Den Finkenbach wieder besucht – wie lange dauert das Larvalstadium des Bachneunauges Lampetra planeri (Bloch, 1784)?

Im November 1989 und von März bis November 1990 wurden im Finkenbach im hessischen Odenwald monatlich Larven des Bachneunauges Lampetra planeri gefangen und vermessen. Ziel war es, anhand von Längenfrequenzen die Anzahl von Altersklassen und damit die Dauer des Larvalstadiums bestimmen zu können. Anhand einer visuellen Analyse der Längenverteilungen wurde 1990 auf eine Dauer des Larvalstadiums von etwas über sechs Jahren, in einigen Fällen vielleicht auch ein Jahr länger, geschlossen. Die 1989/90 erhobenen Daten wurden 2020 mittels generalisierter additiver Modelle erneut ausgewertet. Anhand des Akaike-Informationskriteriums wurde für jeden Monat das Modell bestimmt, das die Anzahl vorhandener Größenklassen am besten beschrieb. Mit diesen Modellen konnten die Einschätzungen von 1990 im Wesentlichen bestätigt werden. Allerdings ist die Annahme, dass das Larvalstadium etwas über sechs Jahre dauert, mit großen Unsicherheiten behaftet. Die Anzahl von Larven in den höheren Längenbereichen ist zu gering, um hier robuste Schlüsse zur Anzahl von Größen- und damit Altersklassen zuzulassen. Weiterhin ist bei anderen Neunaugenarten auch experimentell nachgewiesen, dass die Metamorphose in einem unterschiedlichen Alter einsetzen kann. Das Wachstum von Neunaugenlarven und damit das Alter, in dem die Metamorphose einsetzt, ist von den Verhältnissen in den Gewässern abhängig. Daher wäre es interessanter, den Einfluss von Umweltbedingungen auf das Wachstum in einer Zeit des Klimawandels zu untersuchen, anstatt sich nur auf die Frage des Zeitpunkts der Metamorphose zu beschränken. Finkenbach revisited – how long is the duration of larval life in the Brook Lamprey? Abstract: In November 1989 and from March to November 1990, larvae of the brook lamprey Lampetra planeri were caught and measured monthly in the Finkenbach-River in the Hessian Odenwald in Germany. The aim was to analyse length frequencies to determine the number of age cohorts and thus the duration of the larval life. Based on a purely visual analysis of the length distributions, it was concluded that the larval life lasted a little over six years, and in some cases perhaps a year longer. The data collected in 1989/90 were re-analysed in 2020 with generalized additive models. Using the Akaike information criterion, the model that best described the number of existing size classes was determined for each month. With these models, the assessment was similar compared to the visual analysis in 1990. However, the assumption that the larval stage lasts a little over six years includes a high degree of uncertainty. The number of larvae in the higher length ranges is too low to allow robust conclusions about the numbers of size-cohorts and thus age groups. Furthermore, it has been experimentally shown in other lamprey species that metamorphosis can begin at different ages. The growth of lamprey larvae and thus the age at which they enter metamorphosis depends on environmental conditions. Therefore, it would be more interesting to examine the influence of these conditions on growth in a time of global warming, instead of restricting analyses to the question of the exact age of metamorphosis.

Transcriptomic analysis of gonadal development in parasitic and non-parasitic lampreys (Ichthyomyzon spp.), with a comparison of genomic resources in these non-model species

Lampreys are jawless fishes that diverged ∼550 million years ago from other vertebrates. Sequencing of the somatic and the germline genomes of the sea lamprey (Petromyzon marinus) in 2013 and 2018, respectively, has helped to improve our understanding of the genes and gene networks that control many aspects of lamprey development. However, little is known about the genetic basis of gonadal differentiation in lampreys, partly due to the prolonged period during which their gonads remain sexually indeterminate. We performed RNA-sequencing on gonadal samples from four chestnut lamprey (Ichthyomyzon castaneus) and six northern brook lamprey (I. fossor) to identify differentially expressed genes (DEG’s) and pathways associated with transcriptomic differences in: (1) larvae during early gonadal differentiation versus definitive females (i.e., with oocytes in the slow cytoplasmic growth phase); and (2) females versus definitive males undergoing spermatogonial proliferation. We compared the mapping percentages of these transcriptomes to the two available sea lamprey reference genomes and three annotation files (Ensembl and UCSC for the somatic genome and SIMRbase for the germline genome). We found that mapping the RNA-seq reads to the germline genome gave superior results and, using Trinotate, we provided new putative annotations for 8161 genes in the somatic assembly and 880 genes for the germline assembly. We identified >2000 DEG’s between stages and sexes, as well as biological pathways associated with each. Interestingly, some of the upregulated genes (e.g., DEG’s associated with spermiation) suggest that changes in gene expression can precede morphological changes by several months. In contrast, only 81 DEG’s were evident between the chestnut lamprey (that remains sexually immature during an extended post-metamorphic parasitic feeding phase) and the nonparasitic northern brook lamprey (that undergoes sexual maturation near the end of metamorphosis), but few replicates were available for comparable stages and sexes. This work lays the foundation for identifying and confirming the orthology and the function of genes involved in gonadal development in these and other lamprey species across more developmental stages.

