Ingo Ernst

Concurrent invaders - four exotic species of Monogenea now established on exotic freshwater fishes in Australia

Four species of exotic monogeneans are reported from five species of exotic freshwater fish in Australia: Gytrodactylus bullatarudis from Poecilia reticulata and Xiphophorus helleri in Queensland; Gyrodactylus macracanthus from Misgurnus anguillicaudatus in the Australian Capital Territory; Dactylogyrus extensus from Cyprinus carpio in the Australian Capital Territory; and Dactylogyrus anchoratus from Carassius auratus in the Australian Capital Territory. This is the first published record of described species of monogeneans of the genus Dactylogyrus or Gyrodactylus from Australia and the first report of parasites of M. anguillicaudatus in Australia. The establishment of exotic monogenean populations on Australian native fishes via host-switching is considered less likely than for other parasitic groups due to the generally high host-specificity of monogeneans, combined with the phylogenetic dissimilarity of native and exotic fishes. Similar establishments have occurred elsewhere, however, and the risk of these events increases with each new fish species introduction.

Global spread of the eel parasite Gyrodactylus anguillae (Monogenea)

Gyrodactylus anguillae is known from four continents, and intraspecific variation in the morphology of specimens from each continent is negligible. However, morphological characters alone may not always be adequate for recognizing populations that have been isolated geographically over evolutionary time. We were unable to obtain specimens from Northeast Asia, but G. anguillae collected from North America, Europe and Australia had identical ribosomal DNA sequences (ITS1-5.8SrRNA-ITS2 region). This lack of even minor genetic variation in populations from three remote continents leads us to conclude that they share a contemporary common ancestor. This is one of at least five pests of eels to have invaded new continents as a result of the eel trade in the last four decades; these other known eel pests include Pseudodactylogyrus spp. (Monogenea) and Anguillicola spp. (Nematoda). Their dispersal has probably been secondarily augmented by longshore migration of infected eels, and perhaps also by transportation in ballast waters. The only other known gyrodactylid from eel gills, G. nipponensis from Japanese eel, is not closely related to G. anguillae, because the ITS1 region of the former is short (370 bases) and that of the latter long (685 bases). G. anguillae probably originated in Europe, because it was recorded there over 40 years ago, before the development of the international eel trade. We predict that the worm has a high potential to invade eel populations in East Africa, Southeast Asia, and New Zealand.

Hatching rhythms in the capsalid monogeneans Benedenia lutjani from the skin and B. rohdei from the gills of Lutjanus carponotatus at Heron Island, Queensland, Australia

Spontaneous hatching of eggs of Benedenia lutjani and B. rohdei occurred after incubation for 4-6 days and 6-10 days, respectively, at a constant temperature in the range 22-28 degrees C when exposed to natural illumination or to alternating 12-h periods of light and darkness (LD12:12; light on, 06.00 h; light off, 18.00 h). Under these conditions, hatching of the eggs of both species was rhythmical, all larvae emerging only during periods of illumination. Hatching was not confined to particular times with the illuminated period. Evidence for an endogenous component to the rhythm was revealed by transfer of eggs from LD12:12 to continuous darkness (DD) near the end of the incubation period. Hatching, also occurred only during periods of illumination when eggs of each species were incubated under a DL12:12 regime (i.e. period of illumination 18.00 h to 06.0 h; period of darkness 06.00 h to 18.00 h). When the eggs of B. lutjani and B. rohdei were laid and incubated in DD or continuous illumination (LL), some degree of rhythmicity persisted, raising the possibility that the eggs inherit circadian rhythmicity from their parent. The hatching patterns of these 2 species of monogeneans are discussed in relation to host finding, host behaviour and limited observations on the behaviour of the oncomiracidia after hatching.

The uterine epithelium of Gyrodactylus kobayashii (Monogenea: Gyrodactylidae) : ultrastructure of basal matrices, cytoplasmic membranes and the birth plug, and comparison with other reproductive epithelia

Ultrastructural details of reproductive epithelia in the viviparous monogenean Gyrodactylus kobayashii are described. Specimens of G. kobayashii were fixed for transmission electron microscopy in glutaraldehyde in sodium cacodylate buffer followed by either 1% aqueous osmium tetroxide or 1% aqueous osmium tetroxide reduced with 1.5% potassium ferricyanide. All reproductive epithelia are underlain by a fibrillar basal matrix. The uterine basal matrix is electron-opaque after potassium ferricyanide reduced osmium tetroxide fixation suggesting the presence of carbohydrate-containing materials. With potassium ferricyanide reduced osmium tetroxide fixation, two prominent membrane systems were distinguished in the uterine epithelium. Basal invaginations are short infoldings of the basal membrane. The basal invaginations are common in other reproductive epithelia and tegument and probably enhance transport of materials by these epithelia. Laminated membranes are membrane stacks, resembling endoplasmic reticulum stacks. These membranes were abundant at the apical membrane. The birth plug is a solid cytoplasmic layer, lacking a lumen, and rich in cytoplasmic vesicles. This layer connects the tegument and the uterine epithelium.

