Neptune’s Nurseries: Where Sea Meets Land

The intertwined roots of mangroves help stabilize the substrate, providing a better environment for swamp residents. In such food-rich areas, it probably comes as...

The intertwined roots of mangroves help stabilize the substrate, providing a better environment for swamp residents. In such food-rich areas, it probably comes as no surprise to learn that barracudas and a variety of sharks are frequently encountered in mangroves.

In the blackness before the first glow of sunrise, we entered the mangroves. It was low tide, and we soon found the bottom with the hull of our skiff. From there I put my light meter on my belt and some insect repellent in my pocket, grabbed my 16mm movie camera system, and began hiking in the thigh-deep water. I was the lucky one — the guy with the easy job. I was following Levi and Five, two Bahamian hired hands-turned-pals, who were carrying a 12-foot-/4-m-long stepladder. They were going in front of me to be sure I wouldn’t accidentally step into deep water and take my camera and lens sans housing for an unwanted swim.

Forty minutes later, drenched with sweat, having managed to avoid the unwanted tumble but having met every mosquito and “no-see-um” within miles, we set up the ladder in the shallow water along the shore. Levi and Five quietly retreated, and I put the camera system on my shoulder, climbed the ladder, and began to check out the nearby water and shoreline.

As the sun peeked out over the horizon, I began to make out shapes along the shore. I could see the leaves from mangrove trees dangling next to the water, their shadows flickering across the water’s surface. Not exactly what I was hoping to see, but this game had just begun.

Forty-five minutes later I glanced down to the water next to the stepladder, and I saw a trio of 2-foot-/.6-m-long lemon sharks swimming past. Then came a squadron of small eagle rays and a turtle. A few minutes later, I saw bonefish and a pair of silver jacks rush a tightly packed school of baitfish.

Moments later a 2-foot-long Caribbean reef shark pup cruised by, traveling a nearly identical path as the lemon sharks. Not long after that, an even smaller hammerhead pup cruised past. It struck me as rather ironic that within two hours, while standing high and dry on a stepladder that was in less than 2 feet of water, I was seeing all kinds of things that would be the highlight of most dives.

Yet this was not what I was hoping to film. I was looking for sexually mature nurse sharks. Every year, for thousands of years, adult nurse sharks have entered the shallow water of mangrove swamps in the Bahamas, where they compete for and choose their mates. I stood 12 feet in the air on that ladder in the Bahamian sun until 1:00 p.m., when I was rewarded with the sight of a pair of mating nurse sharks. The male bit the female and flipped her over on her back as the sharks interlocked their tails. Then the male entered the female, and they continued to thrash in the water for several minutes. Camera whirring, I watched the entire event through my flickering viewfinder.

As I climbed down the ladder almost seven hours later, it struck me again just how much marine life and activity I had seen in water that was less than 5 feet/2 m deep. I had been privileged enough to catch a glimpse of the cycle of life, having watched nurse sharks create new life and a school of jacks continue their own lives by devouring other animals. Amazingly, I had seen all this without even getting wet up to my waist.

I had often heard the term “Neptune’s Nurseries” used in reference to mangroves, tide pools and estuaries, but for me the term took on a new reality that day. Like most divers, I have always hurried past the shallows so I could get to the “real” water to do some “worthwhile” diving and exploring. But that day, it really hit home how much I have been missing by overlooking the ocean’s nursery areas. Hopefully, my experience will help you avoid the same oversight.

This article is the first in a four-part series that deals with different ecosystems in the world’s oceans. In this series we will take a look at life in nurseries, sand flats, coral reef communities and the open sea. Gaining fundamental insights into these habitats will provide any diver with a much greater understanding of the richness of the marine kingdom.

Life in the Mangroves

Mangroves is the common name given to 40-plus species of salt-tolerant shrubs and plants that occur in the zone between high and low tide along sheltered coastlines in subtropical and tropical regions of the world. Mangrove swamps are generally found in tranquil waters along the shoreward edge of tropical lagoons and fringing reefs. In some parts of the world, mangroves grow in brackish water estuaries, and in other locales mangroves flourish in sea water that has salinity the same as sea water. But in any case, mangroves rarely grow in water deeper than 7 feet/2 m.

