Sensory Papillae Research Articles

Evolution of novel sensory organs in fish with legs

How do animals evolve new traits? Sea robins are fish that possess specialized leg-like appendages used to "walk" along the sea floor. Here, we show that legs are bona fide sense organs that localize buried prey. Legs are covered in sensory papillae that receive dense innervation from touch-sensitive neurons, express non-canonical epithelial taste receptors, and mediate chemical sensitivity that drives predatory digging behavior. A combination of developmental analyses, crosses between species with and without papillae, and interspecies comparisons of sea robins from around the world demonstrate that papillae represent a key evolutionary innovation associated with behavioral niche expansion on the sea floor. These discoveries provide unique insight into how molecular-, cellular-, and tissue-scale adaptations integrate to produce novel organismic traits and behavior.

Ancient developmental genes underlie evolutionary novelties in walking fish

A critical question in biology is how new traits evolve, but studying this in wild animals remains challenging. Here, we probe the genetic basis of trait gain in sea robin fish, which have evolved specialized leg-like appendages for locomotion and digging along the ocean floor. We use genome sequencing, transcriptional profiling, and interspecific hybrid analysis to explore the molecular and developmental basis of leg formation. We identified the ancient, conserved transcription factor tbx3a as a major determinant of sensory leg development. Genome editing confirms that tbx3a is required for normal leg formation in sea robins, and for formation of enlarged central nervous system lobes, sensory papillae, and adult digging behavior. Our study establishes sea robins as a model organism for studying the evolution of major trait gain and illustrates how ancient developmental control genes can underlie novel organ formation.

Taxonomy of freshwater gobies of the genus Rhinogobius (Oxudercidae, Gobiiformes) from central Vietnam, with descriptions of two new species

Taxonomy of the genus Rhinogobius was investigated for the first time in Central Vietnam. Five species occurring in this region can be distinguished by shape of their head, number of pectoral-fin rays, vertebral counts, arrangements of scales and cutaneous sensory papillae, and coloration. Two species are described as new, Rhinogobius rong sp. nov. and R. nami sp. nov., and the remaining three were identified as R. taenigena, R. leavelli, and R. similis. Though the morphology of R. leavelli and R. similis differed slightly between localities, counts and measurements overlapped, and coloration was variable. This study tentatively identifies all of those populations as R. leavelli or R. similis, but further research using additional specimens from a wider geographic range is required to clear the identity of the Rhinogobius occurring in this region. Rhinogobius rong, R. similis, and R. leavelli are distributed in both Da Nang City and Thua Thien Hue Province, with the first two species occurring widely in coastal areas, and R. leavelli restricted to inland areas. On the other hand, Rhinogobius nami and R. taenigena occur only in inland areas of Da Nang City and Thua Thien Hue Province, respectively.

Lethal effects of praziquantel and albendazole, on the cercariae of Echinochasmus sp. (Dietz, 1909) in-vitro.

Echinochasmidae are considered one of the digenean intestinal parasites of carnivorous mammals and humans. Some larvicidal medications, such as praziquantel and albendazole, were employed to interrupt the life cycle of Echinochasmidae, which may cause harmful and serious effects on the domestic fish, ducks, and humans in our ecosystem. Cercariae of Echinochasmus sp. (gymnocephalus type) were harvested by exposing snails to strong artificial illumination. The emerging cercariae were exposed in vitro to different concentrations of praziquantel and albendazole at the same period of incubation 12 h. Using probit analysis in SPSS version 25, the lethal concentrations 50 and 95% were determined. They were 0.036 and 0.82 ppm, respectively, for praziquantel and 5.3 and 9.2 ppm, respectively, for albendazole. The ultrastructural changes using scanning electron microscope on the tegumental surface of the treated cercariae with the two drugs were compared to the untreated cercariae. The untreated cercariae have a pear-shaped body with a long tail. The oral sucker is armed with a spiny collar and decorated with ciliated and unciliated sensory papillae. The cardinal ventral sucker has a thick, muscular wall. The cercarial tail is decorated with parallel longitudinal tegumental processes and spherical, unciliated papillae. In comparisons, cercariae treated with both drugs lost all healthy morphological features, but in varying degrees and effects between the two drugs. Our findings suggest that the use of both drugs can be recommended during the design of control strategies to combat this type of intestinal parasite.

