Silica reduces the toxicity of aluminium to a tropical freshwater fish (Mogurnda mogurnda)

The toxicity of aluminium (Al) to fish in acidic waters has been well documented. It was therefore expected that Al toxicity would be significant in fish communities in Gadjarrigamarndah (Gadji) Creek, a seasonally flowing stream in tropical northern Australia. This creek receives acidic groundwater containing elevated concentrations of Al from earlier land irrigation of treated mine tailings water from the former Nabarlek uranium mine. It was hypothesised that Al toxicity was reduced by high levels of silica (Si) in the water, and the subsequent formation of Al-silicate complexes. This prompted a laboratory assessment of the toxicity of Gadji Creek water to sac-fry of the native fish, Mogurnda mogurnda, followed by more detailed investigation of the toxicity of Al and the influence of Si in reducing Al toxicity. No mortality of M. mogurnda sac-fry was observed in two toxicity tests using Gadji Creek water collected in August 1997 and September 1998. The majority of Al (80-95%) was calculated to be complexed with humic substances and sulfate, with <1% being complexed with silicate. Assessment of the influence of silica on the acute toxicity of Al in the absence of natural organic complexants (i.e. in reconstituted freshwater, pH 5) revealed that Si reduced Al toxicity. As the molar ratio of Si:Al was increased, the percent survival of M. mogurnda sac-fry increased until there was no significant (P > 0.05) difference from the controls. However, speciation modelling again predicted that little (<3%) Al complexed with silicate, with the speciation and bioavailability of Al remaining constant as the molar ratio of Si:Al increased. Therefore, the original hypothesis that Al-silicate complexes in solution reduced the toxicity of Al to M. mogurnda could not be supported. This potential mechanism, and an alternative hypothesis, that Si competes with Al for binding sites at the fish gill surface, requires further investigation. Crown Copyright (C) 2002 Published by Elsevier Science Ltd. All rights reserved.

Branchial vascular pathways in two species of Tetraodontiformes and the concept of secondary vessels and nutrient arteries

The vascular organisation of the branchial basket was examined in two Tetraodontiform fishes; the three-barred porcupinefish, Dicotylichthys punctulatus and the banded toadfish, Marylina pleurosticta by scanning electron microscopy of vascular casts and standard histological approaches. In D. punctulatus, interarterial anastomoses (iaas) originated at high densities from the efferent filamental and branchial arteries, subsequently re-anastomosing to form progressively larger secondary vessels. Small branches of this system entered the filament body, where it was interspersed between the intrafilamental vessels. Large-bore secondary vessels ran parallel with the efferent branchial arteries, and were found to constitute an additional arterio-arterial pathway, in that these vessels exited the branchial basket in company with the mandibular, the carotid and the afferent and efferent branchial arteries, from where they gave rise to capillary beds after exit. Secondary vessels were not found to supply filament muscle; rather these tissues were supplied by single specialised vessels running in parallel between the efferent and afferent branchial arteries in both species examined. Although the branchial vascular anatomy was generally fairly similar for the two species examined, iaas were not found to originate from any branchial component in the banded toadfish, M. pleurosticta, which instead showed a moderate frequency of iaas on other vessels in the cephalic region. It is proposed that four independent vascular pathways may be present within the teleostean gill filament, the conventional arterio-arterial pathway across the respiratory lamellae; an arterio-arterial system of secondary vessels supplying the filament and non-branchial tissues; a system of vessels supplying the filament musculature; and the intrafilamental vessels (central venous sinus). The present study demonstrates that phylogenetic differences in the arrangement of the branchial vascular system occur between species of the same taxon.

