Portunus segnis (Forskal, 1775) has been referred to as Portunus pelagicus for very long time until Portunus species complex have been recognized based on their molecular distinction (Lai in Raffles Bull Zool 58(2):199–237, 2010). P. segnis are native species in the Arabian Gulf. Inter-moult stage adults were caught during February–November 2016 along the Northern coastal areas of Bahrain to investigate responses of cardiac activity to changes in water temperature normally encountered in their natural environment. Similar body sized individuals of both males and non-ovigerous females were maintained initially at 20∘C in a re-circulating artificial seawater for 2 days prior to recordings of their heartbeats. Recordings were continued over 24–48 h for 39 crabs. Power lab-Ad-Instrument coupled with Impedance Technique adjoined by AC amplifier, Bio-Amp and LabChart-6 were used. The heart rates of P. segnis acutely followed rising or falling temperatures and a stable heart rate at a particular temperature indicated an acclimatization. Temperature coefficient values (Q10) were between 0.1 and 8.6. At a temperature range of (20–30 ∘C), Q10 between 0.4 and 4 were significantly higher (p< 0.05) compared to Q10 of 2–3 calculated at the lower temperature range (10–20 ∘C). The higher values were obtained during either very fast or very low rates at 5–10 ∘C and at ≥ 30–40 ∘C, respectively. The upper lethal limits were 40∘C and 5∘C as the survival becomes limited at these two ‘extreme’ temperatures. Overall, at stable rates, Q10 was between 2.98 ± 2 and 2.26 ± 1 for heart rates at thermal intervals of (20–30 ∘C) and (10–20 ∘C), respectively. Below 5∘C, the heart rate slows down progressively and the crabs become motionless and eventually die. The survival of these species was critical above 40∘C. Warm-adapted species such as P. segnis may be threatened by changes in temperatures as they may already be living at the upper limit of their thermal tolerance during the summer months. Understanding the thermal tolerance of P. segnis would help optimizing conditions for this valuable commercial species in aquaculture. © 2019, King Fahd University of Petroleum & Minerals.

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