Unlocking the Secrets of Non-Calanoid Copepods
A five-year study at the Bermuda Atlantic Time-series Study station reveals the astonishing dominance and ecological importance of these overlooked marine organisms
Imagine an alien world teeming with bizarre creatures, where dragons with feathery antennae hunt their prey, where miniature armored tanks pulse through the darkness, and where the fate of entire ecosystems rests on these nearly invisible beings. This isn't a scene from a science fiction novel—it's the reality of the ocean's zooplankton community, and at its heart are the unsung heroes of the sea: non-calanoid copepods.
For decades, marine scientists focused on the larger, more obvious members of the ocean's drifting community. But beginning in 1995, a landmark study at the Bermuda Atlantic Time-series Study (BATS) station in the Sargasso Sea would reveal an astonishing truth: we had been overlooking some of the ocean's most important inhabitants 1 .
This five-year investigation would uncover the hidden dominance of non-calanoid copepods, creatures so small they had evaded notice, yet so abundant they shape the very functioning of marine ecosystems.
Non-calanoid copepods measure just 0.2-2.0 mm, small enough to slip through standard plankton nets.
The BATS research spanned from 1995-1999, revealing seasonal patterns and ecological importance.
Copepods are small aquatic crustaceans that represent one of the most numerous metazoan groups in aquatic communities worldwide 5 . While their calanoid cousins have received most scientific attention, non-calanoid copepods belong to several different orders including Cyclopoida, Poecilostomatoida, and Harpacticoida 1 .
These tiny creatures typically measure between 0.2 millimeters to 2 millimeters—small enough to slip through the mesh of standard plankton nets, which explains why they remained understudied for so long 1 . Despite their diminutive size, they inhabit every aquatic environment imaginable, from the highest mountain lakes to the deepest ocean trenches 5 .
Non-calanoid copepods inhabit diverse aquatic environments worldwide, from freshwater to marine ecosystems.
| Order | Representative Genera | Size Range | Ecological Role |
|---|---|---|---|
| Cyclopoida | Oithona | 0.2-1.5 mm | Predator on microzooplankton |
| Poecilostomatoida | Oncaea, Farranula, Corycaeus | 0.3-2.0 mm | Detritus feeders, parasite hosts |
| Harpacticoida | Macrosetella, Microsetella | 0.3-1.8 mm | Specialist diatom feeders |
| Mormonilloida | Mormonilla | 0.8-1.2 mm | Rare, deep-water specialists |
| Siphonostomatoida | Various | 0.5-17 mm | Often parasitic forms |
The Bermuda Atlantic Time-series Study (BATS) program represents one of oceanography's most ambitious long-term monitoring efforts. Established near Bermuda in the Sargasso Sea, BATS has collected oceanic data on a monthly basis since 1988, providing scientists with an unprecedented window into the inner workings of marine ecosystems 7 .
The Sargasso Sea, located within the North Atlantic Subtropical Gyre, possesses one of the highest zooplankton diversities in the world's oceans 2 . This region experiences distinct seasonal patterns despite its open ocean location—winter mixing, summer stratification, and fall mixing events—that create a dynamic environment for plankton communities 2 .
The non-calanoid copepod study from 1995-1999 leveraged this unique research platform, collecting samples through oblique net tows from depths of up to 200 meters 1 . The researchers employed multiple sampling techniques, including 200 μm mesh nets for larger specimens and finer 20-35 μm mesh nets to capture the smallest species that had previously escaped detection 1 .
BATS has collected data monthly since 1988
Situated in the North Atlantic Subtropical Gyre
Multiple net sizes to capture diverse species
When Hussain Ali Al-Mutairi meticulously analyzed his samples month after month, he uncovered a stunning reality: non-calanoid copepods dominated the zooplankton community in both abundance and diversity 1 . The cyclopoid genus Oithona emerged as the most abundant, followed by poecilostomatoid families like Oncaeidae, and the genera Farranula and Corycaeus 1 .
Perhaps the most striking finding was the pronounced seasonal signal in abundance. Contrary to expectations that tropical waters would show little variation, the non-calanoid copepod populations surged highest during spring and reached their lowest points during winter 1 . Some specific groups defied this general pattern, peaking in fall or winter instead, revealing a complex tapestry of life history strategies within the community.
