by Whit Gibbons

November 20, 2011

Ever wonder why so many fishes, like minnows and herrings are silver? The simple answer: to be invisible to predators that would eat them. According to a new book, "Fishes: The Animal Answer Guide" (2011, Johns Hopkins University Press) by Gene Helfman and Bruce Collette, such fishes have crystals in their body scales that reflect light. Thus, they look like a mirror in the water, which means a predatory fish sees what looks like the surrounding water rather than a fish it can prey on. "Fishes" is the latest in the excellent Animal Answer Guide series, which includes books on squirrels, turtles, and frogs and toads.

How many kinds of fishes are there? The authors explain that fishes come in three major groups, 115 of them being the so-called jawless fishes, which include lampreys. The cartilaginous fishes, with about 1,300 species, include sharks and rays. By far the largest group, the bony fishes, has about 31,000 different kinds. Whales and dolphins are of course mammals.

Some of the questions people ask about animal groups are what is the largest and smallest species, and how long do they live? Among freshwater fishes, the beluga sturgeon of eastern Europe and Asia probably holds the record, reaching a spectacular length of 28 feet and weighing in at up to 2,900 pounds! Perhaps an even more impressive statistic for this species, which is disappearing in some areas because of overfishing, is that an ounce of beluga eggs (caviar) can cost more than $165 (that's $2,640 a pound). Considering the quantity of eggs carried by a mother sturgeon, a "very large beluga [would be] worth more than a million dollars."

The largest fishes in the world are marine species. Whale sharks (about 40 feet long, 26,000 pounds) and the more slender basking sharks (50 feet long, 8,800 pounds) hold the record for weight and length, respectively. The oarfish, a rarely seen deep water fish that is silver with a red fin running down its back, reaches lengths of more than 26 feet; it is the longest known bony fish. Among the huge game fishes are black marlin (1,560 pounds), blue marlin (1,400 pounds), and Atlantic bluefin tuna (1,496 pounds). The shorter but bulkier ocean sunfish can weigh more than 5,000 pounds.

The tiniest fishes in the world are also the smallest of all vertebrates as adults. The Indian Ocean goby can reach adulthood at a length of less than a third of an inch and seldom gets more than a half inch long. Equally diminutive is the Indonesian minnow, a freshwater species. The pygmy coral reef goby is not only small (three-quarters of an inch) but is said to be "the shortest-living vertebrate known, with a maximum record age of 59 days." During their remarkably short lifespan, they have a turnover of several generations. The ecology of one group of short-lived fishes, African and South American killifishes, is especially interesting, leading to their being considered "annuals," like many plant species that live less than a year. Killifishes lay their eggs in the mud when the temporary ponds they inhabit dry up each year. The adult fish die off but the eggs live on in the mud, hatching when seasonal rains refill the pond.

How short the generation time of fishes is can be accurately measured with careful scientific study. But how old are the longest-lived fishes? Determining the potential longevity of species that outlive humans is a difficult task because of record keeping issues and estimates based on age-related features. Nonetheless, the book notes several species of fishes reported to have lifespans of more than a century.

Common questions people ask about fishes are answered in the book, such as "do fishes sleep?" and "do fishes see color?" Questions most people would not think to ask are also answered. For example, "Is there such a thing as fish leather?" "Do fishes kill ducks in ponds?" The answer to all of the above is yes, and the authors provide accompanying qualifications with each answer.

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