Description: We deployed underwater cameras (GoPro Hero3) to measure the abundance of predators of T. gratilla. According to previous literature, the main fish predators of T. gratilla in this region were Balistapus undulatus (Triggerfish, family Balistidae) and Cheilinus undulatus (Wrasse, family Labridae) (McClanahan and Muthiga, 1989; McClanahan and Shafir, 1990; Eklöf et al., 2009). Abundances of these predators in our study site were the main focus of these surveys. We deployed the cameras in four sampling events: 22.07.2017 (three cameras), 07.08.2017 (seven cameras), 28.08.2017 (six cameras) and 30.09.2017 (six cameras). We deployed three cameras simultaneously, attached to a 50 cm tall wooden pole. Each camera recorded a mean (± standard error) of 53 ± 2 minutes and an area of 10 m2, resulting in a total of ~20 hours of video and a total surveyed area of ~220 m2. We used the maximum count method for counting fishes in the video surveys (Conn, 2011). We counted the fishes as the number of times they appear in the recorded area (i.e. if one fish left the area recorded and appeared again, it was counted as a new fish, as defined by Conn, 2011). This method was used to ensure that any predatory fish of T. gratilla would be detected despite the likely over-estimation of fish counts of abundant fish families. The fishes that appeared in the videos were screenshotted and identified using the fish identification guide developed by Allen et al. (2003). We created one category per fish family identified, two individual categories for the fish species that are predators of T. gratilla (B. undulatus and C. undulatus) and one category for unidentifiable fishes (generally due to suboptimal visibility). The final number of fishes is the individuals count.

Description: To determine the gut content of T. gratilla, we sampled 10 sea urchins along a transect parallel to the coastline and transported them to the Institute of Marine Sciences (Stone Town, Zanzibar). The urchins were collected upon encounter along the transect with the rule of not collecting urchins from the same seagrass patch. This rule was applied to avoid bias towards specific seagrass species. For this analysis, we weighed the sea urchins, measured their diameter to the nearest millimeter with a ruler, and stored them in a 90% ethanol solution. For the gut content analysis, the sea urchins were opened and the content from the whole digestive tract was extracted. The gut content was placed in a Petri dish, and then transferred to a 250 µm mesh to remove the sediment. After the sediment removal, we weighed the gut content fresh weight. Approximately 1 gram (fresh weight) was taken as a sub-sample from the gut content and placed in a new Petri dish to observe under the magnifying glass. Each sample was compared to an example of seagrass material that was prepared specifically for identification after the grazing and digestion process of each species. We imitated this effect by grinding the seagrass species to a similar size and storing them in 90% ethanol solution. Using this method, we could separate the gut contents into the different seagrass species. Contents were then dried in the oven at 60 degrees for 24 hours to obtain their dry weight, and percentage of each seagrass species in the gut was calculated.

Description: Percentage cover and shoot density of seagrasses were measured taking three random points in each plot at each sampling time with 0.25 m2 and 0.01 m2 quadrats, respectively. Percentage cover was included as the only variable for the measurement of benthic macroalgae as macroalgae could not be assessed by shoot density. The Braun-Blanquet scale was used for the cover measurements (Mueller-Dombios and Ellenberg 1974), consisting of a scale of 8 numbers each one referring to an interval of percentage cover. The scale goes as follows: 0.1 = 〈 5% solitary, 0.5 = 〈 5% sparse, 1 = 〈 5% numerous, 2 = cover ≥ 5% - ≤ 25%, 3 = cover 〉 25% - ≤ 50%, 4 = cover 〉 50 - ≤ 75%, and 5 = cover 〉 75%. The cover categories were transformed into the midpoint cover range (Braun-Blanquet 1964). The final units is the percentage cover (%) of each seagrass species and macrolalgae in the quadrat. Shoot density was measured counting the individual shoots of the two seagrass species within the 0.01 m2 quadrat (Number of shoots per 0.01 square meters). The number of shoots were then standarized by square meter to obtain the final shoot density (number of shoots/m2).