The marshes are probably the most important feature, followed by eel-grass beds and mud flats. Warmed at low tide by the spring sun, the marshes and flats raise the temperature of the surface waters, inviting the young fish to enter. Hatching from eggs spawned directly in the marshes, or from offshore spawning, these young fish move into the warm shallow waters of the marshes and find abundant food and protection from large fish-conditions ideal for rapid growth. The fish that are nurtured in the shallows are larger and in better physical condition than those that aren't, and have a much better chance of survival when they return to the estuarine waters in late summer and early fall. Eels must make the same trip, but in the reverse direction-out to the sea as adults and in to the estuary as young elvers.
Diamondback terrapins crawl out of estuarine waters to reach the dunes in which they lay their eggs.
The most successful spawning areas for horseshoe crabs are on sandy banks in estuaries, most notably the Delaware, where their eggs serve as an important food source for shorebirds making the spring passage to northern nesting grounds. We humans too value this abundance of life that depends upon the health of estuaries and make use of it by fishing, crabbing, shellfishing, and bird watching. We enjoy the sheltered waters of an estuary for swimming, sailing, and motor boating. But we also use them for industry, shipping, and waste disposal. The nonextractive and nonpolluting sports are obviously less harmful-but even these can disturb sensitive species and habitats.