Fiddler crabs are semi-terrestrial organisms. They make their burrows in the intertidal zone and at low tide the crabs come out to feed and court. The male crab has one enlarged chelea that is used in ritual combat with other males and also is used in courtship, and one smaller chela, which are used for feeding. The female has two smaller chelea, both of which reach the mouth so can both be used for feeding. These morphological differences would suggest potentially that males are able to pick up organic deposits off the surface sediment at half the rate of the female crab, which uses two claws for feeding. It could therefore be presumed that male crabs would have to feed for twice as long as female crabs. It is on this assumption on which my study was based.

The aim of the project was to determine if there is a difference in the proportion of time spent feeding in male and female crabs during a period of exposure at low tide, and also to determine what other behavioural differences there are between the two sexes.

In order to achieve my aim the behaviour of a single crab through out the whole low tide period was monitored. The data can then be analysed to discover whether the crabs’ behaviour changed through out the period of low tide. Then a comparison of the above data can then be made to determine whether there are differences between the time spent in different behavioural activities between the male and female crabs.

A Sony Hi8 video recorder was used to monitor the crabs’ behaviour. This allowed reanalysis of the data so to produce a detailed study of individual behavioural acts the crabs use.

The study site chosen was a mud beach fringing the mangrove and within this area a 20m² study site was marked out. Within this area, a 1m² quadrat was laid at random, and the study began as the area within the quadrat was uncovered by water. The first adult crab to emerge from its burrow within the quadrat was the crab, which was monitored throughout the whole low tide. The crab was filmed for a 20-minute period, with a 10-minute break between filming periods. This filming schedule was continued until the crab re-entered a burrow and covered the entrance of the burrow over.

At the end of each filming period the density of crabs in the 1m² area surrounding the crab was noted, so access whether the crab move into a higher or lower crab density through out the low tide period.

The path the crab followed was also mapped out in each of the 10-minute break periods approximating the distance the crab moved from its original burrow in meters. This method was relatively inaccurate method of measuring distance but the crabs are extremely sensitive to movement so any attempt to measure movement causes disturbance and the crab to retreat into a burrow. This method of sketching the path taken by the crab allows me to see spatially the area over which the crab followed.

The sex of the crab was alternated each day; i.e. the first male crab to emerge was used one day while the first female crab to emerge was used the following day.

The data will be analysed and the various behaviours seen by both the male and female crabs will first be described in detail. I will then construct an ethogram using the specific behaviours described. The ethogram will then be used to construct a behavioural matrix so to pick out important sequences in behaviours, which are seen repetitively.

I will then test to see if there are differences between behaviours in the male and female crabs. Non-parametric statistical tests such as the Spearman Rank correlation or the Kreskal-Wallis correlation will be used to access the significance of the differences seen between the male and female crabs.

I expected to see differences in behaviour throughout out the low tide period. Primary observations show that the crabs do behave differently though out the low tide period, so much so that they almost seem to have a sequence of behaviours, which they carry out each low tide. The behavioural matrix can be used to assess whether this observation is in fact correct.

Primary observations also indicate that there are differences in the behaviours seen by male and female crabs. Female crabs seem to spent much more of their time around their burrows, while the male crabs seem to move over larger distances to many other burrows. It also seems that the male crabs seem to spend less time feeding than the female crabs. This is not what I originally anticipated seeing, so the data will need to be analysed using the above methods and possibly also determining the feeding rates used by both sexes of crab.

Report

A thesis entitled "A Study of the Behavioural Differences Between Male and Female Fiddler Crabs (Uca) focusing on foraging techniques." will be completed by April 2002 by Kaye Renshaw, University of Durham.