Hypanus brevis: a newly resurrected Eastern South Pacific stingray lineage revealed by integrative taxonomy

This study employs an integrative taxonomic approach to resurrect *Hypanus brevis* as a distinct species from the Eastern South Pacific, separating it from the Eastern North Pacific *H. dipterurus* based on genetic divergence and historical descriptions, while highlighting concerns over a severe genetic bottleneck in the Peruvian population that necessitates urgent conservation measures.

The Gist

The Big Picture: A Case of Mistaken Identity

Imagine you have a family of twins who look so much alike that everyone thinks they are the same person. For over 140 years, scientists thought the Diamond Stingray found in the cold waters of Peru was the exact same animal as the Diamond Stingray found in the warm waters of California and Mexico. They were treated as one big, long family stretching across the Pacific Ocean.

This paper is like a detective story where the authors finally prove that these "twins" are actually two different species that have been living separate lives for millions of years. They are giving the Peruvian version its own name back: Hypanus brevis (the Peruvian Diamond Stingray).

The Investigation: How They Solved the Mystery

The scientists used two main tools to crack the case: Genetic Fingerprinting and Morphology (measuring body parts).

1. The Genetic Fingerprint (The DNA Test)
Think of DNA as a barcode on a product.

• The North American Stingrays (California/Mexico): When the scientists scanned the DNA of these rays, they found a lot of variety. It was like a library with many different books. This suggests a large, healthy population with lots of genetic mixing.
• The Peruvian Stingrays: When they scanned the Peruvian rays, the result was shocking. Every single one of the 27 rays they tested had the exact same barcode. It was like finding 27 identical copies of the same book.
  • The Analogy: Imagine walking into a room of 27 people and realizing they all have the exact same fingerprint. This usually happens when a population goes through a "bottleneck"—a disaster that wiped out almost everyone, leaving only a few survivors who repopulated the area. This suggests the Peruvian population is genetically fragile and vulnerable.

2. The Body Measurements (The Ruler Test)
The scientists also measured the rays like a tailor measuring a suit.

• The Challenge: The two groups look almost identical. They have the same diamond-shaped bodies, the same color (brown/olive), and the same number of teeth. It's like trying to tell apart two identical twins just by looking at their faces.
• The Clue: The only real difference was subtle. The Peruvian rays had a slightly shorter tail and a slightly different shape to their nose compared to their North American cousins. But honestly, without the DNA test, you'd never know they were different just by looking at them.

The History: Why Were They Confused?

The confusion started in 1880. Two scientists, Jordan & Gilbert (in the US) and Garman (in Peru), described these rays almost at the same time.

• The US scientists named the California ray Hypanus dipterurus.
• The Peruvian scientist named the Peru ray Hypanus brevis.
• For over a century, scientists argued about which name was "first" and decided to lump them together under one name. This paper says, "Wait a minute! They are actually different species, so let's bring back Garman's original name for the Peruvian one."

The Evolutionary Story: The Great Split

The scientists used a "molecular clock" (a way to estimate time based on how fast DNA changes) to figure out when the split happened.

• The Timeline: About 3 million years ago, the ancestors of these rays were one group.
• The Event: Something happened (likely changes in ocean temperatures and currents) that pushed the North group up toward California and the South group down toward Peru. The warm waters of the tropics acted like a hot wall that they couldn't cross.
• The Result: They got separated, evolved on their own, and became two distinct species. This is called "anti-tropical speciation"—they split because they moved to opposite ends of the temperature spectrum.

The Warning: A Population in Danger

This is the most critical part of the story.

• The Problem: Because the Peruvian rays all have the exact same DNA, they have very low genetic diversity. In biology, diversity is like a safety net. If a new disease hits or the ocean gets too hot, a diverse population has some members that might survive. A population with no diversity is like a house of cards; one strong wind could knock it all down.
• The Cause: The paper suggests this happened because of a "bottleneck" event (maybe a massive El Niño event that killed off most of them long ago) combined with heavy fishing.
• The Fishing: These rays are caught by local fishermen in Peru. They are sold for money, but there are no strict rules protecting them yet. The scientists are worried that if we keep fishing them without understanding they are a unique, fragile species, we might wipe them out before we even realize how special they are.

