Codium simulans is a green seaweed which was described from the Gulf of California, and ranges from Island Santa Cruz, California to Guerrero, Mexico. In 1917, Collins and Hervey (1917) described a seaweed from Bermuda, as C. isthmocladum var. clavatum, later raised to subspecies status. Codium ishtmocladum is a widely distributed Atlantic species, known from North Carolina to Brazil, and Mauritania to Senegal. A genetic analysis found that C. isthmocladum var. clavatum, clustered closely with C. simulans, that this seaweed was introduced from the Pacific Ocean, A specimen from the Indian River Lagoon, Florida, also clustered with C, simulans. Morphologically identified specimens of 'C. clavatum' in the U.S. National Museum collections have been collected in Atlantic and Gulf Florida and Martinique. Extensive algal blooms occurred off southeast Florida in the 1990s-2000s that were attributed to C. ishmocladium. Further genetic sampling is needed to determine whether C. simulans was actually involved in these bloom.