When a river meets the sea, ecological magic happens. An estuary is formed. According to the National Oceanic and Atmospheric Administration, estuaries "provide habitat for over 75 percent of the U.S. commercial sea catch." Estuaries have little wave action. This provides crucial refuge for all types of freshwater and ocean-dependent animals. The habitats convert the sun's energy, which creates a food source for animals.
The river otter, a cousin to the mink, weasel, wolverine and badger, belongs to a group called Mustelids, which have special scent glands to mark their territory. The river otter feeds on estuary fish, amphibians, crustaceans, snakes, insects, frogs, turtles and any aquatic invertebrates. Harbor seals often bask in the sun on the banks of the water and dive in for herring and salmon. The harbor seal, a cousin of the walrus, spends part of its life in the water but is dependent on estuary land to give birth and raise its young. It preys on estuary fish that include cod, herring, sea bass, shrimp, mollusks, whiting and squid.
Marshes and mangroves provide an essential food source for a variety of birds. They feed on fish, plants and snails as it is very easy to hunt and forage in the shallow waters of estuaries. Ducks hunt in the mud to find food, feeding on shellfish and insect larvae. The great blue heron is a common sight in marshes, agricultural areas and mud flats feeding on fish, small mammals, reptiles and even other birds.
The Pacific and chinook salmon lives in the ocean, migrating upriver to breed and spawn. Eels swim through on their way down the river to breed at sea. The whitebait lay their eggs in estuarine waters. The young fish are swept out to sea and then return swimming upriver where they mature. The Pacific spring lumpsucker spends its time foraging for food on the bottom of estuary waters, eating worms and mollusks. The starry flounder spawns in estuaries near river mouths. It feeds on zooplankton, crustaceans, amphipods and copepods, and changes its color to blend in with the bottom to avoid other preying animals.
One of the oldest and well known creatures on Earth is the horseshoe crab. It thrives in soft sand or estuary mud, foraging and eating worms and mollusks. Over 80 species of mud shrimp thrive in estuaries. They have 10 legs, making them a free-swimming crustacean. They feed on worms, large planktonic organisms, small crustaceans and plant material and sponges. Dungeness crabs are found all over the planet, starting their existence in the eelgrass. Once they get bigger, they move out to estuary waters feeding on clams, fish, worms, squid, snails, sea stars and birds' eggs.
The dragonfly is one of the best-known insects. Baby dragonflies eat tadpoles, fish eggs and other small aquatic animals. Adults consume voluminous amounts of ants, mosquitoes, butterflies, flies and other flying insects. The damsel fly has a long thin body with a distinctive oblong head, short antennae and bulging eyes. It consumes other small aquatic insects by capturing them, while flying, with its hind legs covered in prickly hairs.