The Conclusion: What Now?

The paper calls for a change in how we manage these fish:

1. Give them a name: Officially recognize the Peruvian Diamond Stingray (Hypanus brevis) as a separate species.
2. Protect them: Stop treating them as just "another ray." Because they have low genetic diversity, they need extra protection.
3. Manage the fishery: Fishermen need to be careful not to catch too many, especially the big mothers, to ensure the population doesn't collapse.

In short: We just discovered that the "Diamond Stingray" in Peru is actually a unique, genetically fragile species that has been hiding in plain sight for 145 years. We need to protect it before it's too late.

Technical Summary

1. Problem Statement

The study addresses a long-standing taxonomic confusion regarding the "diamond stingray" in the Eastern Pacific. For approximately 145 years, the lineage has been treated as a single conspecific species, Hypanus dipterurus (Jordan & Gilbert, 1880), which was historically synonymized with Trygon brevis (Garman, 1880) due to nomenclatural disputes.

• Taxonomic Ambiguity: The original descriptions were published nearly simultaneously in 1880. Garman (1913) incorrectly synonymized H. dipterurus under H. brevis based on a misinterpretation of publication dates, leading to decades of confusion.
• Conservation Risks: Treating distinct evolutionary units as a single species masks localized population declines and skews fisheries data. H. dipterurus is currently listed as Vulnerable on the IUCN Red List, but its status relies on aggregated data that may not reflect the true status of specific sub-populations.
• Knowledge Gap: While populations in the Eastern North Pacific (ENP) have been studied, the Eastern South Pacific (ESP) populations (specifically off Peru) lacked genetic assessment, leaving a significant gap in understanding connectivity and speciation across the Eastern Pacific.

2. Methodology

The authors employed an integrative taxonomic approach, combining molecular genetics, morphometrics, meristics, and fisheries data analysis.

• Sampling:
  • ESP Population: 27 specimens collected from Peruvian artisanal fisheries (Lambayeque, La Libertad, Ica, Lima, Tumbes) and Ecuador. Four complete specimens were preserved as vouchers (IMARPE collection).
  • ENP Population: 8 DNA sequences retrieved from GenBank/BOLD (USA and Mexico).
  • Outgroups: Hypanus longus, H. say, H. americanus, H. sabinus, and other stingray genera.
• Molecular Analysis:
  • Markers: Mitochondrial Cytochrome c Oxidase I (COI) and 16S rRNA genes.
  • Analyses:
    • Genetic Diversity: Haplotype diversity ($H_d$), nucleotide diversity ($\pi$), and neutrality tests (Tajima's D, Fu's $F_S$).
    • Population Structure: Analysis of Molecular Variance (AMOVA) and fixation indices ($\Phi_{ST}$).
    • Phylogenetics: Bayesian inference using single-locus (COI) and concatenated (COI + 16S) datasets.
    • Molecular Dating: Relaxed molecular clock analysis calibrated with fossil records (Dasyatidae teeth) and geological events (Isthmus of Panama closure) to estimate divergence times.
• Morphological Analysis:
  • Detailed morphometric and meristic measurements (disc width, tail length, dentition, denticles) of fresh specimens compared against original descriptions (Garman, 1880; Jordan & Gilbert, 1880) and literature data.
• Fisheries Data:
  • Analysis of IMARPE landing records (1996–2024) to estimate catch volumes, economic value, and spatial distribution of the target species in Peruvian waters.

