Tridacnins: Symbiosis-profit or defense-purpose?

In the hemolymph of bivalve clams, the Tridacnids, an anti-galactan specific agglutinin/precipitin system has been found, which may be of importance for symbiosis between Tridacnids and algae. The agglutinating and precipitating “Tridacnin” molecules are regarded as essential for the elimination and utilization of their algal guests, which have become senile and during degeneration have exposed galactan structures on their surfaces. In this connection, the question is discussed, whether the immune-like systems in invertebrates can be generally regarded as defense mechanisms. Evidence is presented, that the lectin system of invertebrates, especially in Tridacnids, serves more for digesting their symbionts and their photosynthetic products.