Fiddler Crab Field Survey

A fiddler crab is a semi-terrestrial, tiny, and fast-moving species that lives in fine marshy mud during the diurnal low tides. Burrows are important in the lives of fiddler crabs because they shield the crabs from being swept away by high tides and help the crabs keep their gills moist on low tide-hot days. The burrow is sometimes used for breeding and to shield the fiddler crab from predators. They are commonly found along the eastern United States’ intertidal marshy shoreline. Fiddler crabs are members of the family Una spp. The fiddler crabs are usually found in huge numbers living in small adjacent territories. The male fiddler usually has one big claw which is used for fighting other males while the females have two small claws. The fiddler crabs have been known to feed on dead organic matter (they are detritovores). Therefore they reduce the organic content along the shoreline and aerate the sediment at the shoreline. The crabs start burrowing at a very early age. Burrowing is usually influenced by various factors such as the ground temperature, tidal periodicity, root material density, mate display activities, stem density and threat posed by the predator ((Quareshi and Saher). In most cases the crabs have been known to choose specific places along the marshy shoreline to make their burrows. These specific habitats have the specific conditions that are favourable to the survival of the fiddler crabs.

Objective

The main aim of this study is to apply various techniques to the field so as to determine the population change of the fiddler crab along the intertidal marsh zone and identify the specific environmental factors that influence the population change of the crabs.

Materials

100m transect tape

Quadrat square large

Small rulers

Penetrometers

PH pen

Field Procedure

Site

The class conducted the fiddler field survey at the Jacksonville Timucuan Trail Waterway. The class was divided into three class groups before the experiment began. The students were to collect qualitative data on the fiddler crabs such as the PH, temperature and compaction of the sediment. Quantitative data such as the size of the burrows was also to be collected.

Procedure

The initial step of the experiment was to use the 100m transect tape to measure 100m and then divide the 100m into three separate sections; 0m, 50m and 100m. Then quadrat squares large was used for testing in the separate sections. Ten holes in each section were selected and small rulers were used to assess the length of each hole.

The final step included the collection of qualitative data on PH, temperature and sediment compaction by the use of the PH pen and Penetrometer.

Results

The air temperature was 720F

Sea water temperature of 650 F

Cloud cover of 30%

Wind Speed of NW miles an Hour

Qualitative data on the holes form the three sections is as follows;

Table 1 showing the data collected on the holes from the three sections from the different quadrats.

Area 1

Area 2

Area 3

Quad1

Quad2

Quad3

Quad1

Quad2

Quad3

Quad1

Quad2

Quad3

0M

50M

100M

0M

50M

100M

0M

50M

100M

0.5

0.5

1

1

1

0.5

1.5

1.5

0.75

1

1

0.5

1

1

0.7

1

0.5

1

1.25

1

2

1.5

0.5

0.3

0.5

1

0.5

1

0.5

1

1.5

0.5

1.5

1

0.5

1

1.25

0.5

0.25

0.8

0.7

1.5

0.5

1

1

1

0.25

2

1

1

1.7

0.5

1

1.5

0.75

0.5

2

1.4

0.5

1

1

0.3

0.7

0.5

0.5

0.25

1

0.5

1.7

2

0.5

0.3

0.5

1

1

0.4

1

1

1.5

0.5

0.5

0.25

1

0.5

1

1.5

0.5

6

0.5

1

Average

0.8

0.675

1.05

0.96

0.95

1.04

0.9

0.73

0.8

pH

6.6

6.91

6.87

7.1

6.95

6.96

6.95

7.42

6.87

Temp

24.9

65.5

69.1

75.2

71.2

71.2

24.4

22.2

22.4

Compaction

150

100

100

40

58

42

45

32

30

Holes

85

25

20

460

400

1140

260

540

960

Discussion Questions

Graph the mean of the number of fiddler crabs` burrow for each quadrat.

Graph 1 showing the average holes per section for each and every quadrat

Does the number of fiddler crabs` burrows change predictably along the marsh intertidal zone?

Yes, the number does change along the marsh intertidal zone. This is because the crabs require certain conditions for survival that are only provided by specific intertidal zones along the marshy shoreline. This means that most of the fiddler crabs population is found in places along the shoreline that are favourable to the crabs.

Make separate graphs that plot the mean number of fiddler crab burrows against: a) sediment temperature, b) sediment compaction, and c) ph.

Graph 4 showing the number of fiddler crab against the sediment pH

Graph 5 showing the number of fiddler crabs burrow against the sediment compaction

Graph 6 showing the comparison between sediment temperature and number of fiddler crab burrows.

Are there any relationships between mean crab burrow number, and any of the tested environmental factors? If so, why might these factors be important to these crabs? If not, what are some other factors that may be important but were not measured in our study?

Yes, there is a relationship between the burrow number and environmental factors. Factors such as pH are always crucial to the survival of all animals. Most of the animals will be favoured by a pH of 7(Neutral) or close to 7. This is because their biological processes inside their bodies will not be affected.

