Eating toxic algae makes plankton speedy swimmers

A meal of toxic algae puts a spring into a tiny ocean-dwelling plankton’s trek. The bad news: That just might send it straight into the jaws of a hungry fish.

Copepods (KO-puh-podz) are relatives of shrimp and lobsters. But very tiny cousins. They grow to be only about 1.5 millimeters (less than 0.06 inch) long. Still, size isn’t everything. Each one can suck in 100 liters (26.4 gallons) of seawater per day.

Ocean algae can sometimes bloom — reproduce rapidly to form huge clouds. Some species of these algae, such as Alexandrium fundyense, contain a harmful toxin that can poison local shellfish. When blooms of these algae occur, fish farmers shut down their operations to make sure that their lobsters and shellfish (and the people who eat them) won’t get sick. If the blooms of algae hang around for a long time, those fisherman can lose a lot of money.

But these algae don’t kill one species of copepod. Their toxin just changes the behavior of this tiny crustacean. It makes the critters swim faster and straighter, a new study shows.

In theory, such zooplankton (ZO-plank-tin) — tiny creatures that eat plant-like algae — could chow down on these toxic algae, reducing the size of a bloom. But they don’t. And that puzzled Rachel Lasley-Rasher. “Why aren’t zooplankton removing this algae?” wondered the marine biologist, who now works at the University of Maine in Walpole. “Do they eat [the algae] and get sick? Do they eat it and die? How does it affect their behavior?”

To find out, Lasley-Rasher and her colleagues collected many copepods from the Gulf of Maine in plankton nets. They looked for the species Temora Longicornis. “It co-occurs in the Gulf of Maine with the algae,” she explains.

At the time, Lasley-Rasher was in graduate school at the Georgia Institute of Technology in Atlanta. So her team packed the fresh-caught plankton in ice and shipped them south to her lab.

There her team divided the copepods into tanks. A share of the tanks got some A. fundyense to eat, mixed in with other, harmless algae. Copepods in other tanks were fed just the harmless algae. One tank of unfortunate copepods got no food at all. After two hours, the scientists moved all the copepods to tanks with normal algae, and let them feed for another 15 hours. This was to make sure that none of the tiny crustaceans were changing their behavior just because they were too hungry. 

At this point, the scientists measured how many copepods had reproduced and died. They also studied the crustaceans’ behavior.

Unlike many other species of copepods, these didn’t die after dining on harmful algae. They produced as many eggs as those getting a healthy diet. The one big difference: They swam straighter and 25 percent faster than normal.

The downside of speed

Swimming fast might seem like a good thing for a prey species, such as copepods. In fact, it’s not. These animals need to swim in slow, lazy loops. “[They] want to swim around but don’t want to expose themselves to predation,” explains Michael Finiguerra. This marine ecologist, who works at the University of Connecticut in Storrs, was not involved in the new study.

Copepods, Finiguerra says, might be compared to a football player who wants to take the ball toward the end zone. If the player runs in a straight line, he faces a high risk of being tackled by the other team. Similarly, a straight-shooting copepod can be tracked and gobbled up by a hungry small fish. But if the player — or plankton — zigs and zags, it becomes harder to track and catch.

That is why copepods normally swim back and forth in unpredictable loops. It’s how they try to avoid becoming a predator’s lunch. The fast, straight lines they swim after eating toxic algae, he notes, likely “tip the balance” towards their becoming lunch.

It’s not, however, just swimming straight that matters. By increasing their speed after eating harmful algae, the copepods make a slightly bigger wake as they plow through the water, Lasley-Rasher explains. This means predators that hunt by sensing the current produced by prey could find them easy pickings.

Her group calculated that after eating toxic algae, the copepods could be 56 percent more likely to encounter a predator — such as a shrimp — that will snatch them up. Lasley-Rasher and her group published their results April 27 in the scientific journal Proceedings of the Royal Society B.

“It was kind of refreshing for [the scientists] to look past just ingestion and survival,” says Finiguerra. Normally, he explains, scientists might think that there’s one pathway: those algae make a toxin, which will kill whatever eats them. But, as Finiguerra notes, this study shows that “it’s much more complicated than that.”

So far, Lasley-Rasher and her group haven’t yet tested the copepods in the presence of predators. “It’s definitely a next step,” she says. But if the copepods really do get eaten more often after a meal of toxic algae, this could be one reason why they don’t eliminate algal blooms. “Predators are more likely to encounter the [copepods] eating the toxin,” she explains. That provides an explanation “for bloom persistence, if the ones that eat [the algae] get removed from the system.”

Power Words

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bloom      (in microbiology) The rapid and largely uncontrolled growth of a species, such as algae in waterways enriched with nutrients.

copepod     A type of small crustaceans found in salt and fresh water. Some species of copepods are plankton, while others spend time on the sea floor. They aren’t limited to oceans — copepods are also found in freshwater, from ponds to puddles. They eat phytoplankton. 

crustaceans    Hard-shelled water-dwelling animals including lobsters, crabs and shrimp.

current     A fluid body — such as of water or air — that moves in a recognizable direction. (in electricity) The flow of electricity or the amount of electricity moving through some point over a particular period of time.

graduate school   Programs at a university that offer advanced degrees, such as a Master’s or PhD degree. It’s called graduate school because it is started only after someone has already graduated from college (usually with a four-year degree).

graduate student     Someone working toward an advanced degree by taking classes and performing research. This work is done after the student has already graduated from college (usually with a four-year degree).

ingest   (n. ingestion) To eat or deliberately bring nutrients into the body by mouth for digestion in the gut.

phytoplankton      Sometimes referred to as microalgae, these are microscopic plants and plant-like organisms that live in the ocean. Most float and reside in regions where sunlight filters down. Much like land-based plants, these organisms contain chlorophyll. They also require sunlight to live and grow. Phytoplankton serve as a base of the oceanic food web.

plankton    A small organism that drifts or floats in the sea. Depending on the species, plankton range from microscopic sizes to organisms about the size of a flea. Some are tiny animals. Others are plantlike organisms. Although individual plankton are very small, they form massive colonies, numbering in the billions. The largest animal in the world, the blue whale, lives on plankton.

predator    (noun: predation) A creature that preys on other animals for most or all of its food.

prey     (n.) Animal species eaten by others. (v.) To attack and eat another species

red tide    A population explosion of certain species of plankton. When enough are present, they can color the water red or reddish-brown. Some secrete a poison that can kill surrounding fish and make people sick.

toxin    A poison produced by living organisms, such as germs, algae, bees, spiders, poison ivy and snakes.

wake    An area of disturbed air or water left behind an object (such as a boat or animal) moving through the it.

zooplankton    Small organisms that drift in the sea. Zooplankton are tiny animals that eat other plankton. They also serve as an important food source for other marine creatures.