The cool water of the Pacific Ocean along Isla Magdalena is very productive. Phytoplankton blooms support rich animal plankton life. The plankton is nutritious food for many species of filter feeding organisms, such as these red megabalanid barnacles attached to a moon snail shell. Although they have a calcareous shell similar to those made by snails and clams, barnacles are crustaceans not molluscs. Megabalanid barnacles capture plankton with a "net" formed by their long, thin setose legs or cirri. The larvae of megabalanid barnacles are opportunistic settlers on any available hard substratum.
The larvae of other species of barnacles are very specific about where they settle. Some, such as Cryptolepas rhachianecti the gray whale barnacle, live only on a particular host animal species. The gray whale barnacles eventually become deeply imbedded in the skin of their host whale. Despite their seemingly irritating placement in the whale skin, the barnacles feed on plankton, just as do their free-living cousins.