High temperature will resulted into the dehydration of the fiddler crabs while low temperature will lead to slow metabolism in the fiddler crab. Both situations results into the death of the crabs.

Graph the mean size of fiddler crab burrows for each numbered quadrat. This will test whether fiddler crab burrow size changes along the slope.

Graph # showing the average size of burrow per section in quadrat 1

Graph# showing the average size of burrow per section in quadrat 2

Graph# showing the average size of the burrows per section in quadrat 3

Do the number of fiddler crab burrows change predictably along the marsh intertidal zone? If not, do you think your results are accurate? Why or why not?

The fiddler crab number changes along the marsh intertidal zone. This is because of the change in environmental factors.

Make separate graphs that plot the mean size of fiddler crab burrows against: a) sediment temperature, b) sediment compaction, and c) pH.

Graph # showing the comparison between the temperature of the sea water and the average size of the burrows in quadrat 1

Graph # showing the comparison between the temperature of the sea water and the average size of the burrows in quadrat 2

Graph # showing the comparison between the temperature of the sea water and the average size of the burrows in quadrat 3

Conclusion

From the above results it is evident that the environmental factors along the marshy intertidal zone affects the presence of fiddlers crabs. Environmental factors such as pH, temperature and sediment compaction clearly affect the survival of fiddler crab. However there are additional factors such as stem density and threat posed by the predators affect the crabs.

Section 2

The fiddler crabs are of importance in the ecosystem at the marshy shorelines of the United States. The crabs usually make burrows which affect the nature of the sediment at the shoreline. The faecal pellets of the crabs have been known to favour the growth of the cord grass (Spartina alterniflora).

During the field survey we encountered the following three species of the fiddler crabs; uca minar, uca pugilator, uca pugnax. However the fiddler crabs are referred by the local people in different common names; uca pugnax is referred to as sand fiddler crab, uca gugilator is referred to as marshy fiddler crab while uca minar is referred to as mud fiddler crab.

The taxonomy of the fiddler crab is as follows; Kingdom (Animalia), Phylum (Arthropoda), Class (Malacostra), Order (Decapoda), Family (Ocypodidae), Genus (Uca), and the species are Uca pugilator, Uca minar and Uca pugnax.

The fiddles crabs acquire their name from the waving movement made by the male crabs with their enlarged claws during courtship. The motion resembles a musician playing the fiddle or the violin. Each species of the fiddle crab has been known to have distinctive waving motion. All fiddle crabs are square in shape.

Ghost crabs are closely related to the fiddle crab. Both the ghost crab and the fiddler crab are found in the Ocypodidae family. Roughly 22 species of the ghost crab are found worldwide. The taxonomy of the ghost crab is; Kingdom (Animalia), Phylum (Arthropoda), Sub phylum (Crustacea), Class (Malacostraca), Order (Decapoda), Intraorder (Brachyura), Family (Ocypodidae) and Sub family (Ocypodinae). The 22 ghost crab species are usually found in two genera; Ocypode and Hoplocypode.

3.

Uca minar

Uca pugilator

Uca pugnax

Enlarged claws red-jointed for males

Enlarged claws for males

Enlarged claws for males

Detritovores

Detritovores

Detritovores

Square carapace

Square carapace

Blue patch carapace

Hard claws

The inside part of claws is soft

Have chelipeds (claws with legs)

Have eight legs

Have ten legs

Legs (pereiopods) have dark bands.

Habits muddy and sandy shores

Habit estuarine and shorelines

Habits shorelines

The determination of sex of the crabs can range from been quite easy to been troublesome. The larger the crab species the easier it is to identify the crab gender. A general method of trying to identify the gender of the crabs is by viewing the underside of the crab. If the crab has a triangular shape on the rear end then it is male, but if it has a broad round marking at the rear end then it is female. However the identification of gender in some species is different from the rest, for example the blue crabs` gender can be identifies b checking the colour of the claw, if the claw is blue then the crab is male, but if the crab has a red-tipped claw then it is female. As for the fiddle crab, the male has an enlarged claw while the female has two small claws.

The crabs usually undergo moulting. This process is crucial since it helps the crabs to regain its lost body parts such as the claw. After the loss of its major cheliped, the small claw of the fiddle crab grows into a large claw will the lost claw grows back into a small claw.

The crabs are of important to the marshy ecosystem since their activities are beneficial to other members in the ecosystem. Birds such as the seagull predate on the crabs therefore the crabs act as a source of food. The faecal matter of the crabs enriches the muddy soil in the marshy area hence favouring the growth of vegetation such as the faecal matter of fiddle crab favours the growth of the cord grass. The crabs, most especially the fiddle crab aerate the sediments of the shoreline by making burrows.

Works Cited

Quareshi, Naureen Aziz and Noor Us Saher. “Burrow Morphology of Three Species Of Fiddler Crab (Uca) Along The Coast Of Pakistan”. www.vliz.be/imisdocs/publications. N.p., 2012. Web. 1 Mar. 2017.