The impacts of climatic change on organisms depend on the interaction of multiple stressors and how these may affect the interactions among species. Consumer-prey relationships may be altered by changes to the abundance of either species, or by changes to the per capita interaction strength among species. To examine the effects of multiple stressors on a species interaction, we test the direct, interactive effects of ocean warming and lowered pH on an abundant marine herbivore (the amphipod Peramphithoe parmerong), and whether this herbivore is affected indirectly by these stressors altering the palatability of its algal food (Sargassum linearifolium). Both increased temperature and lowered pH independently reduced amphipod survival and growth, with the impacts of temperature outweighing those associated with reduced pH. Amphipods were further affected indirectly by changes to the palatability of their food source. The temperature and pH conditions in which algae were grown interacted to affect algal palatability, with acidified conditions only affecting feeding rates when algae were also grown at elevated temperatures. Feeding rates were largely unaffected by the conditions faced by the herbivore while feeding. These results indicate that, in addition to the direct effects on herbivore abundance, climatic stressors will affect the strength of plant-herbivore interactions by changes to the susceptibility of plant tissues to herbivory.

High density populations of the crown-of-thorns seastar, Acanthaster planci, are a major contributor to the decline of coral reefs, however the causes behind periodic outbreaks of this species are not understood. The enhanced nutrients hypothesis posits that pulses of enhanced larval food in eutrophic waters facilitate metamorphic success with a flow-on effect for population growth. The larval resilience hypothesis suggests that A. planci larvae naturally thrive in tropical oligotrophic waters. Both hypotheses remain to be tested empirically. We raised A. planci larvae in a range of food regimes from starvation (no food) to satiation (excess food). Algal cell concentration and chlorophyll levels were used to reflect phytoplankton conditions in nature for oligotrophic waters (0-100 cells ml-1; 0-0.01 μg chl a L-1), natural background levels of nutrients on the Great Barrier Reef (GBR) (1,000-10,000 cells ml-1; 0.1-1.0 μg chl a L-1), and enhanced eutrophic conditions following runoff events (100,000 cells ml-1; 10 μg chl a L-1). We determine how these food levels affected larval growth and survival, and the metamorphic link between larval experience and juvenile quality (size) in experiments where food ration per larvae was carefully controlled. Phytoplankton levels of 1 μg chl a L-1, close to background levels for some reefs on the GBR and following flood events, were optimal for larval success. Development was less successful above and below this food treatment. Enhanced larval performance at 1 μg chl a L-1 provides empirical support for the enhanced nutrients hypothesis, but up to a limit, and emphasizes the need for appropriate mitigation strategies to reduce eutrophication and the consequent risk of A. planci outbreaks.

We examined the long-term effects of near-future changes in temperature and acidification on skeletal mineralogy, thickness, and strength in the sea urchin Tripneustes gratilla reared in all combinations of three pH (pH 8.1, 7.8, 7.6) and three temperatures (22 °C, 25 °C, 28 °C) from the early juvenile to adult, over 146 days. As the high-magnesium calcite of the echinoderm skeleton is a biomineral form highly sensitive to acidification, and influenced by temperature, we documented the MgCO3 content of the spines, test plates, and teeth. The percentage of MgCO3 varied systematically, with more Mg2+ in the test and spines. The percentage of MgCO3 in the test and teeth, but not the spines increased with temperature. Acidification did not change the percentage MgCO3. Test thickness increased with warming and decreased at pH 7.6, with no interaction between these factors. In crushing tests live urchins mostly ruptured at sutures between the plates. The force required to crush a live urchin was reduced in animals reared in low pH conditions but increased in those reared in warm conditions, a result driven by differences in urchin size. It appears that the interactive effects of warming and acidification on the Mg2+ content and protective function of the sea urchin skeleton will play out in a complex way as global climatic change unfolds.

Bibliographic citationBibliographic citation Wolfe, K; Graba-Landry, Alexia; SA Dworjanyn; Byrne, M (2015). Larval phenotypic plasticity in the boom-andbust crown-of-thorns seastar, Acanthaster planci. University Of Tasmania. Journal contribution. https://figshare.utas.edu.au/articles/journal_contribution/Larval_phenotypic_plasticity_in_the_boom-and-bust_crown-of-thorns_seastar_Acanthaster_planci/23002589

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