Changes in precipitation may alter food preference in an ecosystem engineer, the black land crab, Gecarcinus ruricola

Abstract

Gecarcinid land crabs are ecosystem engineers playing an important role in nutrient recycling and seedling propagation in coastal forests. Given a predicted future decline in precipitation for the Caribbean, the effects of dehydration on feeding preferences of the black land crab Gecarcinus ruricola were investigated. G. ruricola were offered novel food items of lettuce, apple, or herring to test for food choice based on water and nutritional (energetic) content in single and multiple choice experimental designs. The effect of dehydration was incorporated by depriving crabs of water for 0, 4, or 8 days, leading to an average body water loss of 0%, 9%, and 17%, respectively, (crabs survived a body water loss of 23% + 2% and 14-16 days without access to water). The results were consistent between the single and multiple choice experiments: crabs consumed relatively more apple and fish and only small amounts of lettuce. Overall, no selective preferences were observed as a function of dehydration, but crabs did consume less dry food when deprived of water and an overall lower food intake with increasing dehydration levels occurred. The decrease in feeding was likely due to loss of water from the gut resulting in the inability to produce ample digestive juices. Future climatic predictions suggest a 25-50% decline in rainfall in the Caribbean, which may lead to a lower food intake by the crabs, resulting in compromised growth. The subsequent reduction in nutrient recycling highlights possible long-term effects on coastal ecosystems and highlights the importance of future work on climate relative behavioral interactions that influence ecosystem function.

Keywords: Caribbean; Climate change; Dehydration; Feeding; Gecarcinus ruricola; Water budget.

Figure 1. Photo of crabs drinking at a pool.

Black land crabs, Gecarcinus ruricola, emerged after rains in large numbers. This was the only time they were observed in the open during daylight hours. The crabs congregated around standing pools of freshwater and were observed drinking by scooping water with the chelae (photograph—Iain McGaw).

Figure 2. Rainfall data.

Boxplot of the number of consecutive days per month without rain in a 0.25° grid surrounding the Cape Eleuthera Institute for the years 1998–2015 inclusive. Data was gathered from the new global scale rainfall product, SM2RAIN-CCI. Mean levels for each month are shown as a solid square and the open circles are statistical outliers (values either greater than upper or lower quartile + 1.5 * interquartile difference).

Figure 3. Water loss in land crabs.

Water loss (expressed as percent body mass loss) of black land crabs G. ruricola held in perforated plastic containers inside the crab hutch (solid lines, n = 8) and in wire mesh containers in the laboratory (dashed line, n = 10). The former treatment was designed to mimic the burrow environment of the crabs, and animals were maintained in these conditions until all had succumb from water loss. The data represent the mean + SEM.

Figure 4. Oxygen consumption rates with dehydration.

Resting oxygen consumption rates (mg O₂ kg h⁻¹) of eight hydrated and eight dehydrated black land crabs G. ruricola over a period of 8 days, followed by 1 day of recovery with free access to water. The data represent the mean + SEM, asterisks denote significant differences between the hydrated and dehydrated crabs (p < 0.05).

Figure 5. Feeding on wet and dry foods.

Boxplots showing amount of (A) fresh lettuce, apple, or fish and (B) dry lettuce, apple, and fish (% dry mass as a function of animal dry mass) consumed by land crabs when offered just one food item after they had been deprived of water for 0, 4, or 8 days. The solid symbols in the bars represent the adjusted means derived from the model coefficients and the smaller open circles are the statistical outliers (values either greater than upper or lower quartile + (1.5 * interquartile difference)). Note the different scales on the y-axis for the fresh and dry food. The data was derived from 14 different individual animals (held in separate experimental feeding chambers) for each food type and dehydration level treatment.

Figure 6. Multiple choice of items offered to land crabs.

Boxplots showing amount of fresh lettuce, apple, or fish consumed (% dry mass as a function of animal dry mass) by land crabs when offered a multiple choice of all three items after they had been deprived of water for 0, 4, or 8 days. The solid symbols in the bars represent the adjusted means derived from the model coefficients and the smaller open circles are the statistical outliers (values either greater than upper or lower quartile + (1.5 * interquartile difference)). The data was derived from 14 different individual animals (each held in a separate experimental chamber) for each dehydration level treatment. All three food items were offered in excess in order to ensure crabs did not consume all of one food item and then just move onto the next item.

Figure 7. Effect of crab size on feeding preferences.

Amount of fresh lettuce, apple, or fish consumed (% dry mass as a function of animal dry mass) of land crabs varying in size between 25 and 475 g when offered a multiple choice of the three food items. Each crab was maintained it a separate experimental container and the food items were offered in excess in order to maintain a multiple-choice of food items throughout the 12 h experimental period. Only fully hydrated crabs were used in this experiment.