Arrow Worms Hold Steady Through El Niño Swings in Mexican Pacific

In the warm coastal waters of Bahía de Navidad, on Mexico’s Pacific coast, a group of translucent, torpedo-shaped predators has been quietly keeping time with the ocean’s rhythms for six straight years. These animals are chaetognaths, commonly known as arrow worms, and despite their small size they are among the most voracious carnivores in the plankton. A new study drawing on 110 consecutive months of sampling has now revealed, in unusual detail, how their communities respond to the twin forces of seasonal climate and the El Niño Southern Oscillation, one of the most powerful climate drivers on the planet. The findings, published in Discover Oceans, come from one of the longest monthly chaetognath time series ever assembled for the tropical eastern Pacific.

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