The role of epifaunal crustaceans on Sargassum spp. at Magnetic Island, Great Barrier Reef, Australia

Martin-Smith, Keith Michael (1994) The role of epifaunal crustaceans on Sargassum spp. at Magnetic Island, Great Barrier Reef, Australia. PhD thesis, James Cook University.

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Generalisations about the community ecology of invertebrates associated with plant surfaces have been developed largely from studies on terrestrial insect-plant systems and by limited studies on temperate marine macroalgal systems. This study was designed to quantify the seasonal variation in populations of a tropical macroalga and its associated epifauna, to investigate the causal factors producing the phenological patterns and to relate these findings to the general area of plant-arthropod relationships. The system investigated was four sympatric species of the brown alga Sargassum and their mobile epifauna, living at Magnetic Island, Queensland, Australia (19°10'S, 146°50'E).

Over two annual cycles all species of Sargassum showed pronounced seasonality in size and reproduction but not in density; three of four species grew annual laterals from perennial axes in spring, reached maximum size in summer, reproduced and subsequently senesced, while the fourth species showed the opposite phenology. Epiphytic algae on the surface of Sargassum were primarily absent during the spring and summer periods of Sargassum growth but attained high abundance during the winter on the residual portions. Epifauna was diverse and abundant on all species of Sargassum, being dominated numerically by gammarid amphipods, sphaeromatid isopods, tanaids, errant polychaetes and gastropods. There were few significant differences between abundance of epifauna on different species of Sargassum and few or no representatives of the reef cryptofauna: this suggested that the epifauna was a distinct algal-associated community. All epifaunal taxa also showed distinct, repeated seasonal changes in abundance. Gammarid amphipods, sphaeromatid isopods, tanaids and polychaetes — together with many of the less abundant taxa — had abundance maxima in winter and minima in summer. Conversely, only one dominant taxon, gastropods, and two less abundant taxa had summer maxima and winter minima. At finer temporal scales, epifaunal abundance was consistent over a time scale of hours and days, and moderately variable over a scale of weeks. There were few significant day-night variations in abundance of epifauna.

Manipulative experiments were run to test hypotheses about factors influencing the abundance of epifauna. Recolonisation experiments showed that the populations of epifauna were extremely dynamic in space and time, equilibrial communities being re-established on defaunated plants in approximately two weeks. The influence of predation by fishes was examined with an eight-week exclusion experiment: no effect of predation was detected although cage artifacts may have obscured abundance changes of small magnitude. The influence of habitat complexity and heterogeneity was examined using artificial plants with and without epiphytic algae: a very significant positive correlation was found between the abundance of epiphytic algae and the abundance of many taxa of epifauna. Analysis of the results at the community level revealed that communities became increasingly similar over the eight weeks of the experiment, as epiphytes accumulated on the originally epiphyte-free artificial plants. It is suggested, therefore, that the seasonal patterns of abundance of epifauna, both at the community and taxon level, are driven primarily by fluctuations in the abundance of epiphytic algae.

A detailed study of the sphaeromatid isopods was conducted to determine whether the above results and hypotheses were applicable at the species level, as opposed to the family or community level. Resolution of the seasonal pattern of abundance for the sphaeromatid family revealed that each of three common genera had distinct, unimodal phenologies: Cerceis and Cymodoce showed autumn maxima while Neonaesa had a winter maximum. Size-frequency distributions of all genera suggested that reproduction occurred continuously over extended periods of time and that adults emigrated from Sargassum upon reaching a certain size. For these isopods the Sargassum and epiphytes acted as a nursery habitat for juveniles, providing habitable space and a potential food source. A series of laboratory and field experiments with artificial substrata revealed that various aspects of habitat structure (size and colour) and habitat architecture (number, size and arrangement of habitable spaces) were important determinants of colonisation by Cymodoce. It is suggested that the observed patterns of abundance for sphaeromatid isopods on Sargassum were produced by the selective colonisation of epiphytes by juveniles in response to a complex set of habitat criteria.

Although complicated at a local scale, broad scale patterns in the Sargassum-epifauna system are similar to those in temperate macroalgal-epifauna interactions. Sargassum and its associated epifauna, in common with these other systems, appears to be a 'passive' system, wherein associations are facultative and unspecialised. This contrasts strongly with 'active' terrestrial systems where plants and arthropods commonly have highly specialised, often obligate relationships. Thus, paradigms developed from terrestrial systems about the role of factors such as habitat structure or secondary compounds will need to be revised before they can be applied to marine plant-arthropod interactions.

Item ID: 24116
Item Type: Thesis (PhD)
Keywords: brown algae; community ecology; crustaceans; epifauna; epiphytes; isopods; life histories; life history; Magnetic Island; Sargassum; seasonal variation
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Date Deposited: 14 Dec 2012 03:46
FoR Codes: 06 BIOLOGICAL SCIENCES > 0603 Evolutionary Biology > 060308 Life Histories @ 100%
SEO Codes: 96 ENVIRONMENT > 9608 Flora, Fauna and Biodiversity > 960808 Marine Flora, Fauna and Biodiversity @ 100%
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