New Data on the Larval Stages of Leptophallus nigrovenosus (Digenea, Plagiorchiata).

(1) Background: Leptophallus nigrovenosus, an esophageal parasite that primarily affects water snakes of the genus Natrix, has a known life cycle that involves snail and amphibian hosts. However, the biological aspects, chaetotaxic patterns, and pathogeny of this parasite in its hosts have not been fully elucidated. (2) Methods: Snails (Planorbarius metidjensis) were collected in Spain and examined for cercaria emergence. The larvae were used to experimentally infect Salamandra salamandra, and metacercariae were isolated. Their chaetotaxy was studied using microscopy and scanning electron microscopy. The eye histology was also examined. (3) Results: The cercariae displayed distinctive morphological characteristics. The results of this study revealed three types of ciliated sensory papillae on the cercarial teguments, suggesting an adaptation for host detection and orientation. The metacercariae isolated from subcutaneous tissues showed oval bodies covered in spines. The chaetotaxy patterns matched those of Leptophallinae species. This is the first report of the presence of L. nigrovenosus in the snail P. metidjensis. Additionally, this study detected metacercariae in the eyes of S. salamandra, emphasizing the need for further research on trematode infections in amphibian eyes. (4) Conclusions: Members of the genus Salamandra can serve as secondary intermediate hosts for L. nigrovenosus, and the presence of metacercariae in amphibian eyes may have implications for the survival and habitat management of these amphibians. Understanding this parasite's prevalence, transmission dynamics, and impacts on host populations is crucial for conservation strategies.

Microanatomy and ultrastructure of the nervous system of adult Renicola parvicaudatus (Digenea: Renicolidae).

The digenean complex life cycle includes various morphological forms with different locomotory and behavioral activities, and the functional specialization of their nervous system is of importance for the transmission of these parasites. Adult digeneans acquire many adaptive features associated with the final settlement in a vertebrate host. Our study describes the general morphology and ultrastructure of the nervous system of the adult renicolid digenean Renicola parvicaudatus parasitizing the renal tubules of herring gulls. Using immunocytochemical and electron microscopic methods, we identified the distinctive characteristics of ganglia and synapses in the studied species. A comparative analysis of the organization of the nervous system of adult individuals and their continuously-swimming stylet cercariae revealed a number of stage-related differences in the composition of ganglia, the distribution of serotonin- and FMRFamide-immunoreactive neurons, the cytomorphology of neuron somata and free sensory endings. Thus, in adults, the presence of FMRFamide-positive neuron somata, accessory muscle bundles in the ganglionic cortex, and eight types of neuronal vesicles was detected, but no glia-like elements were identified. Their neurons are characterized by a larger volume of cytoplasm and also show greater ultrastructural diversity. Although the sensory papillae of adults do not vary in their external morphology as much as those of larvae, their sensory bulbs are more diverse in cytomorphology. Following our previous data on the "support" cell processes related to various tissues of the larvae and considered as glia-like structures, we also briefly present the identified features of the parenchyma, attachment organs and excretory system of adult individuals. The excretory system of adult R. parvicaudatus is characterized by the presence of unique terminal cells with several flame tufts, which are not typical either for the larvae of this species or for other digeneans studied so far. We also used molecular phylogenetic analysis to clarify species identification.

