Environmental Conditions and Human-Induced Pressures Shaping the Population Structure of Ocypode cursor on the Samandağ Coast

Technical Summary: Environmental Conditions and Human-Induced Pressures Shaping the Population Structure of Ocypode cursor on the Samandağ Coast

Problem Statement
Sandy shore ecosystems are increasingly degraded by anthropogenic pressures such as tourism, urbanization, and industrial fishing, threatening the biodiversity of species dependent on these habitats. The ghost crab Ocypode cursor (Linnaeus, 1758), the sole representative of its genus in the Mediterranean, is a protected species (Bern and Barcelona Conventions) recognized as an "ecosystem engineer" and "bioindicator." Despite its ecological importance, there is a lack of comprehensive data regarding how specific environmental features and human-induced disturbances influence its population structure, spatial distribution, and burrow characteristics in the Eastern Mediterranean. This study addresses the gap in understanding the factors shaping O. cursor populations on Samandağ Beach, Turkey's longest sandy beach, to inform conservation strategies.

Methodology
The research was conducted in July on Samandağ Beach, a 14 km stretch divided by the Asi River into a western sector (Area A) and an eastern sector (Area B).

• Field Survey: Researchers established 63 transects (25 m length × 15 m width) at 200 m intervals across both areas. Surveys occurred between 4:00 AM and 10:00 AM to coincide with peak crab activity.
• Data Collection: Observers waited 4 minutes post-arrival to allow crabs to emerge before counting individuals and burrows across three distance intervals from the tide line (0–5 m, 5–10 m, 10–15 m). A total of 1,247 individuals were observed, with 250 captured for morphometric analysis (carapace length, width, weight, and sex). Additionally, 4,836 burrows were documented, with diameters measured for 1,241.
• Disturbance Assessment: Eight anthropogenic and physical factors were quantified: coastal development, beach use, dog presence, fisherman activity, marine debris, vehicle access, slope, and freshwater discharge.
• Statistical Analysis: Data were analyzed using R (v4.3.1). Chi-square tests assessed sex ratios; Pearson correlations evaluated morphometric relationships; Wilcoxon rank-sum and Kruskal–Wallis tests compared groups; and linear modeling (with log-transformed burrow abundance) determined the predictive power of disturbance variables.

Key Results

• Population Structure: The study recorded a statistically significant male-biased sex ratio (154 males to 96 females; ratio 0.62:1, p < 0.001). However, no significant sexual dimorphism was found in carapace length or width, though males showed a slightly, non-significantly higher average body weight.
• Morphometrics: A strong positive linear relationship existed between carapace length and width (r = 0.92). Weight increased exponentially relative to carapace dimensions.
• Spatial Distribution: Area A (West) exhibited higher burrow abundance and greater variability in burrow diameters compared to Area B (East), which showed lower density and a more homogeneous size distribution. Burrows were heavily clustered in the 0–5 m zone from the shoreline, with density declining significantly further inland.
• Impact of Disturbances: While all eight disturbance factors were more prevalent in Area A, linear modeling revealed that only fisherman activity had a statistically significant negative association with burrow abundance (p = 0.047). Other factors, including dog presence, marine litter, vehicle access, and coastal development, showed no significant independent effects on burrow density. Correlation analysis confirmed low multicollinearity among predictors.

Key Contributions
This study provides the first thorough evaluation of potential anthropogenic threats to O. cursor on the Samandağ Coast. It contributes:

1. Baseline Data: Quantitative metrics on population structure, sex ratios, and burrow characteristics for this protected species in the Eastern Mediterranean.
2. Regional Comparison: Evidence of distinct population structures between the western and eastern banks of the Asi River, suggesting that local environmental heterogeneity (potentially sediment characteristics or moisture retention) drives population differences more than the measured anthropogenic variables.
3. Disturbance Specificity: Identification of fisherman activity as the sole significant predictor of reduced burrow density among the tested variables, while noting that other common stressors (e.g., marine debris, vehicle access) did not show measurable impacts on burrow abundance in this specific context.

Significance and Claims
The authors assert that this research is significant for establishing a scientific basis for the conservation of O. cursor and its dune habitat. The study highlights that while O. cursor is sensitive to habitat disturbance, its population dynamics on Samandağ Beach appear to be shaped primarily by local environmental conditions and resource availability rather than a broad spectrum of human pressures, which were found to be relatively low-intensity and spatially uniform in their impact on burrow density.

The paper claims that the bioindicator capacity of O. cursor may be context-dependent, becoming more informative in systems with stronger disturbance gradients. Consequently, the authors argue that effective coastal management for this region should focus on maintaining habitat integrity, preserving natural dune processes, and implementing precautionary measures (such as restricted-use zones) to prevent the system from exceeding critical disturbance thresholds. The findings underscore the need for targeted conservation actions that address specific pressures like fishing activity while acknowledging the role of geomorphological factors in shaping population structure.