Timing specific reproductive events according to changes in the environment often ensures the maximum survival of many organisms and their offspring. Thus, adapting to variable conditions in order to maintain the same timings is crucial to the fitness of the species. Fiddler crabs were chosen to examine the behavior of organisms in timing specific life events due to their well-known reproductive behavior and their abundance. The common timing of larval release of fiddler crabs was found to be during nocturnal high amplitude tides in order to minimize predation and maximize distribution. If incubation and developmental durations are affected by variations in temperature, larval release timings might be mismatched with the targeted event, resulting in lower viability. Variations in temperature also cause cold, nutrient-rich, deep source water to be moved to the surface in a phenomenon called upwelling, which also affects the survival of planktonic larvae. Two fiddler crab species, Uca terpsichores and Uca deichmanni, were studied in order to determine if and how they adapt to such variations in reproductive conditions in order to hit the optimal timing for larval release. By comparing the amount of predation of brine shrimp and larvae during day and night as well as upwelling, it was found that the risk of predation was higher during the day and during upwelling. Through monitoring courtship occurrences across a long period of time and logging the environmental conditions, it was found that U. deichmanni consistently courted at times matching high tides, regardless of temperature and upwelling, whereas U. terpsichores changed its courtship timings between locations and environmental conditions. To determine the ability to keep larval release timings, the incubation periods of U. deichmanni and U. terpsichores were monitored in various temperatures both in the field and in the lab. Although both species were found to have longer incubation periods in colder temperatures as expected, U. terpsichores had low synchrony in larval release timings, while U. deichmanni had synchronized and well-timed larval releases, but released late with very low temperatures. These differences suggest a difference in timing strategies, ultimately resulting in a difference in offspring survival and fitness.
a. The purpose of the research was to determine the ability of fiddler crabs to keep to larval release timings under variable conditions, specifically, by adjusting courtship timings, as well as to determine how two different species of fiddler crabs respond to a change in the environment, in order to maintain an optimal larval release timing.
b. The hypothesis tested by the researcher was that the timing of courtship relative to the timing of large amplitude tides will shift as a function of temperature, and it would change with a change in temperature from either different geographic locations, or different times in the year.
c. To determine the level of courtship at any given time, the researchers counted the number of males courting at low tide for both species. To determine the temperature, the researchers placed temperature data loggers in the sediments and in the water at the habitat of each species. This was done across two years, one being a weak upwelling year and the next being a strong upwelling year. The sampling was done at multiple sites at every low tide daily for six months, each year. In order to see how temperature affected courtship timings, wavelet coherences for courtship and tidal amplitude were studied, in the context of the temperature, to see how strongly courtship and tidal amplitude were correlated at different temperatures.
d. The main results show a strong correlation between tidal amplitude courtship across different temperatures from both years for Uca deichmanni, which indicated that the courtship timings did not change in accordance to temperature and upwelling. However, the correlation was much weaker, and became non existent at very low temperatures for Uca terpsichores at different sites. This indicated that courtship timings were changed in response to a change in temperature for U. terpsichores.
e. The conclusion of the study was that both fiddler crab species studied had their own measures to ensure optimal larval release timings. It was found that Uca deichmanni did not change its courtship timings with variable temperatures, yet still managed to have synchronised and well timed larval releases, while Uca terpsichores changes its courtship timing to adjust for the longer incubation period.
The researchers found that the optimal time for larval release was during the night and not upwelling by collecting data on the amount of predation during the diurnal cycle. By collecting data on the amount of courtship displayed during different times of the year, the researchers were able to find that for Uca terpsichores changed its courtship timings with a change in temperature, and that Uca deichmanni did not. Finally, by collecting data on larval release timings, the researchers were able to find that Uca deichmanni timed larval release very well, except for very low temperatures, which would cause it to release late. Researchers also found that Uca terpsichores released larvae with low synchrony under cold conditions. Thus all the results from the experiments supports the hypothesis that both species were able to adjust to a change in temperature and incubation conditions in order to hit an optimal larval release timing.
a. Phenology is the study of how the timing of events of organisms are affected by changes in their environment, such as climate, or physical changes to its habitat.
b. Organisms change their timing of events for maximal survival based on their environmental cues. However, sometimes when two related events are timed by different cues, those two events might become mismatched. As a consequence of the warmer weather, the oak trees buds burst earlier, and the caterpillars that feed on the leaves emerge sooner. But migratory birds, who use photo periods as a cue, times their arrival differently than the caterpillars, thus they will come at a different time than when the caterpillar population peaks. This mismatch of the two events (arrival of birds, caterpillar population peaking) is thus a consequence of different cues, and has a consequence in the fitness of the bird population as they will have less prey to feed on.
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One strategy that was not discussed in lecture 17 but is employed by the fiddler crabs is avoiding predators in time. Fiddler crabs make sure that the larval release is during the night, when it is shown to have the least amount of exposure to diurnal predators.
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In non upwelling conditions, water temperature was the major factor that affected the courtship timings and larval release.
a. Uca deichmanni would be more vulnerable if the variability in the timing of the upwelling increased. Upwelling can cause very low temperatures to occur, and in a upwelling with not a lot of variability, Uca deichmanni often release late because of the low temperatures. Thus if upwelling just occured very frequently, without an increase in intensity, Uca deichmanni would be more affected as they are vulnerable to very low temperatures. In contrast, if the temperature stayed very low, Uca terpsichores can just court very early, and have a very long incubation period and still release on time, making it less vulnerable.
b. Uca terpsichores would be more vulnerable if variability in the intensity of the upwelling increased. Because U. terpsichores cannot change its incubation length, it times for the optimal larval release timing by changing its courtship timings, thus any temperature variability during incubation will cause it to miss its larval release. An increase in the variability of the intensity of upwelling would mean temperature could get very cold, then become warm again suddenly, which would mean it would cause U. terpsichores to release early if it had started courting early to accomodate the initial cold temperature. In contrast, Uca. deichmanni can adapt to the change in temperature by thermoregulation done by the female, making it less vulnerable to such variability.