Food Competition and Food Preference in Land Hermit Crabs

Transcription

1 Department of Entomology Ecological Methods, January Food Competition and Food Preference in Land Hermit Crabs Authors: Tomas Bernelind David Isaksson Johanna Öhman

2 Abstract Size does matter, at least when it comes to land hermit crabs competing for food. We have been studying the behaviour of land hermit crabs in competition for food and preferences of food. In the feeding experiment coconut, fish and papaya were used as representatives for different sources of nutrients; lipids, proteins and carbohydrates, respectively. No significant difference in preferences was found, but it was obvious that larger crabs easily won competition for food against smaller individuals and were better at making their way through the crab crowd to reach the food. Keywords: Hermit crabs, food competition, food preference, Costa Rica Introduction The composition of nutrients in food is of high importance for most living organisms. Land hermit crabs (Coenobita compressus) are scavengers, but they do not select their foods randomly (Thacker, 199). Thacker (1998) determined that land hermit crabs avoid food of the same kind as they have recently eaten. Land hermit crabs that receive a multiple-item diet will have a higher relative growth than crabs feeding on single-item diets (Thacker, 1998). Accordingly, as some food items are more common and more easily accessible, land hermit crabs should avoid these and prefer the ones that are less common. To test for any preferences in a multiple-item diet, coconut, puffer fish and papaya was used. These food sources were also used for evaluating competition for food. The questions we wanted to answer were: Are larger land hermit crabs better competitors for food than smaller crabs? Are the food preferences of land hermit crabs the same, independent of size? Will, for example, smaller individuals tend to urge more protein in order to grow faster? Are there preferences for any of the nutrient sources lipids, proteins or carbohydrates in the land hermit crab? Materials and Methods Study area and experimental design The study was conducted in December on the shores of the Sirena field station in the National Park of Corcovado, Osa Peninsula, Costa Rica. Land hermit crabs (Coenobita spp.) were studied in a feeding experiment to investigate competition for and preferences of food. For evaluating potential preferences of different nutritional sources, Thacker (1998) used seeds, snails and flowers as sources of lipids, proteins and carbohydrates, respectively. We used coconut, puffer fish and papaya as lures representing the same nutrient sources as Thacker (1998) for studying potential preferences. In addition, we studied the competition for these nutrient sources.

3 The preferences of food were studied through a choice experiment where the hermit crabs could choose between the three food sources. The competition for food was studied using both a no choice and a choice experiment. The no choice and the choice subplots together form a plot and data was originally collected from a total of fifteen plots throughout the beach. In the no choice experiment, the three foods were separated by 1m. In the choice experiment the distance between each of the three food sources was cm (Figure 1). The two subplots were separated with m, while the main plots were divided by m, in order to avoid the same individuals appearing twice. Circles with a cm diameter were marked on the sand beach, parallel to the shoreline and as close to the forest edge as possible. In each circle, one of the three foods coconut, (Cocos nucifera L., Arecaceae), puffer fish (Tetraodontidae) or papaya (Carica papaya L., Caricaceae) was placed. Systematic randomisation was used when positioning the three food sources in each subplot. Each food type was moved one step to the left of the previous position, thereby avoiding complicating factors due to sampling methods. cm 1cm Food placed in center of circle Figure 1. Schematic illustration of the choice subplot. The food was cut into approximately cm pieces that were pinned to the ground to keep them in position. After having been left undisturbed for 1 minutes, all hermit crabs in each of the circles were collected. Morton and Yuen () showed that the hermit crab Diogenes edwardsii (De Haan, Diogenidae) have a feeding time range of.8 to more than min. This, together with personal observations of feeding Coenobita spp. infers ten minutes to be suitable. When collected, the crabs were separated into those feeding on the food and those not feeding, but within the circle. Finally, the width of the thorax exoskeleton was measured with a digital calliper of a two decimal accuracy. We believe that the width of the thorax is a good estimate of the general size of the hermit crabs since the exoskeleton is regularly shed as the body grows. Statistical analyses The number of hermit crabs studied for food preferences in the choice experiment were log-transformed in order to get a normal distribution, thereby fulfilling the criteria of an ANOVA test. Hot and sunny environmental conditions were unfavourable for surveying hermit crabs, resulting in some subplots having few or no hermit crabs. A set level of ten individuals was believed to be enough to perform statistical tests. Therefore, subplots with fewer than ten individuals were removed from the analyses.

4 Results In Figure, the sizes of the hermit crabs in each of the analysed plots are presented as a box plot. The horizontal line in the middle of the boxes represents the median and the edges of the boxes the upper and lower quartiles. Outliers are shown as asterisks (*). 1 Width of thorax (mm) Plot no Figure. Box plot of the sizes of hermit crabs in each plot. Competition experiment In the no choice experiment, where the hermit crabs could not choose between food types, the hermit crabs actually feeding were significantly larger than the rest of the crabs within the cm diameter circle (Figure ). (ANOVA, General Linear Model, F 1, 7 = 17., P =.1). However, no difference in mean size of the crabs was found between the food sources (ANOVA, General Linear Model, F, 7 = 1.97, P =.1). Size of hermit crabs (mm) 1 * * * On food Off food Coconut Fish Papaya Figure. Mean width of thorax in hermit crabs on food and off food for the respective food types in the no choice experiment. Standard deviations are represented by error bars and * represent significant differences between on and off food (P =.1).

