1 Feeding preferences of the sea urchin Diadema 2 setosum (Leske, 1778) in Taklong Island National 3 Marine Reserve, Guimaras, Philippines 4 Jennelle Christianne S. Luza1, Maria Celia D. Malay1, 5 1 Division of Biological Sciences, College of Arts and Sciences, University of the Philippines 6 Visayas, Miagao, Iloilo, Philippines 7 8 Corresponding Author: 9 Jennelle Christianne Luza1 10 Brgy. Inzo Arnaldo Village, Roxas City, Capiz, 5800, Philippines 11 Email address: [email protected] 12 13 14 Abstract 15 Background. Sea urchins are keystone herbivores that greatly influence primary productivity, 16 algal abundance and scleractinian coral recruitment. The long-spined black sea urchin Diadema 17 setosum is widespread and abundant in reef flats throughout the Philippines. Prior studies 18 regarding the feeding preference of D. setosum have been conducted overseas, but little is known 19 about the impact of the echinoid herbivory on reef flat communities in the Philippines. Feeding 20 preferences of D. setosum on four common marine plant species, Halimeda macroloba, 21 Ceratodictyon spongiosum, Padina sp., and Enhalus acoroides were investigated at the 22 University of the Philippines Visayas Marine Biological Laboratory, located in Taklong Island 23 National Marine Reserve (TINMR), Guimaras. 24 Methods. Two food choice experiments were conducted; choice feeding and no-choice feeding. 25 The outcome of choice feeding experiments, expressed as consumption (in g) and percent 26 consumption (%), were used to determine its feeding preferences. The two most preferred feeds 27 determined were then used in no-choice feeding experiment to measure its consumption rate 28 (g⸱echinoid-1⸱hr-1). 29 Results. Results of the choice feeding experiment show that D. setosum significantly prefers C. 30 spongiosum (4.83 ± 2.56 g consumption or 32.2%) and H. macroloba (3.73 ± 2.27 g or 24.8%), 31 and avoids E. acoroides (only 0.17 ± 0.22 g or 1.13%) (F= 5.423, p < 0.05). The no-choice 32 feeding experiment between preferred feeds show H. macroloba was consumed more (0.22 ± 33 0.16 g⸱echinoid-1⸱hr-1) than C. spongiosum (0.15 ± 0.05 g⸱echinoid-1⸱hr-1) although there was no 34 significant difference (p > 0.05) in consumption rate. Results of the no-choice feeding 35 experiment may have been affected by poor water quality and are considered inconclusive. 36 Nevertheless, the study supports the ecological role of D. setosum as an important herbivore that 37 regulates certain macroalgal species in TINMR through its grazing activities. 38 39 Introduction 40 Sea urchins play a vital role in marine ecosystems especially in shallow tropical seas. They are 41 considered keystone herbivores as they effectively influence marine plant populations such as 42 algae and seagrasses, their primary productivity and abundance, and scleractinian coral 43 recruitment by grazing on algae that compete with corals (Shunula & Ndibalema, 1986; Alves et PeerJ Preprints | https://doi.org/10.7287/peerj.preprints.27733v1 | CC BY 4.0 Open Access | rec: 15 May 2019, publ: 15 May 2019 44 al., 2003). Studies about sea urchins peaked after a massive mortality event of Diadema 45 antillarum in the Caribbean in 1983 which caused drastic changes in the coral reef community 46 structure (Lessios et al., 1984). The disappearance of urchins in the area resulted in increased 47 density and diversity of algal species and led to higher algal cover (Solandt & Campbell, 2001). 48 In terms of feeding ecology, grazing urchins operate as either generalists or specialists in 49 a community (Stimson, Cunha & Philippoff, 2007). Some urchins feed on available algae in its 50 environment (Solandt & Campbell, 2001). In Hawaii, studies about the native sea urchin 51 Tripneustes gratilla showed that it can function as biocontrol agent for invasive algae (Stimson, 52 Cunha & Philippoff, 2007; Westbrook et al., 2015). However, Tomas, Box & Terrados et al. 53 (2011) suggested that some sea urchin species like Paracentrotus lividus do not function as 54 control agent of invasive algal species. Other studies report that different species of sea urchins 55 exhibit feeding preferences (Larson, Vadas & Keser, 1980; Hay, Lee & Guieb, 1986; Solandt & 56 Campbell, 2001; Tuya et al., 2001; Stimson, Cunha & Philippoff, 2007; Kasim, 2009; Lyimo et 57 al., 2011; Seymour et al., 2013). 58 Food preferences of sea urchins may be influenced by the distribution and abundance of 59 its food source (Seymour et al., 2013), the chemical and morphological properties (Shunula & 60 Ndibalema, 1986; Solandt & Campbell, 2001; Tuya et al., 2001; Erickson et al., 2006; Souza, de 61 Olivera & Pereira, 2008; Seymour et al., 2013), and caloric content (Larson, Vadas & Keser, 62 1980) of plant species, and the different stages of sea urchin development (Westbrook et al., 63 2015). Hay, Lee & Guieb (1986) also stated that chemotaxis correlated with daytime and 64 nighttime hours affect the feeding behavior of the urchin. Additionally, the preferred and non- 65 preferred feeds of sea urchin differ at different seasons of the year (Larson, Vadas & Keser, 66 1980; Seymour et al., 2013). 