Temporal variations in the air, soil, and fiddler crab (Austruca perplexa) burrow temperatures in southern Thailand

Abstract

Fiddler crabs are intertidal organisms and they live on various habitats such as muddy sandy flats, salt marshes, and mangrove swamps (Crane 1975). They are among the most abundant organisms in their habitats and their densities may exceed 50 crabs/m (Bertness and Miller 1984; McCraith et al 2003; Tina et al 2015a, b). Both males and females construct burrows and stay inside the burrows during high tide (Tina et al 2015 a, b). They are active on the surface and show various activities (e.g., feeding, fighting, waving, burrowing, grooming, etc.) during low tide (Tina et al 2016, 2019, Tina 2020; Tina and Muramatsu, 2020). Their burrowing activity is very important for their habitats since it promotes soil aeration and thus oxidizes hydrogen sulphide. Hydrogen sulphide inhibits plant growth by affecting their nitrogen uptake and growth (Howarth and Teal 1979; Bradley and Morris 1990; Koch et al 1990; Wiessner et al 2005). Crab burrowing activity also reduces salinity by increasing water flow through the sediments. A higher salinity level negatively affects the growth and productivity of mangroves (McKee 1993; Sylla et al 1996; Twilley and Chen 1998). For these reasons, fiddler crabs are known as ecosystem engineers in their habitats (see Jones et al 1994; Kristensen 2008). Moreover, burrows have several advantages for the fiddler crabs. They provide shelters from predators and environmental extremes (e.g., very high or low temperature), they provide water for physiological needs, and they act as sites for moulting and breeding (Crane 1975; Christy 1982, 1987; Keeratipattarakarn et al 2020). The intertidal animals live in a dry and open environment and face several environmental constraints (Chapman and Underwood 1996; Thurman 1998; Somero 2002; Schneider 2008; Miller et al 2009; Allen et al 2012). Among these environmental constraints, temperature is the most important since it affects their behaviour, physiology, growth, and reproduction (Weinstein 1998; Ruscoe et al 2004; Resgalla et al 2007; Allen et al 2012). In the case of fiddler crabs, very high or very low temperature affects their metabolic rates, heart rates, haemolymph osmolality, muscle hydration, ovarian development, and egg hatching success (Vernberg and Vernberg 1966; Eshky et al 1995; Matsumasa and Murai 2005; Colpo and López-Greco 2017; Principe et al 2018; Chou et al 2019). They have developed several physiological, morphological, and behavioural adaptations for dealing with the very high or low temperature (Eshky et al 1995; Thurman 1998; Yoder et al 2005; Levinton et al 2015; da Silva Vianna et al 2020). Moreover, their burrows act as a refuge through maintaining a suitable temperature when the outside temperature is very high or low (Powers and Cole, 1976; Wolfrath, 1992; Keeratipattarakarn et al 2020). Suitable burrow temperature is not only important for the Abstract Fiddler crabs live in an intertidal habitat and face several environmental constraints. Extreme environmental conditions, especially temperature affects their growth and reproduction. They use several strategies to deal with extreme temperatures. Among these, constructing burrows is important. Burrows act as a refuge during very high or low temperatures. This study investigates the temporal variations in air temperature, burrow temperature of large-sized male and female Austruca perplexa crabs, and the soil temperature near their burrows in Nakhon Si Thammarat province, southern Thailand (tropical climate). Air, burrow and soil temperatures were measured every 30 min in a day using temperature sensors. We observed that from 8:00 up to and including 17:30, burrow temperature was lower than soil temperature, but other times, burrow temperature was higher than soil temperature. In the case of air temperature, it was lower than soil or burrow temperatures most of the time in a day. When we compared temperatures among air, soil, and burrows at day (6:00 up to 17:30) and night (18:00 up to 5:30), burrow temperature was lower than soil temperature during the day but was higher at night. The air temperature was lower than soil or burrow temperatures on both day and night. This study shows that A. perplexa crab burrows can modulate the inside temperature and maintain a suitable temperature for the crabs.