Abundance of Ghost Crabs (Ocypode spp.), Zonation Shift, and Burrowing Behavior in Response to Tourist and Thermal Variation on Sandy Beaches of Koh Tao, Surat Thani Province

Abstract

Background and Objectives : Koh Tao is one of the world’s top tourist destinations, with significant construction development to support tourism and a high number of tourists on its sandy beaches coupled with temperature fluctuations, poses a potential threat to biodiversity. Ghost crabs are one of the dominant species inhabiting these beaches and is most affected by human threats. This study was conducted on a single beach divided into four zones with varying levels of tourist numbers and temperatures. Ghost crabs in each zone were expected to respond differently in terms of abundance, zonation shift, and burrow excavation for shelters, refuge from predators, human disturbance, desiccation and temperature changes, reflecting the changing lifestyles of ghost crabs under different environmental conditions. The objective of this study was to develop ghost crabs as bioindicators of habitat loss caused by tourists and  temperature fluctuation on sandy beaches for management and conservation of coastal and marine resources of Thailand.

Methodology: The study was conducted on the sandy beach of Chalok Baan Kao Bay Beach, Koh Tao, Surat Thani Province, (lat. 10°04'01''N , long. 99°49'33''E)  an area with a high number of tourists and a concrete seawall at the uppermost part of the beach adjacent to residential areas. Survey transects and sampling plots were established and divided into four zones from the seawall, including the upper part of high tide zone (A), the lower part of high tide zone (B), the upper part of mid tide zone (C), and the lower part of mid tide zone (D), each zone spaced one meter apart. Sampling was conducted using quadrats made of rope measuring 1 ´ 5 meters placed consecutively along eight transects, totaling 32 sampling stations. The study examined ghost crab species, burrow density, burrow size, burrow depth, and burrow morphology, along with measurements of air and sand temperatures in each zone on the beach, as well as at the burrow entrance and inside the burrow. The numbers of tourist in each zone on this beach were evaluated. The quality of environment and water quality were measured, such as beach slope, grain size, organic matter, moisture, loudness of sound, water temperature, salinity, pH, dissolved oxygen, wind and wave.

Main Results : Chalok Baan Kao Bay Beach was characterized by a low sloping sandy beach composed of coarse sand with 1.52% organic matter. Two ghost crab species were found : the smooth-clawed ghost crab (Ocypode cordimanus Latreille, 1818), which is the dominant species, and the long-horn ghost crab (Ocypode ceratophthalma (Pallas, 1772)). Ghost crabs in the four different zones responded to tourists and temperature variations by having significantly different abundances, crab hole sizes, and crab hole depths (p<0.05). The ghost crabs in the upper part of mid-tide zone (C), as a recreational and transit area for tourists, exhibited the lowest abundance (0.25±0.16 holes/m²), with large burrows (20.43±10.22 mm), the deepest burrow (13.97±6.77 cm) with more branched and complex shapes than other zones. Large crabs shifted their habitat to the upper part of the high-tide zone (A) where there were no tourists, and were the most abundant (0.75±0.40 holes/m²). Small crabs (9.47±5.12 mm) avoided tourists and settled in the lower part of the mid-tide zone (D) with the least deep burrows (8.46±2.41 cm). Temperatures among the four zones were not significantly different (p>0.05), but air temperature, external burrow temperature, and internal burrow temperature were significantly different (p<0.05). Therefore, the ghost crabs responded to temperature variations by digging deeper burrows to avoid the high temperature outside the crab burrow. Water quality in the study area was within normal criteria, with seawater temperature at 31.48±1.34°C, salinity at 32.03± 0.87 psu, pH at 8.12±1.10, and dissolved oxygen at 7.05±0.51 mg/L. The ghost crabs in Chalok Baan Kao Beach in this study responded more to tourists than to temperature fluctuations across zones, due to the short-term temperature increase and the tide level. The response of the ghost crabs in different zones on the same beach in this study was informative similar to previous studies conducted on multiple beaches with varying threat levels. The ghost crabs in Chalok Baan Kao Beach living in different zones responded to tourists and temperature variations, exhibiting decreased abundance, zonation shifts, and significantly increased burrowing depth and complexity. Therefore, the study of the ghost crabs under Chalok Baan Kao Bay Beach, which is divided into 4 zones, can be used to identify the impacts of tourist threats and temperature variations that cause the loss of ghost crab habitats in the sandy beaches. Furthermore, air temperature monitoring on Koh Tao between 2022 and 2023 showed a trend of rising maximum temperatures in 2023, the study period, compared to 2022. Therefore, the effects of long-term temperature changes should be continuously monitored to understand how climate change will impact ghost crab habitat loss in the future.

Conclusion: Ghost crabs inhabiting in four different zones of Chalok Baan Kao Bay Beach, Koh Tao, responded to tourists and temperature variation by altering their abundance, zonation shift, burrow depth, and burrow morphology. This study therefore indicated that ghost crabs on Chalok Baan Kao Bay Beach, have the potential to be developed as  bioindicators for the impacts of human threats and temperature change, which lead to habitat alterations for ghost crabs within different zones of the same sandy beach.