3. Key Results

A. Genetic Divergence and Speciation

• Distinct Lineages: Phylogenetic trees and haplotype networks revealed two completely isolated monophyletic clades: one for the ENP (H. dipterurus) and one for the ESP (H. brevis).
• Genetic Distance: The mean COI genetic distance between the two lineages is 4.0%, well above the standard interspecific threshold for elasmobranchs.
• Population Isolation: AMOVA results showed that 97.54% to 99.25% of genetic variation occurs between the ENP and ESP groups, with $\Phi_{ST}$ values near 1.0, indicating complete genetic isolation and no gene flow.
• Low Diversity in ESP: The ESP population (H. brevis) exhibited extremely low genetic diversity. All 27 specimens shared a single fixed COI haplotype, suggesting a severe historical bottleneck or a founder event. In contrast, the ENP population showed high haplotype diversity.

B. Divergence Time

• Molecular dating estimated the split between the ENP and ESP lineages at approximately 3.09 million years ago (Ma) (Late Pliocene).
• This timing aligns with the final closure of the Central American Seaway and subsequent oceanographic shifts (intensified upwelling, thermal barriers) that drove anti-tropical speciation.

C. Morphological Findings

• Subtle Differences: Morphological overlap is significant, making visual identification difficult. However, subtle distinctions were noted:
  • Disc Shape: H. brevis has a disc slightly wider than long (DL/DW ratio 82.5–87.0%), whereas H. dipterurus is more disc-like (88.9–94.9%).
  • Tail: H. brevis has a shorter tail relative to disc width compared to congeners like H. longus.
  • Denticles: Adult H. brevis specimens showed scattered scapular denticles and enlarged denticles anterior to the caudal spine, a trait variable with age.
  • Coloration: H. brevis is generally brown to olive-brown, sometimes reddish near margins, lacking the distinct spots seen in some other Hypanus species.

D. Fisheries and Distribution

• Distribution: H. brevis is confirmed in the ESP (Peru and likely northern Chile), while H. dipterurus is restricted to the ENP. The "tropical gap" (Central America) likely separates them.
• Catch Data: Between 2015–2024, Peruvian artisanal fisheries landed an average of 46.2 tons/year of H. brevis (historically labeled Dasyatis brevis or Hypanus sp.), valued at ~$62,000 USD annually.
• Fishing Pressure: The fishery is coastal (mostly within 5 nautical miles), dominated by gillnets. While landings have remained stable, the lack of management and the species' low genetic diversity raise concerns.

4. Key Contributions

1. Taxonomic Resurrection: The study formally resurrects Hypanus brevis (Garman, 1880) as a valid species, distinct from H. dipterurus. This increases the recognized species count in the genus Hypanus to 12 (plus 3 cryptic units).
2. Resolution of Nomenclatural Conflict: It clarifies the 145-year-old dispute by confirming that Garman's description of T. brevis from Peru represents a distinct evolutionary lineage, not a synonym of the California-based H. dipterurus.
3. Discovery of a Bottleneck: The identification of a single fixed mitochondrial haplotype across a wide geographic range in Peru indicates a severe genetic bottleneck, highlighting a critical conservation vulnerability.
4. Biogeographic Insight: It provides evidence for an anti-tropical speciation pathway in the Eastern Pacific, driven by Pliocene oceanographic changes and the formation of the Isthmus of Panama.

5. Significance and Conservation Implications

• Conservation Status: The resurrection of H. brevis necessitates a reassessment of its IUCN status. Currently, it is lumped with H. dipterurus (Vulnerable). Given its restricted range, high fishing pressure, and critically low genetic diversity, H. brevis likely faces a higher risk of extinction than previously thought.
• Management Urgency: The study argues for immediate precautionary management in Peruvian waters. The lack of genetic variation reduces the species' adaptive potential to environmental changes (e.g., ENSO events) and overfishing.
• Data Quality: It highlights the danger of aggregating catch data for cryptic species. Accurate species-level identification is essential for effective fisheries management and preventing the collapse of distinct evolutionary units.
• Future Research: The authors call for nuclear genetic studies to confirm population structure and effective population size, as mitochondrial data alone may not fully capture the demographic history of the bottlenecked ESP population.

In summary, this paper uses integrative taxonomy to correct a historical error, revealing a distinct, genetically impoverished stingray species in the Eastern South Pacific that requires urgent conservation attention and specific management strategies separate from its northern relative.