Published 1 September 2022
The parasitic flatworms known as flukes have attained a well-deserved notoriety. Of the two largest subgroups of flukes, most of this notoriety is attached to members of the Digenea as this group includes those species that directly infect humans. However, one should not underestimate the diversity of their counterparts in the Monogenea. Monogenean flukes are mostly parasites of aquatic vertebrates (fishes and amphibians). Unlike digeneans with their complex life cycles alternating between invertebrate and vertebrate hosts, monogeneans have just a single host and lack an ecologically differentiated larval stage.
With over 1000 species described to date, the Dactylogyridae are one of most diverse families currently recognised among the flukes (Kmentová et al. 2022). All known species are parasites of ray-finned fishes, with different species attacking marine and freshwater hosts. The majority of species are ectoparasites on the host’s gills. Others attach to the skin or are endoparasites, inhabiting the nasal cavities, digestive tract or urinary system.
Dactylogyrids are small to medium-sized flukes, the largest growing a couple of millimetres in length. At the head end of the worm is a subterminal mouth and two to five bilateral pairs of adhesive head organs. There are often two or four eye-spots that may help the fluke in locating a host. The rear end of the body forms a large attachment organ, the opisthaptor. In typical dactylogyrids, the opisthaptor is more or less disc-shaped and carries two pairs of large hooks that are used to grasp the host. In some species, however, only one pair of hooks is present or none. Individuals are hermaphroditic with male and female gonads opening via a common genital pore.
As is typical for soft-bodied animals with few systematically useful characters, the classification of dactylogyrids has been beset by uncertainty. Historically, several subfamilies have been recognised but they have often been poorly defined. A broad molecular phylogenetic analysis of dactylogyrids by Kmentová et al. (2022) identified two major clades within the family that the authors proposed recognising as Dactylogyrinae and Ancyrocephalinae. However, neither clade could be clearly distinguished morphologically. Though individual species are typically sensitive to salinity levels (to the extent that changes in tank salinity have been used to control monogenean gill parasites in aquaculture), both marine and freshwater taxa were included in both clades. However, molecular data did indicate that endoparasitic species formed a single clade within Dactylogyrinae.
Gill-parasitic dactylogyrids primarily impact their hosts through damage to the gill tissues, reducing their efficacy in oxygen uptake. Such damage may be induced actively by the worm’s feeding, or mechanically by the action of the opisthaptor, and may take the form of lesions. Strings of sticky parasite eggs may be visible tangled among the gill filaments, doubtless further inhibiting breathing. Efforts to develop an aquaculture industry for dhufish Glaucosoma hebraicum here in Western Australia have been stymied by heavy infestations with the species Haliotrema abaddon (Kritsky & Stephens 2001). Infested fishes suffered poor growth and vitality and were vulnerable to secondary effects such as organ dysfunction or infection by other parasites. The worms spread readily between hosts and even thwarted attempts to prevent their transmission between tanks. Evidently, those dactylogyrid eggs are easily picked up.
Kmentová, N., A. J. Cruz-Laufer, A. Pariselle, K. Smeets, T. Artois & M. P. M. Vanhove. 2022. Dactylogyridae 2022: a meta-analysis of phylogenetic studies and generic diagnoses of parasitic flatworms using published genetic and morphological data. International Journal for Parasitology 52: 427–457.
Kritsky, D. C., & F. Stephens. 2001. Haliotrema abaddon n. sp. (Monogenoidea: Dactylogyridae) from the gills of wild and maricultured West Australian dhufish Glaucosoma hebraicum (Teleostei: Glaucosomatidae), in Australia. Journal of Parasitology 87 (4): 749–754.