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Halloween Science: Fear Makes the Ocean Go Around

Halloween, in a lot of ways, is a celebration of fear.  We dress like ghosts, goblins, and movie serial killers to give ourselves a sense of control over the things we’re afraid of.  It’s also a good time of year to indulge in horror movies, where we can watch ghosts, goblins, and serial killers terrorize other people from the apparent safety of our own homes.

From an ecological standpoint, we have it pretty good.  We’ve more or less tamed most environments on land and only make short forays into the oceans under conditions where we still have quite a few advantages.  Most of the time we have more in common with Jason than his hapless victims.  Imagine being a member of a school of menhaden or a seal that has to make daily trips through Shark Alley.  It would be like spending your whole life as a camp counselor at Crystal Lake, constantly looking over your shoulder and getting picked off the second you let your guard down.  If mortal terror was a regular part of your life, you’d better believe it would affect your daily habits.  And if every member of your species lived with that same fear, there would be places no one in their right mind would go and choices between death by starvation and possible death by being eaten.  After all, fish are always eating other fish.  Let’s take a journey through the low end of the food web and see what horror can teach us about marine ecology.

Don’t go in there!

In the movies, if a house has a reputation for being haunted or people tend to disappear in a certain secluded spot, these places become known as places to avoid.  Inevitably, someone will check one of these places out (after all, it wouldn’t be much of a scary movie if no one actually went to the scary place) and meet a gruesome end.  If the character is on the smarter side, maybe something else compels them to enter the dangerous place, such as a friend in need or being trapped forever unless they can brave the danger zone.

In the wild, any animal that isn’t an apex predator has to make these choices every day.  Getting enough food to survive may entail venturing into open areas where larger fish hunt.  Finding a mate may require making sounds that also attract predators.  Surviving is a constant numbers game where the odds of a grisly death are weighed against the odds of starvation or reproductive success.

A recent paper by Vaudo and Heithaus (2013) looked at habitat choice by rays in Shark Bay, Australia, a place literally named for the large sharks that frequent it.  The rays in the bay would use a variety of habitats during the cold season, but during the warm season would bunch up in the shallows.  This actually took them out of their optimal temperature range and did not make it any easier for them to find food, but cramming together in shallow habitats did coincide with the time of year when tiger sharks migrate into the area in high numbers.  It should be noted that sharks were caught and observed in the general area of the bay during this time of year, but not necessarily exactly where the rays were.  The actual level of interaction between the sharks and rays was fairly low, but the mere presence of sharks in the general area was enough to fundamentally change the behavior of the rays.

The grisly death that awaits stingrays straying from the shallows. Image by John Harding (thejohnharding.wordpress.com).

We have to stick together!

Safety in numbers and working together are just as important to surviving in the ocean as they are to surviving a horror movie.  Splitting up, or even worse, actively fighting each other are behaviors that practically beg the killer to strike (or might be part of the killer’s plan all along).  That said, when a character gets overwhelmed by the situation, freaks out, and turns on the rest of the group it just may buy them some time before the axe falls.

Sticking together is a well-known defense mechanism among marine animals.  Species as diverse as herring, squid, and even higher-level predators like dolphins and sharks will school together for safety, and will bunch even closer together in the presence of predators.  However, marine animals will also turn on each other.  Toscano et al. (2010) found this when they placed competing species of fish (pinfish and pigfish) and crabs (blue and mud crabs) into mesocosms with both sheltered and open habitat with a shared predator, the mighty toadfish.  Competition over food and shelter caused pinfish to push pigfish into the open, and mud crabs to oust blue crabs from protective habitat.  This made the unfortunate pigfish and blue crabs more vulnerable to toadfish attack, and attack they did.  The trials did not last long enough to determine whether the treacherous pinfish and mud crabs would later get their just desserts, so from this study it can be concluded that at the very least those willing to turn on their fellow prey bought themselves some time.

Imagine this face being the last thing you ever see. Image from chesapeakebay.net.

Fear draws the map.

The mere possibility of being eaten is enough to make marine animals leave perfectly good habitat and even turn on each other.  The predators themselves don’t even have to be seen to tell whether they’re in the area; one only has to observe the behavior of their prey.  In fact, the influence of marine predators’ rule by fear can be seen beyond even their immediate prey.  In some cases, fear can cause trophic cascades that affect everything down to where the grass grows.

To see a real trophic cascade in action, we have to go back to Shark Bay.  Aside from large numbers of sharks, Shark Bay is also home to large, healthy seagrass beds and the dugongs and sea turtles that graze on them.  The interior portions of these beds tend to be fairly shallow, though their edges may border deeper channels.  In contrast to the rays discussed earlier, shallow, interior habitat is actually riskier for large grazers.  This is because at the edge they have more water to maneuver in and more potential escape routes, while they’re pretty much stuck if a tiger shark ambushes them in the shallows.

A study by Burkholder et al. (2013) shows that these risk affects can profoundly impact the health of the seagrass beds.  In the less-risky edge habitats, exposed seagrass is grazed practically down to the sand.  In these areas, seagrass within cages mean to exclude large grazers showed faster growth, broader leaves, and generally better health, though there was some evidence of increased competition between seagrass species.  Seagrass condition was similar in the more dangerous interior habitat.  From this, the researchers were able to conclude that life is easier for seagrass in areas where it’s easier to be killed by a tiger shark.

Fear is a powerful motivator, and in the marine environment can literally build the whole ecosystem.  Next time you’re out on the water, take a moment to appreciate the scenery.  That nice SAV bed might be there because large fish patrol the area and scare the grazers away.  Your favorite fishing spot might be loaded with fish because just outside the area schools of sharks and pods of dolphins regularly pass through.  The odds are good that all of that natural beauty is the product of the vast majority of the organisms there being constantly terrified.

References

Burkholder, Derek A., Heithaus, Michael R., Fourqurean, James W., Wirsing, Aaron., & Dill, Lawrence M. (2013). Patterns of top-down control in a seagrass ecosystem: could a roving apex predator induce a behaviour-mediated trophic cascade? Journal of Animal Ecology DOI: 10.1111/1365-2656.12097

Toscano, Benajmin J., Fodrie, F. Joel, Madsen, Shanna L., & Powers, Sean P. (2010). Multiple prey effects: Agonistic behaviors between prey species enhances consumption by their shared predator Journal of Experimental Marine Biology and Ecology, 385, 59-65 DOI: 10.1016/j.jembe.2010.01.001

Vaudo, Jeremy J., & Heithaus, Michael R. (2013). Microhabitat Selection by Marine Mesoconsumers in a Thermally Heterogeneous Habitat: Behavioral Thermoregulation or Avoiding Predation Risk? PLOS One, 84 DOI: 10.1371/journal.pone.0061907