Chaetognaths occupy a pivotal position in marine food webs. Roughly 133 valid species are recognized worldwide, and after copepods they are typically the most numerically dominant group in marine zooplankton communities, accounting for between 5 and 30 percent of total zooplankton abundance. They feed aggressively on smaller plankton, structuring and regulating pelagic ecosystems and contributing to the biological pump, the process by which carbon is transported from the surface ocean to the deep sea. Because different species are associated with specific water masses and hydrographic conditions, chaetognaths also serve as sensitive biological indicators of environmental change, which makes them ideal subjects for tracking how tropical coastal ecosystems respond to climate variability.

The research team, led by M. C. Franco-Gordo of the University of Guadalajara, sampled a coastal oceanographic station in Bahía de Navidad, Jalisco, where the seafloor lies about 150 meters down. Every month from November 2010 to December 2016, before dawn and typically around 4:30 in the morning, the researchers towed a one-meter conical net with 250-micrometer mesh from roughly 40 meters depth to the surface. A calibrated flowmeter measured the volume of water filtered on each tow, while conductivity-temperature-depth profilers recorded vertical profiles of temperature, salinity, and, during the first half of the study, dissolved oxygen. Surface chlorophyll-a concentrations were measured in the laboratory as a proxy for phytoplankton biomass, and coastal upwelling was quantified using the Bakun upwelling index, including a fifteen-day lagged version designed to capture the delay between physical forcing and biological response.

The six-year window was fortuitously timed to capture the full drama of ENSO variability. A prolonged La Niña event dominated the beginning of the series, from November 2010 to March 2012, followed by a long neutral period from April 2012 to October 2014, and then an extended El Niño from November 2014 to May 2016 that included the exceptionally strong 2015–2016 event. A shorter La Niña returned at the end of the series in late 2016. Overlaid on this interannual seesaw was the region’s pronounced seasonal cycle: a mixed season from January to May, when coastal upwelling brings cool, nutrient-rich water to the surface, and a stratified season from July to November, when the water column warms, freshens under monsoon rains, and becomes vertically layered.

In total, the team identified 145,077 chaetognath specimens, representing fifteen species in three families and ten genera. Just three species dominated numerically: Zonosagitta bedoti, Parasagitta euneritica, and Flaccisagitta enflata together accounted for 73.2 percent of total abundance, and with Aidanosagitta septata the top four species made up nearly 90 percent. Remarkably, these dominants persisted across every season and every ENSO phase, a signal of ecologically resilient local populations with broad environmental tolerance. The community was overwhelmingly tropical in character, with about 56 percent of recorded species having tropical biogeographic distributions, and the sampling captured roughly 87.5 percent of all chaetognath species known from the Central Mexican Pacific.

The abundance story unfolded in three acts. During the early La Niña, chaetognath numbers were at their lowest. They climbed steadily through the neutral years, peaking spectacularly in 2013, when the highest abundances of the entire series were recorded between January and July. During the strong El Niño of 2015–2016, abundance declined moderately, but species richness told a different story: it peaked during the warm event, reaching up to 12 species per sampling date. This inversion—more species but fewer individuals during El Niño, and fewer species but booming populations during neutral conditions—contrasts sharply with other zooplankton groups in the region, such as copepods, euphausiids, and fish larvae, which typically decline during El Niño.

To disentangle the environmental drivers, the researchers deployed a battery of statistical tools. Permutational multivariate analysis of variance confirmed significant differences in community structure between mixed and stratified seasons and among the three ENSO phases, although the interaction between the two was not significant, and dispersion tests urged some caution in interpretation. Redundancy analysis, constrained by eight environmental gradients, explained 25.6 percent of the total variance in community structure. Its first axis, correlated strongly with temperature at 10 meters depth, chlorophyll-a, and the lagged upwelling index, reflected the seasonal upwelling cycle, while the second axis captured ENSO-driven interannual variability. Species-level generalized linear models reinforced the picture: chlorophyll-a was the most frequently selected predictor, followed by the fifteen-day upwelling index, underscoring how tightly these predators are coupled to the productivity fueled by coastal upwelling.

The species-specific responses were ecologically telling. Zonosagitta bedoti and Parasagitta euneritica were associated with strong upwelling and low temperatures, thriving in the cool, productive mixed season. In contrast, Serratosagitta bierii, Aidanosagitta septata, Aidanosagitta regularis, and Serratosagitta pacifica were linked to moderate-to-high temperatures. Flaccisagitta enflata and its relative F. hexaptera reached peak abundance during stratified periods and El Niño conditions, coinciding with anomalously warm water, deeper mixed layers, and a subsurface hypoxic layer lying near 45 meters depth. F. enflata is known to tolerate a wide thermal range, from 15 to 30 degrees Celsius, and to adjust its metabolism to ambient temperature, which likely explains its success under warm, low-oxygen conditions. Indicator species analysis added finer texture: S. pacifica typified the La Niña stratified season, A. septata the El Niño stratified season, and F. hexaptera and S. bierii the neutral stratified season.

