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Sexual selection in fiddler crabs 

 Question: Are females attracted to larger males? 

Fig. Female fiddler crabs' attraction to males that are magnified to different sizes is studied in the field. 

Conclusion: Size of male is an important criterion in mate choice. 

Burrow fidelity in fiddler crabs 

On Pulau Hantu, two species of fiddler crabs, Uca annulipes and U. vocans are found in abundance on the lagoonal shore, with pure populations of U. vocans patchy in distribution and U. annulipes, the more abundant species.

 

Uca vocans (seen here in these photographs) and U. annulipes were tracked for a few consecutive days in many low tide periods in the field in order to answer the above question. 

Repeated re-emergence is indicated by the number of sticks next to the burrow.

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Question

Is there a difference in the burrow residency period between Uca vocans (a droving species) and U. annulipes (a non-droving species)? 

Functional morphology of fiddler crabs 

Uca vomeris

Uca perplexa

Fig. Relationship between number of spoon-tipped setae and carapace width in Uca perplexa & U. vomeris 

Question

Do the mouthparts of Australian fiddler crabs, U. vomeris and U. perplexa, from different habitats differ in their setation?

 

Conclusion: Uca perplexa (a sandy habitat fiddler crab) has significantly more spoon-tipped setae than U. vomeris (a muddy habitat inhabitant)   

RESEARCH

 My lab is currently conducting research on the ecology of ocypodoid crabs (fiddler crabs, ghost crabs and soldier crabs), the most prominent wetland and coastal macrofauna. These bioenergetically significant crustaceans play an important role in the nutrient cycling and energy flow in coastal ecosystems, and may be used as indicators of eco-system disturbances. 

 Ocypodoid crab biology & ecology 
 Foraging ecology of fiddler crabs 

 Sexual dimorphism is distinct in fiddler crabs as the male has an enlarged cheliped that is used as a mate attractant and for combat with other males. Only the minor cheliped can be used for foraging! 

 Questions: 

  1. What are the foraging strategies used by the male fiddler crab to compensate for this apparent ‘disadvantage’? 

  2. Do males feed faster to compensate? 

  3. When do males & females leave a foraging patch? 

Uca annulipes

Fig. Mean scoop rate vs carapace width of Uca annulipes 

Question 1

For any given carapace width, female U. annulipes have a faster mean scoop rate than males 

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Conclusion: 

Males, with only one cheliped with which to feed, do not compensate for handicap by feeding faster 

Fig. Relationship between (a) dactyl length and (b) dactyl-pollex width, with carapace width in U. annulipes 

Question 2

For any given carapace width, although slopes and intercepts are not significantly different between the two sexes, males tend to have larger dactyl length & dactyl-pollex width.

 

Conclusion: Males collect more sediment per scoop than females 

Question 3

Sequential patch depletion is studied by comparing the chlorophyll concentration in the surface sediment sampled from each patch. 

 

Conclusion: Males compensate by leaving a patch at a higher threshold of food density

 

Overall Conclusion: Male U. annulipes does not compensate by feeding at a faster rate than females. Male U. annulipes compensates for the apparent ‘handicap’ of having only one cheliped with which to feed by: 

  1. leaving a patch at a higher threshold of food density 

  2. larger scoops of sediment per lift 

  3. traveling shorter distances in between patches 

Fig. [Ch a] depletion in Uca annulipes' first five feeding patches in bare & pneumatophore mangrove habitats. Male U. annulipes have higher threshold of [Chl a] in both PnueH& BareH.

Littorinid snail biology & ecology 

Littorinid ecology and behaviour also fascinate my students and me. We have investigated the strange ‘head standing’ behaviour of periwinkles, compared the enzyme stability in various species of periwinkles when subjected to heat stress and looked at the effects of temperature and density on the aggregation behaviour of periwinkles. In addition, we are also interested in using these gastropods as bioindicators of coastal habitats; studies involving the use of gastropod mucus and radula as non-invasive methods with which to assess environmental pollution have recently started. 

a
b
c
d
e
f

Fig. a-c: Amazing! These Australian littorinids can withstand temperatures of more than 40ºC! Does the ‘head stand’ behaviour help to alleviate heat stress? Fig d-e: Two local littorinids also exhibit this peculiar ‘standing’ behaviour at Tanjung Chek Jawa, Pulau Ubin. Is there a ‘cooling-off’ effect seen in ‘standing’ Echinolittorina malaccana? Why are there more ‘standing’ Echinolittorina vidua than E. malaccana? 

Echinolittorina malaccana
Echinolittorina vidua 
Nodilittorina pyramidalis 
Austrolittorina unifasciata 
Prof. Shirley Lim
C.Wong
Predator-prey interactions in the local mangroves 

Conclusion:

1) Lethal effects in the lowered densities and smaller-sized individuals in the Paracleistostoma depressum (Camptandriidae) population. 

2) Little or no effects observed in Haberma nanum (Sesarmid) population. 

Fig. Crabs were stimulated with the following models ex situ to simulate predator presence 

Conclusion:

Predator presence has varying trait-mediated effects on P. depressum and H. nanum. Paracleistostoma depressum depends on its cryptic colouration for predator evasion, while Haberma nanum depends on flight in addition to crypsis for predator evasion.

Predator presence can have varying consumptive and non-consumptive effects on their respective prey populations.  The consumptive effect of a predator results in the lowered density of its prey and characterises densitiy-mediated interactions.  In addition, the presence of predators alone drives behavioural changes in the prey to lower predation risk.  These non-consumptive effects are known as trait mediated-interactions.  In the Lim Chu Kang mangrove, the presence of two mudskipper species exerts predatory pressures on the crab prey via lethal and nonlethal effects.

Fig. Effect of predation on the mean densities and mean carapace widths of Paracleistostoma depressum.

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