1. WALLACE RESOURCE LIBRARY
Module 03 – Animal Behaviour
D02 – Interspecific differences in fiddler crab feeding rates.

2. Introduction to fiddler crabs
Fiddler crabs are small intertidal crustaceans and usually have one very large claw.
Fiddlers are active on mudflats at low tide where they feed on detritus, algae and bacteria.
During high tide they live in burrows.
There are lots of different species feeding together on the mud and this study looks at the differences between the species in how they feed.
The mudflats studied are in Indonesia
Background
Fiddler crabs are small, intertidal crustaceans, characterized by extreme cheliped asymmetry in males. They are one of the most characteristic groups of animals associated with intertidal shores, in particular with mangrove forests. Fiddler crabs are active on mudflats at low tide; when the tide recedes they come out of their burrows to feed on the sediment surface. During the high tide period they remain in their burrows to avoid being washed out to sea. Fiddler crabs are often found living in groups of hundreds or thousands and the coexistence of species is common. Species are able to coexist if they are not too similar in their resource use and when sufficient resources are available.
The main part of a fiddler crabs’ diet consists of detritus, bacteria and algae that are scraped off of sand particles on and in the sediments. The crabs filter sediment through their mouthparts to obtain food; they use specialised hairs to scrape organic matter off of individual sand particles. Females can feed much more efficiently than males as they can use both chelipeds (claws) to pick up sediment and pass it to their mouths, males however feed using just one cheliped, as it is only the small cheliped that can pick up sediment. All species have specialised setae that they use to filter specific types of sediment, if species can filter different sediment types then they are able to divide the resources in an area.
In the Wakatobi National Park, South East Sulawesi, nine species of fiddler crab have been found coexisting at the interface of a mudflat and a mangrove. This is the highest diversity of fiddler crabs ever to be recorded. How they manage to live in such close proximity and share resources is not yet fully understood. The nine different species can be found living on different parts of the shore, some living at the top of the shore near the mangrove forest and others living towards the low tide line. Due to the tidal cycle the crabs living at the top of the shore will have longer to feed than the ones inhabiting the lower shore. Comparing the feeding rates of different fiddler crab species can determine whether they have different feeding strategies for living at different shore heights.
Further reading
For more background to this project, and to see how it can be used in real life research, please read Lopez and Levinton (1987) which is in this folder. This document is a scientific paper written on deposit feeding animals and contains work on feeding rate and optimising feeding strategies.

3. Research Questions Do feeding rates differ between species
Do feeding rates differ among the same species living at different tidal heights?
These are some of the key questions that will be answered in this lecture, although there will be many more. At the end of the lecture, see if you can answer these questions, and if you can’t then see if you can find out more to help you.

4. Methods
Three species of fiddler crab are observed at 2 different heights along horizontal transects above the low tide mark.
10 individuals from each species are studied by observing individuals through binoculras whilst they feed.
The number of times each crab scooped up sediment into their mouths during 30 seconds was recorded.
These are some of the key questions that will be answered in this lecture, although there will be many more. At the end of the lecture, see if you can answer these questions, and if you can’t then see if you can find out more to help you.

5. Discussion?
The data suggests that each species of fiddler crab has its own rate of feeding. This rate might be related to the time it spends out of water and its tolerance to desiccation and over-heating.
Further analysis shows that within a species, the rate of feeding differs for different heights of the beach above the low tide level – reasons for this are considered.
Answers to the research questions
1. The data suggests that different species of fiddler crab have different feeding rates. Based on our knowledge of fiddler crab feeding strategies, we can suggest that these differing rates may be in response to the amount of time species spend on the sediment surface during the low tide period. Uca crassipes has a slower feeding rate which could be explained by observations showing this species spends most of the low tide period on the sediment surface. This is possible because they have a high tolerance to desiccation (drying out) and a high heat tolerance, whereas other species must frequently return to their burrows to replenish water supplies and to cool down. The differences between species feeding rates may also have something to do with their mouthparts; these are all different shapes and sizes depending on the sediment being filtered. Each crab has specialised mouthparts for sifting certain sediment types, this may alter the amount of sediment filtered “per scoop”, which will then determine how long each “scoop” is sifted for and therefore how many “scoops” per thirty second time period.
2. The data suggests that fiddler crab feeding rates will differ depending on the shore height at which they live. From our knowledge of fiddler crab behaviour we can suggest that this is due to feeding times. The crabs inhabiting the upper shore will have longer to feed due to tidal cycles, whereas the crabs towards the lower shore will have less time to feed and therefore compensate by having a higher feeding rate. The differences are more extreme in some species, such as Uca jocelynae, this could be for many reasons, such as the amount of time spent feeding on the sediment surface, the organic matter content of the sediment or the size and shape of the mouthparts. It should also be noted that the sediment at the lower shore is wetter than the sediment at the upper shore; sediment is easier and consequently quicker to sift when wetter.