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Sandhoppers (amphipods) have long been thought to overwinter in the supratidal zone at high temperate latitudes, buried in a torpid or dormant state described as 'hibernation'. Early correlative studies suggested an association between invertebrate activity and extracellular Mg2+, a well known narcotizing agent. More recently large temperature-related increases in extracellular Mg2+ were recorded in a Scottish population of the sandhopper Talitrus saltator associated with overwintering. Our study tests the idea that at low environmental temperatures large T. saltator enter a deep torpor, distinct from temperature-related reduced activity, and associated with an increase in extracellular Mg2+ related to, but different from, any effect of low temperature on Mg2+.. Haemolymph from buried individuals was sampled in situ Sept 2017 - Feb 2018. Extracellular ions, including Mg2+ were quantified and correlated most strongly with burrow temperature. While there was a statistically significant and predictable increase in [Mg2+] h with temperature reduction (0.19 mmol.L−1.°C) the [Mg2+] h of torpid individuals (3.91 mmol.L−1) was greater than resting individuals ((6.39 mmol.L−1) at equivalent low temperatures (< 4.2 °C). There was no difference in the concentrations of Na+, Ca2+ or Cu2+ as a result of temperature change or torpor. This was confirmed in laboratory experiments where there was a difference in [Mg2+] h between torpid and resting individuals at the same comparatively high temperature (Δ 0.87 mmol.L−1, T = 10 °C). There was a larger difference as a result of temperature reduction from 10 to 2 °C (Δ + 4.78 mmol.L−1). This associates Mg2+ with torpor in T. saltator , as does experimentally blocking the antennal gland, which results in an elevated extracellular Mg2+ (but not Na+) and a cessation of activity even at high temperatures (15 °C). It is clear from the present study that elevated [Mg2+] h in overwintering sandhoppers is associated with torpor and, to a lesser extent, exposure to low temperature, and these responses are different from individuals from a Scottish Talitrus saltator population examined previously. • Magnesium ions act as a narcotizing agent to crustaceans. • Large sandhoppers Talitrus saltator overwinter in a torpid state. • Overwintering/low temperature results in elevated circulatory magnesium in hoppers. • Circulatory magnesium increase in torpid hoppers is independent of temperature. • Magnesium increase in torpor is greater in Scottish than SW England populations. [ABSTRACT FROM AUTHOR]