5 In the choice experiment the hermit crabs actually feeding were significantly larger than those surrounding the food, but still within the cm diameter circle (Figure ) (ANOVA, General Linear Model, F 1, 7 = 17.7, P =.). However, no difference in size was found between the food types (ANOVA, General Linear Model, F, 7 =., P =.). Size of hermit crabs (mm) 1 * * * On food Off food Coconut Fish Papaya Figure. Mean width of thorax in hermit crabs on food and off food for the respective food types in the choice experiment. Standard deviations are represented by error bars and * represent significant differences between on and off food (P =.). Preference experiment To test for any potential preferences of food type, the total number of crabs in each food circle of the choice subplots were used. The logged numbers of hermit crabs in the choice experiment are presented in Figure. No statistical difference in preferences of food type was shown (ANOVA, General Linear Model, F, = 1.8, P =.17). Mean no. of hermit crabs (log) coconut fish papaya Figure. The logged numbers of hermit crabs for the different food types (ANOVA, General Linear Model, F, = 1.8, P =.17).

6 Discussion Our study showed larger land hermit crabs to be better competitors for food than smaller conspecifics (P <.1). However, no differences in crab size were found between the three different sources of food. (Figure and Figure ). When analysing the total number of land hermit crabs on each food, no preference was found (Figure ). The sizes of the hermit crabs measured varied between.7 and 1.78 mm (Figure ). This large size interval suggests the whole population of hermit crabs to be included in the study. The fact that no specific size seems to dominate the data set, makes the analyse of competition and preference valid for the whole population. Ramsay et al (1997a) studied the hermit crab Pagurus bernhardus. They found that the larger the crab, the better it competed for food when food resources were scarce and the density of crabs high. Our study also showed larger crabs to be better competitors. The larger crabs feeding seemed to care less of the competition from the considerably smaller crabs than of those medium sized. Suggestively, the feeding behaviour of the land hermit crab is somewhat similar to that of a pack of lions, where the large males lets cubs have access to the food while eating, not allowing the smaller lionesses the same privilege. This could be due to the smaller crabs not having the same magnitude of food requirements as their larger conspecifics. Also, the smaller crabs are not as much in the way of the larger hermit crabs as the medium sized crab. No differences in size in relation to preferences of food were precluded in this study. This agrees with Thacker (1998), stating that size differences do not affect the formation of odour preferences and that land hermit crabs prefer a multiple-item diet throughout their lives. Thacker (1998) has previously showed that land hermit crabs prefer food different from that of their last meal. Personal observations suggested carbohydrates, in the form of vegetative matter, to be the most easily accessible nutrient source for land hermit crabs on the shore of Sirena. This would rend papaya the least preferred food type, however, our results did not show a significant preference for neither lipids nor protein (Figure ). According to the National Food Administration of Sweden (), the recommended food intake for humans consists of -% carbohydrates. If land hermit crabs as well need relatively large intake of carbohydrates, this could partly explain our results. Although land hermit crabs use olfaction to locate food (Thacker, 199), the behaviour of crabs seems to be highly affected by the behaviour of surrounding individuals, much like the feeding behaviour of vultures. A large pile of land hermit crabs feeding seem to attract more crabs than more easily accessible, untouched food items do. This agrees with the findings of Ramsay et al (1997b), where, despite similar background population densities, the aggregations of the hermit crabs Pagurus bernhardus were particularly variable. This could have brought a random factor into the data set; the choice of the first individual arriving to the subplot becomes very important.

7 Complicating factors, such as the fact that some crabs were frightened when approached, is believed to be of minor importance. Some of the larger crabs feeding would be collected as crabs not feeding, resulting in an underestimate of the result. In addition, the systematically changed order of food sources in relation to surveyors should avoid any sampling bias. Acknowledgements Thanks to Andrés Vega for providing us with professional guidance, papaya, fish, and loads of other stuff. Appreciations to Christer Björkman, Richard Hopkins and Mattias Jonsson for their constructive comments on this paper and their positive attitudes throughout this course. References Morton, B. Yuen, W.Y.. The feeding behaviour and competition for carrion between two sympatric scavengers on a sandy shore in Hong Kong: the gastropod, Nassarius festivus (Powys) and the hermit crab, Diogenes edwardsii (De Haan). Journal of Experimental Marine Biology and Ecology,, pp 1-9. Ramsay, K. Kaiser, M.J. Hughes, R.N. 1997a. A field study of intraspecific competition for food in hermit crabs (Pagarus bernhardus). Estuarine Coastal and Shelf Science,, pp. 1-. Ramsay, K. Kaiser, M.J. Moore, P.G. Hughes, R.N. 1997b. Consumption of fisheries discards by benthic scavengers: utilization of energy subsidies in different marine habitats. Journal of Animal Ecology,, pp Thacker, R. W Food choices of land hermit crabs (Coenabita compressus, H. Milne Edwards) depend on past experience. Journal of Experimental Marine Biology and Ecology, 199, pp Thacker, R. W Avoidance of recently eaten foods by land hermit crabs, Coenobita compressus. Animal Behaviour,, pp 8-9. The National Food Administration of Sweden (Livsmedelsverket) SNR