The Science Team
What Are Harpacticoid Copepods?
The Point Sur Crew
Video
What Are Harpacticoid Copepods?
(And why do they matter?)
by Frank Stephenson
But first, what are copepods?
Our planet is aswarm in copepods, which, for the most part, are very tiny crustaceans, all cousins to shrimp and crabs. Like their bigger kin, copepods are segmented and generally have antennae (two) and bodies that come in a wide variety of shapes.
Taxonomically, copepods belong in the subclass Copepoda, in the great phylum Arthropoda, the largest grouping of animals on the planet, home of the insects. The vast majority of copepods are tiny—a dozen or more of some species could fit neatly on an average shrimp’s eyeball. But their size range is staggering. They range from a millimeter (about four one-hundredths of an inch) in length to a rare, foot-long species that lives exclusively on the hides of finback whales as a parasite.
Nature has cast copepods into a fundamental role in keeping Earth’s waterways clean. Those species that spend their lives on sea bottoms are champion decomposers, literally the eaters of the dead. These animals live off the colossal mass of dead and dying plants and animals the world’s biosphere produces every day.
Biologists also see copepods as bioindicators, species that can be used as tools for measuring the relative health of a given environment. Because copepods tend to be extremely sensitive to any changes in their environments—such as the intrusion of toxins or fluctuations in salinity or dissolved oxygen—when scientists find abundant copepod populations they generally know they’ve found a healthy ecosystem.
Scientists have identified at least seven groups, or orders, of copepods. Two of the largest and most important groups are the Calanoida and the Harpacticoida. By far the biggest, in terms of sheer number of individuals alive at any given time, is Calanoida. Most calanoids live their entire lives swimming in the water column, where they become essential fodder for many commercially important species of fish and also whales.
But in terms of number of species, Harpacticoida—with upwards of 4,500 identified so far—is the most diverse group of copepods. These animals live almost entirely on the bottoms of the world’s seas, gulfs and oceans. Unlike other copepods that spend all their lives floating along as key ingredients of the world’s zooplanktonic soup, harpacticoids aren’t built for swimming. The animals, in essence, hop around on—and burrow into—the bottom in search of food and mates.
Why Study Harpacticoids?
Like all copepods, harpacticoids eat primarily bacteria, algae and organic waste. In this way, they perform a great service to marine ecology—disposing of dead plants and animals and, when eaten themselves, recycling nutrients and energy that otherwise would be lost forever to the oceans’ great depths.
Scientists have documented the role that harpacticoids play in the life cycle of many commercially important fish caught from the world’s shallow saltwater environments, such as estuaries, bays and seas lying above continental shelves. For example, a number of species of grouper and salmon couldn’t survive without harpacticoid diets for their juvenile stages, which can be amazingly picky about what they eat.
But less clear is the role harpacticoids play on the world’s deep seafloors. In these vast, perpetually lightless voids beyond the heavy tides and strong currents, life works in ways not fully understood.
Do the harpacticoid hordes of the deep oceans perform the same roles as their shallow-water counterparts do?
Prof. David Thistle, a biological oceanographer at Florida State University who has studied harpacticoid ecology for nearly 35 years, says that’s a question biologists still don’t know the answer to. There’s no hard evidence, for example, that any species of deep-ocean fish depends on harpacticoids for any crucial part of its life cycle. But there’s circumstantial evidence that’s the case for a number of species caught for the market by deep-ocean trawls, such as the sablefish (Anoplopoma fimbria), which inhabits the muddy seafloors of the Northern Pacific, Thistle said. (The sablefish is but one of a dozen or more slow-growing, deep-water fish that are targets for commercial fishermen these days as stocks of more commonly known species decline globally in the face of over-harvest and pollution.)
Also, do these animals live just about anywhere in the deep oceans, and with similar population demographics? Does a collection of harpacticoids taken from off the southern California coast essentially look the same as one taken off Japan? Or even off northern California? Or do they, like countless terrestrial plants and animals, differ widely in species variation over a large area? If so, why? If not, why not?
These are basic ecological questions about a huge group of marine organisms that no one has yet answered, Thistle said.
“These questions go to the essence of why we’re out here,” Thistle said. He was speaking from the fantail of the R/V Point Sur, a 135’ research vessel stationed at Moss Landing Marine Laboratory just north of Monterey, California. Thistle was into the fifth day of a 21-day, National Science Foundation-funded mission to search for answers to questions that, if they weren’t about animals so terrifically difficult and expensive to catch and study, likely would have been answered decades ago.
