Like mammals and birds, fish have bones and skin rich in connective tissue. But thanks to the cold environment in which fish bodies function, their collagen differs from mammal and bird collagens. Fish collagen is less cross-linked, and so melts and dissolves at much lower temperatures. The collagen and gelatin of warm-water fish like tilapia melt at around 77°F/25°C, that of cold-water cod around 50°F/10°C. So we can extract fish gelatin at cooking temperatures far below the boil, and in relatively short times. The collagen of squid and octopus is more cross-linked than fish collagen, so these molluscs require more prolonged heating at 180°F/80°C to give up much of their gelatin. Most cooks recommend cooking fish stocks for less than an hour so as to avoid making the stock cloudy and chalky with calcium salts from the disintegrating bones. Another reason for short and gentle extractions is that fish gelatins are relatively fragile and more readily break down into small pieces when cooked. And because they associate more loosely with each other, they form delicate gels that melt far below mouth temperature, at 70°F/20°C and lower.