Eelgrass (Zostera marina)

by Prentice K. Stout

To the scuba diver, eelgrass is a jungle, to many marine invertebrates and fish it is a haven and nursery. It is food for ducks and geese, and a fascinating study area for the scientist.

Eelgrass communities are excellent study habitats for those willing to get wet. Situated as they are close to the shores of quiet salt ponds and lagoons, a person with flippers and a snorkel can spend many productive hours swimming over these aquatic fields. A study of these areas heightens an understanding of their value both in the total coastal process and as a habitat for many commercially valuable marine species.

Recognizing Zostera is not hard - they have thin leaves about 12 to 20 inches long (31 to 53 centimeter) with parallel edges and three veins running along their length. While they are alive they are green, but when cast up on the shore they turn black, and eventually grayish-white when bleached by the sun.

Eelgrass is not a seaweed or algae; it is a true flowering plant and a monocot (a plant having a single seed leaf). This places it in a category unique among marine species. During the spring, as the water warms, millions of pollen grains are released and come in contact with the style of the female flowers. Deep in the ovary, fertilization takes place, and in August the eelgrass plants produce hundreds of tiny seeds. These units of reproduction sink to the muddy bottom or are carried away by currents to other sites, where they create new colonies.

Studies reveal that eelgrass communities are valuable as sediment traps that help stabilize the coastal zone. Because their leaves are so closely packed together they also act as dampers reducing the motion of the water. Suspended materials carried by currents move into these areas, where the waters are calmer, and there they sink to the bottom. The closely packed leaves also provide a haven for young fish species such as flounder and mummichogs. Larger, predatory species find it difficult to hunt in this aquatic jungle.

Birds such as geese and ducks consume the leaves of Zostera as a principal food source. In September, the leaves break away from the roots. Some float away, carried by currents; others fall to the bottom where at least 85 percent of them decompose. Certain microscopic organisms called Detritivores begin to break it down into smaller particles, and these become surrounded by bacteria and fungi. They, in turn, are consumed by filter feeders such as clams and scallops. Deposit feeders (sea urchins) and the sediment feeders (worms and snails) also consume this marine "soup." The adult and larval forms of these invertebrates become food for larger life forms such as fish. It is a continuous cycle of life and death and life again. So important is Zostera's role in this food cycle that estimates reveal that more than 20 species of commercially valuable fish species feed in these eelgrass meadows at some point in their lives.

The surface of the leaves form a substrate for many invertebrate species. In 1937, R.C. Stouffer subdivided the eelgrass invertebrate community into four major categories: those on the plants, among the plants, on the mud surface, and in the mud. Perhaps the invertebrates most easily seen in New England waters are two related worms that secrete about themselves a hard tube of calcium carbonate. Spirobus borealis and Spirobus spirillum look alike, but a little study reveals that Spirobus spirillum coils to the right and Spirobus borealis to the left. If placed in a marine aquarium, these worms will emerge displaying delicate plume-like appendages. Another species, the Bryozoans, will appear as a flat crust growing on the blades, but a magnifying glass or microscope will show the individual animal, or Zooecia, that makes up this colony.

A nursery, or shelter, and a food source for animals, eelgrass has also provided many benefits to humans. In some Scandinavian countries eelgrass was used as roof thatch and upholstery. Burned, it gave not only heat but soda and salt. Early historical records indicate that in the United States eelgrass brought $20 to $30 a ton as insulation and sound-deadening material. In the 1920s and 30s, the Samuel Cabot Company (which still markets stains) sold a product called Cabot's Quilt, which consisted of two layers of building paper with a layer of eelgrass stitched between in quilt fashion for insulation. A researcher's studies indicate that a six-inch layer of eelgrass spread to a density of 1.5 pounds per square foot has the insulation efficiency of six inches of fiberglass insulation. Further studies reveal that Zostera will burn if subjected to a flame but will not support combustion by itself.

The vital importance of eelgrass was first noted by Danish biologists in 1890, but it was revealed dramatically in 1931 when a serious fungal disease and a change in ocean currents that brought warmer waters to the extensive Atlantic Zostera meadows teamed up to kill this species. With this catastrophic decline, which killed over 90 percent of the North Atlantic eelgrass population, many species of ducks and geese vanished. In addition, lobster, crabs, scallops, clams, and other invertebrates declined. A vital part of the food chain in coastal areas had been removed, and the decline in Zostera also caused significant problems with coastal erosion. It was not until 1945 that a recovery began. Zostera is now once again abundant.

