**1. Introduction**

Among the myriad anthropogenic impacts on the world's oceans, perhaps the most significant is the increase in temperature associated with production of greenhouse gases [1]. This warming is responsible for large-scale changes in circulation and storm activity through melting of glaciers, warming of air masses, and increased evaporation and salinity [1], and as such, warming may have an indirect effect on marine organisms. However, the majority of marine organisms are ectothermic, and are therefore dependent on environmental temperature to gain adequate energy for their own biological functions. The relationship between the performance of an ectotherm and temperature is non-linear, where performance gradually increases with temperature until it reaches

a thermal optimum after which it rapidly declines (Thermal Performance Curve: [2]). Hence, the effect of increasing temperature on marine ectotherms may be more direct, affecting physiology and metabolism [3–7], which may have implications for growth, motor function, development, reproduction and behaviour [6–15], which in turn may impact species' abundance and distribution [16]. Therefore, increasing temperatures may be the most pervasive climate change factor influencing marine organisms [7,17,18]. Warming can subsequently impact entire ecological communities and ecosystems by differentially impacting individuals and functional traits [19–21].

Coral reefs are one of the most biodiverse ecosystems in the world [22,23]. Even though the rate of increase in sea surface temperature (SST) is 30% less in tropical oceans than the global average [24], coral reefs are also among the most sensitive ecosystems to changes in environmental conditions [22,25,26], and thus, are particularly at risk of thermal stress. Tropical ectotherms have a narrow thermal tolerance range, and their thermal optimum is close to their thermal maximum, as they have evolved under relatively stable thermal conditions [27,28]. As SSTs rise, corals and coral reef associated organisms are being subjected to higher temperatures (29–31 ◦C) for increasing periods of time [24]. As a consequence many tropical organisms are thought to be living at or close to their thermal limits [29]. SSTs are predicted to continue to rise over the coming years [1] and extreme warming events, resulting in global scale coral bleaching, are occurring with increasing regularity and severity [30–34], causing degradation of coral reef habitats [22,30,31,35,36].

In addition to the corals themselves, research on the effects of marine heatwaves has also focused heavily on fishes [37–48], which are also typically included in coral reef monitoring efforts. However, studies have almost completely ignored the myriad of small, cryptic, species, which make up a disproportionate amount of coral reef biodiversity [49–51]. One such group are parasites, which make up the largest consumer strategy globally [52] and comprise an estimated 40% of global biodiversity [53–56]. In addition to host behavior, physiology, and population dynamics, parasitic organisms have been shown to have impacts on interspecific interactions, energy flow, and the structure, ecology, and biodiversity of communities [55,57–60]. Parasites are particularly diverse on coral reefs [61] with an estimate of over 20,000 species on the Great Barrier Reef (GBR) alone [62]. However even with such a large presence in coral reef communities, they are significantly underrepresented in ecological studies [10,55,63]. Coral reef parasites are also ectothermic, and as such, may be affected by changes to their environmental temperature [10,11,64–67]. Some parasites are ectoparasitic and would be highly vulnerable to increased temperatures. Ectoparasites are also likely to be directly impacted by the temperature itself, in addition to being indirectly affected through changes in community structure due to temperature impacts on hosts [68] and other organisms. For example, the life cycle of a monogenean ectoparasite (*Neobenedinia*) was faster and the life span of their larvae (oncomiracidia) decreased as temperatures increased from 22 ◦C to 34 ◦C [69].

Gnathiid isopods are one of the most common ectoparasites in coral reef habitats [70–72]. They are small crustaceans, typically 1–3 mm long, that do not permanently live on their fish hosts [73,74]. In fact, with few exceptions, they associate only long enough to extract a blood meal and may therefore also be referred to as "micropredators" [75,76]. After feeding on tissue and blood from their fish host they return to the benthos to molt and progress to the next developmental stage [73,74]. They are only parasitic during their three juvenile stages, and no longer feed once they metamorphose into an adult. Gnathiids can have significant impacts on their hosts [66]. Direct effects include influencing behavior [77–83], physiology [84], and mortality [85]. Indeed, as few as one gnathiid can kill a young juvenile fish [83,86–90]. Indirect effects include transmission of blood-borne parasites [91] and wounds that can facilitate infection [92]. Gnathiids are also the most common items in the diet of many cleaner fishes, including *Labroides dimidiatus* [93,94], a species with far-reaching ecosystem effects [95–97]. Indeed, environmental perturbations, including a coral bleaching event with water temperatures reaching up to 30 ◦C, resulted in an 80% decline in *L. dimidiatus* at Lizard Island, GBR [48]. However, the processes leading to this decline remain unknown.

A long-term monitoring study of gnathiid isopods off Lizard Island, GBR, revealed a significant decrease in gnathiid abundance during extreme warm-water periods associated with bleaching events, compared with cooler periods in the same year or during non-bleaching years [65]. However, the cause of this decline was unclear. Sikkel et al. (2019) [65] hypothesized that the direct effects of temperature on gnathiid mortality may have partly contributed to the decline in gnathiid abundance. The aim of this study therefore, was to assess the direct effect of a rapid increase in seawater temperatures on mortality of shallow-reef gnathiid isopods. By conducting laboratory experiments on gnathiids in two coral reef regions subject to bleaching, GBR, Australia [31,32,98], and Philippines [99–101], we show that a rapid increase in temperature causes significant increases in mortality.