While mangroves can be found around lagoons, sand spits and other sheltered zones, they tend to flourish in areas where rivers and streams flow into the sea. In places where mangroves have become well-established, they form dense estuarine forests, ideal habitats for thousands of marine species.

The entangled mass of roots and branches that form the underwater portion of the forest combines to form near-vertical emerald walls. The intertwined roots also create a seemingly endless maze of tunnels that surround and protect water-filled channels and creeks in the mangroves from the pounding surf. In addition, the roots help stabilize the substrate, providing a better environment for swamp residents. Above the waterline, mangroves tend to block even rather stiff sea breezes, creating a well-protected habitat.

While the visibility in the mangroves is rarely as good as it is at nearby reefs, the water is extremely rich in nutrients that support an abundance of life. Dissolved organic compounds constantly leach from the mangrove roots and leaves that fall off the mangroves and decompose in the water. As a result, mangrove swamps provide a nutrient-rich habitat that serves as a nursery area for many species of juvenile fishes.

A variety of algae, sea grasses and decaying leaves are often present on bottoms that vary from mud to fine sand. The soft substrate provides ideal habitat for burrowing worms, crustaceans, bivalve mollusk, and upside-down jellyfish. The roots of many mangroves suspended in the water and not penetrating the substrate provide an ideal place for attachment for sponges, some corals, hydroids, bryozoans, oysters, scallops and more. The root system is an excellent hiding place for myriad crabs, shrimp, lobster and fishes.

Mangrove swamps serve vital roles as nursery areas for many juvenile fishes, including many species such as snappers and grunts that as adults occur in many coral reef communities. This is especially true in places where the mangroves are near an extensive, well-developed coral reef system.

It is also common to encounter stingrays, eagle rays, turtles, manatees and even alligators in mangrove swamps. Commercially valuable fishes such as mullet, jacks, red drum, groupers and others commonly enter mangroves in pursuit of prey. In areas where freshwater rivers and streams flow into mangroves, it is not uncommon, especially during the rainy season, for “freshwater fishes” such as gars, sunfish, basses and catfishes to enter mangroves. In such food-rich areas, it probably comes as no surprise to learn that barracudas and a variety of sharks are frequently encountered in mangroves.

In locales where mangroves enclose large bays and lagoons, and where strong tidal currents keep the salinity near that of sea water, large sea grass meadows flourish near the mangroves, encouraging frequent visits by porgies, drums, needlefish, flounders, soles, surgeonfishes, goatfishes, parrotfishes and more. Many of these species also use the sea grass beds and the mangroves as nursery areas. Spring and early summer are the times when most of the hatchlings and juveniles occur. In fall and winter, they tend to move off into the surrounding deeper water.

Many bird species reside in or visit the mangroves. The list of common species includes a surprising number of songbirds, as well as gulls, herons, egrets, oystercatchers and many more species.

The roots of mangroves are a vital key to their success, as well as an Achilles’ heel that makes them susceptible to pollutants and disturbances. Prolonged increases in the amount of suspended sediment in a mangrove swamp and an increase in pollutants, especially sewage and oil-based contaminants, can lead to a sudden demise of an entire mangrove forest.

In many parts of the world where human development has taken place, mangroves have been severely damaged. Scientists warn that the demise of the mangroves is not an isolated occurrence. Studies have demonstrated that the health of mangroves impacts not only nearby reefs, but a variety of other habitats that can be hundreds of miles away. (For more information on mangrove ecosystems, see the October 1999 “Eco-Seas” column.)

Life in the Tide Pools of Rocky Shores

In many areas around the world, rocky beaches can be found where the sea meets the land. As tides rise and fall, different portions of these beaches are covered and then uncovered by sea water. But when the tides ebb and rocky beaches are uncovered, not all of the water spills back into the sea. Some is trapped in rocky depressions known as tide pools. These pools of water form oases of life along rocky shores for an astonishing variety of creatures.

Tide pools range in size from small puddles to small lakes. Found in all shapes and sizes, these basins of life are inhabited by a variety of creatures that have successfully adapted to the rigorous conditions of this narrow band where sea meets land.

The basic needs of tide pool creatures are the same as those of other organisms: food, oxygen, water and sometimes shelter. This might not seem like too much to ask, but meeting these needs is a constant challenge to the creatures that inhabit the rocky intertidal zone. No other habitat in the world is subjected to such dramatic changes over the course of a day, or even an hour, as tides rise and recede.