Among the more striking discoveries was the second recorded occurrence of Sagitta bipunctata in the Central Mexican Pacific, the first having been reported 53 years earlier. This circumglobal species is typically regarded as oceanic, so its appearance in nearshore waters is remarkable, and its sporadic records, along with those of S. pacifica and Pterosagitta draco, suggest that rarer species are more sensitive to hydrographic variability and dispersal constraints than the resilient dominants. The study also documented exceptional chlorophyll-a peaks during the mixed seasons of the neutral years, in April 2013 and April 2014, with zooplankton biovolume tracking the same rhythm, low during stratified periods and comparable across ENSO phases.

The overarching conclusion is that seasonal hydrographic variability, not ENSO, is the primary architect of chaetognath community structure in this tropical coastal system, with ENSO anomalies exerting secondary but clearly detectable effects. El Niño-related stratification appears to open the door to additional species while limiting overall biomass, whereas La Niña enhances productivity but favors a reduced set of dominant taxa. As the authors note, continued long-term monitoring, complemented with genetic tools, ecological niche modeling, and physiological assays, will be essential to clarify the mechanisms behind chaetognath persistence and to predict how these voracious, ecologically indispensable predators will fare as the tropical Pacific continues to warm.

Subject of Research: Long-term chaetognath community structure and its response to seasonal and ENSO-driven environmental variability in the coastal Central Mexican Pacific.

Article Title: Chaetognath community structure in the coastal Central Mexican Pacific: persistent seasonal signals amid ENSO variability (2010–2016)

Article References: Franco-Gordo, M. C., Plascencia-Palomera, V., Godínez-Domínguez, E., Lozano-Cobo, H., Gómez-Gutiérrez, J., Ambriz-Arreola, I., Kozak, E. R., & Suárez-Morales, E. (2026). Chaetognath community structure in the coastal Central Mexican Pacific: persistent seasonal signals amid ENSO variability (2010–2016). Discover Oceans, 3(1), Article 8. https://doi.org/10.1007/s44289-026-00121-9

Image Credits: AI Generated

DOI: 10.1007/s44289-026-00121-9

Keywords: chaetognaths, arrow worms, zooplankton, ENSO, El Niño, La Niña, coastal upwelling, Mexican Pacific, Bahía de Navidad, marine ecology, time series, biodiversity

The study site could hardly be better suited to the question. Islas Marietas National Park sits roughly eight kilometers off the coast of Nayarit in the Central Mexican Pacific, an oceanographic crossroads where the cold, low-salinity California Current meets the warm Mexican Coastal Current and the high-salinity water mass of the Gulf of California. The result is a sharply seasonal regime: sea surface temperatures swing from 18 to 21 degrees Celsius between January and May, then climb to 27 to 30 degrees from July to November. Layered on top of this annual rhythm are internal waves, seasonal upwellings that drop pH and spike nutrients, and the interannual whiplash of the El Niño-Southern Oscillation, which has pushed temperatures above 31 degrees for weeks at a time and, in La Niña years, dragged them a full degree below normal. Any fish community living here must be built for volatility.

The research team, led by Cassandra de Alba-Guzmán, designed their monitoring to capture volatility at every scale. Divers conducted diurnal visual censuses along 25-by-4-meter belt transects at depths of 3 to 13 meters, five transects per site per season per year, across six sites split between Isla Larga and Isla Redonda. Alongside fish counts, they quantified the benthic foundation of the reef, recording the percentage cover of branching and submassive corals, macroalgae, turf, rock, and sand using PVC quadrats placed every four meters. Crucially, the team went beyond simply tallying species. They classified every fish by six functional traits, body length, schooling behavior, diet, mobility, position in the water column, and activity period, and then computed a full suite of functional diversity metrics, from functional richness to functional vulnerability, using the mFD package in R.

The first headline finding is that the two islands, separated by less than a kilometer, host strikingly different fish communities, and the difference is written in the seafloor. Isla Redonda, with its steep slopes, rocky substrates, large caves, and tunnels, supported the highest species richness, with a mean of 36.7 species per transect compared to 32.5 on Isla Larga, along with higher exponential Shannon diversity, functional richness, functional evenness, and functional redundancy. Several species, including the finespotted moray Muraena lentiginosa and the tiger snake eel Echidna nebulosa, were recorded only on Isla Redonda, likely exploiting the island’s unique structural refuges. Isla Larga, by contrast, carries roughly 15 percent more live coral cover, dominated by branching Pocillopora colonies that form continuous plates with narrow, homogeneous spaces between them. That framework shelters juvenile fish, but the low diversity of benthic components limits the variety of niches available, producing assemblages dominated by a few coral-dependent species rather than a broad functional portfolio.

