Gut microorganisms of surgeonfishes (family Acanthuridae)

Clements, Kendall David (1991) Gut microorganisms of surgeonfishes (family Acanthuridae). PhD thesis, James Cook University of North Queensland.

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Abstract

The main aim of this thesis was to describe the intestinal endosymbiotic communities of a range of marine herbivorous fishes, with particular emphasis on the surgeonfishes (Family Acanthuridae). There were three main components to this objective: (a) an examination of the structural features and systematic position of endosymbiotic microorganisms in herbivorous fishes; (b) an examination of the relationship between endosymbiont occurrence patterns and host distribution and feeding; and (c) an investigation of the mode of symbiont transmission between generations of host acanthurids. The study of endosymbiont transmission involved four additional elements: (a) a study of the distribution pattern of juvenile acanthurids relative to adult distribution at Lizard Island; (b) a behavioural study of the juveniles of two species of acanthurids; (c) an examination of the microbiota of juvenile acanthurids; and (d) an aquarium experiment to directly investigate epulo transmission.

The most characteristic element of the acanthurid microbiota was an assemblage of large protists, referred to as epulos. These epulo forms, or types, were characterized by differences in shape, size and mode of reproduction. One of the epulo forms appeared identical to microorganisms previously reported from acanthurids in the Red Sea. Electron microscope sections of epulos from Great Barrier Reef acanthurids revealed that the symbionts are prokaryotes, and thus together with Epulopiscium fishelsoni from the Red Sea represent the largest known forms of this cell type. Features identifying the epulos as prokaryotes include the presence of bacterial-type flagella, a bacterial nucleoid, and the absence of a nucleus or any other membrane-bound organelle. A number of the different epulo types examined were found to share similarities in ultrastructure. The structural similarities and patterns of occurrence of these epulo types suggest that they may represent a suite of ecomorphotypes. A variety of protozoan taxa, including trichomonad, diplomonad, and opalinid flagellates and vestibuliferan and nyctotheran ciliates, were also found to inhabit the guts of herbivorous acanthurids.

Endosymbiotic communities were a characteristic feature of most species of herbivorous acanthurids and the pomacanthid Centropyge bicolor. Although some siganids and pomacentrids sometimes harboured endosymbiont populations, the inconsistency of microbial populations amongst these taxa suggested that the symbioses represented facultative associations. Endosymbionts were not detected in scarids. A range of epulo forms was observed in most herbivorous and detritivorous acanthurids; epulos were not found in planktivorous acanthurids. Epulos were also absent from the herbivorous species Acanthurus achilles, A. leucosternon, A. nigricans, and A. xanthopterus. The ubiquitous occurrence of epulos in several species of herbivorous acanthurids collected from a number of geographical regions suggests the possibility of an obligate relationship. The host/microorganism associations of the protozoan symbionts were found to be more variable than those of epulos, suggesting that the acanthurid/protozoan symbioses may be facultative relationships.

Most species of juvenile acanthurids were found to settle in habitats where adults were common, suggesting that juveniles had access to adult endosymbionts at an early stage. Most acanthurids settled in very low numbers each season relative to adult populations. Juvenile Acanthurus nigrofuscus and Ctenochaetus striatus were found to resemble adult conspecifics in terms of daily feeding pattern, and appeared to be responsive to small-scale variation in habitat structure and possibly to the density of interacting species. Juvenile A. nigrofuscus practise conspecific coprophagy, a behaviour which appears to be a mechanism for the transfer or retention of endosymbionts. This behaviour was not observed in juvenile C. striatus, suggesting that these species may differ with respect to their mode of epulo retention. Newly settled acanthurids did not harbour endosymbionts, but intestinal populations of epulos were rapidly established following settlement. Populations of other endosymbiont taxa, such as flagellates and spirilla, took longer to become established in the host gut. The epulo types found in juveniles differed from those characteristic of adult conspecifics; in general the smaller epulo types predominated in juvenile acanthurids. The results of the aquarium experiment on endosymbiont transmission strongly suggested that newly settled acanthurids may be infected with epulos by exposure to the faeces of infected hosts.

Item ID: 33764
Item Type: Thesis (PhD)
Keywords: Acanthuridae; acanthurus; epulopiscium; epulos; herbivorous fishes; Lizard Island; microorganisms; prokaryotes; surgeonfishes
Additional Information:

Publications arising from this thesis are available from the Related URLs field. The publications are:

Sutton, D.C., and Clements, K.D. (1988) Aerobic, heterotrophic gastrointestinal microflora of tropical marine fishes. In: Proceedings of the 6th International Coral Reef Symposium (3), pp. 185-190. From: 6th International Coral Reef Symposium (ICRS), 8-12 August 1988, Townsville, QLD, Australia.

Clements, K.D., Sutton, D.C., and Choat, J.H. (1989) Occurrence and characteristics of unusual protistan symbionts from surgeonfishes (Acanthuridae) of the Great Barrier Reef, Australia. Marine Biology, 102 (3). pp. 403-412.

Jones, G.P., Kaly, U.L., and Clements, K. (1991) Preliminary records of the coral reef fishes of Tuvalu. South Pacific Journal of Natural Science, 11. pp. 40-57.

Clements, K.D. (1991) Endosymbiotic communities of two herbivorous labroid fishes, Odax cyanomelas and O. pullus. Marine Biology, 109 (2). pp. 223-229.

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Date Deposited: 29 May 2015 04:09
FoR Codes: 06 BIOLOGICAL SCIENCES > 0602 Ecology > 060205 Marine and Estuarine Ecology (incl Marine Ichthyology) @ 100%
SEO Codes: 96 ENVIRONMENT > 9608 Flora, Fauna and Biodiversity > 960808 Marine Flora, Fauna and Biodiversity @ 100%
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