Dietary requirements of seaweed flies ( Coelopa frigida)

The seaweed fly, Coelopa frigida (Fabricius), is mostly found in piles of decomposing seaweed deposited on the seashore which form its only breeding sites. It is shown that C. frigida can complete its life cycle in a wide variety of marine algae, and that the larvae are unable to survive without some, as yet unidentified, consituent of seaweed. The larvae also have a requirement for a microbial gut flora which probably derives from the bacterial flora naturally associated with algae growing in the sea. After deposition of the seaweed on the shore, the bacterial population increases enormously, and is ingested by the feeding Coelopa larvae. The dietary requirement for bacteria can be satisfied by a variety of pure bacterial cultures of marine origin, and also by pure cultures of Escherichia coli, Bacillus subtilis and Saccharomyces cerevisiae. It is suggested that the microbial cells are being used by the larvae as their principal source of energy. The bacterial populations naturally found on stranded seaweed are grazed by the feeding larvae. It is the combined activities of microbial and insect populations that result in rapid decomposition of the seaweed. The ecological relationships between marine algae, the microbial flora, and dipteran larvae are discussed.