Agri Kultuur December / Desember 2015 | Page 13

Steelhead Trout Hatchery cluding fish, arose. Today there are over 24 000 different types of fish (some of the latest estimates place this figure close to 30 000) all descended from the ancient fishlike creatures of the middle Ordovician period about 480 million years ago. Over time some have evolved into weird and wonderful shapes and colours to the extent that they now hardly look like fish at all. Others, however, have remained virtually unchanged. They are the so-called “living fossils” the most famous of which are undoubtedly the coelacanths, whose present-day representatives are almost identical to the last fossils known from deposits da- Public information centre ting back some 70 million years. Fish are remarkable creatures. During the course of evolution they have developed physical and behavioural characteristics that have allowed them to inhabit most of the world’s bodies of water, except for a few places where no life can possibly exist. They have spread to such diverse habitats as coral reefs, dark abysses, deep caves, thermal springs, arctic waters, seasonal pools, raging torrents, mid-ocean expanses, and numerous other watery worlds. As a result, the forces of evolution have moulded fish into myriad configurations that make it possible for them not just to tolerate such conditions Live transport truck but to thrive and breed in them, often in spectacular ways. Fish Evolution: It is generally accepted that life began in water some 3 billion years ago. For many millions of years after that there appears to have been no major development leading to the emergence of complex organisms. Indeed, it took around 2.4 billion years before the first animals that could be regarded as invertebrates began to evolve. From then on the pace accelerated, and it only took a relatively short time of about 120 million years before the first vertebrates evolved. These aquatic creatures, known from fossils dating Raceway tanks for grow-out