A case of abnormal swimming patterns in juvenile Oblada melanura naturally infected with Philometra obladae (Nematoda: Philometridae) in the Tyrrhenian Sea off Sicily, Italy.

The saddled seabream, Oblada melanura (L.), is a common seawater species present in the Mediterranean. Between July and August 2023, during diving activities along the Sicilian coast of Italy, we recorded with a digital camera several specimens of O. melanura showing an abnormal pattern of swimming in the water column. The unusual swimming behaviour was characterized by fast, uncoordinated directional changes and isolation from the remaining fishes in the shoal. Four dead fish were found and collected for necropsy and parasitological evaluation/examination. Upon gross examination, all fish showed an evident coelomic distension; the coelomic cavity of two fish was filled with nematodes that dislocated the coelomic organs, and the other two had degraded nematodes and a conspicuous quantity of fluid in the coelomic cavity. All collected parasites were identified as Philometra obladae (Nematoda: Philometridae) according to morphological criteria. Here, we describe the unusual swimming behaviour of O. melanura naturally infected with Ph. obladae and the results of the examination of dead fish infected with this parasite. Future studies are needed to better evaluate and describe the dynamics and the epidemiology of Ph. obladae infection in wild O. melanura.

Phylogenetic relationships of philometrid nematodes (Philometridae Baylis & Daubney, 1926) inferred from 18S rRNA, with molecular characterisation of recently described species.

Nematodes of the family Philometridae Baylis & Daubney, 1926 (Dracunculoidea Stiles, 1907) are generally poorly known, and there are many taxonomic issues within the family. Philometrids are parasites of fish and are found in various locations throughout the host, including within the subcutaneous tissues and musculature, the abdominal cavity and gonads; vast sexual dimorphism often means the males are not collected, leading to many species being described solely on female characteristics. Although there have been a number of studies utilising molecular data, the vast majority of species are yet to be sequenced. This study undertook genetic sequencing of 15 recently described species of philometrids across 4 genera, many of which were from specimens collected from waters off Australia. All of the sequences obtained were closely related with representatives of the family Philometridae. Species were found to be distributed in the phylogenetic trees within 4 clades based on a combination of site of infection within the host and host habitat. Family of host and geographical location was not as important for position within the trees. Clade A contained philometrids collected from the abdominal cavities and head tissues of South American freshwater fish. Clade B contained philometrids primarily from the abdominal cavities of freshwater European cyprinids. Clade C contained philometrids primarily from the ovaries of marine fish. Clade D contained philometrids from the body tissues of marine and freshwater fish. The potential co-evolutionary patterns between philometrids and their fish hosts are highlighted as an area of future research. This research also highlighted the importance of correct identification of any sequenced specimen.

Phylogeny of dracunculoid nematodes (Chromadorea: Rhabditida: Spirurina: Dracunculoidea) from some Eurasian freshwater fishes.

Sets of small ribosomal DNA (SSU rDNA) and large ribosomal DNA (LSU rDNA) sequences were obtained for Philometroides moraveci Vismanis Yunchis, 1994, Philometra kotlani (Molnár, 1969), Philometra rischta Skrjabin, 1923, Philometra cf. obturans (Prenant, 1886) (Philometridae), Sinoichthyonema amuri (Garkavi, 1972), Agrachanus scardinii (Molnár, 1966), Kalmanmolnaria intestinalis (Dogiel Bychowsky, 1934) and Skrjabillanus tincae Shigin Shigina, 1958 (Skrjabillanidae). Phylogenetic analysis of SSU rDNA data shows that dracunculoid nematodes are divided into two well-supported clades designated as Clade I and Clade II, respectively. Clade I includes the type species of the genus Philonema Kuitunen-Ekbaum, 1933, some species from the family Daniconematidae Moravec Køie, 1987 and two subfamilies of skrjabillanids, Skrjabillaninae Shigin Shigina, 1958 and Esocineminae Moravec, 2006. Clade II unites species from the families Dracunculidae Stiles, 1907, Micropleuridae Baylis Daubney, 1926 and Philometridae Baylis Daubney, 1926. Within the Philometridae, there are several well-supported groups of species, one of which unites freshwater Philometra spp. from the Palearctic cyprinids, identified as P. kotlani, P rischta, P. ovata (Zeder, 1803) and P. cyprinirutili (Creplin, 1825). However, the phylogenetic relationships of most philometrids are unresolved. An analysis of partial SSU and LSU rDNA sequences indicates that there is no direct phylogenetic relationship between Agrachanus Tikhomirova, 1971 (type species Skrjabillanus scardinii Molnár, 1966) and Skrjabillanus Shigin Shigina, 1958 (type species Sk. tincae), which means that the genus Agrachanus can be resurrected. Our study confirms that Philonematinae Ivashkin, Sobolev Khromova, 1971 should be elevated to the family rank. We formally establish the family Philonematidae Ivashkin, Sobolev Khromova, 1971 stat. nov. We also suggest combining the superfamilies Dracunculoidea Stiles, 1907 and Camallanoidea Railliet Henry, 1915 into the infraorder Camallanomorpha Roberts, Janovy Nadler, 2013.