Comparing the metabolic and behavioral resilience of a native mullet (Mugil cephalus) and invasive mullet (Osteomugil engeli) to marine heat waves

Abstract

Marine heatwaves (MHWs) have increased in both frequency and intensity since pre-industrial times, with tropical ecosystems emerging amongst the most vulnerable. Rising temperatures cause thermal stress to many stenothermic (thermally specialized) ectotherms, while also increasing their cost of survival due to thermally driven rises in metabolic demand. Concurrently, MHWs is thought to facilitate the spread of eurythermic invasive species which exhibit broad ranges of thermal tolerance and a capacity to rapidly colonize vulnerable habitats. Here, we compare the impacts of moderate (+2 °C) and intense MHW conditions (+4 °C) on the juveniles of two Mugilid species found in Hawaiʻi, the native striped mullet (Mugil cephalus; ʻamaʻama) and the invasive Marquesan mullet (Osteomugil engeli; kanda). These species compete for habitat throughout their life cycle, with O. engeli increasingly dominating mullet assemblages across the Hawaiian Islands. Given its invasive status, O. engeli was hypothesized to exhibit a greater tolerance to MHW temperatures. Comparisons included seven metrics critical for ecological fitness, including five physiological traits spanning standard metabolic rate (i.e. metabolic cost of survival, SMR), Q10 of SMR (i.e. thermal sensitivity), maximum metabolic rate (MMR), aerobic scope (AS, i.e. maximum surplus capacity for aerobic activities), and critical thermal maximum (CTmax, i.e. upper thermal limit for survival). We also compared two behavioral responses: preferred temperature (Tpref and maximum avoidance temperature (Tavoid). Individuals (Mugil cephalus: 82, Osteomugil engeli: 95) were randomly assigned to a temperature treatment representing ambient temperatures and 5-day MHW conditions typical for Hawaiian reefs (ambient summer temperature 27.5±0.1°C, current moderate summer MHW 29.5±0.1 °C, intense summer MHW 31.5±0.1 °C). Contrary to expectations, native M. cephalus showed a 49.0% increase in aerobic scope (AS) at 31.5 °C, while invasive O. engeli experienced a 27.5% decrease from 29.5 °C to 31.5 °C, denoting strong thermal sensitivity in the capacity of physiological functions. Both species had CTmax values above intense MHW temperatures (40.6–41.2 °C for M. cephalus, 40.4–42.0 °C for O. engeli), with O. engeli showing significantly higher thermal tolerance (p < 0.005). Temperature preference (Tpref) did not differ across treatments for either species. Contrary to expectations, these results suggest M. cephalus may be more resilient to MHWs than its invasive competitor and highlight the importance of combining physiological and behavioral metrics when predicting MHW impacts on native and invasive species.