Molecular characterisation of selected parasites from commercially important fish in the south-eastern region of Australia

Parasites infecting edible fish can exert a broad and multi-faceted impact on fish health and production and it can even be a human health concern. The economic impacts of mass mortality events, and the loss of consumer confidence due to blemishes in fish for commercial sale, can be significant. In Australian waters, research using combined morphological and molecular methods of parasite identification has been slow, particularly for parasites that infect
commercial edible fish.
This study investigated nematode and monogenean parasites in selected species of commercially sourced and popularly consumed fish from south-eastern Australian waters. Fish-borne nematodes are internal parasites that may impact fish health and production. Additionally, many nematodes are zoonotic and consuming infected fish is a human health concern. Monogenea cling to the external surface of fish, particularly the gills, and can significantly impact fish health in intensive aquaculture systems. The popularity of fish as part
of a healthy diet and the expansion of cultural cuisines have increased the necessity to better understand the infection dynamics of nematodes and monogeneans in Australian fish.
In this Thesis, 695 fish were dissected from nine species: Australian anchovy Engraulis australis, Australian pilchard Sardinops sagax, Australian snapper Pagrus auratus, blue mackerel Scomber australasicus, eastern school whiting Sillago flindersi, blue-spotted flathead Platycephalus caeruleopunctatus, dusky flathead Platycephalus fuscus, sand flathead Platycephalus bassensis, and tiger flathead Platycephalus richardsoni. All the fish were identified using both molecular (cox1 gene) and morphological methodology.
Overall, 5,404 nematodes were collected. These included larvae and adults which were identified using molecular (ITS region) and morphological methodology as Anisakis types I, II & III, Contracaecum type II, Echinocephalus sp. larva, Hysterothylacium types IV, VI, VIII, XIV & XVIII, H. zhoushanense larva, Terranova types I & II; and Capillaria sp., Dichelyne cf. pleuronectidis, and Dichelyne sp. 1.
Overall, 2,112 monogeneans were collected from the fish. There were 13 Monogenea species identified using molecular (28S rRNA and cox1 genes) and morphological methodology as Allogastrocotyle bivaginalis, Bivagina pagrosomi, Choricotyle australiensis, Gastrocotyle kurra, Kuhnia scombercolias, K. scombri, Lamellodiscus pagrosomi, Mazocraes australis, Microcotyle bassensis, Platycephalotrema bassense, Polylabris australiensis, Polylabris
sillaginae, and Pseudokuhnia minor.

Eight separate investigations were conducted across four experimental chapters. Experimental Chapter 1 investigates pelagic fish, Australian anchovy, pilchard, and eastern school whiting. The first investigation describes nematodes, particularly those with zoonotic potential. Nematodes were identified as Contracaecum type II, Hysterothylacium types IV, VIII, XIV & XVIII and Terranova type II. Whilst in the second investigation, Monogenea infection in the
same fish species is described. Monogenea were identified as Mazocraes australis, Polylabris australiensis, and Polylabris sillaginae.
Experimental Chapter 2 investigates pelagic blue mackerel. The third investigation describes nematode infections. Nematodes were identified as Anisakis types I & II, Contracaecum type II, Hysterothylacium types IV, VI, VIII & XIV, Terranova types I & II, and an adult Capillaria sp. Seven nematodes were identified for the first time in Australian blue mackerel. Nematodes
that are confirmed as zoonotic were Anisakis and Contracaecum, and Hysterothylacium and Terranova with zoonotic potential. The fourth investigation describes Monogenea. Five species were identified as Allogastrocotyle bivaginalis, Gastrocotyle kurra, Kuhnia scombri, K.
scombercolias, and Pseudokuhnia minor.

Experimental Chapter 3 investigates the highly popular demersal Australian snapper which is often served raw as sashimi in Australia. Investigation five focuses on the nematodes of zoonotic importance. Zoonotic Anisakis pegreffii was identified as a dominant species. This investigation also identified A. brevispiculata, Dichelyne cf. pleuronectidis, Dichelyne sp. 1, and Terranova type II. Investigation six describes Monogenea. Bivagina pagrosomi, Choricotyle australiensis, and Lamellodiscus pagrosomi were identified infecting the gills of
snapper.
Experimental Chapter 4 focuses on the highly popular demersal Australian flathead fish. Investigation seven focuses on the nematodes that infect three species of flathead; the sand flathead, the tiger flathead, and the dusky flathead. Nematodes were identified as Anisakis types I, II & III, Contracaecum type II, Hysterothylacium types IV, VI & VIII, and Terranova types I & II, Echinocephalus sp. larva along with an adult Capillaria sp. Statistical analysis showed that infection data can vary between flathead species in similar geographic areas. Investigation eight focuses on the Monogenea in the blue-spotted flathead. Monogenea were identified as Microcotyle bassensis and Platycephalotrema bassense.