Morphology Features and Microanatomy of The Lingual Papilae of The Eonycteris Spelaea (Dobson, 1871): Scanning Electron Microscpy and Light Microscopy

Background: Eonycteris spelaea is a sizable nectar-feeding bat that falls within the taxonomic classification of Order Chiroptera and Family Pteropododae. The form and structure of the tongue play a crucial role for bats in the intake and digestion of food in their mouth. Each papilla's morphology, dimensions, spatial arrangement, and physiological role exhibit variations among different animal species, contingent upon their respective lifestyles. Aim: This research attempts to examine the morphology and microstructure of the Eonycteris spelaea's lingual papillae, collected from Timor Island, East Nusa Tenggara, Indonesia. Methods: This study will achieved a scanning electron microscope and a light microscope in the presence of haematoxylin eosin staining and employed a sample of 6 sexually indiscriminate adult Eonyteris spelaea bats. Results: The tongue of Eonycteris spelaea separated into three distinct parts: the apex, corpus, and radix. The structure's apex contains filiform papillae, which come in many varieties, such as scale-like filiform papilla, enormous trifid papilla, and little crown-like papilla. Additionally, there is a cluster of fungiform papillae on the outside edge of the highest point and transitional papillae connecting the large trifid papillae with the smaller crown-shaped papillae. The corpus section comprises two papilla types: filiform papilla (leaf-shaped filiform papilla and big crown-shaped papilla) and fungiform papilla. The radix comprises the elongated conical papilla, rosette-shaped filiform papilla, short conical papilla, transitional papilla, and three circumvallate papillae at the back of the tongue. Conclusion: Lingual papillae E. spelaea comprise wide variety of mechanic papillae and also sensory papillae which specific dietary regimen on their living habitat.

A NEW SPECIES OF THE BRISTLETAIL GENUS <i>CTENOLEPISMA</i> (ZYGENTOMA, LEPISMATIDAE) FROM SOUTHWESTERN KAZAKHSTAN

A new bristletail species is described from Kazakhstan: Ctenolepisma capsicum sp. n. It seems to be especially similar to such congeners common in Kazakhstan and Central Asia as C. mauritanicum (Lucas 1846), C. turkestanicum Kaplin Shakula 2022 and C. przewalsky Kaplin 1982, which all share a medium-sized and weakly to moderately pigmented body, two pairs of abdominal styli, the absence of bristle combs from abdominal sternites I and II, and of medial combs from all abdominal sternites; and urosternites III–VIII and the posterior edge of thoracic tergites each with 1 + 1 sublateral bristle combs. The first tergite of their abdomen bears 1 + 1 lateral, the second 2 + 2 or 3 + 3, terga III–VII each with 3 + 3, and tergite VIII with 2 + 2 bristle combs. These species differ well from each other in the number of sensory papillae on the apical article of the labial palps, the length ratios of the distal part of the prosternum occupied by bristle combs, and in the total length of the prosternum, the chaetotaxy of the second tergite of the abdomen, thoracic sternites, and the number of ovipositor articles.

A revision of the gudgeon genus Hypseleotris (Gobiiformes: Gobioidei: Eleotridae) of northwest Australia, describing three new species and synonymizing the genus Kimberleyeleotris.

Species within the northwest Australian clade of Hypseleotris (six species) and the genus Kimberleyeleotris (two species) are reviewed following the recording of new populations in the region and a molecular study of the group that identified three undescribed candidate species. Based on the analysis of extensive morphological and nuclear and mitochondrial molecular datasets, Kimberleyeleotris is here formally synonymised with Hypseleotris. Furthermore, three species from the Kimberley region, Western Australia, are described to science: Hypseleotris maranda sp. nov., Hypseleotris wunduwala sp. nov., and Hypseleotris garawudjirri sp. nov. The presence of, or number of scales across the head and body, the pattern of sensory papillae on the head, fin ray counts, dorsal and anal fin colouration (particularly in breeding males), and body depth, can be used to distinguish the members of the northwest Australia lineage. Furthermore, the newly described species were genetically separated from all northwest Australian congeners by K2P distances ranging from 7.8-11.3% based on the CO1 gene, and 7.7-16.3 % based on the entire mitochondrial genome. Two of the new species, H. maranda sp. nov. and H. wunduwala sp. nov., have extremely narrow ranges being found in single sub-catchments of the Roe and King Edward Rivers respectively. On the other hand, H. garawudjirri sp. nov. is moderately widespread, being found across the Charnley, Calder, and Sale rivers. While the conservation risk to H. maranda sp. nov. and H. wunduwala sp. nov. is inherently high due to their small range, there are currently no obvious local threatening processes to either of these species given their remote locations that are little impacted by human activities.

