This study focuses on the Irukandji jellyfish Carukia barnesi Southcott, 1967. Little is known about the general ecology of C. barnesi; however, the medusa stage is considered oceanic, planktonic, has been found around coral reefs or islands, and under certain conditions, on beaches. Carukia barnesi is a relatively small box jellyfish species (bell size up to 35 mm), that are typically present during the summer monsoonal summer months between November and May in Queensland, Australia, which is commonly referred to as the 'stinger' season. Although not well defined, the distribution of this species is considered along the Great Barrier Reef and adjacent coastline, between Lizard Island and Fraser Island. There is evidence that the length of the Irukandji season in the Queensland region has progressively increased over the last 50 years, based on annual sting records, from 15 days long historically to over 150 days long currently, which has been speculated to be attributed to increased seawater temperatures. Similarly, there have been anecdotal reports that the southern distribution of C. barnesi has also increased over the last 50 years.
A sting from C. barnesi commonly results in Irukandji syndrome, which is often severely painful, potentially fatal, and frequently requires hospitalisation for treatment. The direct cost associated with treating envenomed victims, and the negative impact this species has on the Australian tourism industry through reduced revenue, are substantial (i.e., an estimated 65 million dollars in lost tourism revenue in 2002 alone). Further exacerbating the impact of this species is the simple fact that there are currently no methods in place for mitigating stings when this species is present other than through beach closures. Stinger exclusion nets are commonly used along the north-eastern coast of Queensland; however, these nets are designed to exclude large cubozoan species, primarily Chironex fleckeri, and do not exclude small species such as C. barnesi. Also, this species occurs with substantial spatial and temporal variability during the monsoonal summer months. Therefore, understanding the factors that contribute to this variability may facilitate the ability to model, and therefore predict, when and under what circumstances this species may be more prevalent. Currently, the ecological data required to produce a predictive model does not exist. Prior to the commencement of this research project, the early life history of C. barnesi had never been observed, or described, and nothing was known about the thermal and osmotic tolerance, or preference, of any of its life stages.
This study first describes the early life history C. barnesi, from egg fertilisation through to medusa production, and elucidates that this species develops an encapsulated planula stage that remains viable for six days to over six months. The polyps of C. barnesi asexually reproduce ciliated swimming polyps and produce medusae through monodisc strobilation. This study resulted in the first verified culture of C. barnesi polyps. With the polyp stage of the life cycle in culture, the opportunity to conduct manipulative temperature and salinity experiments were pursued, which provides new insights into potential polyp habitat suitability. Primary findings revealed 100% survivorship in osmotic treatments between 19‰ and 46‰, with the highest proliferation at 26‰. As salinity levels of 26‰ do not occur within the waters of the Great Barrier Reef or Coral Sea, it is concluded that the polyp stage of C. barnesi are probably found in estuarine environments, where these lower salinity conditions commonly occur.
With the relationship of temperature and salinity on the polyp stage known, focus was shifted to exploration of these factors on the medusa stage. The thermal and osmotic tolerance of C. barnesi medusae were investigated to determine if environmental parameters drive the marked seasonality of this species. By exploring oxygen consumption over a range of temperatures, the minimum thermal requirement for C. barnesi was estimated at 21.5ºC, which does not explain the seasonal occurrence of this species. The optimum temperature for swimming pulse rate was determined to occur between 27.5ºC and 30.9ºC and the optimum temperature was estimated at 29.2ºC, which encompasses the typical summer thermal regime in situ. This research concludes that reduced fitness associated with environmental temperatures that departure from optimum may better explain the seasonal pattern of this species. Conversely, departure from optimum temperature did not explain the southern distribution limits of this species, suggesting that C. barnesi could theoretically persist further south than their loosely defined southern distribution limits. The optimum salinity of C. barnesi medusae was estimated at 35.8‰ and fitness was reduced as salinity levels reduced below 29‰, adding further support that C. barnesi medusae are oceanic and cannot persist in estuarine environments, where low salinity conditions commonly occur. The respiration rate of C. barnesi was significantly suppressed at night, providing evidence that this species is less active during night conditions, presumably to conserve energy.
Further exploration of the diurnal behaviour pattern of C. barnesi medusae revealed that during light conditions, this species extends its tentacles and 'twitches' them frequently. This highlights the lure-like nematocyst clusters in the water column, which actively attract larval fish that are consequently stung and consumed. This fishing behaviour was not observed during dark conditions, presumably to reduce energy expenditure when they are not luring visually oriented prey. Larger medusae were found to have longer tentacles; however, the spacing between the nematocyst clusters was not dependent on size suggesting the spacing of the nematocyst clusters is important for prey capture. Additionally, larger specimens twitch their tentacles more frequently than small specimens, which correlate with their ontogenetic prey shift from plankton to larval fish. These results indicate that adult medusae of C. barnesi are not opportunistically grazing in the water column; instead, they utilise sophisticated prey capture techniques to specifically target larval fish.
This thesis also discusses the results of each of the experiments as a whole, and highlights areas where future research is required to predictively model the occurrence of this species. The overall focus of this thesis was to better understand the ecology and physiological limitations of C. barnesi to elucidate the factors that may contribute to the observed seasonal and distributional patterns. This research has also produced the baseline data for future research to build upon, with the expectation that the synthesis of these and future data will facilitate the ability to model, and therefore predict, the occurrence of this species in order to reduce the number of people stung.
LINK TO RESEARCH (published 2017)