Smell

Living in a Sea of Smells
(Published by The Quoddy Tides April 26, 2002)

We humans tend to take our sense of smell for granted, but for many creatures inhabiting Cobscook's shores, the ability to 'smell' or sense chemical cues in the surrounding environment is a life and death matter.

Snails and sea urchins, limpets and sandworms, barnacles and hydroids - these are among the many animals that have the ability to sense and respond to chemicals in their environment. And this improves their odds of surviving the harsh realities of life at the edge of the sea. This ability to 'smell' may serve different functions. It can be used to find food or to avoid being eaten by another animal, to reproduce, or to find a place to settle and grow.

Sara Lindsay, Assistant Research Professor of Marine Sciences at the University of Maine, studies the sensory biology and ecology of marine invertebrates.

"Many invertebrates, like lobsters, crabs, sea stars and dogwhelks, use chemical cues to locate prey. Others respond to chemicals that indicate danger or produce their own in order to avoid being preyed upon," says Lindsay. "Lobsters provide a good example. Their antennules are incredible sensory organs. Each ring has hundreds of sensory hairs, all of which link directly to the brain. The chemicals that they sense in the water are compared back and forth between antennules so they know which way to move to locate prey."

Acorn worms, worm-like animals called hemichordates, make a nasty smelling compound and store it in their body as a chemical defense.

Lindsay explains, "Think of an organic chemistry lab in the mud. The acorn worms produce brominated phenols and brominated pyrroles, both of which belong to a family of fairly toxic compounds." This compound is concentrated in their tails, and for good reason.

"Acorn worms keep their head down in the mud, but need to put their tail above the mud in order to release waste. They often lose their tails to predators, so this is one way to reduce the chances of that happening," says Lindsay.

Several species of marine worms produce similar compounds to avoid being eaten. Chemical defenses are common in shoreline animals that cannot flee from predators.

Bryozoans, sometimes called sea-moss animals, are colonies of tiny individuals that form a sheet of crust often found on blades of kelp and other algae.

"Sea slugs, or nudibranchs, like to eat bryozoans; but once the nudibranch begins munching on it, the bryozoans detect the predator by smell and start growing protective spines. A chemical induces a physical response to predation," says Lindsay.

Reliance on chemical signals is a common theme when it comes to reproduction.

Lindsay states, "Humans use cologne and perfume to make themselves attractive, and we all have hormones that fluctuate over time. The same thing happens in the ocean. For example, sandworms that paddle through mud or sand release a pheromone that coordinates mating. In at least one case, males and females have a complex mating dance and a specific pheromone prevents worms from mating with the wrong species.

In previous Soundings articles, we reported that both sea urchins and scallops were shown to rely on a combination of biological cues to begin the reproductive cycle. A combination of water temperature and the presence of chemicals associated with an initial release of sperm and eggs sets off a mass spawning event.

Sea cucumbers release a mucus that triggers the mass release of eggs and sperm. Synchronized spawning increases the chances of successful reproduction. For some organisms, especially sea stars, there are also different chemical compounds that guide the sperm to the egg.

Lindsay's specialty is in habitat selection, where marine invertebrates use particular chemical cues to find the right place to settle on rocks, seaweed, or other surfaces.

"Mud flats are mine fields full of disturbance because of the many different animals that live in the mud. Burrowing sandworms, snails on the surface, surface-feeding clams - all alter the environment in minute ways that leave a signature in the chemical of the mud for other animals to recognize," states Lindsay.

"Juvenile polychaete worms, including common lugworms, and several species of juvenile clams will reject a site that has been disturbed by humans or by another animal. Our experiments indicate that there is a chemical cue that they sense, but we are still trying to figure out how they detect that cue," says Lindsay. She adds, "These chemical cues are important when clams or worms are trying to settle in a particular place because they want to avoid being buried by an adult that releases waste above them or avoid being eaten by the animal that caused the disturbance."

Barnacles also use smell to decide where they are going to settle. Settling barnacle larvae avoid dogwhelk mucus trails, and are attracted to areas where adult barnacles are found in abundance. Limpets, the oval shaped, flat snails with a patterned shell, follow their own mucus trails to find their home scars on rocks after feeding.

Chemical communication is not limited to animal organisms. Lindsay explains, "Seaweeds have evolved a chemical defense in response to grazing by periwinkles, sea urchins, fish, and other animals. Smaller algae grazing organisms such as amphipods and nudibranchs are commonly immune to seaweed chemical defenses, and use seaweed cover as a defense against predators. The seaweeds concentrate these compounds in cells near the plant surface and in young tissue and increase chemical defense in the summer when grazing pressure is highest."

The natural system of Cobscook Bay is made up of not only a great diversity of marine organisms, but also a complex web of interactions driven by chemical communications. We are beginning to understand the ways in which some marine animals rely on chemicals in their environment and those released from their own bodies. The more we know, the better we will be able to understand how the changes we make to the environment will affect the health of this system.

This column was prepared by Cheryl Daigle. Cobscook Bay Soundings was a monthly column produced by the Maine Chapter of The Nature Conservancy. Its purpose was to share what is known about the workings of the Cobscook Bay marine environment, so that all who make decisions about the use or care of the bay have the best available information.

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