Grey parrot (Psittacus erithacus) beak papillae and nerves identified using novel 2-D and 3-D imaging modalities.

The avian beak is a complex organ containing bone, neurovascular tissue, and keratinized covering (rhamphotheca). Nerve-rich papillae extend through bone into rhamphotheca providing sensory input from the beak tip. Beak trimming is a common procedure in avian species and is used for corrective, cosmetic, and behavioral modification purposes. Yet, practitioners are not well versed in complete beak anatomy, and therefore, beak trimming often disregards neurovasculature, injuring the patient and hampering recovery. Here, using comprehensive anatomical description, we aim to provide recommendations on how to safely perform beak trimming without damaging underlying sensory papillae. Here, we evaluated beaks of 2 deceased grey parrots (Psittacus erithacus). In one, we used a novel stain and microcomputed tomography to visualize papillae in the upper and lower beaks. In a second, we hand isolated the upper and lower beak dermal papillae and used high-resolution photography plus traditional paraffin histology. Papillae and their nerves were easily identified in these 2- and 3-dimensional approaches. This allowed us to determine the approximate lengths of papillae within the upper and lower beak. Based on these findings, the authors recommend lateral radiographs of the bird's head and beak to identify the location of the underlying bone relative to the overlying rhamphotheca before performing beak trims. Specifically in grey parrots, the authors recommend the upper and lower beak should not be trimmed closer than 8 to 10 mm from the underlying bone. Further work is needed to support these recommendations and provide guidelines for other species.

Morphology, molecular systematics and phylogeography of the Spine-cheek Gudgeon, Eleotris acanthopomus (Teleostei: Eleotridae) from the north-western Indian Ocean

General morphology, cephalic sensory papillae pattern, postcranial osteology, phylogenetic placement, phylogeographic pattern, and distribution of Eleotris acanthopomus Bleeker, 1853 (Teleostei: Gobiiformes: Eleotridae) from the north-western Indian Ocean (Arabian Sea) are described. The cephalic papillae pattern follows the ‘open’ opercular pattern, and the cheek pattern ‘2.4’. The vertebral column includes 10 abdominal and 14–15 caudal vertebrae, for a total vertebral count of 24–25. Corroborating the morphological results, DNA barcoding based on mitochondrial COI confirmed that the specimens collected from the Arabian Sea coastal area are conspecific with E. acanthopomus from other Indo-Pacific localities. Eleotris acanthopomus from the Arabian Sea shows the closest phylogenetic relationship to E. pellegrini (11.9% K2P genetic distance). The presence of E. acanthopomus in the coastal area of the Arabian Sea represents the first record of this genus and species for the northern Indian Ocean and markedly expands its known geographical distribution range. A preliminary phylogeographic analysis of the newly collected E. acanthopomus haplotype from the Arabian Sea and archived haplotypes from other Indo-Pacific localities suggests that extensive dispersal occurs among regional populations, most probably during the pelagic larval stage.

