Century-Old Fish Parasite Gets Its First DNA Portrait in Landmark Study

Hidden on the gills of sheepshead and pinfish in the warm coastal lagoons of Mexico’s Yucatán Peninsula lives a tiny parasite that science has known for more than a century, yet almost entirely by sight alone. Lernanthropus caudatus, a copepod first described in 1922 by the American parasitologist Charles Branch Wilson, has now been given a complete modern makeover. A team of researchers led by Juan F. Espínola-Novelo, Betzi F. Pérez-Ortega, Raúl Castro-Romero and Gerardo Pérez-Ponce de León has published a detailed redescription of the species together with the first-ever molecular characterization of its DNA, an integrative analysis that appeared in the open-access journal Acta Parasitologica. The work combines painstaking light microscopy of dissected appendages with cutting-edge mitochondrial DNA sequencing, offering the most complete portrait of this gill-dwelling crustacean ever assembled and adding sequences from its type host, the sheepshead Archosargus probatocephalus, for the very first time.

The story began during a broad survey of the metazoan parasites of marine and estuarine fishes on the Yucatán Peninsula. Between 2023 and 2026, the researchers collected 122 individuals of three sparid fish species: 31 sheepshead, Archosargus probatocephalus; 31 grass porgies, A. rhomboidalis; and 60 pinfish, Lagodon rhomboides. The fish came from two coastal lagoons in the southern Gulf of Mexico, La Carbonera and Celestún, with additional specimens sampled offshore of Celestún. All fish were obtained through commercial capture, dissected, and their gills examined under a stereomicroscope. From these hosts the team recovered dozens of specimens of Lernanthropus, including 21 copepods from sheepshead, two from grass porgy, and 36 from pinfish across the two lagoons, with infection intensities ranging from a single parasite to as many as eleven per infected fish. Specimens were preserved in 100 percent ethanol for both morphological and molecular work, and voucher specimens were deposited in the Colección Nacional de Crustáceos in Mexico City.

Morphologically, the copepods proved a treasure trove of detail that earlier generations of taxonomists had never captured. The females, measured from 14 voucher specimens, range from roughly 2,955 to 5,456 micrometres in total length, with a cephalothorax narrowed anteriorly and a broad, sub-quadrangular trunk bearing a distinctive projection on each side. The well-developed dorsal plate covers only part of the fourth legs, leaving the uropods visible. Among the newly documented traits are the seven-segmented antennules bearing a long aesthete, the subchelate antennae lacking a parabasal flagellum, a mandibular blade armed with seven teeth, and the fine architecture of the maxillules, maxillae and maxillipeds. The team also documented, for the first time in this species, a spermatophore connected to the spermatophore reservoir and onward to the female genital area, a structure essential for sperm transfer that had gone unrecorded in both the original 1922 description and a 2003 redescription from Brazil.

The males, smaller at roughly 1,661 to 1,964 micrometres in total length, received equally thorough treatment. Their antennules are five-segmented and armed with eight setae of varying sizes plus an aesthetasc, while their antennae are bi-segmented with a strongly curved distal claw. The male mandible emerges from within an elongated labrum as a long blade bearing a small tooth, and the fourth legs extend as long as the trunk itself. These appendage-level details matter enormously in copepod taxonomy, because species boundaries within the genus Lernanthropus, which contains 103 described species worldwide, often hinge on subtle differences in the shape and armature of legs, mouthparts and genital structures. By documenting features absent from earlier accounts, the authors have provided the anatomical vocabulary needed for future comparative and phylogenetic studies across the family Lernanthropidae.

On the molecular side, the team extracted total genomic DNA from egg sacs and amplified a partial sequence of the mitochondrial cytochrome c oxidase subunit I gene, the workhorse barcode marker for animal species identification, using the classic Folmer primers LCO1490 and HCO2198. Four isolates of L. caudatus were successfully sequenced, one from sheepshead at 662 base pairs, one from grass porgy at 652 base pairs, and two from pinfish at 629 to 671 base pairs. The new sequences were deposited in GenBank under accession numbers PZ844629 through PZ844632 and compared against published sequences of 18 isolates spanning five genera of the family Lernanthropidae, with the caligid copepod Caligus cybii serving as the outgroup.