A specialist in the morphology of harpacticoids, Thistle is serving as chief scientist for this mission. His first explorations of the deep silt-and-clay bottoms off California’s continental shelf came in the 1970s as a doctoral student at Scripps Institute of Oceanography, at San Diego. He’s one of a handful of oceanographers in the world that has taken a long-term interest in studying the smallest benthic (bottom-dwelling) animals found in the deep oceans.
“The deep sea is really under-studied for harpacticoid copepods,” he said. “The samples we’re getting are probably the first collected for this purpose at these depths along the west coast. ”
Home on the Abyssal Range
Once brought back to his lab at Florida State, the samples will require at least a year to fully analyze. Thistle already is confident of at least one thing—his team will be adding much to the scientific literature. He expects to find dozens, if not hundreds, of harpacticoid copepod species completely new to science.
“Our guess is that 90 to 99 percent of the animals we collect will be new to science. Because they have been so little studied, very few of them have been identified as to species, with formal scientific names.”
But perhaps equally as important as what he finds on this trip will be where he finds it. Thistle said this mission is really an experiment in biogeography, a branch of biology popularized in the late 1960s by the famed naturalist E.O. Wilson of Harvard and his colleague, the late Robert MacArthur.
Biogeography essentially is the scientific acknowledgement that there’s a certain pattern in how nature distributes plants and animals across a given environment over time. If it’s true that nature always finds a way, it’s equally true that it’s not always obvious how it goes about it. Why trees, mushrooms, bacteria, sponges, apes, salamanders and amoebae naturally live where they do and when isn’t trivial science. Biogeography is a systematic way of getting clues to how nature fills available environmental niches with life. It has become a fundamental tool in such fields as conservation biology and landscape ecology.
“How extensive is the range of individual harpacticoid copepod species out here? Right now, we have no idea,” Thistle said. “Do they have ranges of hundreds of kilometers or just a few? We hope to find that out.”
One practical, ecological reason for why it matters where benthic (bottom-dwelling) copepods live and what species they are touches on humankind’s historic use of the oceans as dumping grounds. Few of the world’s deepest places have escaped this insidious insult from modernity. Untold tons of highly lethal chemicals, including some of the most lethal gases ever made—from sarin to mustard—along with radioactive waste litter Earth’s seafloors. The deep waters off the U.S. west coast contain them all—some charted, some not.
If a species of harpacticoids is found to have a great range, such dumping may have only localized impact; whereas if the same species is confined to a small area, toxins dumped on top of them could wipe them out. This is equally true of all bottom-dwelling life forms, not just copepods. “The wider the ranges, the less we have to worry about driving some species to extinction,” Thistle said.
But Thistle is 
quick to justify such research on other grounds, namely, what it means for the generation of oceanographers poised to succeed him. Experiences offered by NSF-supported research missions such as this are invaluable to his field’s future. “You can’t train oceanographers in a lab—you must go to sea,” he said. “What we’re doing is for the scientists of tomorrow.”
He points to his two doctoral students aboard, Erin Easton, of Crown Point, Indiana, and Melissa Rohal, of Columbus, Ohio, as good examples. Erin, a team leader for this project, already has shown promise in identifying copepods with both microscopes and DNA sequencing technology. She envisions a career in teaching. Melissa is drawn to deep-sea exploration in general and is seeking her niche in research. Both young women are intensely focused and are making the most of this rare training experience at sea, the first extended research cruise for both.
Thinking Small
Biologists who spend their careers studying large things, E.O. Wilson has suggested, shouldn’t nurse any conceits about the importance of their work. Wilson should know—he built a reputation as one of the world’s greatest naturalists by studying ants. Are the obscure, minuscule, denizens of the deep studied by Thistle and his students any less worthy of study than annoying visitors to a picnic?
In his masterful biography, The Naturalist (1995, 2006), Wilson reflected on the small things that make biology the endlessly fascinating science it is. Although written mainly for the life terrestrial, his words apply equally well to the life aquatic:
“The key to taking the measure of biodiversity lies in a downward adjustment of scale. The smaller the organism, the broader the frontier and the deeper the unmapped terrain. Conventional wildernesses of the overland trek may indeed be gone. Most of Earth’s largest species—mammals, birds and trees—have been seen and documented. But microwildernesses exist in a handful of soil or aqueous silt collected almost anywhere in the world. They at least are close to a pristine state and still unvisited. Bacteria, protistans, nematodes, mites, and other minute creatures swarm around us, an animate matrix that binds Earth’s surface. They are objects of potentially endless study and admiration, if we are willing to sweep our vision down from the world lined by the horizon to include the world an arm’s length away. A lifetime can be spent in a Magellanic voyage around the trunk of a single tree.” — F.S.
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