The kidney of the giant clam Tridacna maxima contains algal arsenic compounds, probably as a consequence of the clam's symbiotic relationship with unicellular algae (zooxanthallae). Extraction of Tridacna kidneys yielded three novel arsenic compounds, N-(5′-deoxy-5′-dimethylarsinoyl-β-D-ribosyloxycarbonyl)glycine 5, (2S)-3-(5′-deoxy-5′-dimethylarsinoyl-β-D-ribosyloxy)-2-hydroxypropanoic acid 4a, and (2R)-3-(5′-deoxy-5′-dimethylarsinoyl-β-D-ribosyloxy)-2-hydroxypropanoic acid 4b, in addition to four previously reported dimethylarsinoylribosides. The structures of the three new Compounds were assigned chiefly from NMR data, and those for compounds 4a and 4b were confirmed by synthesis. Extraction of a second batch of Tridacna kidneys gave, in addition to compounds identified in the first extraction, (2S)-3-[(5′-deoxy-5′-trimethylarsonio-β-D-ribosyloxy)-2-hydroxypropyl]sulfate 18 and two novel compounds: an arsenic-containing nucleoside, 9-(5′-deoxy-5′-dimethylarsinoyl)-9H-adenosine 16 and N-[4-(dimethylarsinoyl)butanoyl]taurine 21. Syntheses of compounds 16 and 21 are reported. The presence of the nucleoside 16 in Tridacna, as a consequence of algal metabolism, supports a proposed pathway for the biogenesis of arsenic-containing ribosides by algae involving methylation and adenosylation by S-adenosylmethionine. A biogenetic pathway for compound 21 involving donation of the 3-amino-3-carboxypropyl moiety of S-adenosylmethionine is also proposed. The presence of both compounds 16 and 21 in Tridacna may represent the first example of donation, by S-adenosylmethionine, of all three of its alkyl groups to a single acceptor (arsenic) within one organism.