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

Marine sponges are notoriously difficult to identify. Many species lack fixed, easily readable external characteristics, and their morphological traits can be ambiguous, variable with environment, or shared misleadingly among unrelated lineages. This has produced a long history of taxonomic confusion, misidentifications and inconsistent classification, particularly among so-called cryptic species that look nearly identical but differ genetically. In recent years, sponge systematics has been transformed by integrative approaches that pair careful morphological description with molecular tools, allowing researchers to delimit species and reconstruct evolutionary relationships with far greater confidence than morphology alone permits.

The new study focuses on the order Scopalinida, a group erected and redefined only in the last decade using both morphological and molecular evidence. Scopalinida contains a single family, Scopalinidae, which in turn holds just three genera: Scopalina, Stylissa and Svenzea. The genus Svenzea, named in honor of the Colombian spongiologist Sven Zea, was established in 2002 for reef-associated sponges whose classification had long bounced between the former order Halichondrida and the family Dictyonellidae. Its defining features include distinctive microanatomy, the arrangement of skeletal elements, the composition and shape of spicules, and the presence of granular cells observed in both adults and larvae, along with a larva of extraordinary size. The larvae of Svenzea zeai, at up to six millimeters long, are regarded as the largest documented for the entire phylum Porifera.

Despite decades of study, only seven species of Svenzea had ever been described, and none had been recorded anywhere along the Eastern Tropical Pacific. That gap made the Mexican Pacific an intriguing frontier. Between July and November 2024, a team led by Eric Bautista-Guerrero of the Universidad de Guadalajara collected samples by SCUBA diving at depths of three to eight meters in the coral community known as Plataforma Pavonas, within Islas Marietas National Park in Bahía de Banderas. The site, where live pocilloporid coral cover is roughly eleven percent and sponges account for only about one percent, proved to host an abundant, widely distributed sponge growing over semi-shaded substrates, cave walls and vertical rock formations.

Underwater, the sponge is unmistakable: a thickly encrusting, irregularly massive animal ten to twenty-five centimeters across and two to three centimeters high, glowing bright orange in life and fading to beige in alcohol preservation. Its surface is smooth but microscopically hispid, densely pierced with incurrent pores and marked by bifurcated exhalant channels that lead to elevated, translucent chimney-like oscula. Under the microscope, the internal architecture revealed a cavernous choanosomal skeleton of disorganized, multispicular tracts, bundles of three to six needle-like spicules cemented by spongin fibers, ascending toward the surface. The spicules themselves, slender styles in two size categories measuring roughly 413 to 551 micrometers long, proved to be significantly larger than those of any previously known Svenzea species, and the complete absence of oxea, a second spicule type common in relatives, provided another decisive clue.

The team did not stop at adults. Using plankton nets towed by a diver around the coral community, they captured free-swimming larvae in full planktonic condition and raised them for observation. The larvae are bright orange, elongated to ovoid, and slightly flattened at the posterior pole, giving them a pyriform, or pear-shaped, appearance. Measuring 710 to 766 micrometers in length, they are uniformly covered in fine cilia about 24 micrometers long and swim in counterclockwise spirals, sometimes pausing or sinking before resuming their corkscrew journey. Their clear anterior-posterior polarity and cylindro-conical body plan echo the unusual parenchymella larvae documented in other scopalinid sponges, although the new species’ larvae are far smaller than the giant larvae of the Caribbean Svenzea zeai, which reach over six millimeters in length.

To place the new species on the sponge tree of life, the researchers extracted DNA from three adults and two larvae and amplified two independent genetic markers: the mitochondrial cytochrome c oxidase subunit I gene, COI, and the nuclear small-subunit ribosomal RNA gene, 18S. Sequences were aligned against a comprehensive set of Scopalinidae sequences from public databases, and phylogenetic trees were reconstructed using both maximum likelihood and Bayesian inference methods. The two approaches produced congruent topologies, and both markers placed Svenzea marialmae firmly within a well-supported clade containing Svenzea, Scopalina and Stylissa, confirming its membership in the family Scopalinidae and its distinction from the family Dictyonellidae, to which some of its relatives were once assigned.