It is easy to see that eelgrass and the organisms that live in its grassy confines do not exist in isolation; each species is involved in a number of relationships and interactions. These relationships and interactions have two consequences: a flow of energy from the autotrophs (green plants and algae that make their own food) to the heterotrophs (organisms that eat green plants or each other) and the continuous cycling of inorganic materials which move through living (biotic) organisms and back to the environment. Such a complex combination of living and nonliving elements in a natural setting is referred to as an ecosystem. Ecosystems are large and complex. We can narrow this complex down to "habitat": that portion of an ecosystem in which a particular organism lives. Eelgrass lives in the salt pond ecosystem but has its own habitat within that ecosystem. A further refinement can be made by stating that organisms sharing a common habitat and interacting with each other create a community.

Salt: In the Oceans and in Humans

by Prentice K. Stout

Salt is an essential component of human diets. All fish, reptiles, amphibians, birds, and mammals carry within their veins the elements of sodium, potassium, and calcium in almost the same proportions as the oceans. The "sea" within us has the same saltiness as the Precambrian seas of three billion years ago. Rachel Carson, in her book The Sea Around Us, gives us a clue to our origins: "When the animals went ashore to take up life on land, they carried part of the sea in their bodies, a heritage which they passed on to their children and which even today links each land animal with its origins in the ancient sea."

For humans, salt is as essential as water. We can perish from too little salt as we can of thirst. Salt regulates the exchange of water between our cells and their surrounding fluids. One component of salt, sodium (Na), is involved in muscle contraction including heartbeat, nerve impulses, and the digestion of body-building protein. Humans contain about eight ounces of salt. The amount of salt is regulated in our bodies by our kidneys and by perspiration.

What is salt? It is a compound and has a cubic crystalline form when seen under a scanning electron microscope. Its chemical formula is NaCl, or sodium (Na) chloride (Cl). The chlorine part (ion) accounts for 55% of the dissolved solids in sea water, while the sodium accounts for 30%.

The combined oceans contain about four and one-half million cubic miles of salt—enough to cover the entire depth of the United States to a depth of one mile.

Where did all this salt come from? Part of the salt content of earth came from the breakup of rocks by frost and erosion. The salt was locked in these rocks, and as the rain fell the dissolved salts were carried into the sea. The balance of the sea's salt was leached form the rocks beneath the sea's surface.

The sea is about 3.5% salt. Stated another way: for every 1,000 pounds of water, 35 pounds are salt; or we can say that the average salt content is 35 parts per thousand. In areas closer to the shoreline, this figure will vary because of evaporation and dilution. There is evidence that the salinity of the oceans has changed little since their formation. Through the process of evaporation and freshwater input, the salinity balance stays the same. In addition, much salt returns to land through salt spray or through the salt crystals that form nuclei for raindrops. It then finds its way back to the oceans.

Salinity is vital for animals that live in the ocean. Most marine organisms in the open ocean have body fluids whose salinity closely approximates that of the water around them. However, problems arise for marine organisms who live in the coastal environment. In the intertidal zones and estuaries, wide fluctuations demand mechanisms that will increase water uptake or remove salt. Some animals protect their internal salinity with shells or scales. Oysters thrive in low-salinity waters of 7 to 18%, but their predators (mainly starfish) cannot. Some species of fish can readily adapt to rapid changes in salinity, such as the common mummichog or killfish (Fundulus heteroclitus), which can tolerate a wide range of salinity. Perhaps this adaptation has permitted it to thrive in these stressful habitats. Particularly susceptible to salinity fluctuations are the eggs of many vertebrates and invertebrates.

Myths have arisen about salt. To spill salt at the table requires throwing a pinch over the left shoulder for good luck.

There are some examples of art that celebrates salt. In the Kunsthistorische Museum in Vienna resides a magnificent sixteenth-century Golden Salt Cellar, product of the craftsmanship of Benvenuto Cellini. Perhaps because of the myth about spilled salt, Leonardo da Vinci's famous "Last Supper" has a spilled saltcellar in front of Judas.

Salt could be used in Central Africa in the 1800s to purchase a bridge. The French, who detested the salt tax, the gabelle, began a revolution that helped to repeal the tax. In India, Mahatma Gandhi led a march to the ocean in protest against the British law that forbade Indians from making their own salt. "Why," reasoned Gandhi, "should they not march two hundred and forty miles to the shore where the salt from the sea was free?"

Poland's Wieliczka salt works is a museum made up of chambers with sculptures and chandeliers in salt 400 feet underground. Near Hutchinson, Kansas, a storage company operates the world's largest warehouse in a salt mine 650 feet beneath the surface—it covers 300 acres.

Some 14,000 uses have been found for salt. It is used in adhesives, batteries, explosives, meats, and metals. Before the days of refrigeration, people used to cure meats, thus preventing spoilage.

The next time you shake out some salt remember that it demands your attention. We all spent the first months of our lives in a sac of saline solution. We are dependent on this humble compound.

