We are all familiar with the salmon’s journey. From the ocean, it enters rivers and makes a miraculous journey upstream to spawn and then die. In the Caribbean, there are many animals that find a life between the land and sea, and a home in fresh and salty waters.

On St. Martin, one is never far from the sea. Most obviously, we find it at the edge of every beach and at the bottom of each seaside cliff. But the sea also has ways of invading the island itself.

The sea seeps into the land through the porous limestone, adding its salty essence to well water. It washes upstream in the few spots where fresh water running down a gut reaches the sea. And, of course, salty water from the sea fills many of our salt ponds.

These brackish waters—neither part of the sea, nor totally separate from it—are a rich and unique habitat. The creatures that live here must adapt to the changing conditions of this zone: a rainstorm pushing the balance towards freshness, a dry spell pushing it to salty.

For some, life between two worlds is just a passing phase. Many juvenile fish use brackish mangrove wetlands as a nursery. In the shallow water, sheltered in mangrove roots, they find a safe place to grow. They then swim out to the coral reef to live. Some freshwater species—like the Mountain Mullet and many freshwater shrimp—float in the sea as eggs. After hatching they travel with the current, then swim into fresh water to mature.

Fish like the Crested Goby spend a lifetime on the borderline. They often live around mangroves, digging out a hollow in the sand or finding a root-sheltered hiding spot. They also live in estuaries where streams and rivers empty into the sea. Unlike most fish, they can live perfectly fresh water, pure seawater and anything in-between.

The Crested Goby is also flexible when it comes to food. Algae is on the menu, but so are crabs, insects, snails and even small fish. It is an integral part of the wetland community that captures nutrients washed down from the island. It plays a part in keeping the seas both clean and full of life.

The adaptable Crested Goby has found a niche that allows it to occupy the cracks and crevices between two worlds. In doing so, it has also turned its back on life in the open ocean. It is a creature of the sea, tied forever to the edge of the land.