A project undertaken at the School of Biological Sciences, The University of Queensland, and supervised by Assoc. Prof. Thomas H. Cribb and Dr Scott C. Cutmore
Trematodes (flukes) of the superfamily Hemiuroidea are major endoparasites of commercially important pelagic fishes such as tunas and marlins. Hundreds of species of the families Didymozoidae, Hirudinellidae, Hemiuridae and Lecithasteridae are known from these fishes.
Hemiuroid trematodes have complex life-cycles involving 3–4 hosts, however, these are poorly known with complete or fragmentary life-cycles available for only a handful of species. Thus, although the life-cycles of some near-shore fish-infecting hemiuroids are known, no life-cycles for those hemiuroids infecting pelagic fishes (i.e. those that live in the open ocean) have ever been elucidated. Especially, the identities of their first intermediate hosts remain a mystery.
Fig. 1: New didymozoid species from the fins of Ladyfish, Elops hawaiensis. A: anterior extremity showing oral and ventral suckers, pharynx, uterus, seminal duct and alimentary tract; scale=160 μm. B: posterior extremity showing uterus, digestive caeca, vitellarium and saccular excretory vesicle; scale=130 μm.
We hypothesize that almost all pelagic hemiuroids use pelagic oceanic molluscs as first intermediate hosts. Pelagic molluscs include pleustonic (surface-floating) snails (Epitoniidae) and “blue dragons” (Glaucidae), and holoplanktonic molluscs of the superfamily Pterotracheoidea and the order Pteropoda. In total, this molluscan fauna comprises 17 families and over 330 species, forming a significant and globally-distributed element of the marine plankton. These characteristics make them ideal hosts for parasite transmission to large pelagic fishes.
Globally, trematodes in planktonic molluscs have been very rarely documented. This is due to the logistical difficulty of finding these molluscs, their patchy and seasonally-dependent distribution, and typically low prevalence of infections. In addition, in the absence of molecular sequence data, identifying trematode larvae to species has been impossible due to their lack of distinctive morphological features.
Therefore, our main goal has been to find and characterise hemiuroid infections from planktonic and pleustonic organisms via molecular and morphological tools, in order to elucidate life-cycles of pelagic hemiuroids and enhance our understanding of trematode evolution. We have not succeeded in this, but we are not beaten yet!
An important and completely unexpected finding was made. Large numbers of hemiuroid metacercarial larvae from three families (the Accacoeliidae, Didymozoidae and Hemiuridae) were found to use pleustonic organisms as second- or third-intermediate hosts.
In addition, by serendipity, first-stage infections previously collected from a Moreton Bay bivalve were found to match perfectly with a new adult species of Didymozoidae from the ladyfish Elops hawaiensis from Moreton Bay (drawings of the new species shown here), and with metacercarial larvae from juvenile whitings (Sillago sp.) from the same area. Moreover, first-stage infections from a vermetid gastropod (“worm snail”) from Moreton Bay were molecularly identified as belonging to the Didymozoidae. All these findings are novel and greatly enhance our understanding of hemiuroid host-usage and evolution.
In light of these results, we aim to:
- comprehensively document the richness of the trematode fauna infecting planktonic and pleustonic molluscs in Australian waters using molecular and morphological data;
- understand the ecological importance of planktonic and pleustonic molluscs for trematode parasite transmission;
- explore host-parasite interactions in the pelagic environment in the context of hemiuroid life-cycle completion;
- document the novel use of both bivalve- and gastropod hosts by hemiuroids of the family Didymozoidae using molecular data.