Paula Marcotegui

The impact of climate change on the distribution and sustainability of pelagic fish remains poorly understood, even though it can cause drastic shifts in species distribution, reproduction, migratory behavior, and recruitment. In turn, parasites can reveal important details about the biology of their hosts and the changes they undergo. The aim of this study was to evaluate whether changes occurred in the parasite community structure of the Argentine anchovy, Engraulis anchoita, after three decades, during which environmental changes have been documented in the Argentine Sea. Parasite assemblages of anchovies captured throughout their distribution in the Argentine Sea in 2022 were examined and compared with those of anchovies collected in the same regions between 1993 and 1995. Multivariate analyses were performed by assigning fish to three spatial groups (belonging to two different stocks), identified from the first sampling period (Northern Buenos Aires, Southern Buenos Aires, and Patagonian). The dataset included 42,290 metazoan parasites belonging to 13 species. Samples differed significantly among groups and between periods. A decrease in parasite abundances was observed for most species across all three groups, resulting in similar patterns of dissimilarity between periods. The congruence of these changes across three groups over a broad geographic scale suggests that such variations are likely driven by large-scale processes, possibly climate change, rather than by random, short-term, cyclical, or local variability.

It is often postulated that natural systems are expected to suffer an increasing risk of infectious disease outbreaks as climate change accelerates. In the northern Argentine Sea, the rise of ocean temperature has produced a tropicalization of demersal megafauna since 2013. This rapidly warming hotspot provides an excellent model to test whether fish parasites have increased, declined, or remained stable in the region. Cardiocephaloides physalis a parasite of penguins Spheniscus magellanicus as adult and suspected to parasitize anchovies Engraulis anchoita as larvae is here used to compare their occurrence and abundance between samples composed by 1752 fish of variable age caught at different latitudes during 1993–1995 and 2022 and between 20 juvenile birds and literature data. In the present work, the identity of metacercariae as C. physalis is confirmed genetically, as well as a net decline of population parameters of the parasite to its effective disappearance in anchovies from northern areas and to extremely low levels in fish from southern regions and penguins. After analysing possible causes for such changes in a scenario of rapid regional tropicalization, a direct effect of increasing temperature on parasites arose as the main causal candidate for the observed decline in their populations over the last decades. Beyond the biological and ecological consequences of global change on them, parasites offer excellent systems for measuring and monitoring such effects. The almost local extinction of C. physalis in a marine hotspot of global warming seems to be one of the first examples of such processes.

A new species of Ceratomyxa (Ceratomyxidae, Myxosporea) was found infecting the gall bladder of the Argentine croaker Umbrina canosai Berg 1895 (Sciaenidae, Perciformes) from the Argentine sea. Using an integrative taxonomic approach that combines morphological, bioecological, and molecular analyses, we provide evidence that clearly differentiates this species from known taxa and formally describe Ceratomyxa fialai as a new species. This study is the first to apply landmark-based geometric morphometrics (GM) in myxozoan research, providing a detailed analysis of conspecific morphometric variation of ceratomyxid myxospores, examining their natural variation within and among different ceratomyxids infecting the gall bladder of U. canosai. Using GM analyses, we successfully capture and quantify phenotypic variation at the organismal level. Our results suggest that myxospore shape variation may be driven by both developmental noise and phenotypic plasticity. The work highlights the utility of GM in advancing the understanding of myxozoan morphology and its evolutionary implications and emphasizes the need for further research on myxospore shape evolution and its ecological and adaptive significance in natural populations.

In this study, we compared the efficacy of geometric morphometric techniques, including outlines and landmark-based approaches, to support the differentiation of Trichodina bellottii from three co-occurring killifish species. Both methods were able to differentiate trichodinids from different host species. However, discriminat analyses and MANOVA results based on landmarks had greater accuracy possibly because these analyses only provide information on certain points defined by the researcher, while the analyses based on outlines take into account points with less taxonomic information.

Trichodina centrostrigeata Basson, Van As et Paperna, 1983 from Oreochromis mossambicus (Peters) and O. niloticus (Linnaeus) from different host populations from Argentina, Mexico and South Africa was reviewed. Although T. centrostrigeata has a distinct denticle structure that makes morphological taxonomic inferences uncomplicated, variation of the denticles within and among individuals and populations were still observed. While traditional taxonomy of mobilines is heavily reliant on morphometrics, and recently even more so on molecular analysis, this paper proposes the use of geometric morphometry, specifically elliptical Fourier analysis, to address morphological conflicts that arise when comparing different populations. By applying this technique, combined with traditional taxonomy, it was found that T. centrostrigeata in this study can be grouped into two separate morphotypes, the first (type a) from aquaculture farms in Argentina and Mexico and the second (type b) from a natural habitat in Glen Alpine Dam, South Africa. This study supports the validity of geometric morphometry as an additional technique to distinguish not only between species but also evolutionary plasticity of the same species from different localities and habitats.

The finding of Pomphorhynchus sphaericus in new localities from La Plata River allowed the reevaluation of the species using a taxonomic integrative approach. The newly found specimens in Pimelodus maculatus from Samborombon Bay differ from P. sphaericus by the roots of hooks 1–6 which not form a wide sheet split into 2 apophysis, the slender, separated and equatorial testicles, the position of the cement glands, the shape of the proboscis, the shape and length of lemnisci, and the eggs size. Despite the notorious observed morphological differences, the COI mtDNA analysis confirmed that Pomphorhynchus individuals are the same conspecific, and showed that there is a high phenotypical plasticity in this species. Pomphorhynchus sphaericus is the first South American species analyzed to a DNA level (COI mtDNA, ITS, and 18S rDNA genes). The molecular analysis relates P. sphaericus to P. bulbocolli and P. purhepechus.

Tilapia is the common name for a wide spectrum of cichlid fishes usually selected for aquaculture activities. However, some aspects of the natural history of these fishes, such as the diversity of certain ectoparasitic protistan groups remain understudied. In order to understand the diversity of ciliates of the family Trichodinidae parasitizing tilapia around the world, a database with all available accounts was assembled. This information, along with records derived from our own recent research, allowed us to generate a checklist containing all the records for tilapia-Trichodinidae associations. The checklist is presented as a host-parasite list containing 44 nominal taxa from 29 countries on four continents, and it also presents the first data from Argentina, where tilapia culture is still an incipient, but growing activity. The observed high species richness of trichodinids is probably derived from a set of specific taxa for tilapia, along with an important component of species acquired from the new environments where tilapia have been introduced. Data presented here could be used along with existing information of metazoan parasites to build a general view about diseases that affect tilapia.