This contrast delivers a counterintuitive lesson: more coral is not automatically more fish diversity. The researchers argue that what matters is the diversity of benthic components, not just their total cover. When a single coral morphofunctional group expands and outcompetes hydrocorals, sponges, and octocorals, the reef becomes structurally and functionally simpler, even as raw coral percentage climbs. On Isla Redonda, the mosaic of algal turfs, hydrocorals, octocorals, crustose coralline algae, and articulated calcareous algae creates microhabitats that allow species with different trophic roles to coexist, reducing interspecific competition and boosting functional redundancy, the ecological insurance policy that keeps a reef running when individual species falter.

The temporal record is where the decade’s drama unfolds. The most significant break in fish composition came after 2016, following the El Niño event of 2015-2016, when sea temperatures exceeded 31 degrees Celsius for more than six consecutive weeks. The schooling planktivore Apogon pacificus, a cardinalfish that had been abundant, declined sharply, and the composition of the assemblage shifted markedly. Extreme heat does not merely discomfort fish; it disrupts larval development, suppresses recruitment, and can cause outright mortality, while simultaneously degrading the benthic habitats and food resources on which reef-associated species depend. By 2018 and into 2019, species richness had fallen from 36.58 to 31.83 species on average, a decline the authors attribute primarily to habitat homogenization as benthic diversity eroded.

Then the pendulum swung the other way. The La Niña event of 2020-2022 brought abnormally cool temperatures, down as much as 1.0 degree Celsius, which drove subsurface influxes of nutrient-rich water and fueled plankton proliferation. Planktivorous species responded dramatically: the abundance of the blue-and-gold chromis Azurina atrilobata and the goatfish Mulloidichthys dentatus surged between 2019 and 2020, with M. dentatus jumping by more than 1,200 percent in functional entity terms. Functional dispersion rose in two significant steps, in 2020 and again in 2022, reflecting a community increasingly spread across functional space, with a few dominant species pulling away from the pack. Meanwhile, the benthic data showed Pocillopora coral expanding after 2020, further squeezing out other benthic components and narrowing the ecological stage.

Seasonality, too, left its fingerprints on the assemblage. Apogon pacificus, Abudefduf troschelii, and Mulloidichthys dentatus peaked in abundance during the cold season, when the California Current delivers nutrient-rich waters that boost primary production and the planktonic food web. Azurina atrilobata, the spotted grunt Haemulon maculicauda, and the grouper Cephalopholis colonus dominated the warm, oligotrophic months. The researchers link these cycles to reproductive timing, since temperature and currents regulate spawning aggregations and larval survival, and to foraging migrations, exemplified by the mackerel scad Decapterus macarellus, which vanished from every warm-season survey as it tracked favorable feeding grounds elsewhere.

What kept the system from unraveling, the authors suggest, is connectivity. Functional divergence and functional vulnerability, the metrics that measure how exposed a community is to losing its rare functions, showed no significant changes across the entire decade. With the two islands less than a kilometer apart, individuals, larvae, and nutrients flow freely between them, and niche complementarity spreads risk across the park as an integrated whole. The coexistence of the two most abundant species, the planktivorous Azurina atrilobata and the omnivorous wrasse Thalassoma lucasanum, illustrates how dietary differentiation defuses what should be fierce competition between similarly sized fish. But the researchers are explicit that this buffering capacity is conditional: if natural and anthropogenic stressors intensify, the connectivity that currently stabilizes the park could be overwhelmed.

The stakes extend well beyond the park’s boundaries. Coral reefs support food security and livelihoods for more than 500 million people worldwide, and reef fish are the engines of those services, grazing macroalgae to clear space for coral settlement, preying on echinoderms and crustaceans to steady the trophic web, and even contributing calcium carbonate to the reef framework itself. Islas Marietas is also a tourist hotspot, facing pressure from boats, snorkeling, diving, and illegal fishing, which compounds the climatic stress documented here. The study’s authors recommend that future monitoring incorporate direct measurements of habitat complexity, potentially through 3D photogrammetry, and trophic network analyses to sharpen predictions. Their broader message is sobering and actionable in equal measure: protecting reefs means protecting the diversity of the seafloor’s building blocks, not just counting coral, because when benthic variety collapses, the fish community’s functional architecture follows, and the resilience that ecosystems like Marietas have shown over this decade may prove to be a loan rather than a gift.

Subject of Research: Decadal taxonomic and functional changes in coral reef fish assemblages at Islas Marietas National Park, Mexican Pacific

Article Title: Decadal changes in the fish assemblage structure and function within an insular coral community

Article References: de Alba-Guzmán, C., Rodríguez-Troncoso, A. P., Cupul-Magaña, A. L., Rodríguez-Zaragoza, F. A., Cabral-Tena, R. A., & de Jesús Adolfo Tortolero-Langarica, J. (2025). Decadal changes in the fish assemblage structure and function within an insular coral community. Discover Ecology, 1(1), Article 15. https://doi.org/10.1007/s44396-025-00015-8

Image Credits: AI Generated

DOI: 10.1007/s44396-025-00015-8

Keywords: coral reef fish, functional diversity, El Niño, ENSO, Mexican Pacific, Islas Marietas, benthic habitat, long-term monitoring, species richness, ecosystem resilience, marine protected area, planktivores