In Vitro Anthelmintic Effect of Ferula assa-foetida Hydroalcoholic Extract Against Flukes of Fasciola hepatica and Dicrocoelium dendriticum

Background: Dicrocoeliasis and fascioliasis are foodborne parasitic diseases of the biliary tract, resulting from Dicrocoelium dendriticum and Fasciola hepatica causing extensive financial losses and serious health problems in ruminants. Due to low-performance medications, drug delivery is a tremendous project to improve interventions available for these diseases. Objectives: This study aimed to determine the anthelmintic properties of Ferula assa-foetida extract against F. hepatica and D. dendriticum using in vitro assay. Methods: The effects of diverse concentrations of F. assa-foetida extract (400 - 1000 µg/mL) for 12-24 hours were examined for the treatment of D. dendriticum and F. hepatica. The anthelmintic efficacies were evaluated using scanning electron microscopy (SEM). The MTT assay was carried out to evaluate the cell viability of all cells in culture media. Results: The SEM images of treated worms by F. assa-foetida extract (200 µg/mL) confirmed excessive damage, which included an entire lack of sensory papillae and destruction of distinguished network structures and tegument vesicles. Variables of duration and concentration presented a considerable effect on both the mortality rate and the anthelmintic properties of F. assa-foetida; accordingly, as the time and concentration increased, the mortality rate became higher. Based on the MTT assay, the toxicity of F. assa-foetida at 800 µg/mL concentration was 8.7%. Therefore, it can be argued that F. assa-foetida had anthelmintic properties. Conclusions: This is the first study that evaluated the impact of F. assa-foetida on liver flukes of D. dendriticum and F. hepatica. Therefore, it paved the way for further studies on the control of those trematodes. It is recommended to document and look into the indigenous understanding of viable medicinal plants to provide evidence for their use.

Revisiting Acanthocephalus dirus (Van Cleave, 1931) (Acanthocephala: Echinorhynchidae) Using SEM and Energy Dispersive X-Ray Analysis (EDXA) for the First Time

Acanthocephalus dirus (Van Cleave, 1931) Van Cleave and Townsend, 1936 is the most widely distributed and studied species of acanthocephalans in North America. Acanthocephalus jacksoni Bullock, 1962 from New England and Acanthocephalus parksidei Amin, 1975 from Wisconsin are junior synonyms. Line drawings were used in these and all subsequent descriptive accounts since. We are reporting SEM images of A. dirus for the first time from 2 Wisconsin populations from Catostomus commersoni Lacépède showing new details that could not have been seen only with optical microscopy. The new SEM observations include new features that were not previously reported such as apical sensory pores, micropores variably distributed on the trunk and on the proboscis and hooks, hooks of variable size, and bursa with 2 circles of sensory papillae and 2 pedunculated suckers. We are also providing a detailed elemental analysis of middle hooks using Energy Dispersive x-ray analysis (EDXA) showing high levels of sulfur in all samples especially hook tips.

Oropharyngeal morphological aspects of Arapaima gigas (Schinz, 1822)

The study of the functional anatomy of the digestive system of fish, in particular the oropharyngeal cavity, is of great importance because it allows inferences about the feeding habit, mechanisms of capture, selection, and processing of food carried out by different species. Thus, the aim of this study was to describe the anatomical adaptations of the oropharyngeal cavity of the pirarucu (Arapaima gigas Schinz, 1822) using scanning electron microscopy (SEM) techniques. The oropharyngeal cavity of six specimens of pirarucu was collected in juvenile phase, from Aquaculture Research Center at the Universidade Federal de Rondônia (UNIR), created for commercial purposes. The anatomical pieces were fixed in 10% buffered formalin and processed for SEM analysis. Anatomically, the oropharyngeal cavity of the pirarucu is composed of five pairs of branchial arches, apical portion of the tongue, floor of the tongue, lower pharyngeal area, and upper pharyngeal plate. In SEM, we observed that the mucosa of the apex of the tongue and the upper pharyngeal roof have a smooth texture and are covered by squamous cells with numerous small openings scattered over the surface. The portions of the floor of the tongue and the lower pharyngeal area, on the other hand, have adaptations in the form of a projectile and numerous sensory papillae, giving a rough texture to the region. Thus, the oropharyngeal cavity of pirarucu is adapted for the capture, apprehension, and swallowing of its prey, with signs of carnivory.

