Size, Shape, Scope and Strength of skeletons, Evechinus chloroticus, New Zealand

• Main Author: Goodwin, Shannon Louise
• Other Authors: Smith, Abby, Lamare, Miles
• Format: Thesis
• Language: English
• Published: University of Otago 2020
• Subjects: Evechinus chloroticus; Ocean acidification; Skeletal allometry; Spine strength; Carbonate mineralogy; Echinoid; New Zealand; White Island
• Online Access: http://hdl.handle.net/10523/10521

Ocean acidification will affect calcifying organisms as calcium carbonate saturation levels decrease due to climate change. Echinoids are important components of the coastal ecosystem and use magnesium in their skeletal calcification. Magnesium-calcite is highly susceptible to dissolution and the effects of lowered pH on the skeletal system of echinoids could be severe. This thesis examines the differences in allometry (shape and size), biomineral composition and flexural strength of Evechinus chloroticus skeletal components from six separate populations around New Zealand, including Whakaari White Island as a proxy for future ocean acidification conditions. I measured 20 parameters for 64 individuals from six different locations. Individual skeletal elements within individuals and populations exhibited little variation in size and shape, particularly in those elements comprising the Aristotle’s lantern. Using a standardised measurement to compensate for size of the individual, there was no obvious trend noted amongst locations except for weight of Aristotle lantern components, demonstrating a linear trend of increasing weight with increasing latitude. Evechinus produces skeletons formed of magnesium-calcite (range=3.2–11.9, average=8.6 wt % MgCO3 in calcite  2.01 SD, N=90); here I compared magnesium content in skeletal elements, which showed little variation within and between individuals of the same population. Variation among populations was also minimal. Magnesium content in test plates and Aristotle's lantern components was 9.5 wt % MgCO3 ( 0.4 SD, N=54), whereas spines were in general lower in magnesium (4.9,  0.23 SD, N=18). Flexural strength of primary spines, measured by a two-point bending test was (average=112.0,  37.0 SD, N=640). The data exhibited a broad latitudinal trend with strength increasing with latitude, presumably linked to temperature. Spines from Fiordland, however, were weaker than expected; and those from White Island were stronger, likely due to the influence of seawater chemistry and/or growth rate. Skeletal elemnts of Evechinus chloroticus around New Zealand exhibit minimal variations in response to different abiotic conditions. Tight morphological constraints on parameters, for example the Aristotle’s lantern minimise variations exhibited by individuals and populations, while other parameters like spine morphology are thought to result from biotic pressures. Individual variation in biominerals are minimal and differences observed in magnesium content are expected to arise from different chemical pathways being utilised to offset conditions each calcium structure is exposed to. Strength is the ultimate result from differences in morphology and biominerlisation, affecting the locomotion and defence mechanisms of Evechinus chloroticus. Latitude, and subsequent temperature is linked to spine strength; however, using White Island specimens as a proxy for individuals in future climate change scenarios, there was not an expected decrease in strength below what is anticipated from temperature differences. It is accepted that individuals and populations of Evechinus chloroticus around New Zealand have adapted to maintain skeletal conditions. Keywords: Evechinus chloroticus ▪ Ocean acidification ▪ Skeletal allometry ▪ Spine strength ▪ Carbonate mineralogy ▪ Echinoid