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Lim, Shirley S. L.
The temperature profiles of 50 burrows of Ocypode ceratophthalmus at Changi Beach Park Park, Singapore and 30 burrows of O. quadrata at East Beach, San Salvador, Bahamas were determined at 2 cm-intervals until the maximum depth in the burrows using a multi-channel thermometer. Wax casts were then made of the burrows sampled and the prevalence of the different burrow shapes was compared for both study sites. J-shaped burrows and Y-shaped burrows were the most prevalent in Changi Beach Park Singapore and East Beach, San Salvador, Bahamas respectively.
The temperature difference between the burrow opening and the maximum depth of the J-, U-, Y-shaped and straight burrows ranged between 7 °C and 10 °C, with the maximum temperature at the burrow opening and sand surface recorded as 43 °C and 48 °C respectively for both study sites. Pearson’s correlation results showed that light intensity, sand temperature and air column temperature of the burrow opening were positively correlated (p < 0.05). The negative correlation between burrow depth and mean temperature was similar for burrows of both O. ceratophthalmus and O. quadrata. J-shaped burrows excavated by O. ceratophthalmus and straight burrows excavated by O. quadrata were the coolest. This probably suggests that simple or straight burrows serve the main function of thermal stress alleviation. The mean burrow depths of juveniles were shallower as compared to adult O. ceratophthalmus. Predators could easily dig into the shallow burrows; hence, having Y- shaped burrow with an escape route is crucial for juveniles’ survival.
The effect of temperature on heart rate was investigated in 18 juveniles (carapace width, CW < 27.1 mm) and 17 adults (CW ≥ 27.1 mm) of O. ceratophthalmus. The heart rate of these ghost crabs were recorded at 25 °C, 30 °C, 35 °C and 40 °C, using a non-invasive heartbeat detector. Repeated measures Analysis of Variance (ANOVA) showed the interaction ‘crab stage’ (juveniles / young adults) × ‘temperature treatment’ was significant: the mean heart rate of adult O. ceratophthalmus increased with temperature while the reverse trend was observed for juveniles. Mean heart rate of juvenile and young adults were 196.1 ± 12.6 and 102.7 ± 1.89 beats per minute at 25 °C respectively. Mean heart rate of adult O. ceratophthalmus was 149.17 ± 6.13 beats per minute, while mean heart rate of juvenile O. ceratophthalmus was 186.32 ± 2.74 beats per minute at the highest temperature of 40 °C. Heart rate of adult ghost crab increases with increase in temperature and the reverse trend was observed in juvenile. The reverse trend might suggest that juvenile have acclimated to heat stress. No significant interaction was observed for Q10 values, with subsequent main factors ANOVA showing that adults have significantly higher mean Q10 (1.50 ± 0.03) than juveniles (0.94 ± 0.06) regardless of temperature treatments. The relatively lower Q10 generally indicates that juveniles are physiologically adapted to temperature fluctuations. This study also provides a better understanding on how adult and juveniles O. ceratophthalmus have their own physiological and behavioural adaptations to overcome thermal stress.
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