All fish share two traits: they live in water and they fish dinners for two a backbone—they are vertebrates. Apart from these similarities, however, many of the species in this group differ markedly from one another. Fin fish like salmon have gills, are covered in scales, and reproduce by laying eggs.

Eels, by contrast, have worm-like bodies and exceedingly slimy skin. Whale sharks, the largest fish, give birth to live young and eat only tiny fish, squid, and plankton. Some species, such as the weedy sea dragon, are so bizarre they seem almost unreal. Fish have developed special senses, too. Because water transmits sounds, disperses chemicals, and conducts electricity better than air, fish rely less on their vision and more on their hearing, taste, and smell. Many can detect motion in the water using a special row of scales with sensors known as the lateral line.

Others can find their prey and even navigate by detecting electrical charges. One reason fish are so diverse is that 70 percent of the planet is covered in water. The animals in this group live in a variety of habitats ranging from coral reefs and kelp forests to rivers, streams, and the open ocean. Another is that fish are very old on the evolutionary scale. According to fossil records, they have been on Earth for more than 500 million years! Can Portuguese sardines make a comeback?

There are dozens of reasons to add more fish to your diet. It’s a healthy source of protein and many types are lean on calories and fat. But the real perk is all the omega-3 fatty acids, essential fats that our bodies don’t produce autonomously. What’s the Healthiest Fish to Eat? It’s a weeknight mainstay for a reason. Salmon is beloved for its versatility, quick cook time and mild flavor. Even fish haters can get down with its subtle taste, especially if it’s in a creamy sauce or coated in spices or fruit salsa.

Tuna is another popular choice, thanks to its mild but luxurious flavor and versatility. The light canned kind—which is typically skipjack tuna—is the leanest and lowest in mercury. Due to its minimal fishiness and subtle flavor, tilapia is our go-to recommendation for people who don’t think they like fish. Because its natural taste is so bland, tilapia can seamlessly lend itself to a wide range of ingredients and flavors. It’s one of the leanest and most versatile options on the market, as well as one of the lowest in mercury. If you’re searching for a low-calorie protein that cooks up in a flash, look no further than cod. Every three-ounce serving contains less than 100 calories and a single gram of fat.