Urea based osmoregulation and endocrine control in elasmobranch fish with special reference to euryhalinity

Since the landmark contributions of Homer Smith and co-workers in the 1930s there has been a considerable advance in our knowledge regarding the osmoregulatory strategy of elasmobranch fish. Smith recognised that urea was retained in the body fluids as part of the 'osmoregulatory ballast' of elasmobranch fish so that body fluid osmolality is raised to a level that is iso- or slightly hyper-osmotic to that of the surrounding medium. From studies at that time he also postulated that many marine dwelling elasmobranchs were not capable of adaptation to dilute environments. However, more recent investigations have demonstrated that, at least in some species, this may not be the case. Gradual acclimation of marine dwelling elasmobranchs to varying environmental salinities under laboratory conditions has demonstrated that these fish do have the capacity to acclimate to changes in salinity through independent regulation of Na+, Cl- and urea levels. This suggests that many of the presumed stenohaline marine elasmobranchs could in fact be described as partially euryhaline. The contributions of Thomas Thorson in the 1970s demonstrated the osmoregulatory strategy of a fully euryhaline elasmobranch, the bull shark, Carcharhinus leucas, and more recent investigations have examined the mechanisms behind this strategy in the euryhaline elasmobranch, Dasyatis sabina. Both partially euryhaline and fully euryhaline species utilise the same physiological processes to control urea, Na+ and Cl- levels within the body fluids. The role of the gills, kidney, liver, rectal gland and drinking process is discussed in relation to the endocrine control of urea, Na+ and Cl- levels as elasmobranchs acclimate to different environmental salinities. (C) 2003 Elsevier Inc. All rights reserved.

Moravecia australiensis n. g., n. sp (Dracunculoidea : Guyanemidae) from the gills of the green porcupine fish Tragulichthys jaculiferus (Cuvier) in Australia

A new dracunculoid genus and species, Moravecia australiensis, is described from gill-filaments of the green porcupine fish Tragulichthys jaculiferus (Cuvier) (Tetraodontiformes: Diodontidae) from Moreton Bay, Queensland, Australia. Abundant mobile larvae and a few adult males with females occurred in the gill-filament between the epithelial basement membrane and efferent artery. Gills of all 69 fish examined contained larvae. Eleven harboured adult nematodes of a previously undescribed species belonging to the family Guyanemidae. The new species is placed within a newly proposed genus because it differs from the four existing genera in the family in possessing fine cuticular transverse striations, two forward protruding cephalic elevations, a circumoral elevation, a small triangular mouth surrounded by six cephalic papillae arranged in two lateral clusters of three each and a pair of large lateral amphids. Males have two pairs of pedunculate caudal papillae supporting the caudal alae. A key to the genera of the Guyanemidae is presented.

Anatomy of the pharyngeal jaw apparatus of Zenarchopterus (Gill) (Teleostei : Beloniformes)

The structure of the pharyngeal jaw apparatus (PJA) of Zenarchopterus dispar and Z. buffonis, carnivorous estuarine and freshwater West-Pacific halfbeaks, was investigated using dissection, light, and scanning electron microscopy as part of a comparison with estuarine and marine herbivorous confamilials. The Zenarchopterus PJA differs from published descriptions of hemiramphid PJAs in that the otic capsules are less pronounced; the pharyngocranial articulation facet is trough-like; the third pharyngobranchials are ankylosed; the second pharyngobranchial anterior processes are relatively hypotrophied; all pharyngeal teeth except the posterior teeth in the fifth ceratobranchial face posteriorly; the muscularis craniopharyngobranchialis 2 posterior is short; the muscularis craniopharyngobranchialis 2 anterior is lacking, as is its insertion site, the inferior parasphenoid apophysis; the protractor pectoralis is well developed; the pharyngocleithralis internus originates dorsal to the level of the fifth ceratobranchial bony process; the fifth ceratobranchial bony processes are directed ventrolaterally; the opposing upper and lower tooth fields appear not to occlude erosively; and the muscular portion of the pharyngohyoideus is well developed anteriorly. The extent of these differences and their implications for the function of the PJA support recent molecular studies that suggest that the Hemiramphidae is polyphyletic. (C) 2004 Wiley-Liss, Inc.