The infected source fish came from a floating cage in Tamaulipas, Mexico, where chronic presence of L. floridanus had been reported. At the beginning of the experiment, the five infected juveniles had a mean intensity of 228 ± 86 parasites per fish, based on examination of a sample of five fish. The uninfected fry came from a hatchery considered free of most external parasites, and examination of five fish confirmed the absence of L. floridanus. After two weeks of acclimatization in separate tanks, researchers again checked a sample of 10 uninfected fish and found no parasites immediately before the trial. The infected and uninfected groups were then separated within each experimental aquarium by a 2,000-micrometre mesh screen. Its openings exceeded the reported 307–600-micrometre body length of adult L. floridanus, allowing the parasite to cross the barrier without allowing fish to do so.

The experiment was conducted in triplicate under a continuous flow of well water, with no fish mortality during the 29 days. Mean water-quality measurements remained within a range described as suitable for channel catfish: dissolved oxygen was 6.0 ± 0.50 milligrams per litre, temperature 25.06 ± 1.60 degrees Celsius, un-ionized ammonia 0.0018 milligrams per litre, pH 7.50 ± 0.12, alkalinity 324.0 ± 3.22 milligrams per litre as calcium carbonate, and hardness 297.60 ± 3.80 milligrams per litre as calcium carbonate. Fish were fed a commercial diet twice daily, ad libitum, under a natural light-dark cycle. One uninfected fish was removed from each aquarium on each of 14 collection dates, beginning daily through day 5 and then every three days through the end of the trial. Because sampled fish were not replaced, 42 uninfected fish were examined in total across the three aquaria.

No parasites were found on the gill arches of the uninfected fish during the first two days of cohabitation. On day 3 and at the subsequent sampling times, the proportion of infected fish was 100%, apart from days 23 and 26, when it was 0.66. A sigmoid function provided the best description of prevalence over the full trial, with an r-squared value of 0.90. The observed transition therefore occurred between the day-2 and day-3 samples, when the measured proportion changed from zero to one. The curve summarizes this measured time course, but the sampling interval does not reveal the precise hour at which individual fish became infected. The experiment also cannot separate the contribution of parasite eggs, oncomiracidia or any other transferable stage to the observed rise, because the dimensions of the eggs and oncomiracidia were not available to the researchers.

Mean abundance followed a different pattern from prevalence. It remained at zero on days 0 through 2, rose sharply, and reached its main peak of 127 parasites per fish on day 5. During the 10 days after that peak, abundance fell to eight parasites per fish. In the second half of the experiment, mean abundance fluctuated between 11 and 35 parasites per fish. These observations were described by a dome-shaped response curve with an r-squared of 0.76, whose steep left side represents the early increase in abundance and whose more gradual right side represents the subsequent decline. The two fitted trends distinguish two outcomes within this experiment: the proportion of fish carrying at least one parasite rose rapidly, while the number of parasites per fish peaked early and then varied at lower levels.

Researchers assessed how extensively the parasites occupied the fish’s respiratory surface by removing the gill arches and counting L. floridanus under a stereoscope. They recorded both the total parasite count per fish and the percentage of gill arches carrying parasites. The percentage of infected arches increased asymptotically with mean abundance, and the fitted relationship had an r-squared of 0.98. The data indicated a threshold at approximately 40 parasites per fish above which all gill arches were infected. Abundance exceeded that threshold during roughly days 4 to 14. The result shows a strong association between the number of parasites present and the proportion of available arches colonized; because the experiment did not manipulate attachment space or dispersal, it does not establish which mechanism produced the pattern.

The authors considered host defenses as one possible contributor to the decline in abundance after day 5. Monogenean infection can stimulate immune activity, and teleost gill mucus contains compounds including lysozymes, complement-system molecules, antibacterial peptides and lectins that can degrade or neutralize pathogens. Mucus may also affect the survival of invading monogeneans, potentially reducing an ectoparasite population during later stages of infection. The measured decline is consistent with developing host resistance, but the trial did not directly quantify immune responses, so the results do not establish immune activity as the cause of the reduction. The authors also considered parasite reproductive timing. In related monogeneans, warmer temperatures can shorten egg-hatching periods, and eggs held near the approximately 23 degrees Celsius used in previous examples reportedly hatched in about three days. That comparison may be relevant to the appearance of L. floridanus on day 3, but no egg or oncomiracidium measurements were available for this parasite in the present study.

Increasing occupation of the gills could likewise reflect several processes occurring together. Dispersion across more arches could reduce competition for attachment space among parasites of the same species, while a broader distribution could create more opportunities for encounters associated with reproduction. Monogeneans can exhibit aggregated distributions for reproductive purposes, although some studies have also recorded wider dispersion as abundance increases. Differences in susceptibility among gill microhabitats could contribute to the observed relationship. The asymptotic model captured the measured increase in infected arches as parasite abundance rose, but the experiment did not independently test these proposed biological explanations or identify a causal mechanism for the threshold.

The controlled setting also defines the limits of the findings. The trial used small fry, one measured source of infection and aquaria supplied continuously with well water, rather than fish held in commercial hatcheries or floating cages. It lasted 29 days, and the repeated removal of sampled fish without replacement could have changed local host and parasite densities during the observation period. Although the infected and uninfected groups shared water, the physical barrier and controlled environmental conditions simplified exposure and habitat structure. The fitted sigmoid, dome-shaped and asymptotic models describe the experimental observations with r-squared values of 0.90, 0.76 and 0.98, respectively, but they should not be interpreted as forecasts of infection trajectories in farm ponds or cages.

Within those limits, the experiment provides a short-term quantitative description of L. floridanus infection in channel catfish fry. Prevalence changed from 0% on day 2 to 100% on day 3, mean parasite abundance peaked at 127 per fish on day 5, and the proportion of infected gill arches reached 100% when abundance exceeded approximately 40 parasites per fish. The measured trajectory then shifted from an early outbreak to lower, fluctuating abundance over the second half of the trial. The study identifies rapid population-wide infection, an early intensity peak and increasing occupation of gill habitat as its central findings, while leaving questions about the parasite’s reproductive patterns and longer-term population dynamics for further study.

Subject of Research: Short-term gill-parasite infection dynamics in experimentally exposed channel catfish fry

Article Title: Short-term infection dynamics of the monogenean parasite Ligictaluridus floridanus ( Monogenea : Dactylogyridea : Ancyrocephalidae ) on channel catfish Ictalurus punctatus ( Actinopterygii : Siluriformes : Ictaluridae )

Article References: Sánchez-Martínez, J. G., Montelongo-Alfaro, I. O., Rábago-Castro, J. L., Pérez-Castañeda, R., Vázquez-Sauceda, M. D. L. L., Benavides-González, F., & Loredo-Osti, J. (2025). Short-term infection dynamics of the monogenean parasite Ligictaluridus floridanus (Monogenea: Dactylogyridea: Ancyrocephalidae) on channel catfish Ictalurus punctatus (Actinopterygii: Siluriformes: Ictaluridae). Acta Ichthyologica et Piscatoria, 55, 45-49. https://doi.org/10.3897/aiep.55.144960

Image Credits: AI Generated

DOI: 10.3897/aiep.55.144960

Keywords: Ligictaluridus floridanus, channel catfish, monogenean parasite, gill infection, parasite prevalence, parasite abundance, controlled aquarium trial, host-parasite dynamics, aquaculture, Tamaulipas Mexico, Short-term, infection