The macroecology of globally-distributed deep-sea jellyfish

Abstract

Macroecology provides a framework for understanding how local- and regional-scale processes interact, allowing us to understand how the biological and ecological traits of individual species influence large-scale patterns in diversity. The majority of macroecological studies to date have been centred on the terrestrial environment where large databases on species ranges, body size and associated environmental variables are readily available. Due to the inaccessibility of the deep sea, coupled with its relatively recent exploration, deep sea macroecology is the least represented within marine macroecology as a whole. Jellyfish, a significant constituent of the zooplankton, form important and often conspicuous components of marine ecosystems. Jellyfish studies covering large spatial scales are mostly focused on the shallow-water, bloom-forming species that have more apparent anthropogenic interactions, such as Aurelia aurita. The structural simplicity of jellyfish permits the rapid adaptation to changing environments. Plasticity in traits such as feeding, physiology, reproductive output, somatic growth and size are common; and as such allow populations to persist. The coronate medusae Periphylla periphylla Péron and Lesueur, 1810 and Atolla spp. are the most recognised deep-sea jellyfish, and both have cosmopolitan distributions. Little remains known about these genera beyond the early descriptions of the species, particularly relating to their macroecology and the expression of plastic traits according to varying environments. This study presents a large volume of morphological data using museum collections genera in order to better describe morphological variation on a global scale and to examine what factors might drive such variation.

P. periphylla and Atolla spp. exhibit cosmopolitan distributions across the global dataset, found at depths ranging from 0 to 5486 m and 4900 m respectively. Across the global oceanic dataset and case study areas of the Iberian Basin and Porcupine Abyssal Plain, P. periphylla demonstrate no morphological plasticity across temporal or spatial scales, with variation in tentacle number observed within a number of Atolla species, A. gigantea, A. parva, A. vanhoffeni and A. russelli. This may be indicative of the genetic distance between the two species within Coronatae. The first comprehensive comparison between fjord and oceanic P. periphylla populations is described. Contrasting patterns were observed between fjord and oceanic environments, with larger males than females within the fjord population, and larger females than males within the oceanic population. Larger fjord specimens were observed across all sample seasons. The oceanic population provides evidence of the prioritisation of reproductive output within the more variable oceanic environments. Novel methods to further the understanding of volumes of statoliths within medusae statocysts were explored, with specimens of P. periphylla of varying sizes demonstrating a range of statolith numbers, sizes and crystalline morphologies. Statoliths form part of the sensory organs and represent the only hard structures within medusae. Both the number and size of statoliths is proposed to be indicative of the individual medusa age. This thesis shows that macroecological scales are important to consider when comparing globally-distributed species, and highlights the potential of using historical collections to identify ecological patterns. By combining the morphological data with molecular analyses, it would be possible to further the understanding of ecological divergence and speciation in deep sea coronates. Rearing specimens of a known age would also further the understanding of sclerochronological processes and help to determine age in medusae which are believed to be long-lived.

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ORCID for Catherine Lucas: orcid.org/0000-0002-5929-7481

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