general field use, a hand lens is can be a great help but inevitably some specimens need collecting and identifying using a microscope. Measurements of cell sizes and reproductive bodies may be needed to confirm an identification and so binocular and a monocular microscopes fitted with graticules are useful tools. It is useful to build up a pressed collection of seaweed specimens both as an identification reference and for verifying identifications.

The best available texts for identifying red seaweeds are the Seaweeds of the British Isles series of books which are a collaborative venture between the British Phycological Society and Natural History Museum. Another useful text is the Field Studies Council AIDGAP publication ‘A Field Key to the British Red Seaweeds’ by Sue Hiscock.