Diversity and Distribution of Fiddler Crabs (Crustacea: Brachyura: Ocypodidae) around the Arabian Sea.

Nine species of fiddler crabs (Crustacea: Ocypodidae: Gelasiminae) are known from the Arabian Sea and adjacent waters (Red Sea, Gulf of Aden, Gulf of Oman and Arabian/Persian Gulf): five species of Austruca, one Cranuca, two Gelasimus and one Tubuca. COI sequence data match morphological species boundaries and shows high connectivity within each. The fauna is highly endemic, with three species of Austruca (A. albimana, A. iranica and A. sindensis) confined to this region, and four others restricted to the Indian Ocean. restricted to the Indian Ocean. Austruca albimana and A. iranica speciated locally and now have narrowly overlapping ranges in Oman. Our results confirm the westernmost distributions of Austruca annulipes and Tubuca alcocki are Pakistan and the Red Sea, respectively. A key for the nine species is also provided to help identification.

Fiddler crab electroretinograms reveal vast circadian shifts in visual sensitivity and temporal summation in dim light.

Many animals with compound eyes undergo major optical changes to adjust visual sensitivity from day to night, often under control of a circadian clock. In fiddler crabs, this presents most conspicuously in the huge volume increase of photopigment-packed rhabdoms and the widening of crystalline cone apertures at night. These changes are hypothesised to adjust the light flux to the photoreceptors and to alter optical sensitivity as the eye moves between light- and dark-adapted states. Here, we compared optical sensitivity in fiddler crab (Gelasimus dampieri) eyes during daytime and night via three electroretinogram (ERG) experiments performed on light- and dark-adapted crabs. (1) Light intensity required to elicit a threshold ERG response varied over six orders of magnitude, allowing more sensitive vision for discriminating small contrasts in dim light after dusk. During daytime, the eyes remained relatively insensitive, which would allow effective vision on bright mudflats, even after prolonged dark adaptation. (2) Flicker fusion frequency (FFF) experiments indicated that temporal summation is employed in dim light to increase light-gathering integration times and enhance visual sensitivity during both night and day. (3) ERG responses to flickering lights during 60 min of dark adaptation increased at a faster rate and to a greater extent after sunset compared with daytime. However, even brief, dim and intermittent light exposure strongly disrupted dark-adaptation processes. Together, these findings demonstrate effective light adaptation to optimise vision over the large range of light intensities that these animals experience.

Larder Hoarding Versus Immediate in situ Food Consumption in Two Fiddler Crab Species: Is it an Evolutionarily Stable Strategy?

Larder hoarding behavior in central place foragers has been widely studied in vertebrates, albeit, not as extensively as scatter hoarding. However, scant information is available for the invertebrate taxa, especially aquatic species. We investigated this phenomenon via an in situ food supplementation experiment in a community of two sympatric fiddler crabs, Austruca annulipes (n = 80; 40 males and 40 females) and Gelasimus vocans (n = 60; 30 males and 30 females), in a Singapore mangrove patch with an intermediate resource level. As the semiterrestrial intertidal crabs can only forage after emergence from their burrows during exposure period, the duration of time available for feeding is finite and constitutes an important constraint in the optimization of food intake. The activity budget (in terms of time spent on feeding activities, all above-ground non-feeding activities, and burrow-sequestration) as well as the occurrence (if any) of larder hoarding behavior in these two species after they first emerged were recorded by hourly intervals (three hours of observation) to determine the effect of time left for foraging on larder hoarding. Regardless of species, A. annulipes and G. vocans spent most of the time feeding when the tide was out, despite overall significant behavioral heterogeneity (multivariate analyses using ANOSIM) indicating that both species prioritized hunger satiation over other activities. Our results also showed that although the two sympatric crabs live in the same mangrove area with similar food resource levels, only A. annulipes larder hoarded. The propensity to larder hoard did not differ significantly between the sexes, nor among the three time periods of the feeding duration. Gelasimus vocans, one of the species of crabs known to form feeding droves, did not larder hoard at all. We propose that A. annulipes is a species that can deploy larder hoarding as a foraging strategy when it encounters valuable food resources, and such a strategy is highly advantageous for the species as it generally inhabits sandy habitats that are poor in nutrient levels. Hence, larder hoarding can be considered a mixed evolutionarily stable strategy (ESS) adopted by A. annulipes. In contrast, G. vocans that commonly inhabits muddy sediments-with a high level of food resources-did not larder hoard, even when provided with supplemented food, suggesting perhaps that its mixed ESS is droving behavior.

Diversity and Distribution of Fiddler Crabs (Crustacea: Brachyura: Ocypodidae) in Vietnam.

Based on recently collected material and records in the literature, 14 species of fiddler crabs (Crustacea: Ocypodidae: Gelasiminae) are reported from Vietnam. DNA barcoding analyses using the mitochondrial gene COI (cytochrome c oxidase subunit I) was performed to identify examined materials and their precise distributional range. Thirteen species-level taxa are identified and, with the exception of Galsimus borealis and G. vocans, have minimum interspcific divergences of at least 7.27%. The identified species include seven species of Tubuca Bott, 1973, three of Austruca Bott, 1973 and three of Gelasimus Latreille, 1817, and one Paraleptuca Bott, 1973. Two new records of Vietnam are herein reported: Tubuca rhizophorae and T. dussumieri. The southernmost distribution limits of East Asian G. borealis, T. acuta and T. arcuata are in northern Vietnam, A. lactea in central Vietnam, whereas northernmost limit of Southeast Asian T. rhizophorae and T. forcipata in southern Vietnam. A dichotomous key to identify the 14 Vietnamese species is provided.

First Zoeal Stage of 15 Species of Fiddler Crabs (Crustacea: Brachyura: Ocypodidae) from Taiwan.

Fiddler crabs (Brachyura: Ocypodidae: Gelasiminae) in Taiwan consist of 5 genera and 15 species, but knowledge of their larval development is limited to just 3 species, namely Austruca lactea, Tubuca arcuata, and Xeruca formosensis. In our study, the morphology of the first zoeal stage (zoea I) of the 15 species is described and compared to previous studies. The results show that the characters of zoea I can be used to distinguish the five studied genera and most species (except three groups, "Gelasimus borealis, G. jocelynae and G. vocans", "Paraleptuca crassipes and P. splendida" and "Tubuca arcuata, T. coarctata and T. paradussumieri"). The lateral spines on the carapace in zoea I are suggested to be a unique character in the Ocypodinae because they are absent in the Gelasiminae and Ucidinae, which supports the current systematics of the Ocypodidae.