**E**ff**ect of Acute Seawater Temperature Increase on the Survival of a Fish Ectoparasite**

**Mary O. Shodipo 1, Berilin Duong 2, Alexia Graba-Landry 3, Alexandra S. Grutter <sup>2</sup> and Paul C. Sikkel 4,5,\***


Received: 17 July 2020; Accepted: 27 September 2020; Published: 4 October 2020

**Abstract:** Extreme warming events that contribute to mass coral bleaching are occurring with increasing regularity, raising questions about their effect on coral reef ecological interactions. However, the effects of such events on parasite-host interactions are largely ignored. Gnathiid isopods are common, highly mobile, external parasites of coral reef fishes, that feed on blood during the juvenile stage. They have direct and indirect impacts on their fish hosts, and are the major food source for cleaner fishes. However, how these interactions might be impacted by increased temperatures is unknown. We examined the effects of acute temperature increases, similar to those observed during mass bleaching events, on survivorship of gnathiid isopod juveniles. Laboratory experiments were conducted using individuals from one species (*Gnathia aureamaculosa*) from the Great Barrier Reef (GBR), and multiple unknown species from the central Philippines. Fed and unfed GBR gnathiids were held in temperature treatments of 29 ◦C to 32 ◦C and fed Philippines gnathiids were held at 28 ◦C to 36 ◦C. Gnathiids from both locations showed rapid mortality when held in temperatures 2 ◦C to 3 ◦C above average seasonal sea surface temperature (32 ◦C). This suggests environmental changes in temperature can influence gnathiid survival, which could have significant ecological consequences for host-parasite-cleaner fish interactions during increased temperature events.

**Keywords:** Gnathiidae; Isopoda; coral reefs; climate change; ocean warming; coral bleaching; Great Barrier Reef; Coral Triangle
