Statoliths of cubozoan jellyfishes: their utility to discriminate taxa and elucidate population ecology

Mooney, Christopher James (2014) Statoliths of cubozoan jellyfishes: their utility to discriminate taxa and elucidate population ecology. PhD thesis, James Cook University.

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Abstract

Collectively known as the box jellyfishes, cubomedusae have been documented around the world up to latitudes of 42° but are primarily found within the tropics. Cubozoans are unique among the Cnidaria for their complex sensory systems including camera type lens eyes, their strong directional swimming ability and orientation behaviour, yet perhaps best known for their powerful venom with envenomation from some species fatal to man. Two distinct syndromes including the delayed symptomatic Irukandji syndrome from some carybdeids, and the immediately painful and potentially fatal envenomation from large chirodropids, are well known from Australian tropical waters and have gained increasing global interest over recent times. Due to the gelatinous nature and rarity of most species identification has been difficult, and as not all cubozoans are uniformly venomous to humans reliable identification is critical for informed risk management. The population structure of cubozoans is poorly understood at spatial scales that range from hundreds of metres to thousands of kilometres. Further, despite their recognised mobility there are few descriptions of how far they move over days. A greater knowledge of the ecology of these elusive creatures would also provide invaluable information for risk management in areas where cubozoan envenomations are of concern. The objective of this thesis was to utilise the shape and elemental chemistry of cubozoan statoliths to gain knowledge on identification, population structure and sources and dispersal of cubomedusae. Morphometric analysis of hard structures and elemental chemistry assays of calcified structures have been used for identification and population assessment investigations in other marine phyla. The approach taken in this thesis was to use the only hard, calcified structure in cubomedusae (the statolith) to address the following issues: can statolith shape be used to discriminate among species of cubozoans (Chapter 2); can statolith shape discriminate among populations of medusae within a metapopulation (Chapter 3); can elemental chemistry of statoliths be used to discriminate among populations (Chapter 4); and can within statolith variation in elemental chemistry be used to determine the movement of cubozoans between water masses of different salinity (Chapter 5).

Statoliths provided a hard structure for robust morphometric analysis. In Chapter 2 traditional morphometric Length to Width ratios (L: W) and modern morphometric Elliptical Fourier Analysis (EFA) were applied to proximal, oral and lateral statolith faces of 13 cubozoan species to determine if statolith shape could discriminate among taxa. Both L: W and EFA were successful in quantitatively discriminating among species with EFA outperforming L: W as L: W gave no account of the curvature of the statolith. Best discrimination was achieved when analysing all three statolith faces in combination, with Canonical Discriminant Analysis (CDA) of Normalised Elliptical Fourier (NEF) coefficients classifying 99% of samples to their correct species group. The species that showed greatest differentiation in shape from other species were Chironex fleckeri and Copula sivickisi with more elongated shaped statoliths. Statolith shape was not dependent on statolith size in all species except for Malo genus, yet this had little effect on the successful classification of samples to Malo species. Similarities were seen in statolith shape within families and statolith shape agreed with currently accepted Cubozoa taxonomy.

Little is known on cubomedusae population structure and what is known for many species is mostly from a metapopulation perspective, corresponding to the documented range of species. Chapter 3 tested whether statolith shape analysis could be used to distinguish among cubomedusae populations. Twenty medusae of Carukia barnesi, Copula sivickisi (Carybdeida) and Chironex fleckeri (Chirodropida) were each collected from three distinct populations, separated by kilometres to hundreds of kilometres, around northern Queensland, Australia coastline and nearshore islands. CDA was performed on NEF coefficients for statolith proximal, oral and lateral faces and combinations of statolith faces for each species to determine the morphometrics that were best for discriminating populations of cubozoans. Significant discrimination of sampling populations was achieved in two of 21 CDAs (C. sivickisi statolith proximal face and C. fleckeri oral + lateral faces). As such statolith shape analysis was capable of successful discrimination among sampling locations but was not a robust technique among all species.

The use of natural geochemical signatures based on elemental composition of calcified structures is a common tool for investigating population structure or connectivity in marine systems. Chapter 4 used Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICPMS) to extract elemental composition of statolith zones of core, edge and whole statoliths at varying spatial and temporal scales to investigate C. fleckeri medusae population structure; univariate (element/Ca ratios) and multivariate (multi-element/Ca signature) comparisons were used. Best discrimination among statoliths was found with whole statoliths. A nested ANOVA found significant differences among regions (separated by hundreds of kilometres) and sites (separated by kilometres) within those regions in some elemental ratios. Sr/Ca and Fe/Ca were found to significantly differ between sites within regions for all statolith zones. CDA of multielement/ Ca signatures successfully distinguished among regions for all statolith zones and sites within regions for whole statoliths. Differences in statolith elemental chemistry over four seasons, or within a season, were generally small, however Sr/Ca significantly differed at both temporal scales. A positive relationship was found between statolith Sr/Ca and temperature.