Female preferences for the spectral content of advertisement calls in Cope's gray treefrog (Hyla chrysoscelis).

Amphibians have inner ears with two sensory papillae tuned to different frequency ranges of airborne sounds. In frogs, male advertisement calls possess distinct spectral components that match the tuning of one or both sensory papillae. Female preferences for the spectral content of advertisement calls can depend on signal amplitude and can vary among closely related lineages. In this study of Cope's gray tree frog (Hyla chrysoscelis), we investigated the amplitude dependence of female preferences for the spectral content of male advertisement calls, which have a "bimodal" spectrum with separate low-frequency (1.25kHz) and high-frequency (2.5kHz) components. In two-alternative choice tests, females generally preferred synthetic calls with bimodal spectra over "unimodal" calls having only one of the two spectral components. They also preferred unimodal calls with a high-frequency component over one with the low-frequency component. With few exceptions, preferences were largely independent of amplitude across both a 30dB range of overall signal amplitude and an 11dB range in the relative amplitudes of the two spectral components. We discuss these results in the context of evolutionary lability in female preferences for the spectral content of advertisement calls in North American tree frogs in the genus Hyla.

Records of the eel-goby, Taenioides snyderi (Actinopterygii: Gobiiformes: Gobiidae), from Taiwan

Four gobiid specimens collected from the western coast of Taiwan were identified as Taenioides snyderi Jordan et Hubbs, 1925, being characterized by 51–55 (total) dorsal-fin rays, 44–46 (total) anal-fin rays, 17–19 pectoral-fin rays, 10 + 21 = 31 vertebrae, 3 paired barbels on the chin surface, the pelvic-fin base fused by membrane to the abdomen, and the pattern of the dermal folds with sensory papillae on the head and body. Taenioides snyderi has been recorded to date only from Japanese waters, although the likelihood of a wider distribution has been suggested by molecular analysis. This study represents the first specimen-based records of the species outside Japan, with data adding to our knowledge of a poorly known fish group.

Microscopic Observations on Form and Structure of the Worm Enchytraeus buchholzi (Clitellata: Enchytraeidae)

BackgroundThe worm Enchytraeus buchholzi is a new record species for Shaanxi, China, and a key pest on American ginseng Panax quinquefolium. To distinguish the species, the authors prepared its whole mounts and paraffin-embedded sections, and microscopically observed, photographed and measured. Besides, we conducted an experimental study on its DNA barcode. ResultsCells, tissues and organs related to nervous, digestive, circulatory, excretory and reproductive systems were found, photomicrographed and described, including: prostomium, peristomium, segments, clitellum, pygidium, lateral and ventral chaetae; brain, cranial nerves, sensory papillae, ventral nerve cord; pharyngeal pad and glands, retractor muscles and muscular bundles, peptonephridia, esophagus, intestine; dorsal, lateral, ventral and intestinal parietal vessels, coelomocytes, coelomic cavity; nephridia, chloragogen cells; ovaries, groups of germ cells with developing oocytes, mature eggs, spermathecae; testes, seminal vesicles, sperm funnels, penial bulbs. Their shapes and sizes were given, and functions discussed briefly. The visual effect of staining specimens with hematoxylin plus eosin ranked the first, and that with acetocarmine the second. ConclusionsThe supplementary and objective descriptions, with the microphotographs as forceful pieces of evidence, have expanded biological knowledge in aspects of the form, structure and function of the worm, which is helpful for professionals to recognize and understand this species and provide a solid basis for its integrated pest management.

Neodontobutis lani, a new sleeper fish of the family Odontobutidae (Teleostei: Gobiiformes) from Guangxi, southern China.

