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

Abstract

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.