New chlorophyll-a-related compounds chlorophyllone a [1], chlorophyllonelactone a [2], and chlorophyllonic acid a methyl ester [3] as well as pyropheophorbide a [4] were isolated as the main antioxidants from the edible parts of the short-necked clam, Ruditapes philippinarum. Our continuing studies on new antioxidants have resulted in the isolation of the analogous compounds purpurin 18 [6], purpurin 18 methyl ester [7], and 13²-oxopyropheophorbide a [8]. Compound 5, an epimeric isomer of 1, together with 2, 3, and 4, was isolated from the viscera of the scallop, Patinopecten yessoensis. The presence of 1, 2, and 3 in the viscera of the oyster Crassostrea sp. and 1 in the mixture of attached diatoms (Fragilaria oceanica, Fragilaria cylindrus, Nitzschia closterium, Nitzschia seriata, Cocconeis pseudomarginata, and Hyalodiscus stelliger are predominant) cultured for seedling production of juvenile abalone was also confirmed by spectroscopic evidence. Organisms are well known to have defense mechanisms against oxidation, because peroxides of polyunsaturated fatty acids have shown many kinds of toxicity including mutagenicity (1,2). We focused our research interests on marine organisms containing a large amount of polyunsaturated fatty acids and measured peroxide value (POV) by the conventional antioxidant test (3) and mutagenicity by the rec assay (4) of their organic solvent extracts. Positive correlation was observed between POV and mutagenicity of their extracts. Among the extracts of viscera of various kinds of marine fish and bivalves, the extract of the short-necked clam, Ruditapes philippinarum Jay (Veneridae), showed a very low POV and mutagenicity. Hplc analysis revealed that this extract contained amounts of tocopherols too small to show the strong antioxidant activity. These observations encouraged us to survey new antioxidants responsible for this low POV of the extract. We have already communicated the isolation and structure elucidation of pyropheophorbide a [4] and new pheophorbide a (PB a) derivatives 1 (5) and 3 (6) from R. philippinarum collected in Hamana Lake, Japan (7). Our continuing study revealed the presence of the other similar antioxidants 2, 6, 7, and 8 (Figure 1). As compounds 1, 2, 3, and 8 were found to be new PB-a-related compounds, we were interested in the origin of these compounds. We examined the viscera extracts of the scallop Patinopecten yessoensis Jay (Pectinidae) and the oyster Crassostrea sp. (Ostreidae), which are also plankton feeders. From the extracts of the scallop collected in Hokkaido, compounds 2, 3, and 4 as well as a new member 5 of this type of compound were isolated. The extract of the oyster was found to contain compounds 1, 2, and 3. As 1 was also isolated from the attached diatoms, which were microscopically found to be predominantly composed of Fragilaria oceanica, Fragilaria cylindrus, Nitzschia closterium, Nitzschia seriata, Cocconeis pseudomarginata, and Hyalodiscus stelliger, it is particularly interesting to clarify the origin of these isolated compounds. We describe herewith the isolation and structure determination of these PB-a-related compounds 1-8 in detail, and the biogenetic considerations on these compounds.