A Preliminary Illustrated Field Guide to The Common Marine Algae of the Cook Islands (Rarotonga and Aitutaki)

Seaweeds or marine algae comprise a grouping of very diverse photosynthetic organisms whose relatively simple vegetative structure is called “ thallus ”. They have been traditionally set apart from higher plants, whose more complex level of organisation is differentiated as stems, leaves and roots, through which run sap-carrying conductive tissues.

Algae are autotrophic organisms, which are able to manufacture their own organic molecules from elements containing carbon and nitrogen. Their energy is obtained directly from sunlight, which is trapped by the pigment chlorophyll during the process of photosynthesis. Water and mineral nutrients are directly taken from their environment through their cell surfaces.

While seaweeds are an essential part of the marine ecosystem, they are mostly overlooked as they are overshadowed by their more conspicuous neighbours such as corals, fishes and molluscs. Nevertheless, they occupy the bottom of the food chain and are direct food for a large number of marine organisms, and also for humans in many parts of the world. A host of useful products are also extracted from marine algae, with applications ranging from cosmetics to medicine and the food industry.

Disruptions in seaweed growth patterns caused by unnatural sources such as pollution and deliberate introduction can have quite harmful consequences on the marine ecosystem. For instance, in the Mediterranean sea, the tropical green alga Caulerpa taxifolia has in recent years become a pest, as it clogs waterways and displaces other marine organisms after having escaped from an aquarium in Monaco. In the Cook Islands, fluctuations in the numbers of the patito or sea hare (Stylocheilus longicauda) has been linked to the disappearance and reappearance of the blue-green alga Lyngbya majuscula, on which it primarily feeds. These fluctuations have been also linked to blooms in the green alga Boodlea kaeneana, which could be caused by an increase in organic nitrates flushed into the lagoon (for instance from piggeries) or an increase in global seawater temperatures caused by climatic factors such as the greenhouse effect and El Niño.

It is hence important to recognise seaweeds in their natural environment, and realise their major role in the balance and well-being of the coral reef community, on whose biodiversity and health the livelihood of many island communities such as the Cook Islands closely depends