Extreme morphological disparity within Mollusca has long confounded efforts to reconstruct a stable backbone phylogeny for the phylum. Familiar molluscan groups—gastropods, bivalves, and cephalopods—each represent a diverse radiation with myriad morphological, ecological, and behavioral adaptations. The phylum further encompasses many more unfamiliar experiments in animal body-plan evolution. In this work, we reconstructed the phylogeny for living Mollusca on the basis of metazoan BUSCO (Benchmarking Universal Single-Copy Orthologs) genes extracted from 77 (13 new) genomes, including multiple members of all eight classes with two high-quality genome assemblies for monoplacophorans. Our analyses confirm a phylogeny proposed from morphology and show widespread genomic variation. The flexibility of the molluscan genome likely explains both historic challenges with their genomes and their evolutionary success.

Background: The monogenean Benedenia seriolae parasitizes fishes belonging to the genus Seriola, represents a species complex, and causes substantial impact on fish welfare in aquaculture systems worldwide. This study reports, for the first time, the complete mitochondrial genome of B. humboldti n. sp., a new cryptic species from the South-East Pacific (SEP). Methods: The mitogenome of B. humboldti n. sp. was assembled from short Illumina 150 bp pair-end reads. The phylogenetic position of B. humboldti n. sp. among other closely related congeneric and confamiliar capsalids was examined using mitochondrial protein-coding genes (PCGs). Morphology of B. humboldti n. sp. was examined based on fixed and stained specimens. Results: The AT-rich mitochondrial genome of B. humboldti is 13,455 bp in length and comprises 12 PCGs (atp8 was absent as in other monogenean genomes), 2 ribosomal RNA genes, and 22 transfer RNA genes. All protein-coding, ribosomal RNA, and transfer RNA genes are encoded on the H-strand. The gene order observed in the mitochondrial genome of B. humboldti n. sp. was identical to that of B. seriolae from Japan but different from that of B. seriolae from Australia. The genetic distance between B. humboldti n. sp. and B. seriolae from Japan was high. Minor but reliable differences in the shape of the penis were observed between Benedenia humboldti n. sp. and congeneric species. Conclusions: Phylogenetic analyses based on PCGs in association with differences in the shape of the penis permitted us to conclude that the material from the South-East Pacific represents a new species of Benedenia infecting S. lalandi off the coast of Chile. The discovery of this parasite represents the first step to improving our understanding of infestation dynamics and to develop control strategies for this pathogen infecting the farmed yellowtail kingfish, Seriola lalandi, in the South-East Pacific.

The relationships between mean intensity and prevalence of infestation of Pseudione humboldtensis and the size and sex of the hosts, the squat lobste

The monogenean, Zeuxapta seriolae, is a host-specific parasite that has an extensive geographical distribution on its host, Seriola lalandi, and is considered highly pathogenic in farmed fish. In recent years, developing cultures of S. lalandi in different coastal localities in Southeastern Pacific Ocean (SEP) have been affected by moderate and heavy infections of this parasite, attributed to contagion from wild to farmed fish. Here, we evaluated the pattern of genetic variations and biological traits of Z. seriolae in a spatial and temporal scale across its geographical distribution in SEP to determine its genetic status and biological traits, which could affect its transmission dynamics from wild to farmed fish. Wild fish and their parasites were sampled from fisheries in the northern Chilean coast (NCC: 24°S-30°S) and Eastern islands (JFA: ca 33°S; 80°W) between 2012 and 2014. Fragments of 816 bp of the cytochrome c oxidase subunit I (COI) gene was sequenced for 112 individuals from NCC and 63 from JFA and compared using AMOVA. Prevalence and intensity of Z. seriolae were calculated for each area. The parasite body size, fecundity and size at sexual maturity were estimated for 177 parasites from NCC and 128 from JFA, and significant differences were evaluated using GLM. Geographical genetic structuring was detected for Z. seriolae across SEP, with a population in NCC and the other in JFA, both with the same high haplotype diversity. Neutrality tests and mismatch analyses indicated that both Z. seriolae populations are stable. Parasite biological traits such as fecundity, body size, and size at sexual maturity, and population parameters varied significantly between geographical areas. Two genetic groups of Z. seriolae were detected in wild fish across SEP. Because of the seasonal migration of wild host and temporal contact with farming, quantifying the genetic diversity and level of gene flow or isolation between parasite populations is useful for fish health management in farming. The smallest size of sexual maturity in parasites from NCC is predictive of shorter life cycles, and their high genetic diversity suggests high evolutionary potential and high transmission of this parasite to farmed hosts.