Abstract
Species of the genus Euphausia dominate the euphausiid biomass of the Southern Ocean, the three largest being Euphausia superba, E. triacantha and E. crystallorophias. We measured a number of morphological features to identify differences between, and within, these species to obtain ecological insights. Interspecifically, the greatest difference was carapace size, with that of E. superba being by far the largest and most variable. This likely reflects its prolific spawning capacity compared with other euphausiid species. E. triacantha exhibited an extended sixth abdominal segment that could facilitate greater levels of thrust in the tail flip escape response. The pleopods, which provide propulsion in forward swimming, were more than 50% larger in E. superba, indicating a greater capacity for directional movement at high velocities. E. crystallorophias had eyes that were almost double the size of those in E. superba and E. triacantha, which may help retain visual resolution within its under-ice habitat. Intraspecifically, we found the above morphological features differed little between sexes and developmental stages in E. crystallorophias and E. triacantha, but differed significantly in E. superba. Compared to females and juveniles, male E. superba had significantly larger eyes and pleopods, whilst the carapace in males became shorter as a proportion of body length during growth. These features indicate a greater capacity for searching and swimming in males, which, we hypothesise, increases their ability to locate and fertilise females. This morphological specialisation in male E. superba is indicative of comparatively greater inter-male competition resulting from its tendency to form large, dense swarms.
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Data Availability
All raw data on which this study is based are published as an appendix (Appendix 1) to the present manuscript.
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Acknowledgements
We thank the officers and crew of the RRS John Biscoe and RRS James Clark Ross for their assistance in the collection of specimens. We are also grateful to laboratory supervisors Barry Penrose from the University of Hull and Paul Geissler and Guy Hillyard at BAS for facilitating collection of measurement data. GAT carried out this work as part of the Ecosystems programme at the British Antarctic Survey. We are grateful for the comments and suggestions of the three anonymous reviewers in helping to improve this manuscript.
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The research was supported by the British Antarctic Survey, Natural Environment Research Council and by the University of Hull.
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CH, MLJ and GAT devised this study with supporting ideas from JFL. All measurements were performed by CH. GAT wrote the manuscript with support from all authors.
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All work was completed in compliance with British Antarctic Survey (BAS) procedures, following the Antarctic Treaty Environmental Protocol (1996), which requires the prior assessment of all activities in the Antarctic Treaty Area, and is applied by BAS with equal rigour to South Georgia. Work on Antarctic krill (Euphausia superba) and other euphausiid species is exempt from the UK Animals (Scientific Procedures) Act 1986, but all work was compliant with recommended procedures in the handling and treatment of specimens.
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Tarling, G., Hobbs, C., Johnson, M. et al. Comparative morphology of Southern Ocean Euphausia species: ecological significance of sexual dimorphic features. Polar Biol 43, 2043–2058 (2020). https://doi.org/10.1007/s00300-020-02764-6
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DOI: https://doi.org/10.1007/s00300-020-02764-6