Ultrafast PCR Test Identifies Lookalike Sea Squirts in Under an Hour

Two sea squirt species that look nearly identical on the plate can now be told apart in as little as fifty minutes, thanks to a new set of DNA-based tests developed by researchers at South Korea’s National Institute of Food and Drug Safety Evaluation. The study, published in Food Science and Biotechnology, compared conventional PCR, real-time PCR, and an ultrafast real-time PCR platform for identifying Styela clava and Styela plicata, two edible tunicates that have become commercially important in East Asian seafood markets. The work addresses a persistent problem for regulators: once these animals are cleaned, salted, or otherwise processed, the visual features that distinguish one species from the other largely disappear, opening the door to accidental confusion or deliberate substitution in the supply chain.

Styela clava, often called the clubbed tunicate or Asian sea squirt, and Styela plicata, the pleated sea squirt, are sessile marine invertebrates that attach themselves to rocks, docks, aquaculture gear, and hulls. Both have spread far beyond their native ranges and are now established in harbors and marinas across Europe, North America, and Asia, where they are notorious as fouling organisms that smother shellfish beds and compete with farmed mussels and oysters for space and food. Yet the same traits that make them ecological nuisances, including tolerance of wide salinity and temperature ranges and a knack for colonizing new substrates, have also made them candidates for cultivation. In South Korea, sea squirts are a recognized fishery product, and production has grown substantially over recent decades, with both species appearing in domestic markets and in processed seafood products.

The commercial rise of these tunicates has created an authentication challenge. Because the two species differ in texture, flavor profile, and market value, knowing exactly which species is in a package matters to buyers, traders, and food safety authorities. Morphological identification depends on features such as the texture of the outer tunic and the arrangement of internal organs, characteristics that are destroyed or obscured by processing steps like peeling, brining, and freezing. The research team, led by Yun Seo Jung and corresponding author Ho Soo Lim, set out to build a molecular identification method robust enough to work on simply processed seafood products, where visual inspection fails and DNA may be the only reliable evidence of species identity.

The core of the new method lies in the mitochondrial genome. The researchers designed species-specific primers targeting two mitochondrial genes: NADH dehydrogenase subunit 5, known as ND5, and cytochrome c oxidase subunit I, or COI. Both genes are widely used in animal DNA barcoding because they accumulate sequence differences between closely related species at a useful rate while remaining conserved within a species. By aligning sequences from S. clava and S. plicata and identifying positions where the two species consistently differ, the team designed primer pairs that anneal only to the DNA of one species and not the other. This primer design strategy exploits interspecies sequence variation so that amplification itself becomes the identification step: a positive signal for the S. clava primer set means S. clava DNA is present, and a positive signal for the S. plicata set means S. plicata DNA is present.

Specificity is the make-or-break property of any species identification assay, and the team tested all three platforms against a panel of non-target species to check for cross-reactivity. In every case, the assays amplified only their intended target. No signal appeared when the primers were challenged with DNA from other organisms that might plausibly co-occur in seafood samples. This clean specificity profile matters because seafood products are complex matrices that can contain DNA from multiple organisms, including associated microorganisms, and a test that fires spuriously would generate false positives in routine inspection settings. The absence of cross-reactivity across conventional, real-time, and ultrafast formats suggests the primer design, rather than the amplification chemistry, is doing the discriminating work, which bodes well for transferring the assays to other instruments.

Sensitivity testing revealed how little DNA the assays need to succeed. Both the real-time PCR and the ultrafast real-time PCR formats reliably detected genomic DNA at concentrations as low as 1 x 10 to the minus 3 nanograms per microliter for both species. That level of detection corresponds to vanishingly small amounts of starting material, which is critical for processed foods where DNA is often fragmented and degraded by heat, salt, and storage. Conventional PCR, which relies on post-amplification gel electrophoresis to visualize products, was also functional but slower and less convenient for high-throughput screening. Real-time PCR adds the advantage of fluorescence-based detection during amplification, eliminating the need to open tubes after the reaction and thereby reducing contamination risk, a well-known hazard in diagnostic laboratories.

