The subgenus Celuca contains a large number of species from both the Americas and the Indo-West Pacific regions. Celucans are characteristically small to medium sized, broad-fronted fiddler crabs that use lateral claw-waving displays (Crane, 1975). Until recently, the two Celucan species in this analysis,
U. mjoebergi and U. perplexa, were classed as subspecies of U. lactea (Crane, 1975), although today they are treated as species in their own right (Rosenberg, 2001). Although relatively little is known about the Australian species, other
Celucan species from the Americas and Indo-Pacific region have been the subject of much research, including U. beebei (Christy, 1987, 1988a, b; Christy & Schober, 1994; Backwell et al., 1995; deRivera et al., 2003), U. annulipes
(Walker, 1972; Backwell & Passmore, 1996; Jennions & Backwell, 1996, 1998; Zeil & Al-Mutairi, 1996; Zeil, 1998; Backwell et al., 1999) and U. lactea
(Yamaguchi, 1970, 2000; Murai et al., 1987; Goshima & Murai, 1988; Severinghaus & Lin, 1990; Kim et al., 2004; Yamaguchi & Tabata, 2004).
Uca mjoebergi (Rathbun, 1924)
[Named after the Swedish zoologist Dr. Eric Mjoeberg]
Distribution:
West Pacific: New Guinea, North to Northwest Australia (George & Jones, 1982; Rosenberg, 2005). Current data from East-Point Reserve, Darwin.
Appearance:
Broad-fronted, relatively small fiddler crab (male carapace: 16.4 ± 6.7 mm, n = 122; female carapace: 13.8 ± 7.5 mm, n = 80; George & Jones, 1982). The male major claw is uniformly yellow in colour and the carapace is mottled brown-grey- black, providing camouflage against the mudflat surface (Fig. 3.3A; Detto et al., 2006). The legs and carapace also have varying levels of red colouration. Claw colour is known to be used as a cue for mate choice and species recognition (Detto et al., 2006; Detto, 2007).
Habitat:
Dense, mixed-sex colonies within mangrove clearings in the mid to upper regions of inter-tidal mudflats. Often sympatric with U. signata.
Behavioural ecology:
U. mjoebergi are predominantly burrow-mating fiddler crabs. During the reproductive phase of the semi-lunar cycle, females leave their burrows to wander through the colony sampling different males, and finally, choosing one to mate with (Reaney, 2007; Reaney & Backwell, 2007a). Like in other predominantly burrow-mating species, U. mjoebergi occasionally employ surface-mating tactics, in which males approach burrow-owning females and mate on the mudflat surface (reviewed by Nakasone & Murai, 1998). However, unlike many other species of fiddler crab, the semi-lunar reproductive phase of Darwin populations of U. mjoebergi is restricted to the late neap tide (Reaney &
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Backwell, 2007b). U. mjoebergi is a highly territorial species and males use their large claws for signalling and as weapons during territorial disputes (Morrell et al., 2005). There is also evidence to suggest that males form coalitions with their neighbours to help defend their territories (Backwell & Jennions, 2004).
Claw-waving signal
U. mjoebergi uses a highly conspicuous lateral claw-waving signal for mate attraction and courtship, similar in form to the displays of other Celucan
species (e.g., chapters 4 and 5; Salmon, 1967). The display starts with an initial wave, in which the major claw is laterally unflexed, then lifted and dropped in a circular trajectory (Fig. 3.3B-F). This initial claw movement lasts 1.54 ± 0.17 s (n = 46 waves from 6 individuals). During this part of the display the claw manus is raised (Fig. 3.3A; for a guide to fiddler crab anatomy see appendix A) and the claw-tip is lifted well above the level of the eyestalks (Fig. 3.3D). This initial wave is typically followed by one to three similar waves of decreasing amplitude (two in Fig. 3.3).
The separation of the different motion components during the display is clearly represented in the kinetograph (Fig. 3.3B). The kinetograph starts with a small leftward motion, corresponding to the initial unflexion of the claw, and is then followed by the subsequent lifts and drops of the major claw and body in each iteration. As well as claw movements, the display is accompanied by body and leg lifts. The upstroke and downstroke motion maps (Fig. 3.3C) demonstrate that, from the perspective of a crab signal receiver, these body and leg movements contribute substantially to the overall motion signal. The minor claw is also raised during waving, but generally only during high intensity courtship signalling (note that the minor claw is not raised in the example of figure 3.3C). No territorial or agonistic claw-waving signals were observed in this species.