Variation in the egg cell forming region of Gyrodactylus kobayashii Hukuda, 1940 (Monogenea: Gyrodactylidae)

The egg cell forming region of Gyrodactylus kobayashii from goldfish (Carassius auratus) is a thin nucleated cytoplasmic layer surrounding the developing egg cell (= oocyte). The cytoplasm contains numerous elougate membranes. As parasites age, the egg cell forming region becomes electron lucent. The apical membrane of the egg cell forming region becomes disrupted in places. A basal matrix is indistinct in new-born and young worms, becoming more evident as worms grow older. Numerous pits (= basal pits) are found along the basal plasma membrane of worms with a mature male system. These pits appear to be stable components of the membrane and resemble hemidesmosomes. Basal pits were co-incident with sperm in the egg cell forming region in 3 of 5 worms examined. The function of the basal pits of G. kobayashii could not be determined. It is postulated, however, that they either assist sperm to traverse the egg cell forming region to fertilize the egg cell or stabilize the egg cell forming region against damage by sperm traversing this layer. The egg cell forming region encloses a large egg cell and 1 or more smaller differentiating egg cells. The ripening egg cell has a large nucleus and extensive cytoplasm. The cell has a thickened membrane. Large vacuoles and invaginations at the periphery of the egg cell appear to engulf cytoplasm of the egg cell forming region.

Experimental susceptibility of some reef fish species to Benedenia lutjani (Monogenea: Capsalidae), a parasite of Lutjanus carponotatus (Pisces: Lutjanidae)

Abstract The susceptibility of species of lutjanid, lethrinid and serranid fish to infection by either larval or post-larval (juvenile and adult) specimens of the capsalid monogenean Benedenia lutjani Whittington and Kearn (1993) was examined experimentally. Four species of lutjanids became infected when exposed to larvae of B. lutjani, but three species of lethrinids and four species of serranids were not susceptible to larvae under the same conditions. Variability in the intensity of infection by larvae occurred within and between lutjanid species. Few post-larval specimens of B. lutjani transferred between individuals of the specific host Lutjanus carponotatus (Richardson 1842) in 60-l aquaria and none transferred between specimens of L. carponotatus in a 7,500-l concrete tank. These results indicate that transfer of post-larval B. lutjani between individuals of the specific host is unlikely to occur in the wild. Other lutjanid species did not become infected when exposed to specimens of L. carponotatus infected heavily by post-larval B. lutjani, but two lethrinid species were susceptible to infection under the same conditions. These data indicate that different factors may mediate host-specificity for larval and post-larval B. lutjani.

A new genus of viviparous gyrodactylid (Monogenea) from the Great Barrier Reef, Australia with descriptions of seven new species

Acanthoplacatus gen. nov., a new genus of viviparous gyrodactylid, is described from the fins and skin of siganid fishes from the Great Barrier Reef, Australia. The genus is characterized by a muscular, tube-like haptor with 16 marginal hooks on the posterior margin. The ventral lobe of the haptor is located anteriorly relative to the dorsal lobe and contains a pair of hamuli and a ventral bar with posteriorly-projecting ventral bar membrane. A dorsal bar is absent. Five pairs of posterior gland cells surround the posterior terminations of the gut. The male copulatory organ is a muscular, non-eversible bulb with several spines around the distal opening. Species of Acanthoplacatus have a bilateral excretory system consisting of six pairs of flame cells and a pair of excretory bladders. Seven new species are described: Acanthoplacatus adlardi sp. nov. and A. amplihamus sp. nov. from Siganus punctatus (Forster, 1801), A. brauni sp. nov. from S. corallinus (Valenciennes, 1835), A. parvihamus sp. nov. from S. vulpinus (Schlegel and Mueller, 1845), A. puelli sp. nov. from S. puellus Schlegel, 1852, A. shieldsi sp. nov. from S. lineatus (Valenciennes, 1835) and A. sigani sp. nov. from S. fuscescens (Houttuyn, 1782). Species can be discriminated by shape and size of the hamuli, marginal hooks and ventral bar and by male copulatory organ sclerite morphology. Three species (A. brauni sp. nov., A. shieldsi sp. nov. and A. sigani sp. nov.) were assessed for seasonal variation of sclerite size. Ten of thirteen morphological characters showed seasonal variation in size for at least one of the species. The characters were longer in winter except dorsal root tissue cap width. Only one character, marginal hook length, showed significant seasonal variation for all three species. Species of Acanthoplacatus were observed to attach using only the marginal hooks and the role of hamuli in attachment is unclear. The dorsal fin of the host is the preferred site for most species but the anal fin, caudal fin and body surfaces are preferred by some species. Prevalences for species range from 57 to 100%.