67 Diadema setosum, a black and long-spined sea urchin having distinct white dots on its 68 body, is widespread along Indo-Pacific regions including Philippines and is thought to be 69 ecologically important in shallow subtidal ecosystems. Their gonads serve as a delicacy in many 70 local communities and are targeted as wild fishery. Diadema setosum forages at night in the 71 tropics to avoid predators (Lawrence & Hughes-Games, 1972). Studies have reported feeding 72 preferences of D. setosum on a specific macroalgal species vary in different areas around the 73 world (Shunula & Ndibalema, 1986; Moore et al., 2019). Tatsuya, Miyuki & Akira (2016), also 74 reported that grazing and high densities of D. setosum control algal coverage and density on the 75 seaweed bed ecosystems along the central coast of Japan. However, in Singapore reefs, D. 76 setosum is not an important component of the herbivore guild (Goh & Lim, 2015). Seasonal 77 changes have also been reported in the size of the gut of some sea urchins related to changes in 78 food availability (Lawrence, Lawrence & Watts, 2013), there were no changes in D. setosum in 79 Red Sea or on Kenyan reefs (Pearse, 1974; Muthiga, 2003). Diadema setosum was also found 80 out to be a key symbiont of cardinalfish Pterapogon kauderni in Indonesia (Moore et al., 2019). 81 Interestingly, the work of Coppard & Campbell (2007) in Fiji show that sea urchins under the 82 same genus, Diadema setosum and D. savignyi exhibit selective grazing, with distinct feeding 83 preferences. 84 While many studies about the feeding ecology of D. setosum have been conducted in 85 different countries, very little research has been undertaken in Southeast Asia especially in the 86 Philippines. 87 The study was conducted to determine if D. setosum exhibits a preference for different 88 PeerJ Preprints | https://doi.org/10.7287/peerj.preprints.27733v1 | CC BY 4.0 Open Access | rec: 15 May 2019, publ: 15 May 2019 89 feeds (seagrass and macroalgae species) presently dominating in Taklong Island National Marine 90 Reserve (TINMR), Guimaras. The study also determined the rate of consumption of D. setosum 91 for different feeds. Feeding preference assay was done by (a) determining the consumption (g) of 92 the urchins when offered a choice of feeds, to see what the sea urchin preferred, then present the 93 values in percent consumption (%), and (b) investigating how much of the preferred feeds can D. 94 setosum consume in a given amount of time (consumption rate) using single diet experiment. 95 Since sea urchins mainly feed on micro- and macroalge, and others on seagrasses, detrital 96 particles and corals (Cabanillas-Teran et al., 2016), these grazing herbivores can be implicated as 97 a driver of phase shifts in marine environments (Kriegsch et al., 2016). Ecologists are greatly 98 interested on the feeding preferences of sea urchins as it not only determines the phase shifts in 99 an ecosystem, such as shifting from a coral-dominated to macroalgal-dominated system, but can 100 also provide trophic links in community food webs. Aquaculturists also use feeding preferences 101 to determine the quantity and quality of food ingested to determine the optimal physiological 102 condition of sea urchins. Understanding the feeding preference and feeding rate of D. setosum 103 will help predict the impact of herbivory on coral and seagrass communities since feeding 104 preferences interact with plant competitive abilities, life histories, and physical tolerances in 105 determining the impact of a grazer on the marine benthic community (Coppard & Campbell, 106 2007). This study will also help in sustainable management of both the sea urchin and marine 107 plant species in an ecosystem. 108 109 Materials & Methods 110 Experimental organisms 111 Twenty (20) Diadema setosum sea urchins were collected from the rocky shore areas in the UPV 112 Channel separating Taklong Island from mainland Guimaras, located within the Taklong Island 113 National Marine Reserve (TINMR), Guimaras. Utmost care was taken in collecting D. setosum 114 since its long, black spines are fragile. A custom-made sea urchin scooper made from thin rebar 115 was designed for properly collecting the echinoids (Fig. 1). Animals were then placed in large 116 bins (450 l) with aerated sea water, held just outside the laboratory. The sea urchins were starved 117 for at least 48 hours prior to use in assays to acclimatize and to overcome any possible period of 118 ingestive conditioning (Solandt & Campbell, 2001). The tanks were shaded from direct sunlight 119 and were exposed to natural photoperiod. 120 121 122 Figure 1. Custom-made tool for scooping sea urchins (design provided by Harilaos Lessios). PeerJ Preprints | https://doi.org/10.7287/peerj.preprints.27733v1 | CC BY 4.0 Open Access | rec: 15 May 2019, publ: 15 May 2019 123 124 A preliminary survey was done on the forereef of UPV Channel to determine the most 125 common marine plant species by snorkeling around the area.