Very little is known on the sources and movements of potentially fatal C. fleckeri medusae which are found around estuary mouths and beaches along tropical coastlines of Australia, yet largely anecdotal evidence suggests an alternating season of polyps in protected estuaries during the dry season and medusae emerging from estuaries to feed along beaches with the onset of the monsoonal season. It has been hypothesised that elemental signatures in statoliths could provide a Sr – based measure of movements in statoliths. In Chapter 5 an experiment was conducted on young wild caught C. fleckeri medusae to establish how elemental incorporation into statoliths was affected by salinity. A critical salinity test revealed medusae inhabit salinities > 20, as they die in lower salinities. Medusae were held in salinities of 22, 26, 30 and 34 for a duration of four days. LA-ICPMS was used to analyse experimental areas of statoliths and solution based-ICPMS (SO-ICPMS) used for analysing water samples taken from each treatment. Mg/Ca and multi-element/Ca signatures significantly differed among treatments but Sr/Ca or other elements/Ca did not. Partition coefficients were element dependent with D(S)ᵣ, D(Mg) and D(L)ᵢ 2.62 x 10⁻⁶ – 0.81 and D(B)ₐ, D(Mn), and D(Zn) 1.87 - 431. An a posteriori comparison of statolith Sr/Ca and water temperature exposure suggested that strong significant patterns seen in statolith Sr/Ca (Mooney & Kingsford 2012; Appendix B) were the result of changes in temperature exposure and not salinity; statolith Sr/Ca may reveal patterns of movement based on temperature profiles.

The studies in this thesis have shown that the cubozoan statoliths are an invaluable tool for identifying medusae and investigating population structuring. Statolith shape was species specific and can quickly assign specimens to at least family by basic L: W of statolith faces which has great potential to assist in studies on phylogeny and risk management in areas where box jellyfish envenomations are a concern as the severity of envenomation is family dependent. Although only 13 of the viii 41 currently accepted cubomedusae were studied here statolith shape is likely to be a useful taxonomic tool for the Class and statolith proximal, oral and lateral faces should be included in descriptions of all species. Statolith shape also successfully discriminated among sampling locations for two of three species: C. sivickisi at ~ 250 km; C. fleckeri at ~ 15 km. The ecological niche, and associated ecological pressures, of some cubozoan species (e.g. C. barnesi) may not change enough for differences in statolith shape among locations to occur within a species. Statolith shape and elemental chemistry in combination with other stock identification techniques may assist investigations into population structure of cubomedusae. My study concurs with evidence based on patterns of abundance that C. fleckeri medusae populations are most likely highly localised at spatial scales of kilometres and mixing between local populations appears minimal. C. fleckeri statolith Sr/Ca was not significantly affected by salinity changes over the salinity range in which they inhabit but Mg/Ca and multielement/ Ca signatures were. Statolith microchemistry shows promise for determining the environmental conditions that jellyfish experience if local salinity and thermal gradients are known. The techniques applied here could assist in identification and population studies of cubomedusae worldwide.

Item ID: 40730
Item Type: Thesis (PhD)
Keywords: box jellyfish; chironex fleckeri; cubomedusae; cubozoa; cubozoan jellyfish; elemental chemistry; identification; jellyfish; jellyfishes; marine ecology; medusa; morphometrics; otoliths; population dynamics; populations; statoliths
Additional Information:

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

Appendix A: Kingsford, Michael J., and Mooney, Christopher J. (2014) The ecology of box jellyfishes (Cubozoa). In: Pitt, Kylie A., and Lucas, Cathy H., (eds.) Jellyfish Blooms. Springer, Dordrecht, The Netherlands, pp. 267-302.

Appendix B: Mooney, C. J., and Kingsford, M. J. (2012) Sources and movements of Chironex fleckeri medusae using statolith elemental chemistry. Hydrobiologia, 690 (1). pp. 269-277.

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Date Deposited: 14 Oct 2015 01:55
FoR Codes: 06 BIOLOGICAL SCIENCES > 0602 Ecology > 060205 Marine and Estuarine Ecology (incl Marine Ichthyology) @ 100%
SEO Codes: 96 ENVIRONMENT > 9610 Natural Hazards > 961002 Natural Hazards in Coastal and Estuarine Environments @ 100%
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