Fish Schooling

By Prentice K. Stout

Schooling of fish has very little to do with their education. It does have much to do with their ability to survive and reproduce in sufficient numbers. Schools are composed of many fish of the same species moving in more or less harmonious patterns throughout the oceans. A very prevalent behavior, schooling is exhibited by almost 80 percent of the more than 20,000 known fish species during some phase of their life cycle. Many of the world's fishing industries rely on this behavior pattern to increase their catch size, especially for species such as cod, tuna, mackerel, and menhaden.

Aristotle, over 2,400 years ago, observed this behavior in fish. Perhaps he sparked the interest humans have had in this fascinating trait of certain fishes.

Why school? For one, there are ecological advantages. Some species of fish secrete a "slime" that helps to reduce the friction of water over their bodies. Also the fish swim in fairly precise, staggered patterns when traveling in schools, and the "to-and-fro" motion of their tails produces tiny currents called "vortices" (swirling motions similar to little whirlpools). Each individual, in theory, can use the tiny whirlpool of its neighbor to assist in reducing the water's friction on its own body.

Another advantage is the safety factor against predators. A potential predator breeding hunting for a meal might become confused by the closely spaced school, which can give the impression of one vast and frightening fish. Additionally, there is the concept of "safety in numbers"—a predator cannot consume and unlimited quantity of prey. The sheer number of fish in a school allows species to hide behind each other, thus confusing a predator by the alteration of shapes and colors presented as the school swims along. Of course, those on the outside edges of the school are more likely to be eaten than those in the center. Predatory fish also gain from schooling because it gives them the ability to travel in large numbers in search of food. Bluefish (Pomatomus saltatrix) in pursuit of menhaden are a good case in point.

Schooling fish respond quickly to changes in the direction and speed of their neighbors. Anyone who has swum in a school of fish can attest to their ability to change direction swiftly while still retaining their closely knit swimming pattern. They can move from one configuration to another and then regroup almost as one unit.

When young, most fish species do not exhibit the schooling pattern. As they mature, they begin to swim in pairs and then in larger and larger clusters until they attain the classic parallel pattern. Thus, schooling can be said to be a formed behavior pattern imprinted on the genetic material. Research leads us to believe that as the sense organs of the young mature, their schooling behavior strengthens. The first sense used is that of sight, which begins to function immediately after birth to allow for feeding. Fish eyes cannot focus directly forward because they are located on the sides of the head. This placement does, however, permit the eyes to be especially sensitive to lateral movement—a very helpful attribute in schooling. The fish can see what other members of the school are doing in relationship to themselves and respond accordingly.

Of interest is the acoustico-lateralis, the much-studied lateral line system on the sides of some fish. This is a line of special neuromast cells that runs down either side of a fish body. The scientific name for these lines gives us a clue to their function: "acoustico" means sound, and sound waves produce pressures; and "lateralis" alludes to the sides of the fish's body. These two lateral lines are highly sensitive to movements and the displacement of water as the fish swims close to its neighbor. They aid in keeping the fish in a neat, orderly pattern. Some fish do not have lateral lines, nor the sensitive cells, and thus rely on their eyesight. Research suggests that if fish are blinded and their lateral lines cut, schooling does not take place; but if the lateral lines are left in place, the fish are still able to school. The lateral lines are especially important to fish living in the highly murky waters of the estuarine environment where sight is not particularly useful. The silver strip of the Atlantic silversides (Menidia menidia) affords us a good representation of the lateral lines of a typical species.

We are now ready to attempt a definition of schooling—no easy thing to arrive at. An accepted version could be a "grouping of fish based on mutual attraction and exhibiting a geometrical relationship." We say "mutual attraction," for fish of different species are almost never found intermingled. Fish stay with their own kind in a schooling configuration.

Schooling has some other interesting aspects. In the spring along the New England coast the alewife (Pomolobus pseudoharengus), in response to an ancient biological urge to reproduce, begins to form in large schools. They begin just where the rivers pour into estuaries and then, seemingly without a central signal, they migrate up the rushing currents. So large are their numbers that the bottom of the stream cannot be seen, and the whole picture is one of wriggling, bluish bodies swimming against the current. Alewives are "anadromous" fish that, much like salmon and shad, mature in salt water but spawn in fresh water. (Eels, on the other hand, are "catadromous," meaning just the reverse—they grow in fresh water and spawn in the sea.) the shad (Alosa sapidissima), first cousin to the alewife, also schools in large numbers in the spring. They are the source of the highly prized "shad roe," the millions of eggs that will not reach maturity because they are frying in our breakfast pans.