The phylogenetic analysis, performed with Maximum Likelihood methods in RAxML on the CIPRES Science Gateway under the best-fitting TIM2 plus I plus G substitution model selected by the Akaike Information Criterion, delivered a clear verdict on species identity. The four isolates of L. caudatus clustered together in a strongly supported monophyletic clade with 99 percent bootstrap support, and their average intraspecific genetic divergence was just one percent. Between L. caudatus and its closest relative in the dataset, L. tylosuri, the genetic distance jumped to 21 percent, and against all other species of Lernanthropus it ranged from 20 to 24 percent. These deep genetic gulfs, far exceeding the shallow within-species variation, confirm that the Yucatán specimens belong to a distinct biological lineage and demonstrate the power of DNA barcoding to anchor century-old morphology-based names in a modern molecular framework.

Not everything in the tree resolved neatly, however, and the authors are candid about the limitations. The cox1 phylogeny showed very low resolution overall and recovered the genus Lernanthropus as paraphyletic, a reflection of how sparse the genetic library remains for this group. Only a fraction of the family’s diversity has been sequenced, and no species of Lernanthropus has yet been characterized for the nuclear ribosomal marker 28S, which the authors recommend as a priority for future work. Interestingly, the related genus Lernanthropinus did emerge as a well-supported clade with moderate nodal support of 86, even though only three of its ten valid species have been sequenced, with interspecific divergences among those three ranging from 14 to 23 percent. The message is clear: without more representative sampling, the evolutionary relationships and classification of these parasites will remain murky.

The redescription also sharpened the diagnostic boundaries between L. caudatus and its closest look-alikes in the wider Caribbean and Gulf of Mexico region. Lernanthropus caudatus can be readily distinguished from L. longilamina, which also parasitizes sheepshead in the Bahamas, by the narrow anterior body region just below the cephalothorax, the shorter dorsal plate relative to body length, and the fourth legs extending beyond the distal margin of the dorsal plate. Against L. amplitergum, known from pinfish in Florida, the differences are equally decisive: L. caudatus has an elongated cephalothorax bearing lobes lateromedially rather than a suborbicular one, a posterior cephalothoracic region that is wider with an anterior projection, and uropods that extend beyond the dorsal plate rather than falling short of it. Such comparisons underscore why exhaustive anatomical documentation remains indispensable even in the genomic era.

Yet the study also revealed meaningful geographic variation within the species itself. Compared with specimens redescribed from Brazil in 2003, the Yucatán animals show a handful of differences: the Brazilian females reportedly possess a fifth leg, which is absent in the Mexican material; the caudal branches are not visible in dorsal view in the Brazilian specimens but are easily observed in the Yucatán ones; the endopod of the second leg lacks a distal spine in Brazil but bears one in Mexico; and the fourth legs are relatively smaller in Brazilian material from red porgy. The authors interpret these discrepancies as intraspecific morphological variability arising from geographic distance and population isolation, noting that while the host fishes extend their range southward to Brazil, the parasite has only been confirmed on a limited set of sparid hosts, and host association combined with isolation may be shaping local morphological divergence.

Beyond its taxonomic significance, the research fills a conspicuous gap in Mexican parasitology. Very few species of Lernanthropus have been reported from Mexican marine waters, with three known from the Pacific coast and only two from the Atlantic seaboard, and no species of the genus had ever been recorded from sparid fish in Mexican waters until now. By uniting classical taxonomy with molecular systematics, the study sets a template for how obscure, long-named parasites can be dragged into the twenty-first century, one gill at a time. As the authors conclude, far more representative sequencing across the Lernanthropidae, ideally including ribosomal markers, will be needed to untangle the true evolutionary tree of this remarkably diverse but poorly sampled lineage of fish parasites.

Subject of Research: Integrative taxonomic redescription and first molecular characterization of the parasitic copepod Lernanthropus caudatus from sparid fishes of the Yucatán Peninsula

Article Title: Redescription and First Molecular Characterization of Lernanthropus caudatus Wilson, 1922 (Copepoda: Lernanthropidae): A Comparative Integrative Analysis

Article References: Espínola-Novelo, J. F., Pérez-Ortega, B. F., Castro-Romero, R., & Pérez-Ponce de León, G. (2026). Redescription and First Molecular Characterization of Lernanthropus caudatus Wilson, 1922 (Copepoda: Lernanthropidae): A Comparative Integrative Analysis. Acta Parasitologica, 71(5), Article 207. https://doi.org/10.1007/s11686-026-01383-1

Image Credits: AI Generated

DOI: 10.1007/s11686-026-01383-1

Keywords: Lernanthropus caudatus, Copepoda, Lernanthropidae, fish parasites, cox1 DNA barcoding, Yucatán Peninsula, sparid fish, morphology, phylogenetics, redescription, Gulf of Mexico, Acta Parasitologica