Sometimes tide pools are completely devoid of water, only to be flooded a short time later. Temperatures can vary from below freezing to extremely hot. Salinity increases quickly in exposed pools, as the heat of the sun causes the collected water to evaporate, concentrating the salts. This means that the ability for creatures that inhabit tide pools to osmoregulate, or control the exchange of body salts with their surroundings, is critical.

Dissolved oxygen can quickly become a scarce commodity. Competition for space is ruthless. Wave after wave mercilessly pounds the rocks and their inhabitants for days, weeks or even months on end. In addition, predation by both marine and terrestrial predators is often intense.

Yet, as harsh as conditions are, these relatively tiny strips are among the most densely populated regions on Earth. The creatures that inhabit them display an amazing variety of survival adaptations.

The intertidal area of a rocky beach can be divided into four life-sustaining zones. They are the:

• Splash zone: the area above the high tide mark that is impacted only by the splash and spray of waves, except in very rare instances of extremely high tides.

• High tide zone: the area between the average and high tide levels.

• Middle tide zone: the area between the local average sea level and average low tide level.

• Low tide zone: the area exposed to air only during the lowest tides.

Despite the physical proximity of these four zones, the environmental conditions in each zone differ considerably. As a result, different organisms tend to congregate in each zone, the one that best suits their needs. The resulting striated pattern of populations is referred to as vertical zonation. Exactly where the boundaries between the zones are drawn and whether or not other zones should be added remain points of scientific contention. Certainly, the boundaries for the zones are artificial in the sense that their residents frequently cross them.

Creatures living within the splash zone include a variety of algae, rock lice, periwinkle snails, limpets and some species of barnacles, such as acorn barnacles. Although these organisms live the majority of their lives above the waterline, intertidal animals must remain moist in order to survive. Without question, the threat of desiccation, or drying up, is the biggest threat that splash zone organisms face. The organisms that inhabit the splash zone combat this problem in a variety of ways.

Some algae are especially adept at preventing desiccation, due to their ability to form gelatinous masses that trap and store water. However, the same algae are much less able to defend themselves from grazers such as snails, limpets, isopods and crabs that inhabit the splash zone. Periwinkles clamp tightly to a rock and seal the edges of their shell to the rock in order to retain moisture during low tide. Limpets tend to cut grooves into rocks where they live. The grooves enable their shells to seal against the rocks, trapping moisture.

Barnacles are filter feeders, but amazingly those species that inhabit the splash zone can survive by feeding only a few hours a month, when the tides are high enough to cover them. During those precious moments, barnacles extend their feathery feet from volcanolike shells to trap tiny planktonic food items in the water. Between feedings, barnacles seal their shells using a calcareous “door” known as an operculum, thereby preventing desiccation during the long periods of fasting and exposure to air.

Of course, the presence of all these animals, especially the mollusks and crabs, attracts great numbers of birds that come to feed and to gather food for their young.

The creatures that inhabit the high tide zone must be able to survive in both air and underwater, as they are awash several times every day. Nevertheless, this zone is populated by far more species than the splash zone. In fact, the farther you move down from the splash zone, the more life forms that are likely to be present. Crabs, snails, limpets, mussels, barnacles and many more animals are commonly found in the high tide zone. Given their mobility, many of these species also inhabit the splash zone and the lower intertidal zones as well. The most significant difference between the populations of the high tide zone and the splash zone are that far more snails and other mollusks, as well as crabs, shrimps and other arthropods, tend to be found in the high tide zone.

The middle tide zone teems with life. Permanent tide pools, not normally found in the upper zones, protect hermit crabs, snails, anemones, nudibranchs, octopi, and even a variety of small fishes from exposure and the pounding waves. A variety of algae are abundant, as are mussels, chitons, barnacles, sea stars and more. All these animals have relatively flexible, rounded bodies, low profiles and a means to attach themselves to the substrate to minimize the threat of the ever-pounding surf.

Many residents of the middle tide zone also possess large gills that increase oxygen absorption, a feature that helps them overcome the still-present threat of oxygen depletion during low tides.

