Life-cycles of hemiuroid trematodes of pelagic fishes

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.

We hypothesize that almost all pelagic hemiuroids use pelagic oceanic molluscs as first intermediate hosts. These include neustonic (surface-floating) snails from the family Epitoniidae (genus Janthina), 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.

We aim to:

  • comprehensively document the richness of the trematode fauna infecting pelagic molluscs in Australian waters using molecular and morphological data
  • understand the ecological importance of pelagic molluscs for parasitic transmission
  • understand the biology and evolution of trematodes infecting pelagic fishes
  • explore the role of trematodes as indicators of predator-prey interactions in the pelagic environment.



 

Figure 1. The net is towed between 8 and 12 meters-depth in order to collect the highest diversity of pelagic molluscs (photo: Scott C. Cutmore).


Figure 2. The highest numbers of pelagic molluscs were obtained in spots where the highest plankton diversity and abundance were observed (photo: Scott C. Cutmore).


Figure 3. The project involves isolating and analysing all pelagic molluscs collected under a binocular microscope in order to find trematode infections; here at Lizard Island (photo: Scott C. Cutmore).


Figure 4. A heteropod of the family Atlantidae photographed under a  binocular microscope at Lizard Island (photo: Clarisse Louvard).