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Size and characteristics of aggregations of moon jellyfish (Aurelia SP.) in Tasmania, Australia


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Crawford, CM, Moltschaniwskyj, NA and Willcox, ST 2011 , 'Size and characteristics of aggregations of moon jellyfish (Aurelia SP.) in Tasmania, Australia' , Papers and Proceedings of the Royal Society of Tasmania, vol. 145 , pp. 9-15 .

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Moon jellyfish, Aurelia spp., are found in marine and estuarine
environments worldwide and can concentrate into dense
aggregations within enclosed or semi-enclosed water bodies
(e.g., Lucas et al. 1997, Purcell et al. 2000). Aggregations
are often promoted by physical properties of the water body
in which they occur and are commonly believed to facilitate
sexual reproduction, rather than act as a defence against
predation ortargetingfoodsources (Graham etal. 2001, Lucas
2001). The presence of!arge-scale aggregations of medusae, as
a function offavourable conditions, has substantial ecological
and economic consequences (Purcell et al. 2007).
The distribution, abundance and life history characteristics
of the genus Aurelia are highly variable spatially and
temporally (e.g., Schneider & Brehrends 1994, Lucas et al.
1997). The pelagic medusa stage generally occurs seasonally
and lives for several months (Lucas & Williams 1994, Miyake
et al. 1997), but in some populations, medusae will live for
12 months or more (Kinoshita et al. 2006).
Aurelia medusae are voracious feeders and are capable of
modifying the seasonal composition and abundance of the
planktonic community (Schneider & Brehrends 1994, Lucas
et al. 1997). Secondary effects of high grazing pressure include
increased phytoplankton biomass through reduced grazing
pressure by copepods (Lindahl & Hernroth, 1983, Olsson
et al. 1992, M0ller & Riisgard 2007a) and decreased food
availability for other zooplanktivores, which can have impacts
through the food chain (Purcell & Arai 2001). Given their
widespread distribution, occurrence in large aggregations and
capacity to alter trophic dynamics, jellyfishes are potentially
important consumers and transformers of energy and
nutrients in the marine ecosystem (e.g., Watanabe & Ishii
2001, Pauly et al. 2009, Pitt et al. 2009).
Determining the abundances and sizes of jellyfish in
the oceans has proved difficult because of their large sizes,
fragility and patchy distributions, both horizontally and
vertically, and because their gelatinous bodies are difficult
to tag (Purcell 2009). Also, the high water content of their
tissues makes acoustic sampling difficult, although combined
acoustic soundings and video recordings can monitor
relatively reliably some jellyfish species, provided the target
species can be distinguished from other co-occurring species
acoustically (Bamstedt et al. 2003, Alvarez Columbo et al.
2009). Consequently, estimates of the extents, causes and
effects of jellyfish blooms have rarely been conducted on a
large scale (Purcell 2009).
Aurelia sp. medusae periodically occur in dense monospecific
aggregations in the sheltered waterways of southeast
Tasmania, Australia (pl. 1). The medusae are morphologically
similar to Aurelia aurita; however, they are genetically distinct
from other species of Aurelia and have been designated as
Aurelia sp. 7 (Dawson et al. 2005).
Growth, survival and reproduction of the sessile, asexual,
colonial phase of this species has been found to be regulated
by a combination of density-dependent factors and
environmental conditions, which are consequently important
to the formation of jellyfish aggregations (Willcox et al.
2008). These aggregations, however, were largely unnoticed
until they caused the deaths of cultured Atlantic Salmon
in Tasmania, valued at millions of dollars. 'The objective
of this study was to describe the biological characteristics
of this species, as part of a larger study investigating
mechanisms driving the intermittent occurrence of Aurelia
sp. aggregations in southeast Tasmania. This included growth
and reproduction of individuals in the aggregations, and
estimating aggregation size and total biomass of medusae in
the system. To achieve this, we developed a method to assess
the abundance of jellyfish by combining aerial photography,
underwater video photography and net sampling.

Item Type: Article
Authors/Creators:Crawford, CM and Moltschaniwskyj, NA and Willcox, ST
Keywords: Royal Society of Tasmania, RST, Van Diemens Land, natural history, science, ecology, taxonomy, botany, zoology, geology, geography, papers & proceedings, Australia
Journal or Publication Title: Papers and Proceedings of the Royal Society of Tasmania
ISSN: 0080-4703
Collections: Royal Society Collection > Papers & Proceedings of the Royal Society of Tasmania
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