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AN INVESTIGATION INTO THE FIDDLER CRAB SPECIES UCA CHLOROPTHALMUS, U.TETRAGONON, U. VOCANS & U.DUSSUMIERI WITH SPECIAL REFERENCE TO SEDIMENT CONTROL AND BURROW/CRAB ASSOCIATIONS

Introduction

Fiddler crabs inhabit intertidal sand and mud flats.  As deposit feeders they survive upon the faecal pellets at the sediment surface produced by suspension feeders e.g. according to Brafield 1978, resuspended particles from the deposit feeders in the sea water, can clog up the filters of suspension feeders so deposit feeders often predominate in very fine sediments such as those found on mudflats, while suspension feeders predominate in coarser sediment like those found further down shore.  They burrow into the sediment in order to avoid excessive wave action, surface predators, high temperatures and desiccation.  Burrows increase he depth to which oxygen can penetrate ad the digging of them bring lower sediment to the surface.  Males have one enlarged claw, which they use to compete against other males for females rather than for breeding sites as food resources are so abundant.  With the abundance of food resources population density of the fiddler crabs can reach 50 per m2.

 

Aims

This study aims to examine the density of crab burrows and their diameter in association with crab width, length and weight.  Secondly this study will look at crab biomass in relation to sediment chlorophyll, nitrates, phosphates and ammonium, and thirdly it will be looking at the association between crab proportions and the relevant burrow dimensions.

  1. Establish crab abundance and species density over the study site
  2. Relate average burrow diameter to crab size
  3. Relate crab biomass to burrow diameter in order to compare with chlorophyll, nitrates, phosphates and ammonium concentrations within the soil
  4. Relate burrow dimensions to crab size for males and females of Uca chloropthalmus and Uca tetragonon.
  5. Relate the structure of the burrow to the eco-physiology of the crab

 

The hypotheses for the study are:

  1. There will be areas of the intertidal zone within our study site in which each of the four main Uca species will have significantly increased abundance
  2. Crab burrow diameter will correlate significantly with crab dimensions
  3. Nutrient concentration will vary significantly with crab density
  4. There will be significant differences in the burrow dimensions between Uca chloropthalmus and Uca tetragonon and between the males and females within these species.

 

Methodology

10, 1m2 quadrats were taken every 1.5 meters down the shore, across three transects, 4 meters apart. Samples were taken at tide levels below 19m on the tidal scale.   Within each quadrat, species density was recorded by visual observation for 15 minutes onto a tally chart.  The diameter for each burrow within the quadrats was measured three times in order to calculate an average; several crabs from these burrows were caught and brought back to the lab for analysis.  

Three sediment samples within each quadrat were taken using a cylindrical corer of diameter 3.14cm and depth 1cm for later lab analysis of nitrate, phosphate and ammonium concentration.  Three subsequent samples were taken of half of the corer size to assess sediment chlorophyll content.  Back in the lab the nitrate, phosphate and ammonium analysis sediments were re-suspended with 4mls of 0.2% (final concentration) Zinc Chloride, mixed thoroughly and then stored in a dark container in a fridge at 4oC until transported back to the lab in the UK for further analysis.  The sediment samples for chlorophyll analysis had 4mls of 10% formalyn added to them which were again mixed thoroughly and placed in a dark container in the fridge until transportation back to the UK where cell counts would be performed. 

Male and female crabs from each of the two species under investigation were caught. In order to prevent damage to their burrows it was necessary to wait until the crabs had emerged and moved away from the burrow entrance before they were captured. Casts were made by removing excess water with a syringe and then pouring fiberglass boat resin into the burrow and allowing it to set. The casts were then removed from the soil and taken with the relevant crab back to the lab for analysis.

 

Preliminary results

There is a positive correlation between the width of the crab carapace and the average burrow diameter, while there appears to be no correlation between crab weight and burrow diameter in males or females. The species density results show that water content of the sediment affected distribution of the 4 main fiddler crab species.

No analysis of the contents of the sediment could be performed until arrival back in the UK.

Initial analysis of the casts has shown significant differences between the cast dimensions of male and female crabs within a species and between species. Further analysis will be carried out in the UK. 

Report

A dissertation entitled An investigation into the fiddler crab species Uca chloropthalmus, Uca tetragonon, Uca vocans, Uca dussumieri with specific reference to sediment content and burrow/crab associations will be produced by Laura Green & Carol Barr, University of Southampton by May 2003.