The standout result, however, came from the ultrafast real-time PCR platform. By combining rapid thermal cycling hardware with the optimized primer sets, the researchers achieved complete species identification within fifty minutes. Conventional workflows, including DNA extraction, amplification, and gel-based analysis, typically stretch over several hours and require well-equipped laboratory facilities. An assay that delivers a definitive answer in under an hour changes the practical calculus for food inspectors, port authorities, and customs officers who need results while a shipment is still in hand. The ultrafast format also aligns with a broader trend in food authentication research, where portable and microfluidic PCR devices are being paired with streamlined DNA extraction to enable on-site testing at markets, processing plants, and border checkpoints.

The implications extend beyond routine food labeling. Both Styela species are documented invasive species with significant ecological and economic footprints. S. clava, native to the Northwest Pacific, invaded European waters in the twentieth century and devastated shellfish farming in areas such as the Bassin de Thau in France, where it fouls cultivated oysters and competes for resources. S. plicata has similarly established populations in warm-temperate harbors worldwide, including along the Mediterranean coast of Israel, where researchers have studied its heat tolerance as an indicator of its invasive potential. Molecular tools that can rapidly and unambiguously identify these species have dual value: they protect seafood authenticity and they support marine biosecurity surveillance, where early detection of invasive tunicates in ballast water, hull fouling, or environmental samples can inform management responses before populations become established.

The Korean research context gives the work particular urgency. Statistics Korea data show that sea squirt production has grown into a meaningful component of the national fishery, and investigative reporting has previously highlighted deliberate seafood mislabeling in South Korean markets. Regulatory agencies, including the Ministry of Food and Drug Safety, which funded this study through grant 24191MFDS053, have an interest in validated, standardized methods for verifying species claims on processed products. The new assays were developed with such applications in mind, and the authors position the ultrafast real-time PCR format as a practical tool for on-site detection in simply processed seafood, meaning products that have undergone minimal transformation and still yield amplifiable DNA.

The comparative design of the study also offers a template for future authentication work. By evaluating the same primer sets across three amplification platforms, the researchers could weigh trade-offs that laboratories face when choosing among methods: conventional PCR is inexpensive and equipment-light but slow and contamination-prone; real-time PCR is sensitive, quantitative, and closed-tube but requires more sophisticated instrumentation; ultrafast real-time PCR compresses the timeline dramatically but depends on specialized rapid-cycling hardware. For a food inspection laboratory deciding where to invest, the demonstration that a fifty-minute, species-specific, highly sensitive assay is achievable for Styela species makes a compelling case for the ultrafast format, particularly as portable devices bring the technology closer to docks and processing floors. As seafood supply chains grow longer and more global, and as consumers increasingly demand verified provenance, rapid molecular authentication tools of this kind are likely to move from research papers into the standard toolkit of food safety authorities, ensuring that what the label says about a humble sea squirt is exactly what is in the package.

Subject of Research: Development and comparative evaluation of PCR-based assays for species identification of the sea squirts Styela clava and Styela plicata in seafood authentication

Article Title: Comparative evaluation of conventional, real-time, and ultrafast real-time PCR assays for accurate identification of Styela clava and Styela plicata

Article References: Jung, Y. S., Yi, J. S., Cho, A., Lim, H. S., & Chang, M. I. (2026). Comparative evaluation of conventional, real-time, and ultrafast real-time PCR assays for accurate identification of Styela clava and Styela plicata. Food Science and Biotechnology. https://doi.org/10.1007/s10068-026-02323-5

Image Credits: AI Generated

DOI: 10.1007/s10068-026-02323-5

Keywords: Styela clava, Styela plicata, PCR, real-time PCR, ultrafast PCR, seafood authentication, food inspection, mitochondrial DNA, ND5 gene, COI gene, tunicates, species identification