The genetic evidence was strikingly specific. Based on COI sequences, the new species is closest to the Caribbean sponge Svenzea cristinae, with a genetic distance of just 0.052, followed by an undescribed Svenzea and Svenzea zeai, while showing much larger distances from all species of Scopalina and Stylissa. The 18S data told the same story, yielding the lowest interspecies distance, a mere 0.005, between Svenzea marialmae and Svenzea cristinae. Intriguingly, the two markers disagreed on one point: COI recovered Svenzea as monophyletic while 18S suggested it is paraphyletic, a discrepancy the authors attribute to the scarcity of Scopalinida sequences in public databases and to the possibility that some Scopalina species, such as S. goletensis and S. kuyamu, may have been misidentified. Resolving this will require additional genetic markers, but the congruence of the adult and larval sequences within a single clade provides strong evidence that both life stages belong to the same new species.

Morphologically, the new sponge walks a fascinating line between its named relatives. Its skeletal architecture, prominent dendritic spongin fibers cored by long styles over a basal spongin plate, resembles that of Scopalina species, and its thin cushions, conulose surface and bright orange color closely recall the Caribbean Scopalina ruetzleri. Yet it lacks the oxeas and other spicule modifications seen in that species, and it differs sharply from the erect, flabellate Stylissa, whose choanosome is supported by confusedly plumoreticulate spicule tracts. Against its congeners, the comparisons are equally decisive: Svenzea tubulosa is tubular with smaller styles, Svenzea flava bears blunt-ended styloids, Svenzea germanyanezi is a tiny cave-dweller with two categories of oxea, and Svenzea zeai carries short styles and a purple-brown, volcano-like form. Only the combination found in the Mexican specimens, giant styles, no oxeas and vivid orange pigmentation, defines the new species.

Beyond its taxonomic significance, the discovery carries a dedication with deep personal meaning. The species epithet marialmae honors María del Rocío Troncoso González, mother of co-author Dr. Alma Paola Rodríguez-Troncoso, a marine biologist who has spent two decades conserving and restoring the coral communities of Islas Marietas National Park and strengthening biodiversity management in this Marine Protected Area. The type specimens are deposited in the Colección de Esponjas del Pacífico Mexicano at the Institute of Marine Sciences of the National Autonomous University of Mexico, and all genetic sequences have been archived in GenBank, making the data freely available for future studies.

The broader implications reach well beyond one sponge. By raising the global count of Svenzea species to eight and establishing the first documented record of the genus in the Eastern Tropical Pacific, the study fills a conspicuous biogeographic gap and hints at unrecognized evolutionary connections across ocean basins, the new species’ closest relative after all lives on the far side of the American continent in the Caribbean. It also validates the combined use of mitochondrial and ribosomal markers alongside morphological and reproductive characters as a robust framework for testing phylogenetic hypotheses in Demospongiae, reducing the uncertainty of misidentifications that has long plagued sponge taxonomy. For the coral reefs of the Mexican Pacific, the message is clear: even in a marine protected area studied for decades, an abundant, brightly colored animal can remain formally unknown to science. As integrative taxonomy spreads to underexplored regions, researchers expect many more such surprises, each one refining our understanding of how sponge diversity evolved and how these ecologically important filter feeders are distributed across the world’s tropical seas.

Subject of Research: Taxonomic and molecular description of a new marine sponge species of the genus Svenzea from coral communities in the Eastern Tropical Pacific

Article Title: New insights into the diversity of Scopalinida (Porifera: Demospongiae) in the Eastern Tropical Pacific

Article References: Bautista-Guerrero, E., Marin-Ramirez, M. F., Carballo, J. L., Rodríguez-Troncoso, A. P., & Santiago-Valentín, J. D. (2026). New insights into the diversity of Scopalinida (Porifera: Demospongiae) in the Eastern Tropical Pacific. Discover Ecology, 2(1), Article 13. https://doi.org/10.1007/s44396-026-00031-2

Image Credits: AI Generated

DOI: 10.1007/s44396-026-00031-2

Keywords: marine sponges, Svenzea marialmae, Scopalinidae, Eastern Tropical Pacific, new species, integrative taxonomy, phylogenetics, COI, 18S rRNA, coral reefs, Islas Marietas, biodiversity