The most astonishing discovery came when researchers compared samples from different net meshes. The 20-35 μm nets revealed abundances more than an order of magnitude higher than those captured by standard 200 μm nets 1 . At least four species of oncaeid copepods and the harpacticoid Microsetella norvegica had been virtually invisible to previous studies, suggesting marine ecosystems were far more densely populated than anyone had imagined.
| Taxonomic Group | Relative Abundance | Seasonal Pattern | Notes |
|---|---|---|---|
| Cyclopoida (Oithona) | Most abundant | Spring peak | Dominant cyclopoid genus |
| Poecilostomatoida (Oncaeidae) | Second most abundant | Variable by species | Includes Farranula and Corycaeus |
| Harpacticoida | Order of magnitude less | Late summer-fall peak | Dominated by Macrosetella gracilis |
| Mormonilloida | Much lower numbers | Consistent presence | Frequently encountered but rare |
| Siphonostomatoida | Much lower numbers | Consistent presence | Frequently encountered but rare |
The ecological significance of these tiny creatures extends far beyond their numbers. The BATS study revealed that miraciid copepods (a family within the Harpacticoida) consume an average of 359 μg C m⁻² d⁻¹ and regenerate approximately 55 μg N m⁻² d⁻¹ derived specifically from Trichodesmium cyanobacteria 1 . These grazing and regeneration rates peaked from late summer through early winter and reached their lowest points in spring and early summer 1 .
This finding was particularly significant because it demonstrated a direct link between these copepods and nitrogen fixation—a process essential for fertilizing nutrient-poor oceanic waters. By consuming Trichodesmium, which converts atmospheric nitrogen into usable forms, these copepods help distribute this essential nutrient throughout the ecosystem.
| Parameter | Average Rate | Seasonal Peak |
|---|---|---|
| Carbon Consumption | 359 μg C m⁻² d⁻¹ | Late Summer - Early Winter |
| Nitrogen Regeneration | 55 μg N m⁻² d⁻¹ | Late Summer - Early Winter |
| Nitrate Respiration | 0.09 nmol N copepod⁻¹ d⁻¹ | Not specified |
| Ecological Function | Mechanism | Significance |
|---|---|---|
| Carbon cycling | Grazing on phytoplankton and cyanobacteria | Transfers carbon through food webs |
| Nitrogen regeneration | Consumption of Trichodesmium and other nitrogen-fixers | Makes fixed nitrogen available to ecosystem |
| Microbial hosting | Providing microhabitats for specialized bacteria | Enables novel nitrogen transformations in oxygenated waters |
| Organic matter transport | Diel vertical migration and fecal pellet production | Moves carbon and nutrients to deeper waters |
| Food source | Prey for fish larvae and other plankton feeders | Supports commercial fisheries and higher trophic levels |
Recent research has uncovered an even more surprising role for copepods in ocean chemistry. Studies have revealed that copepods host specialized bacteria in their guts and on their exoskeletons that can perform unusual chemical transformations . In 2018, scientists discovered Gammaproteobacteria associated with copepods that respire nitrate even in oxygenated water—a process previously thought impossible in such environments .
These bacteria possess genes for both the periplasmic (Nap) and membrane-associated (Nar) dissimilatory nitrate reduction pathways, allowing them to transform nitrogen in ways that challenge our understanding of ocean nutrient cycles . This copepod-associated nitrate respiration represents a previously unaccounted nitrogen transformation in oceanic surface waters, with rates of up to 0.09 nmol N copepod⁻¹ d⁻¹ .
Understanding these nearly invisible creatures requires specialized equipment and methods. The BATS program employs a sophisticated array of research tools that have evolved over decades of observation:
Modern genetic approaches that identify species by sequencing the 18S V9 hypervariable region of DNA 2 . This method has revealed hidden diversity that traditional microscopy misses.
Arrays deployed at 150, 200, and 300 meters to measure the flux of organic material sinking from surface waters, helping quantify the copepods' role in carbon export 2 .
Instruments that measure Conductivity, Temperature, and Depth, providing critical context about the physical environment the copepods inhabit 2 .
Advanced imaging techniques including High Definition (HD) imaging and Scanning Electron Microscopy (SEM) for detailed morphological examination 4 .
The integration of these tools—from traditional nets to cutting-edge genetic techniques—has transformed our understanding of these critical components of marine ecosystems.
The humble non-calanoid copepods of the Sargasso Sea teach us a profound lesson about the natural world: significance doesn't always correlate with size. These minute creatures, long overlooked in favor of their larger planktonic neighbors, have emerged as essential players in ocean ecology, influencing everything from the productivity of fisheries to the global carbon cycle.
The BATS research has revealed that the Sargasso Sea, rather than being a stable, unchanging environment, pulses with seasonal rhythms and complex biological interactions centered around these tiny crustaceans 1 2 . Their dominance in zooplankton communities and their critical roles in nutrient cycling force us to reconsider how marine ecosystems function.
As climate change alters ocean temperatures, acidity, and circulation patterns, understanding these fundamental components of marine food webs becomes increasingly urgent. The silent work of copepods—grazing on phytoplankton, hosting specialized bacteria, and transporting nutrients—represents a delicate biological machinery that our planet depends on. The continued research at the BATS station ensures we will better understand these vital relationships, helping us protect the intricate web of life that sustains our oceans and our planet.
Copepods influence carbon cycling on a planetary scale
These delicate ecosystems face threats from climate change
BATS continues to monitor these vital organisms