Invertebrate Use of Smell
Many reef Invertebrates, especially those that lack a good sense of vision, are acutely sensitive to telltale chemicals in the water. Sometimes even the faintest trace, perhaps consisting of just a few molecules of a key chemical, can trigger amazing behavioral responses. Some chemical odors, such as those that carry the scent of food or mates, are especially worth seeking out; others forewarn of danger, providing any animal with a sufficiently keen sense of smell a few extra precious moments to seek shelter.
How do invertebrates detect smells?
For a "smell" to be detected by any animal, a few molecules of chemical must come into contact with specialized cells, known as chemoreceptors. There are many different types of chemoreceptor, each tuned in to a particular kind of molecule. Some chemoreceptors pick up food molecules, others detect predator molecules, and so on. When a sufficient number of these molecules is detected in the water, the chemoreceptors trigger a response in the animal's brain -- it smells the chemical. To acquire plenty of chemical information, animals must sample their environment.
Human chemoreceptors are located mostly in our noses and mouths and detect molecules in the air as we breathe. Similarly, animals such as clams and snails have a nose-like structure, called an osphradidium, across which they can pass water, sampling it for chemicals. Crustaceans, such as shrimps, wave their antennae around in their habitat; these are covered in chemoreceptors, so they smell and taste with them. Other animals, including crabs and octopuses, are covered in chemoreceptors, making them super-sensitive to waterborne chemicals.
What do inverts use their sense of smell for?
Animals use smell for four main purposes: to find a home, a mate, food, or to obtain advance warning of a predator or rival.
If attacked, anemones rapidly withdraw their vulnerable tentacles to safety. If the anemone is damaged in the attack, it releases an alarm chemical to which all the other anemones in the area can respond. In addition, the chemical tags the attackers, such as predator sea slugs, making it difficult for them to sneak up unnoticed for several days. Sea slugs themselves seek out their prey, including anemones and coral polyps, by following odor trails, sniffing them out with their incredibly sensitive chemosensory abilities.
Corals, meanwhile, can detect the presence of their neighbors chemically, allowing them to launch an attack if the neighbor starts to muscle in on their territory and overgrow them.
Male snapping shrimps are acutely sensitive to the scent of females and use subtle chemical cues to work out whether they might be about to molt and thus be sexually receptive. The first male on the scene gets the girl, so the rewards are great.
The presence of pheromones in the water is thought to be one of the key factors in triggering the mass spawning events of reef invertebrates. It seems that the spawning of the first corals on the reef acts as a signal to other corals, as well as to clams and some echinoderms, such as sea urchins and starfish. By all spawning at once, these reef invertebrates swamp the ability of predators, which feed on the rich bonanza of their eggs, to make any serious impact on their breeding efforts.