A new species, Neodontobutis lani (Odontobutidae) is described from the Zuojiang River, a tributary of the Xijiang River of the Pearl River basin, at Longzhou Town, Guangxi Zhuang Autonomous Region, Southern China. This species can be distinguished from other Neodontobutis species by following characters: anterior head flat, with interorbital width / eye diameter = 1.41.9 (vs. less than 1.4); several rows (vs. single row) of transforming ctenii on posterior edges of body scales; sensory papilla on lower jaw arranged in two oblong clusters (vs. two single lines). It can be distinguished from Odontobutis species by: separated right and left gill membrane (vs. joined); barbel-like projection present on sensory papillae. Molecular phylogenetic analysis of 2,076 nuclear coding loci indicates that N. lani is a sister species of N. hainanensis, the only Neodontobutis species that has been described from China.

Redescription of the goby Glossogobius tenuiformis Fowler, 1934 (Teleostei: Gobiidae) and assignment of Oman Glossogobius populations: a morpho-molecular approach.

Glossogobius giuris was originally described as Gobius giuris from the Ganges River, India. However, based on the uncertainty of its type locality and its apparent widespread distribution, a comprehensive study is being carried out to define the correct taxonomic status of this taxon. The South African population of the Glossogobius from St. Lucia Lake, KwaZulu-Natal was described as Glossogobius tenuiformis by Fowler in 1934. It has usually been considered as Glossogobius giuris but Hoese and Hammer indicated that it was a distinct species (2021: 83). Here, based on morphological characters and a molecular data set, we redescribe Glossogobius tenuiformis and assign the Oman populations of Glossogobius to this species. Glossogobius tenuiformis is distinguished from G. giuris by the absence or very short (and often only a few papillae long) sensory papilla line 6; presence of single rows of the sensory papillae in each cheek line, 1216 pre-dorsal scale counts, and 1+1+911, usually 1+1+10, gill rakers on outer face of first arch. Glossogobius tenuiformis is also distinguished by molecular characters. Glossogobius tenuiformis shows 16% genetic distance with other members of this species complex, that includes G. laticeps from Vietnam, Bangladesh and China, G. giuris from South Africa and India and giuris C from India, Myanmar, Nepal, Bangladesh and Vietnam, and the highest genetic distance (24%) with G. circumspectus.

Search, find, and penetrate: ultrastructural data of furcocercariae of Cardiocephaloides longicollis (Digenea, Strigeidae) explain their transmission and infection strategy into fish hosts.

The present study provides an overview of the structures linked to fish host finding, recognition, and invasion of one of the most commonly occurring morphotypes among trematodes, furcocercariae. For this, we use free-swimming cercariae of the strigeid Cardiocephaloides longicollis (Rudolphi 1819) Dubois, 1982. Their elongated cercarial body and bifurcated tail are covered by a tegument with an irregular surface, showing numerous folds arranged in different directions and a typical syncytial organization. Both the body and the bifurcated tail are covered with short spines, rose-thorn shaped, as well as four types of sensory papillae, distinguished by the presence or absence of a cilium, its length, and their position on the cercarial body. These papillae are especially important for free-living stages that rely on external stimuli to locate and adhere to the host. A specialized anterior organ is located at the anterior part of the cercariae and is encircled by a triangle-shaped group of enlarged pre-oral spines followed by a transverse row of enlarged post-oral spines that, together with the sensory papillae, allow active finding, recognition, and penetration into fish. The ventral sucker, covered with inner-oriented spines, sensory papillae, and cilia, helps during this process. The cercariae of C. longicollis possess three types of gland cells (a head gland and two types of penetration glands), each containing different types of secretory granules that play a role in host invasion. The protonephridial excretory system consists of an excretory bladder, a system of collecting tubules, flame cells, and two excretory pores in the middle of each furcae, which serve to control osmoregulation in their marine environment, as well as to eliminate metabolic waste. Together with the four types of sensory endings, the central ganglion forms the nervous system. Our results add novel information on the ultrastructure of strigeid furcocercariae, being essential to interpret these data in relation of their functional role to better understand the transmission and penetration strategies that cercariae display to infect their fish hosts.