Immunochemical Methods: Immunochemical methods can detect species-or population-specific molecules (antigens) on the surfaces of cells or in cell extracts and have been used to identify bacteria, phytoplankton, zooplankton, eggs, larvae, and adult marine organisms. One application of this technique has been for identification of aquatic nitrifying bacteria (Ward and Perry, 1980) and nitrogen-fixing microorganisms (Currin et al., 1990) and mapping of their distribution over large areas. In addition to the identification of species and life history stage, immunochemical methods have been used to describe community structure and predator-prey interactions, to identify the partially digested remains of prey organisms from within the digestive systems of their predators (Theilacker et al., 1986; Grisley and Boyle, 1988; Feller et al., 1990), and to detect parasites (Ohman et al., 1991).

Nucleic Acid Hybridization: Nucleic acid hybridization, like isozyme analysis, can be used to discriminate among organisms that lack distinguishing morphological features, such as microorganisms and larval forms of complex multicellular organisms. DNA hybridization can be designed to discriminate among major groups of organisms or to detect individual species within a group of closely related organisms (Woese, 1987). Probes that target unique DNA sequences are prepared and reacted with DNA from the organisms of interest. This technique has already been used to identify species in marine microbial populations (Delong et al., 1989; Sanghoon and Fuhrman, 1990).

RFLP Analysis: RFLP analysis is used to characterize the genetic composition of organisms. Digestion of genomic DNA, mitochondrial DNA, or chloroplast DNA by enzymes that target specific DNA sequences (restriction enzymes) generates DNA fragments that can be separated by electrophoresis to form distinctive patterns that can then be analyzed by standard statistical techniques. Comparison of the similarity among patterns has been used to identify the geographic origin of individuals and to study the taxonomic relationships among species. Information gained by RFLP analysis is especially useful in fisheries management, conservation of endangered species, transport studies of plankton, and studies of evolutionary relationships. RFLP analyses have also been used to study the genetic diversity of a variety of marine and freshwater organisms, including bacteria, marine macrophytes, phytoplankton, zooplankton, eels, oysters, sea turtles, and fish.

Polymerase Chain Reaction (PCR): This technique has provided a revolutionary approach for the synthesis, detection, and characterization of specific sequences of DNA. The PCR procedure employs a heat-stable