Answers To Questions About The Underwater World

By Tony Rice

Curious about the underwater world below your boat? An oceanographer has fascinating answers to some of your common questions.

Colorful coral reefCorals generally live in large colonies. These animals play host to other marine plants and animals. (Photo: Thinkstock/Romolo Tavani)

What are red tides?

Phytoplankton, the drifting communities found throughout the surface layers of the ocean, perform the crucial role of capturing the energy in sunlight to produce complex molecules in the process of photosynthesis. In doing so, they provide the basic food on which almost all the oceans' animal communities depend. But sometimes you can have too much of a good thing, and red tides — or algal blooms — are one of the results.

"Red tide" is a general term for the phenomenon in which the tiny phytoplankton cells become locally so numerous that they cause discoloration of the water. The discoloration is caused by the concentration of pigments in the algal cells, and although these are often red, sometimes extremely so, they can also appear as orange, yellow, brown, or green, depending on the species.

Algal blooms can often be harmful. The harmful effects are usually the result of the production of natural toxins by the algae, which can have serious consequences for both wildlife and man, particularly because the toxins can accumulate in commercially exploited Mollusca shellfish such as oysters and mussels. In addition to these toxic effects, some algal blooms can be so concentrated that they clog the gills of fish and shellfish, and their decaying remains can deplete the oxygen concentration in the water to lethal levels.

Satellite view of algal bloom on Lake St. ClairAlgal bloom on Lake St. Clair. (Photo: NASA Earth Observatory images by Joshua Stevens, using Landsat data from the U.S. Geological Survey)

The cause of algal blooms is often uncertain, though a sudden and local increase in the availability of nutrients is usually involved. Like their terrestrial equivalents, marine plants, including phytoplanktonic algae, are dependent on sufficient sunlight, adequate warmth, and the availability of nutrients such as phosphates and nitrates. In a red tide situation, the injection of nutrients is either too rapid or at the wrong time to allow herbivorous animals to control the growth.

The source of the nutrients can be a result of human activity. For example, algal blooms in freshwater lakes are often caused by runoff from agricultural lands carrying fertilizer into the water body. In the same way, rivers flowing through agricultural landscapes can carry high levels of nutrients to the sea and stimulate algal blooms as a consequence. Some blooms, however, seem to be entirely natural and caused by local wind and water conditions churning up nutrients into the upper water column.

Are corals animals?

Yes! But it took scientists a long time to realize this. It was only a couple of hundred years ago when most people thought corals were strange underwater plants. Corals belong to Cnidaria phylum. The name comes from the Greek word "knide," meaning "nettle." Almost all cnidarians possess stinging structures called "cnidocytes," which they use to capture and subdue their food and for defense against potential predators.

The Cnidaria contains about 9,000 known species, all aquatic, and mostly found only in the oceans. Although they can assume the most amazing shapes and colors, the basic body plan of all cnydarians is extremely simple: two skin layers with a jelly-like substance, called mesoglea, between the two layers.

Cnydarians come in two different body forms — a mobile type that swims freely in the water and a stationary type that lives attached to the sea floor or another animal or plant. The mobile forms include all those creatures known broadly as jellyfish, while the stationary ones include the sea anemones, sea pens, and corals.

Corals are mostly — but not exclusively — colonial, meaning that hundreds of thousands of individual organisms are joined together as a single living unit. The ones that build the familiar tropical coral reefs secrete a hard external skeleton made of calcium carbonate. As the colony grows, the underlying stony mass grows. In all cases, the living tissue is restricted to a very thin surface veneer, with the bulk of the underlying material being the nonskeletal remains of former generations.

Although coral catch food particles with their tentacles, the tropical reef-building ones get some of their food from tiny, specialized photosynthetic algal cells called zooxanthellae, living inside the coral. This mutually advantageous relationship provides the alga with a secure place to live, and the coral with a home-grown food supply.

What are barnacles?

Anyone who has ever walked over a rocky shore will be only too familiar with barnacles. Most intertidal barnacles are no more than a centimeter across and a similar height, but in some areas, like the North Pacific, the shore barnacles can be over an inch tall, while deeper still, they can reach a length of 4 inches or more.

Goose barnaclesGoose barnacles. (Photo: LT. Elizabeth Crapo, NOAA Corps)

Barnacles come in two basic forms: acorn barnacles, the ones we've been discussing, which are stuck directly to rocks, pilings, or ships' bottoms and have a cone-shaped shell; and goose barnacles, attached via a long stalk, which are edible and considered something of a delicacy in many Mediterranean countries. Goose barnacles are often found on floating objects, like wood, buoys, and so on, but they can be attached to rocks, just like their acorn barnacle relatives.

Barnacles are related to crab, shrimp, and lobster, though the resemblance is not very obvious. The giveaway is their legs, normally completely hidden inside the shell when the tide is out. But if you see them underwater, you get a completely different impression. Every couple of seconds, a little hand-like object pops in and out of the hole at the apex of the shell. Under a microscope, you'll see that the "hand" is made up of a series of hairy little jointed bristles. These are the barnacle's legs, and they are built like shrimp or crab legs. In the barnacle's case, they are not built for walking or swimming, but for feeding, because the barnacle uses its legs to filter tiny food particles drifting past it in the water, effectively kicking them into its mouth. Goose barnacles are a bit different, because they spread their hairy legs outside their shells like a net to catch food particles as the water flows through them. They periodically withdraw the "net" and lick off any collected food. 

Tony Rice is a retired oceanographer who lectures on all aspects of the oceans to cruise ship passengers. Can Squid Fly? and his previous collection, Do Whales Get the Bends? are available from Sheridan House.

— Published: August/September 2018

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