In many ways fish schools are much like herds of land animals or flocks of airborne birds. There is that undefined need to stay together. In some instances this herding has been the undoing of certain species. The now-extinct passenger pigeon flocked in such staggering numbers that it was rather a simple task in the predawn hours to take a club and sweep a branch of roosting birds into a sack for future eating. There are stories of the sky being darkened by the passage of these relatives of today's mourning doves. For centuries, wildebeest and antelope have formed huge herds that have crossed the endless African plains in search of greener pastures or to migrate to their ancestral breeding grounds. Indeed, if one looks at the huge cities of today's society one wonders if we humans are not prone to schooling. We live and move in vast numbers controlled by the technology of our society.

Zooplankton

by Prentice K. Stout

Zooplankton (zoon - Greek for animals and plankton, a word derived from Greek meaning wanders) are divided into two types: those that spend their whole lives as small plankton and those that drift as plankton only in their young stages. This second type will mature into various larger life forms.

Some of the permanent members of the zooplankton community are forminiferans, radiolarians, and copepods. The first two are roughly comparable to the diatoms and flagellates of the phytoplankton community. Forminiferans (hole-bearers) are organisms that live within small shells made of calcium carbonate, a substance similar to limestone, marble, and chalk. The protoplasm, a complex chemical "soup" found in living cells of all animals, flows out through the holes in their shells, forming a sticky, food-catching network.

As they outgrow their shells, the old abandoned ones are shed and the animal grows a new one taking the calcium carbonate from the surrounding water. Dead animals and discarded shells sink to the deep ocean bottom, there to be dissolved into calcium and carbon, available for reuse. In shallower waters, where this breakdown cannot take place, their remains built up thick layers called "globigerina ooze," named for the most common forminiferans, the globigerina. Over the 500 million years these animals have been in existence, and have been deposited on the ocean floor, forces have shifted the seabed and the presence of these new and ancient deposits of ooze has enabled scientists to estimate the age of such sedimentary deposits in different, separate parts of the world. This has given us a good fossil record. The chalk beds of Georgia and Mississippi as well as the white cliffs of Dover, England, attest to the vast quantities of these tiny animals.

Radiolarians are similar to their relatives, the forminiferans, both in skeletal shapes and protoplasmic food trapping ability. But their skeletons are made of a more resistant material called silica, a valuable filter and abrasive agent used in such products as toothpaste and for filtering certain liquids. Their bodies litter the ocean floor covering approximately 3 million square miles of tropical oceans. Copepods are so numerous that it is estimated that they compose seven out of every ten zooplankters. Their huge numbers have led some biologists to suggest that there are more of these animals in the world than all the multi-cellular animals combined.

Treading water by beating their limbs (from which they get their Greek name "oar-footed") as many as 600 times a minute, they use a vast amount of energy. this causes them to be ravenous eaters, having to consume their own weight in food each day. All of their food consists of phytoplankton. They are related to crustaceans, among whose members are crabs, lobsters, shrimp, and barnacles. For all their smallness, some large inhabitants of the oceans are dependent on them for food. The 45 foot long Basking Shark and the 60 foot long Whale Shark head the list of those in the oceanic food chain that feed on these copepods.

Dr. A.A. Benson, at the Scripps Institute of Oceanography, has stated that half of the world's photosynthetic product is converted, for a time, into wax by these tiny animals. Their fat is converted into a polyunsaturated liquid wax which is stored for the animal's use, but when the animal is eaten by sardines, herring, and anchovies, their wax is converted back into common fats by these large predators. Thus, we can see the importance of the copepod in the oceanic food chain.

Members of the zooplankton community migrate within the water column vertically each day. The phytoplankton, which do not migrate in this fashion are doomed to be wafted about the seas, by wind and waves. During the daylight hours, depending on the individual species preference, the zooplankters confine themselves to a narrow vertical range. As the fading light announces the onset of evening, the plankton begin a haphazard upward drift until, with night upon them, the entire zooplankton community is on the surface. Here they feed on phytoplankton and are in turn eaten by other larger species. As daylight approaches they drift down to their respective levels in the water column. The causes of this vertical migration have eluded scientists as to the precise answers.

We have mentioned zooplankton in the food chain. We can best illustrate this if we take 10,000 ponds of producers, the phytoplankton. This would support 1,000 pounds of first-order consumers or the zooplankton. IN turn this would support 100 pounds of second order consumers such as herring or anchovies. This would support 10 pounds of third order consumers such as the larger fish species, which would then support 1 pound of the fourth order consumers, for example, a seal. So we can see that for a seal to gain one pound, he would indirectly have to consume 10,000 pound of phytoplankton, the ultimate producers in the sea. Thus anything that interrupts this food chain can have serious consequences to the wellbeing of these consumers up the food chain ladder.

References

Hammer, William M. 1974. "Blue Water Plankton" National Geographic Magazine, October.

Gaskell, T.F. 1964. World Beneath the Oceans, The Story of Oceanography. The Natural History Press, Garden City, New York.