Multiple stable isotopes identify sediment organic matter as the primary nutritional source to American brook lamprey larvae along a stream gradient

Multiple stable isotope ratios (δ2H, δ13C, δ15N, and δ34S) of American brook lamprey (Lethenteron appendix) ammocoetes and their potential sources were measured along a stream gradient. Isotopic ratios were successful at differentiating three potential sources: terrestrial organic matter (TOM; upland plants), sediment organic matter (SOM; benthic organic material), and algae. Bayesian mixing models (MixSIAR) utilizing δ13C, δ15N, and δ34S estimated ammocoetes were highly reliant on SOM at all sites (>95%), but inclusion of δ2H increased the importance of both algae and model uncertainty. Ammocoete δ2H was more depleted than anticipated even after lipid extraction and after accounting for trophic water contributions, suggesting δ2H in consumer tissues (especially for long-lived animals) is still incompletely understood. Model sensitivity testing with trophic fractionation values did not substantially change the results. Regardless of the mixing model investigated, none indicated more algae in ammocoete diets in downstream rather than upstream sites, in contrast with expectations about river productivity. Stable isotopes suggest SOM, and not algae, is nutritionally most important to ammocoetes, even in downstream reaches of rivers.

Riverscape genetics in brook lamprey: genetic diversity is less influenced by river fragmentation than by gene flow with the anadromous ecotype

Understanding the effect of human induced landscape fragmentation on gene flow and evolutionary potential of wild populations has become a major concern. Here, we investigated the effect of riverscape fragmentation on patterns of genetic diversity in the freshwater resident brook lamprey (Lampetra planeri) that has a low ability to pass obstacles to migration. We also tested the hypotheses of i) asymmetric gene flow following water current and ii) admixture with the closely related anadromous L. fluviatilis ecotype having a positive effect on L. planeri genetic diversity. We genotyped 2472 individuals, including 225 L. fluviatilis, sampled in 81 sites upstream and downstream from barriers to migration, in 29 West-European rivers. Linear modelling revealed a strong positive relationship between the distance to the source and genetic diversity, consistent with expected patterns of decreased gene flow into upstream populations. However, the presence of anthropogenic barriers had a moderate effect on spatial genetic structure. Accordingly, we found evidence for downstream-directed gene flow, supporting the hypothesis that barriers do not limit dispersal following water flow. Downstream L. planeri populations in sympatry with L. fluviatilis displayed consistently higher genetic diversity. We conclude that genetic drift and slight downstream gene flow mainly drive the genetic make up of upstream L. planeri populations whereas admixture between ecotypes maintains higher levels of genetic diversity in L. planeri populations sympatric with L. fluviatilis. We discuss the implications of these results for the design of conservation strategies of lamprey, and other freshwater organisms with several ecotypes, in fragmented dendritic river networks.

Novel “omega muscle units” in superficial body-wall myotomes during metamorphosis in the northern brook lamprey (Ichthyomyzon fossor)

Lampreys transform from sedentary filter feeders to more mobile adults through a dramatic metamorphosis that includes remodeling of head muscle and skeletal systems. Metamorphic modifications of body-wall myotomes that could support changes in swimming behavior from larvae to adults have not been previously reported. Thus, transverse sections of northern brook lamprey (Ichthyomyzon fossor Reighard and Cummins, 1916) in larval (n = 4), metamorphosing (n = 3), and adult (n = 2) stages were used to investigate the architecture of body-wall muscle and to detect whether Pax7 and MyoD, proteins important in myogenesis, were co-localized in any muscle nuclei. In addition to myotomal complexity of muscle units composed of parietal and central fibers, there was a novel pattern of omega-shaped muscle units with curves of muscle fibers in the superficial mid-body myotome in metamorphosing lamprey. Small satellite-like cells were identified on central fibers in metamorphosing and adult lamprey muscle using routine histology and immunolocalization of Pax7 and MyoD with antibodies that specifically detect mammalian and teleost proteins. Transient “omega muscle units” may be a marker for impending myotomal growth and increasing swimming efficiency during maturation, possibly restricted to metamorphosis. Finding satellite-like cells suggests that Pax7 and MyoD may have distinctive roles in lamprey myogenesis.