The Sea Urchin Centrostephanus Tenuispinus (Clark, 1914) is an Important Bio-Eroder on a High Latitude (32o S) Coral Reef.

31 Sea urchins are keystone herbivores in many marine benthic habitats. They can significantly influence 32 coral-algae phase shifts and impact on reef carbonate budgets through grazing. Hall Bank reef in 33 Western Australia is unique among other reefs in the region being high latitude with a high 34 hermatypic coral cover but lacking macroalgae and soft corals. Since the reef status is thought to 35 result from high densities of the urchin Centrostephanus tenuispinus limiting the growth of 36 macroalgae, the present study was focused on evaluating their role as bio-eroders. Monthly samples of 37 26 urchins were collected from 2014-2016 and gut composition was analyzed. Gut evacuation rates 38 were calculated using 50 urchins dissected at time intervals (0, 4, 8, 16, 24, 36, 48, 60, 72 and 96 h). 39 Reworked calcium carbonate was calculated using 30 urchins maintained in five cages in a seagrass 40 bed adjacent to the reef site. Mean percentages for organic component, calcium carbonate and other 41 siliceous components were 86.29 ± 3.23%, 10.32 ± 2.76% and 3.39 ± 1.52% respectively. Gut 42 evacuation rates for autumn, winter, spring and summer were 0.70, 0.24, 0.48 and 0.72 (day -1 ). Bio- 43 erosion rates were significantly higher in Summer-16 (3.52 g CaCO 3 m -2 day -1 ) and lower in winter 44 (1.32 g CaCO 3 m -2 day -1 ) (F=101.580, p<0.000). High erosion rates were recorded for large urchins 45 (F= 37.789, P<0.001. Annual urchin bio-erosion was 1017.69 g CaCO 3 m -2 a -1 . Differences in food 46 ingestion rates in response to seawater temperature changes are thought to be the main cause for the 47 significant differences in seasonal bio-erosion rates. 48


Introduction
Sea urchins play a vital role in many marine benthic habitats as grazers, bio-eroders, producers of particulate inorganic matter and habitat providers (Scheibling 1986;Glynn 1988; urchins can be immense as they prevent the growth of all organisms other than encrusting algae. High m -2 a -1 ) compared to reef flats (0.5 kg CaCO3 m -2 a -1 ) (Mokady et al. 1996). Thus, the extent of bio-91 erosion can vary due to habitat structure. The extent of sea urchin bio-erosion mainly depends on 92 species, test size/diameter and population density (Bak 1994). Diadema antillarum has been reported 93 to cause erosion of 4.6 kg CaCO3 m -2 a -1 in a patch reef at St. Croix, Virgin Islands, US (9 individuals 94 m -2 ), and 5.3 kg CaCO3 m -2 a -1 in fringing reefs at Barbados (23 individual m -2 ). Carreiro-Silva and 95 McClanahan (2001) stated that, despite similar body sizes, bio-erosion can vary due to different 96 feeding behaviours (species-specific feeding strategies). The outcomes of these studies imply the need 97 for species-specific studies on bio-erosion in unique sea urchin-dominated habitats. Currently, all 98 studies focused on bio-erosion have been centred on tropical reefs, mainly Caribbean and Indo-pacific 99 regions; reefs in higher latitudes have not been studied.

100
Among other high latitude temperate reefs, Hall Bank reef (32° 2.002´S, 115° 42.957´E) in 101 southern Western Australia is unique due to its high hermatypic coral cover (mean = 52.6 ± 4.65%), 102 and scarcity of soft coral and macroalgae (Thomson and Frisch 2010      The initial mean dry gut-weight in summer and winter was 7.125 ± 2.18 g and 5.198 ± 1.40 199 g, respectively. The gut evacuation experiment revealed that 50% of the gut is emptied within 24 h in

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It is evident that the density of the urchin population has an immense impact on structuring

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Most previously conducted studies on bio-erosion focused on tropical urchins; mainly 287 Diadema, Echinometra and Echinothrix species, and these species are considered to be the most 288 impacting bio-eroding agents in tropical reefs (Bak 1990;Mokady et al. 1996). Environmental 289 conditions, specifically water temperature, impact the urchins' physiology, directly influencing 290 ingestion and gut evacuation rates. Since there were no significant differences in dry gut-weight 291 across the seasons sampled in this study, differences in ingestion rate are critical for determining the 292 rate of bio-erosion. Significant differences in seasonal ingestion rates are mainly in response to the 293 changes in seawater temperatures, from 17°C in winter to 22°C in summer. The gut evacuation rate 294 was highest in summer and lowest in winter. Calculations based on mean seawater temperatures in 295 autumn and spring indicated gut evacuation rates were 0.70 day -1 and 0.48 day -1 . Food ingestion rates 296 were higher in summer due to increased metabolism at higher temperatures; thus the erosion rates 297 were higher as well. Centrostephanus tenuispinus in the current study were subjected to lower water temperatures (17°C) in winter than in summer (23°C). A difference of 6°C directly impacted on physiology and activity levels, and therefore changes in ingestion rates between winter and summer.

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The highest ingestion rate recorded in this study was 5.02 ± 1.33gut dry weight individual -1 day -1 in 301 summer 2016, which is higher than. Diadema savygnyi from tropical waters with a mean ingestion rate of 3.11 ± 0.70 g gut dry weight individual -1 day -1 (Carreiro-Silva and McClanahan 2001). At the 303 same time D. setosum has shown an ingestion rate of 8.36 ± 1.16 g day -1 (Carreiro-Silva and 304 McClanahan 2001). Most urchin species in the tropics experience water temperatures over 25°C all 305 round year, and therefore have consistently high ingestion rates. Since the rate of bio-erosion is based 306 on food ingestion rates, any factor affecting food intake indirectly affects the bio-erosion rate as well.

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The influence of water temperature on feeding has been recorded for other diadematoids as well 308 (Coppard and Campbell 2005).

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Sea urchin feeding, and hence bio-erosion rate, is influenced by space, nature of habitat, 334 habitat structure and water temperature. Interaction of these factors causes higher bio-erosion in some 335 habitats. It is also essential to calculate annual reef accretion rates to determine the impact of bio-336 erosion on reef carbonate budgets.