There are about 100 species in the abalone genus Haliotis; they have a single low-slung shell, and the largest grow to 12 in/30 cm and 8 lb/4 kg. In the United States, the red abalone, Haliotis rufescens, is now farmed in offshore cages and onshore tanks, reaching 3.5 in/9 cm across and yielding 0.25 lb/100g meat in about three years. Abalone meat can be quite tough, in part because they apparently accumulate connective-tissue collagen as an energy reserve! Either very gentle or prolonged heating is essential; the meat toughens badly when it exceeds 120°F/50°C, and the collagen shrinks and compacts the tissue. Once this happens, continued simmering will eventually dissolve the collagen into gelatin and make the meat densely silken. Japanese cooks simmer abalone for several hours to obtain a more savory flavor (free amino acids apparently react to form taste-active peptides).