Figure 3.3. The claw-waving display of Uca mjoebergi. A) Photographs of an adult male. B) Kinetograph of an example claw-waving display. C) Motion maps of the upward (left) and downward (right) display components of the same display. D) Claw-tip path, E) elevation and F) vertical speed during the claw-waving display. Shaded area in D represents the resting position of an average-sized male of the species (according to George & Jones, 1982). Grey lines represent the mean for each individual and black lines represent the overall pooled mean. N = number of individuals and n = number of claw-waves contributing to the overall mean. Blue numbers in B, D, E and F indicate the peak of each of three declining waves.
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Uca perplexa (Milne Edwards, 1852) [latin: “perplexing” fiddler crab]
Distribution:
West Pacific: Thailand, Indonesia, Philippines, Taiwan, Papua New Guinea, Japan, Torres Strait, North-eastern Australia (George & Jones, 1982; Rosenberg, 2005; Nakasone & Murai, 1998). Current data from Bowling-Green Bay, Queensland.
Appearance:
Broad-fronted, relatively small fiddler crab (male carapace: 14.9 ± 4.2 mm, n = 75; female carapace: 11.9 ± 4.9 mm, n = 25; George & Jones, 1982). Both males and females have speckled-brown camouflaged bodies, and the male major claw is yellow, whitening at the claw-tip (Fig. 3.4A). During the reproductive phase, the frontal area of the male body bleaches white, providing strong contrast against the brown mudflat background.
Habitat:
Similar to U. mjoebergi. Mangrove clearings and open sandy mudflats in the upper regions of inter-tidal mudflats. Although in these locations the mudflat is relatively flat, pneumatophores from nearby mangrove trees often break up the visual horizon. Occasionally sympatric with U. polita, U. seismella and U. vomeris.
Behavioural ecology:
U. perplexa employs two different reproductive strategies (Nakasone & Murai, 1998). The predominant strategy is burrow-mating, in which wandering females are attracted by males to their burrow and mating occurs underground. Occasionally, males approach neighbouring females and mate with them at the entrance of the female’s burrow (surface-mating). U. perplexa differs from U.
mjoebergi in the timing of reproductive activity, confining mating activity to several days around the spring tide (How, personal observation).
Claw-waving signal
Similar to other Celucan species, U. perplexa uses a lateral claw-waving display in which the claw moves in a roughly circular trajectory (Fig. 3.4B-C, and D-F left). The display consists of three main stages, a fast lateral unflexion, a slow claw lift, and a fast drop of the claw back to the resting position (Fig. 3.4B, E, F, blue numbers 1-3; chapters 4 and 5; Nakasone & Murai, 1998; Murai & Backwell, 2006). Unlike U. mjoebergi, however, the display is not followed by subsequent diminishing waves. Figure 3.4D-F (left) illustrates the average structure of the lateral claw-wave signal. The entire display lasts 1.08 ± 0.30 s (n = 553 waves from 9 individuals), although signal structure and timing are known to alter according to the fine temporal and spatial context of the courtship interactions (see chapter 4). As in U. mjoebergi, at the peak of the wave the claw manus is lifted and the claw-tip reaches well above the level of the eyestalks (Fig. 3.4D). The display is accompanied by a series of leg, body and minor claw movements, which contribute substantially to the overall motion signature (Fig. 3.4C). Whilst the claw-tip trace is similar to the initial wave produced by U. mjoebergi, the kinetograph motion signatures are very different (compare Fig. 3.3B with 3.4B).
In addition to the lateral courtship waving signal, U. perplexa also use a vertical claw-wave, consisting of a fast up-down claw movement (Fig. 3.4D-F, right). Vertical waves are 10 times shorter than lateral waves, taking only 0.11 ± 0.016 s to complete (n = 59 waves from 8 individuals). This signal is frequently used in agonistic interactions, but also forms part of the close-range courtship display (see chapter 4).
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Figure 3.4. The claw-waving display of Uca perplexa. A) Photographs of an adult male. B) Kinetograph of an example claw-waving display. C) Motion maps of the upward (left) and downward (right) display components of the same display (Note that the male in this example is left-handed). D) Claw-tip path, E) elevation and F) vertical speed during the claw-waving display. Shaded area in D represents the resting position of an average-sized male of the species (according to George & Jones, 1982). Grey lines represent the mean for each individual and black lines represent the overall pooled mean. N = number of individuals and n = number of claw-waves contributing to the overall mean. For (D-F), left - lateral wave; right - vertical wave. Blue vertical lines in B, E and F delimit stages 1 to 3 of the lateral claw-waving display (for details see chapters 4 and 5).
3.3.2. Subgenus: Australuca (Crane, 1975)