Adult and cystacanth forms of the acanthocephalan Serrasentis sagittifer from Australian coastal waters are redescribed and verified as the same species using both molecular and morphological data. This study provides the baseline 18S rDNA, 28S rDNA, and cox1 sequence data to serve as genetic barcode for S. sagittifer . The validity of the currently recognized species of Serrasentis is discussed. The most recently described species are junior synonyms of either Serrasentis nadakali or S. sagittifer , and a number of species are species inquirenda . When using morphological characters to distinguish the species of Serrasentis , consideration needs to be given to the maturity of the specimens, since the trunk elongates and the number and distribution of the ventral combs changes as worms mature, although the proboscis armature itself does not change. A simple key to assist in the identification of species of Serrasentis is provided. Adult S. sagittifer appear to be highly host specific to the cobia, Rachycentron canadum , in northern Australian waters, whereas cystacanths have been reported from a wide range of fish species. The relationship between host length and number of cystacanths shows that most paratenic infections are acquired as young fish, most likely via a crustacean intermediate host.

Cystacanths of 4 species of Acanthocephala are reported for the first time from various species of fish belonging to the Order Pleuronectiformes from waters of the western Gulf of Carpentaria and the central coast of Queensland, Australia: Corynosoma cetaceum Johnston and Best, 1942 (Family Polymorphidae), Serrasentis cf. sagittifer (Linton, 1889) and Rhadinorhynchus sp. (Family Rhadinorhynchidae), and Gorgorhynchoides sp. (Family Isthmosacanthidae). Approximately 32% of the 515 individual fish belonging to 24 species were infected with at least 1 cystacanth. Serrasentis cf. sagittifer was the most-commonly encountered, infecting a total of 18 species of fish across both regions. Gorgorhynchoides sp. infected 7 fish species in the Gulf of Carpentaria only while Rhadinorhynchus sp. (1 fish species) and C. cetaceum (7 fish species) were only found on the central coast of Queensland. Most fish were infected with a single cystacanth of any species. There was no relationship between total length of fish and intensity of infection for any species. This paper provides information on parasite infections in fish hosts commonly caught as by-catch and outlines the need for further studies on these fish to be able to determine sustainability of such fish stocks.