Sea anemones living in the tide pools of the middle tide zone possess an amazing ability to prevent desiccation and cope with greatly elevated body temperature. In fact, studies of the aggregate anemone found in California waters have shown that they can withstand internal body temperatures up to 55˚F/31˚C above ambient temperature. To prevent the fatal loss of water from body tissues during low tide, aggregate anemones retract their tentacles and cover themselves with lightly hued rocks, shells and other debris that tends to reflect heat rather than absorb it.

Diversity of species as opposed to dominance by a few best describes the low tide zone. In this zone are an incredible mix of animals, including many fishes, sponges, nudibranchs, sea hares, octopi, snails, crabs, shrimps, lobsters, worms, sea stars, brittle stars, sea cucumbers, sea urchins, hydroids and more. Juveniles of many species can often be found in the low tide zone, including an occasional small shark, such as a horn shark. A variety of sea grasses and algae are also present.

The low tide zone is constantly being pounded by the surf, making it difficult for many organisms to permanently attach to the substrate. As wave intensity increases, patches of available space often occur. Competition for this space is fierce, and life in the low tide zone is anything but constant. Populations of mussels, barnacles, sea stars, snails, and more increase and decrease as conditions fluctuate, and factors that favor or disfavor each species change over time. The constant changes contribute to the overall health and amazing biological diversity of the tide pools.

Life in the Estuaries, Marshes and Mud Flats

Estuaries are semi-enclosed areas where saline ocean water is diluted by freshwater runoff from land, as water from dew, fog, rain and melting snow eventually work their way to the sea. The diluted salinity, the fluctuating levels of available oxygen, and the constantly changing conditions imposed by the ebb and flow of tide make survival difficult in estuaries. As a result of the constantly changing and harsh conditions, most estuaries are occupied by considerably fewer species than the nearby waters of the open coast. However, some species are very abundant.

The plants and animals that occur in estuaries exhibit a variety of strategies to cope with the range of salinity. Some, known as osmotic conformers, like tunicates and anemones, have little to no capability to regulate the concentration of salts in their internal fluids. They simply do their best to tolerate a wide range of conditions.

Others, like oysters, scallops and snails, actively regulate the exchange of dissolved salts and water in their tissues. Still others, like many crabs, are osmotic conformers when the salinity is near that of sea water and are osmo-regulators in more dilute water.

Estuaries are a particularly important habitat to juveniles of many marine species that hatch and/or develop within relatively calm, protected waters. This is true for many invertebrates and fishes alike. The richness of estuaries makes them among the most productive habitats in the world, but unfortunately, they are also among the most threatened.

The marshes that border many estuaries also provide vitally important habitat. Myriad salt-tolerant plants that form the foundation of thriving communities dominate salt marshes. Estuaries and marshes are among the most highly productive biomes in the world, providing food and shelter to disproportionate numbers of endangered plants and animals, including many migratory waterfowl. Some fishes, such as a variety of flounder, spawn in marshes, while smelt and striped bass swim from the ocean through the estuaries to spawn in fresh water.

Of the fish and shellfish that are commonly harvested in the United States, more than 50 percent spend at least some stage of their development within an estuarine habitat. But the importance of estuaries and salt marshes to humans is not limited to commercially valuable species. These biomes also minimize flood damage and are natural water purifiers.

Mud flats are an intertidal habitat found along the shores of estuaries. They appear where waves and currents are too weak to carry away deposits of clay and silt. Mud flats provide ideal habitat for crustaceans, polychaete worms, and other organisms that thrive on organic debris that becomes trapped in the mud.

Animals that live in mud flats face unique problems owing to the nature of the habitat. Because the physical spaces between the tiny sediment grains are so small, the water in these spaces is not sufficiently flushed by the tides. In order to survive in these conditions, animals must be able to tolerate much higher concentrations of their own wastes than do inhabitants from other areas. In addition, oxygen in mud flat water is quickly used up and not replaced until the next tidal flush. To make living conditions even tougher, usually only a very thin surface layer of the mud ever contains a significant amount of oxygenated water. Below this layer are darker zones dominated by types of bacteria that do not require oxygen. The decomposing activities of these bacteria are primarily responsible for the foul smell of sulfur that is usually associated with this sediment.

