Ecological Patterns and Processes in Mangrove Ecosystems Ii Problem Set

Topics: Mangrove, Fiddler crab, Intertidal zone Pages: 7 (2182 words) Published: August 24, 2013

During our Field Trip to the Cairns Airport Mangrove Boardwalk on 7th August, 2013, I observed a distinct distribution pattern in the locations of the Orange-Clawed Fiddler Crabs (Uca coarctata). Upon further investigation, I noticed that the larger males of the species (easily identifiable as those with one large claw) colonised the open areas located beside the Middle Creek viewing platforms, that were in full sun for a larger part of the day, and offered no protection from predators. Conversely, the smaller females colonised the shadier and more forested area on the banks of the small stream, located between the two platforms, approximately 50 and 90 paces either side respectively, along the boardwalk. I was intrigued as to why this would be so.

The male Orange-Clawed Fiddler Crab (Uca coarctata) avoids shaded and vegetated habitats that hinder visibility. Inhabiting open and less structured mud-flat areas enable the male Fiddler Crab better visibility for predator detection, and improved scope for the use of courtship signals. Male Fiddler Crabs rely upon their large claws for the provision of visual aids in many aspects of their day to day existence. They use their claws in territorial defence in order to ward off antagonistic males. Their complex signals also aid females in detection of, and their approach to, males of the same species. Wandering burrow-mating females may visit up to 100 males before making a choice of mate. Interestingly, those males who lose their claws, will regenerate another claw that requires less energy to function, thus making it more effective at signalling, and possibly leading to more successful meetings with females. As the male Fiddler Crab is quite dependent upon visual cues, it needs to create its home in an open, flat environment with few structures able to obstruct his view of others, and their view of him. Competition and predation by other species of crabs may be higher in more vegetated areas of the mangrove, making it much more dangerous for conspicuously waving males to inhabit these areas. Whereas the female Fiddler Crabs, who do not wave and do not draw predators attention to themselves , may be better equipped to live in a more complex environment, with its higher concentration of competitors and predators.

Orange-Clawed Fiddler Crab eyes are raised high above their bodies like periscopes, enabling them to have an excellent visual range with full panoramic field of view without the need for eye movements (Zeil & Hemmi, 2006). As a result of this, potential predators are seen as being above their horizon, and other crabs below their horizon (Layne et al., 1997; Layne, 1998). The male Uca uses complex ‘waving’ patterns in order to attract females during courtship (Crane, 1975). The waving of their claws occurs as a break in their visual horizon (Christy, 1995; Land & Layne, 1995a), in the area between prey flying above, and other crabs below - be they potential mates or rival. This waving behaviour may make them more obvious to potential predators. Osborne and Smith (1990) hypothesised that Uca species only colonise clearings or open canopy forest habitats. As their visual resolution is poor, the open mudflat habitat provides the crab with a calm visual environment, as opposed to the highly vegetated regions of the mangrove, with its complicated motion patterns (Zeil & Hemmi, 2006). Uca may also avoid shadier areas because this change in light levels might reduce the effectiveness of their waving display. (Crane, 1975) proposes that darker forests may not be conducive to visual displays. Teal (1958) and Hyatt (1975) found that spectral sensitivity varies between Uca species depending on shade levels in their habitats. If crabs have...
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