One of the highlights of my recent voyage in Mexico’s waters was an unexpected face-to-face encounter with a whale shark, which I described in this column as a giant, harmless, plankton-eating fish.

Regarding that, California reader and former Hawaii resident Bud Barnard wondered via email why they’re called sharks.

"I suppose it’s some kind of biological species conformity," he wrote, "but probably many ‘ignorant landlubber nonbiologist types’ like me are mentally asking the same question."

Common and scientific animal names are often designated with logic similar to that of SpongeBob SquarePants.

The scientific name of the largest whale in the world, the blue, for example, means little mouse. In a common name conundrum, Spanish sailors of old called orcas whale killers, which somewhere along the line became killer whales. The reversal of terms implies a threat to humans. Orcas in the wild, though, do not kill people.

In the case of the whale shark, there’s a smidgeon of reason behind its name: The fish is in all ways a shark, but it’s as big as a whale.

In the world of fish classification, biologists divide jawed fish (hagfish and lampreys are jawless) into two groups: those with skeletons made of bone and those with skeletons made of cartilage. Sharks are the latter, along with rays.

With some exceptions, sharks share several other traits. Rather than having a single gill opening on each side covered by a bone plate, such as that of bony fish, sharks have five to seven uncovered gill slits.

Another distinction is in the eyes. Bony fish don’t have eyelids and, therefore, can never close their eyes. Most sharks, however, have a membrane that covers and protects the eye during feeding. The great white shark lacks such an eyelid but can roll its eyes into the eye sockets as protection against struggling prey.

Skin type is another difference between sharks and bony fish. Rather than overlapping scales, shark skin consists of tiny sandpapery nodes similar to teeth. That’s why, if humans rub a shark the wrong way, we can end up with skin abrasions.

People have long considered sharks to be primitive. Some are, but only in terms of evolution, meaning they have changed little from their earliest state. Horn sharks, not found in Hawaii but common along the coasts of California, the Galapagos Islands, Japan, South Africa and Australia, have remained the same for 160 million years.

Other shark species are highly evolved. Fossil records of great whites and makos date back only about 50 million years.

The oceans of the world host about 400 shark species. The smallest, at 8 inches long, is a deepwater Caribbean dogfish. The largest shark, growing up to 60 feet long, is the harmless, even friendly, whale shark.

Because scientific names are hard to pronounce and difficult to remember, I prefer fish names dating back to the Pixar Period: Nemo, Marlin, Dory and Bruce.

Another name I like is Bud Barnard, who swam with wooden goggles in Waikiki in the 1940s. Thanks, Bud, for the fun emails and good questions.

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Reach Susan Scott at www.susanscott.net.