Nevertheless, a variety of species overcome the daunting challenge of surviving in the mud flats by living in holes, tubes and burrows. Clams such as geoducks and other bivalve mollusks extend siphons to the surface in order to take in oxygen-rich water and filter food particles. Serving as snorkels to the surface, the tubes and burrows of a variety of worms and shrimp enable these creatures to acquire life-sustaining food and oxygen from surface water. When oxygen content is too low, many of the tube dwellers and burrowers stop feeding and seal their holes or shells in order to establish a more constant environment. The name of the game at that point is just to hang on until a fresh supply of oxygen comes in with the next high tide.

A visit to a mud flat will almost always reveal a number of species of shorebirds and migratory waterfowl that prey upon shrimp, worms, mollusks and crustaceans. If you watch for even a short time, you are likely to see birds quickly picking their way through the mud in search of prey. If possible, try visiting a mud flat at various stages of a tidal flow, and you will see how the activity level changes dramatically with the water level. As the tide rises, mud flats are often visited by shore crabs, shrimps, birds and fishes, as well as raccoons, foxes and other terrestrial predators. Their stay is typically temporary, and most depart as the tide falls.

Unfortunately, estuaries, salt marshes and mud flats are also prime real estate. According to some estimates, more than 90 percent of estuaries, salt marshes and mud flats have been severely impacted or destroyed by human development.

Considering the list of animals that live in Neptune’s nurseries, it is easy to understand how vitally important the health of these areas is to many natural systems. Gaining a basic understanding of the importance of these areas to so many natural systems can only help give us all a better appreciation for the natural world — and for our role as its protector.

A Mangrove Moment

Seeking refuge from the strong winds and big seas, we pulled into a channel in the mangroves, where we dropped the hook. Standing on the bow of the boat, I looked out and saw the dark, sinister shape of a shark making its way across the shallows. This one was a big guy, a 7-foot-/2-m-plus long tiger shark.

A dozen or so yards in front of the shark, I saw another shadow. This one was smaller and more rounded. Within a few more seconds, I realized I was looking at a medium-sized loggerhead turtle. The chase was on!

I knew that turtles are a favorite prey item of sharks, especially tiger sharks, and clearly this shark was in pursuit. I knew that tiger sharks were often reported to follow prey in an attempt to wear it down prior to making their attack, but I had never seen a tiger shark in action. The turtle surfaced for air several times in only 30 seconds or so as the pair reached shallow water, with the shark continuing to close the gap. What happened next was anything but what I expected.

The loggerhead turned on the tiger shark. Within the blink of an eye, the turtle bit the shark on its gill slits and tore out a chunk of flesh. The shark flinched and gave ground, but returned and circled the turtle. When the shark closed in, the turtle once again went on the attack. After several more confrontations and near confrontations, the shark swam off and the turtle crawled up onto the beach, rather tired perhaps, but not looking too much the worse for wear. It seemed rather obvious that the turtle had been quick to recognize its mortal enemy, and that the reptile seemed to know that the best defense in this case was a good offense.

Observers tell me that tiger sharks fail a lot. They just don’t give up very easily. Surely next time, this predator will be a little wiser in the real-life game of predator versus prey. It will need to be to take on a master like this loggerhead.

As for me, I learned that the mangroves are filled with big animals and even bigger surprises.

The Importance of Turtle Grass Beds

Turtle grass is an important seed-bearing plant that inhabits the protected, shallow waters in estuaries, lagoons and mangroves throughout the Caribbean. Beds of turtle grass often border coral reefs. The plants possess broad leaves that restrict the flow of current, thus creating superb habitat for a wide variety of algae, as well as encrusting sponges, hydroids, bryozoans and other small invertebrates that live on the grass blades. Scavengers such as sea cucumbers and sea urchins feed upon the decaying blades of turtle grass that settle to the bottom after the grass dies. The abundance of all these smaller creatures attracts many fishes and other larger predators to the turtle grass beds.

In essence, turtle grass serves as the mainstay of a very important ecosystem within the Caribbean and associated waters, as a great many species depend on the existence of the beds for food, shelter and habitat. As divers, we rarely visit the turtle grass, but their importance as breeding grounds, habitat for young to grow and feeding areas for many species ranging from tiny invertebrates to lemon sharks, blacktips, Caribbean reef sharks, sharpnose sharks, barracudas and many other species of fishes should not be underestimated.

Throughout the Caribbean, commercial development has eliminated and threatened many turtle grass beds, but in recent years scientists, governments and developers have been working together more closely to try to protect these environmentally sensitive, yet vital, areas.

By Marty Snyderman