The pelagic propagule’s toolkit: An exploration of the morphology, swimming capacity and behaviour of marine invertebrate propagules

Montgomery, Emaline M. (2017) The pelagic propagule’s toolkit: An exploration of the morphology, swimming capacity and behaviour of marine invertebrate propagules. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

The pelagic propagules of benthic marine animals often exhibit behavioural responses to biotic and abiotic cues. These behaviours have implications for understanding the ecological trade-offs among complex developmental strategies in the marine environment, and have practical implications for population management and aquaculture. But the lack of life stage-specific data leaves critical questions unanswered, including: (1) Why are pelagic propagules so diverse in size, colour, and development mode; and (2) do certain combinations of traits yield propagules that are better adapted to survive in the plankton and under certain environments? My PhD research explores these questions by examining the variation in echinoderm propagule morphology, locomotion and behaviour during ontogeny, and in response to abiotic cues. Firstly, I examined how egg colour patterns of lecithotrophic echinoderms correlated with behavioural, morphological, geographic and phylogenetic variables. Overall, I found that eggs that developed externally (pelagic and externally-brooded eggs) had bright colours, compared to the typically pale colour intensity of internally-brooded eggs. Additionally, my analysis suggested geographic location as a potential driver of the evolution of colour diversity through the selection of better-adapted pigments in response to ecological pressure. I then undertook a critical assessment of swimming capacity and sensory ability in propagules from four co-occurring North Atlantic echinoderms with two different types of pelagic development: the sea stars Asterias rubens (planktotrophic) and Crossaster papposus (lecithottrophic), the sea urchin Strongylocentrotus droebachiensis (planktotrophic), and the sea cucumber Cucumaria frondosa (lecithotrophic) at two different temperatures. Propagule swimming speed increased with ontogeny in two of the four species (the sea stars A. rubens and C. papposus) but did not uniformly increase with temperature. Contrary to initial assumptions, some lecithotrophic propagules emerged as the fastest swimmers (e.g., 1.2 mm s⁻¹ in the brachiolaria of C. papposus). Lastly, in a study of phototaxis involving the same focal species, variation in swimming speed and trajectory were detected when propagules were exposed to three different light colours. Taken together, the data generated by my PhD work provide a framework to assess the adaptive value of pelagic propagules to benthic animals, to examine the trade-offs of complex lifehistory strategies, and to enhance modeling of larval dispersal in the marine environment.

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